I \ I I Uf\L.nUL.

IC/T^/33IUTCERNAL REPORT

CLimited distribution)

International Atomic Energy Agency

and

United Nations Educational Scientific and Cultural Organization

INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

ELECTRICAL BEHAVIOUR 0? LAKGMUIR FILMS:

A R E V I E W *

V.K. Agarwal

International Centre for Theoretical Physics, Trieste, Italy.

MIRAMARE - TRIESTE

May

* To "be submitted for publication.

ABSTRACT

The electrical behaviour of thin films obtained by a variety of

processes, e.g. thermal evaporation in vacuum, have been extensively studied.

But the study of organic mono- and multilayer films obtained by the Blodgett-to

Langmuir technique (commonly referredAas Langmuir films) has gained considerable

momentum only during the past decades. Unlike in evaporated films, the

striking features of these organic films are their controllable thicknesses

down to one monolayer (™ 25 A) and the possibility of obtaining them free

from holes and conducting imperfections .The aim of this paper is to describe

the film deposition techniques, some of the .properties of the films so obtained

and to review their electrical behaviour. It has been intended here to make

this review a comprehensive and up-to-date source of information for the

workers vho are either already engaged in this field or planning to adopt

Langmuir films for future investigations. Nevertheless, for greater details,

to suit one's particular interest, original papers must be consulted.

In this survey, emphasis has also been laid on the possible problems

worth further study to get more insight into the basic properties of theselatter

films. Further, since the^ possess some interesting electrical properties,

this paper may prove useful in the assessment of our depth of knowledge about

them and in reducing the existing gap between basic research and technological

applications. Their potential usefulness in developing devices has also been

discussed. It is very likely that further exploitation of Langmuir films should

prove to be one of the productive enterprises for the advancement of thin

film technology.

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I. INTRODUCTION

Insulating thin films in the thickness range 100 A to 20 ,000 A have

been a subject of varied interests among the scientific community because of

their potential applied significance for developing devices, such as, optical,

magnetic, electronic, etc. Some of the unusual electrical properties

possessed by thin films which are unlike those of bulk materials, led to

think about their technological applications and,consequently, interest in thin

film studies grew rapidly. Earlier studies did not prove to be very inspiring

because the films obtained always suffered with the presence of pin holes,

stacking faults and other impurities, etc., and hence the results were not

reproducible. In the last few decades many sophisticated methods have become

available for the production and examination in thin films and reproducibility

of the results could be achieved and controlled to a greater extent. Never-

theless, the unknown nature of inherent defects in a wide variety of thin

film systems still complicates the interpretation of many experimental data

and thus limits their use in the devices.

In a recent bibliographical survey on breakdown conduction in thin

films the author found that the major subject of investigation have been

the films prepared by thermal evaporation under vacuum or such like methods.

It was thus realized that the Langmuir films, which are the subject of the

present paper, have remaine.d less known among the workers of this field. In

fact, films formed by oil on water surface vere known as early as 17T1* when, •2)

Benjamin Franklin reported their presence. Lord Rayleigh was the first who

proposed that these films were only one molecule thick. Rayleigh's hypothesis

of monomolecular spreading thus gave birth to a new "branch of science involving

3) Mthe study of physics and chemistry of surfaces . Subsequently, Langmuir

introduced the simple technique of transferring these monolayers onto a

substrate by moving the latter across the film-covered surface. This technique

of transferring the monolayers onto the solid surface by repeated dipping and

withdrawal process was later modified by Blodgett and Langmuir * and has

become the basis of a new field of thin film investigations. This novel

technique of "building up" mono- and multilayer films is commonly known as the

Blodgett-Langmuir (henceforth BL ) technique,and the films thus formed are

many times referred to as Langmuir films.

Using this simple technique, it is now possible to obtain monomolecular

spreading of a number of organic substances, such as, fatty acids and their

soaps, branched and unsaturated fatty acids, esters, dyes and many proteins,

etc. It appears that this is the only technique "by which it is possible to

deposit films on smooth substrates which are uniform, structurally oriented

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and whose thicknesses are accurately known and controllable. It is now

well established that films which are largely free from defects and other

conducting imperfections can "be obtained and a wide range of electrical

characteristics can be studied in a variety of organic materials in thin

film form. However, we shall here be concerned only with "solid" monolayers

in which the molecules are closely packed and which can be deposited on solid

surfaces. Among the many substances which form a stable monolayer, the

straight chain fatty acids and their metal soaps are used for most electrical

measurements. This is, probably, because the fatty acids precipitated by

calcium or barium ions in the solvent were used initially ' * and the

technique was perfected for them.

Although, in principle, any substance forming an insoluble;stable and

condensed raonolayer can be used for building up multilayers, it is not

necessarily so. For example, the molecules containing a hydrophilic group

such as -OH or -COOH at one end of the chain and a hydrophobic group such

as CH, at the other have been found to form good monolayers, the molecules

being arranged almost vertically on the water surface and becoming close

packed when the monolayer is compressed. But not all such substances behave

this way, If the molecule is too short, the Van der Waal forces along their

lengths would be insufficient to hold them together as a "solid" film and,for

long molecules, the buckling of the molecules themselves may give rise to a

crumpled film on compression. Therefore, only the molecules of -inter-

mediate chain length can be used to obtain an oriented close-packed solid

monolayer on the water surface. However, there are no clear cut criteria

for the choice of substances forming good Langmuir films. Reference should bev)

made to the excellent text of Gaines for the choice of material,and a

recent review of Srivastava is a comprehensive source of information for

beginners in this field. In addition, the reader is referred to several moreQ)-lM

reviews and texts that have appeared before and cover the vast subject

on such films.

Langmuir films were successfully prepared as early as 1935 "but 3Ome

of their dielectric properties studied in around the same period did not yield

reproducible results. Renewed attempts were made in the Sixties but it was

not until the late Sixties that reliable results could be obtained. This is

because many further advances in the original B-L technique have "been made

during the last decade to obtain a good film for reliable .electrical measure-

ments. A handful of experimental data on electrical behaviour of Langmuir

films during the last few years yielded some very interesting electrical

characteristics which will be discussed exhaustively in this paper. There

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is no '..-ther useful survey available in the literature covering an up-to-date

account of their electrical behaviour, Even the books and reviews published

recently or in the past on the whole subject of thin films do not mention the

work on Langmuir films. The one review devoted to Langmuir films carries

only limited ini'ormation about their electrical characteristics.

Essentially, the two crucial factors governing thin film properties are

their uniform thicknesses and structural perfection. Particularly, all the

electrical properties are very much sensitive to these parameters and thus,for

a judicious experimental study, one requires films of accurately known thick-

ness and free from gross defects. The Langmuir films, which concern us in

the present paper, are known to fulfill these requirements to a far greater

extent than other types of thin films. For instance, Mann and Kulin • have

recently shown through their electrical studies that the films obtained by

the modified B-L technique were free from even a small fraction of holes and

conducting imperfections. Secondly, the uniform thicknesses of these films

are accurately known,as measured by standard optical methods like Tolansky's

multiple beam interferometry, * and may be controlled and reproduced

successfully to a lower limit of one molecule thick (about 25 A ). In addition

to many other features described in the text of the paper, these two constitute

a sound basis for studying these films. Control of these two parameters in

other film systems is notoriously difficult to achieve.

To fulfill the aim of this paper, the film deposition technique of

Elodgett and Langmuir will "be discussed in detail. The recently proposed1 o \

evaporation technique to obtain such organic films down to one molecule

thick and with better purity than the evaporant, will also be described. The

physical properties of these films, which are relevant in the context of their

electrical behaviour, are described in the next section. Each of the following

sections covers a particular aspect .of the electrical behaviour and carries

a detailed account of the experimental and theoretical work (if any). To

make this survey more substantial, it has been thought desirable to discuss

briefly the existing theories about conduction mechanisms, "breakdown

behaviour, etc.,in the respective sections. This vould certainly prove useful

for the better understanding of the subject and may further help the researchers

to select out some suitable problems for their studies on Langmuir films.

Nevertheless, efforts have also been made to point out some of the possibilities

for future investigations on such films which require the specific attention

of the workers.

-k-

For the very fact that many reviews and "books have already been published

on the -whole range of electrical measurements in thin films, this paper is

devoted to Langmuir films only. However, to adopt a broad viewpoint, some

pertinent references have been cited just to make a comparison between the

results on Langmuir films and on other film systems. To summarize for a

wider readership and to make the survey an up-to-date source of information,

all the possible references since 1937 (when the earliest work on electrical

properties was reported) have been provided.

In the end, some space has been reserved to discuss briefly the future

potential of these films in the development of devices. In spite of the fact

that further electrical studies on Langmuir films would prove useful for a

"better understanding of the underlying electrical phenomenon in thin films,

it is also very likely that the use of such ordered film systems in developing

devices should lead to a major "breakthrough for the advancement of thin film

technology.

II. FILM DEPOSITION TECHNIQUES

1. Blod^ett-Lanamuir technique

This technique requires a very -simple apparatus consisting of a

long narrow trough T (Fig.l) usually made of inert materials such as teflon*

perspex, etc. Metals such as copper or aluminium are not used "because they

may contaminate the solution with metallic ions whose presence would stop the

deposition. The inside of the trough is heavily waxed. Waxing of the

trough is necessary in order to obtain a higher water level at the edges,

which ensures that there is no leakage of the monolayer past the barriers.

The barriers, rectangular in shape, such as glass slides, must also be

waxed all around to render them hydrophobic. The trough is first levelled

and then filled to the brim with doubly distilled or deionized water (specific

resistance ~ 6 x 10 ohm cm) having a low concentration of barium by adding

3 x 10 ^ M barium chloride or such like. For easier deposition, the pH of

the solution is adjusted to greater than 6 by the addition of a specific

quantity of an alkaline substance. For example, KHCO- at a concentration of,

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k x 10~ M , thus holding the pH at 7-2. This results in smooth deposition

of Y-type films. The surface of the liquid is now thoroughly cleaned by

sweeping the waxed Carrier B across the long edges of the trough and finally

leaving it resting on the trough end (Fig.la). Another Carrier, A ,rests at

the other end of the trough which is used to hold the vaxed silk thread S

placed upon the water surface as shown between A and E . The thread is

vaxed so that it does not vet and sink into the vater and it is carefully made

to touch the vater surface at every point to avoid any possibility of leakage

of the spread monolayer. The technique here is specifically described for

barium stearate films and must be followed similarly for all other substances

eligible to form Langmuir films.

Stearic acid (or other material to be deposited) dissolved in benzene

(concentration = 3 x 10 by weight) is now placed by the micrometer syringe

on the vater surface near D. in the form of a small droplet vhich spreads

spontaneously and pushes the thread outwards to take the shape as shown in

Fig.lb. (if the first drop does not spread quickly, it is an indication

that the surface is not perfectly clean.) As soon as the spreading is

complete, the thread is fastened to the trough edges by two small clips C .

At this stage the divalent barium ions present in the solution undergo surface

reaction with stearic acid molecules to form the barium stearate soap

and the solvent (benzene) slovly evaporates leaving behind a monolayer on the

surface, its boundary being confined by the thread. If a second drop is put

near D after the spreading is complete, it would not spread and remain as

residue vhich is an indication that the available surface has been covered

with the monolayer. The stearate molecules at this stage remain standing

upright with their reactive carboxyl groups touching the water surface but

some of them also tend to bend over the water surface due to the relatively

large space available for them. A Email drop of oleic acid called "piston oil"

is now placed on the surface at P which thus exerts a constant lateral

compression of 30 dynes/cm on the spread monolayer. It has been found that

this much surface pressure ( <v 30 dynes/cm) is sufficient to compress a mono-

layer of stearic acid into a solid phase and the molecules remain relatively

closely packed. The thread boundary nov separates the whole surface area

between barriers A and B into two regions with a stearic acid monolayer

on one side of the thread and oleic acid on the other, which press each other

to give an equilibrium shape to the thread (shown in Fig.lc). The transfer

of monolayer onto the"conditioned" slide may nov be accomplished by moving the

dipper near D up and down repeatedly across the monolayer covered surface.

"Conditioning"of the slide here means to render it hydrophobic initially. This

••• nn be achieved simply by rubbing the slide surface with a waxy solid, e.g.

ferric stearate has been found satisfactory. It has "been found that a'pure

aluminium or1 silver film deposited on the cleaned slide by thermal evaporation

also behaves as a hydrophobic surface for good multilayer deposition.

The monolayer is thus transferred on the "conditioned" slide on both

its downward and upward journey, and the process may "be continued till the

thread S moves forward through an area to attain the shape shown in Fig.Id.

Every time the thread moves forward through an area equal to that of the slide

on which the film is deposited and the thread motion takes place because of

the constant compression provided by the "piston oil". This visible motion

of the thread is a very clear cut indication that the layers are being trans-

ferred on the slide every time on dipping or vithdrawal of the latter. The

film thus built up is termed Y-type and contains an even number of layersaccurarely

outside water. Film thickness can then be calculated/by counting repeatedly

the number of monolayers and multiplying it with the known monolayer thickness.

To obtain an odd number of monolayers outside the water surface, it is necessary

first to dip the slide in the solution and then to spread the stearic acid in

the stage shown in Fig. lls. In this case, the first layer will be transferred •

when the slide is moved upwards and finally an odd number of layers will be

obtained. Even to obtain one monolayer, this process has been found satisfactory

by the author in his studies with these films. To obtain X type of films,

it is necessary to make the solution more strongly alkaline (pH = 9) and then

the transfer would follow only during the down trips of the slide. It is

necessary to maintain the ambient temperature at 20 - 22 C during experi-

mentation and the substrate should be raised or dipped across the monolayer

very slowly and smoothly. To obtain a large number of layers on a solid

substrate, i.e. thicker filons , the author has found that it is not convenient

to use a trough of large dimensions and thus a large surface area for monolayer

spreading. This is due to the perfect cleaning of the surface becoming more

cumbersome as well as the molecules developing some tendancy to collapse. More

satisfactory results have been obtained by using a trough of lesser dimensions19)

{say 29 x 20 x 6 cm, as used by Nathoo yi) and using a,fresh solution after

every deposition cycle is completed. It is also advisable to

prepare the solution once in a large quantity to avoid any possible changes

in its pH value, etc., which may affect the nature of monolayer deposition.

The technique described above appears attractively simple in principle,

however,it requires one's scrupulous attention and experimental skill to obtain

structurally stable and smooth filma. It is therefore necessary to mention

some of the important precautions to be taken during experimentation,and these

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follow. For necessary details references should further "be made to the

text "book of Gaines and original papers of Blodgett and Langmuir

One can also make use of commercially available highly sophisticated film

balances for film deposition.

a) List of precautions

i)The wax and chemicals used in the experimentation must "be of highest

grade quality and of extreme purity. Chemicals with E. Merck grade have been

found satisfactory. For still higher purity, the solvents (benzenej etc.)

should be fractionally distilled.

ii)The water used should be good quality conductivity water. For best

results deionized water should be treated with KMnO, (10 m) and NaOK (10 m)

and after heating it for about 10 hours at 70°C, it must be doubly distilled

in a quartz apparatus, It must "be remarked here that deionized water, if

used directly, may contain some organic impurities.

iii) Greatest care must be taken against dust contamination or the intro-

duction of any foreign material from the outside which may cause disruption

of the film structure. It is thus necessary to cover the whole experimental

set-up with a suitable transparent shield box and to design it in a way that

all the operations needed during experimentation, e.g. the movement of the dipper,

be operative from outside.

iv) Any sort of mechanical vibrations or other disturbances during

deposition may result in the cracks in the deposited film giving rise to voids

and inhomogeneities in the film, For this, the experiment should, preferably,

be performed on a rigid foundation or,if a table is used, antivibi-ation

mountings must be fixed to the four legs.

v)The specific concentrations of the substances must he added in the

bath solution to hold the required pH value of the solution. Any change in

its pH may markedly affect the nature of the deposited film, Specifically,

barium ion concentration should be properly maintained to avoid the presence

of free stearic acid molecules in the deposited film.

vi)Since the hands of the workers are themselves the most likely source

of greasy contamination, it is necessary to clean them as thoroughly as

possible. Specific attention is also required to avoid any surface active

contamination.

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vii) Th<? film transfer should "be accomplished under as high a lateral

compression aa possible. The higher the surface pressure, the more compact

will the film be. Nevertheless,the use of oleic acid as "piston oil" ensures

a satisfactory film deposition without any voids in it.

viii) The first mnolayer should be transferred as slowly as possible to obtain

a smooth surface, because it acts as the foundation stone for "building-up of

the multilayers. It is also to "be observed that the slide emerging from the

water is completely dry "before being re-immersn:.

Since the whole phenomenon is on a molecular level, even small con-

centrations of impurities or contamination in any form may affect the results

markedly. Therefore, conclusively the key-words in the preparation of Langmuir

films are the purity and cleanliness of all the components used, as is in the

case of some surgical operation. It would be "best for greater details to refer19)to the recent work of Kathoo who has described at -length the preparation of

Langmuir films for electrical studies. One novel feature which has been19)introduced by Uathoo is the use of a device to raise and lower the substrate

across the monolayer covered surface. The PTFE piston used for the device is

pulled upwards by suction of a pump and falls dovn under gravity when the

suction is reduced appropriately. This device could control the speed of the

dipper in the range 0.1 mm/a to 1 mm/s.

b) Selection of substrate and cleaning procedure

Obviously, one of the major factors governing the degree of uniformity

of the deposited film in the above process, is the smoothness of the substrate

used. Therefore, considerable attention must be paid to the selection of a

substrate with smooth surfaces and subsequent thorough cleaning to minimize

the appreciable variations in the electrical properties. Microglass slides

having no scratches are first selected and rinsed thoroughly with deionized

distilled water. The planeness and the smoothness of the selected slides is

then checked by the standard method of matching their surfaces vith a master21)optical flat . On proper illumination, the formation of reasonably straight,

equidistant, parallel and smooth fringes show that the slide surfaces do not22)

have much curvature and are almost plane , The smoothness of such selected

microglass slides can be of a much higher degree than that of the usual optical

flat . This is, for example, revealed by the absence of "wriggle" in the

fringes of equal chromatic order (FECO) in extensive film thickness measurements

on a k layer "barium stearate film using "Gold Seal" micro-slides. The author

himself has used "Gold Seal" microslides in the electrical measurements and

found them satisfactory for obtaining uniform Langmuir films.

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The selected microglass slides are then treated vith chromic sulphuric

acid (cold saturated), rinsed vith doubly distilled water and allowed to remain

in a caustic soda solution for several hours. Finally» these are thoroughly

washed using a strong jet of doubly distilled water and dried by a current of

warm air. Ultrasonic cleaning with Ultrax cleaning fluid and then washing

with distilled water, also results in satisfactory cleaning. Both of these23)

methods are used "by Bucher et al. in their work for building up of the

monolayers of dye stuff - arachidic acid. The degree or cleanliness of a

slide may then be checked by immersing it through a cleaned water surface

covered with a small amount of talcum powder> lycopodium or such like.

Presence of any greasy contamination on the slide would be detected by the

grains of talcum being pushed away while the slide is immersed. This simple2*0test was devised by Langmuir and may also be applied to control the

cleanliness of all the glass components used for the experiment. Absence of

any "breath figures" on the slide after highly breathing on it is also a good

test of the,cleanliness.

2. Evaporation method

18} ,The recent attempt by Baker to deposit thin films (.of the order of

one monolayer thick) of organic solids by thermal evaporation in vacuum opens

up a new field in respect of organic films. • It was reported "by Luff and25)

White that thermal degradation of high molecular weight materials takes

place during vacuum evaporation and thermal hazard is much lower in the case

of lower molecular weight solids. Thus, it was realized that thermal de-

composition of stable compounds is less likely to occur. Making use of these

arguments, Baker was successful in depositing thin films of stearic acid

(mol. wt — 28*1.5) and melissic acid (mol. wt 1+66.8) by thermal evaporation

in vacuum. The unchanged composition of both the evaporated films and the

evaporant residue thus evidenced for the absence of thermal degradation in

this case. He could obtain very thin films of the order of one monolayer in

thickness by maintaining a slow controlled rate of evaporation and other

experimental requirements. It has also been pointed out "by him that the

purity of the remaining evaporant and evaporated films was slightly higher than

the originally used acid salts and has ascribed it to be due to the "outgassing"

of the acid prior to deposition. This is a remarkable achievement indicating

the absence of any foreign impurity in these organic films unlike in the case

of other materials evaporated thermally. Whereas, this technique is of

particular interest in the field of tribology because of the possibility to

produce a monolayer film of boundary lubricant, the author feels it to be of

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equal importance for the physicists to study the electrical characteristics of

such evaporated films. Although, Baker has not mentioned explicitly the

defect nature of these evaporated organic films, the absence of thermal de-

composition and reported "better purity are, evidently, the two important

features. None of the other works on evaporated film systems seems to have

reported these features. After this successful attempt, undoubtedly, future

workers will introduce many more refinements to obtain evaporated films of

several other organic solids and studies of their electrical behaviour would

reveal many hidden facts.

III. PHYSICAL PROPERTIES OF LANGMUIR FILMS

Many interesting physical properties of Langmuir films have been in-

Vestigated and these have been reviewed well in the existing literature

In this section, only those properties which have relevance in their electrical

behaviour are discussed briefly. Nevertheless, it has been thought desirable

to introduce this section because some of them require specific attention of

the workers in future investigations.

1, Nature and orientation

The nature and orientation of the deposited film is mainly governed by

the angle of contact between the monolayer covered solution and the solid

surface and hence the condensed monolayers may be obtained with different

molecular orientation, at will. These monolayers are characterized as X,

Y or Z type and their molecular arrangements are shown in Fig.2. The

molecules in the case of X and Z type of films are oriented in the same

direction and thus the surface of the film will be composed of a carboxyl

and methyl groups, respectively. On the other hand, in a Y type film, the

molecules in adjacent layers are oriented oppositely and the film surface is

composed of methyl groups. Of these three types of films, the one which has

been studied is the Y type in which the monolayer transfer takes place "both

ways, i.e. on each dipping and withdrawal of the slide across the surface.

On the other hand, in X films, transfer takes place only when the slide is

being dipped and,for Z type, it takes place only when the slide is withdrawn.

The orientation is termed "exotropic" when the methyl groups touch the solid

surface and the carboxyl groups remain away from it in the first monolayer. If

the orientation of these two groups is reversed, the monolayer is termed

"endotropic". Thus, an X type film is made up of a aeries of exotropic

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layers; Y type is made up of a series of alternating exotropic and endo-

tropic layers and the Z type is made up of a series of endotropic layers.

However, it must be pointed out that Z type films are rather uncommon.

2. Uniformity and thickness

Two crucial factors for making satisfactory electrical measurements on

films are their uniformity and accurately known thicknesses and both of these

requirements are fulfilled to greater extent if one -uses Langmuir films.

Metrical thicknesses of several fatty acid monolayers have "been measured using

the best known optical methods. For example, the significant contribution in

this context has been due to Srivastava and Verme , They have

employed Tolansky's standard multiple beam interferometric • technique with

fringes of equal chromatic order (FECO) which ensured absence of any error

in their measurements due to surface irregularities. They thus obtained

extremely sharp and smooth fringes and determined thicknesses with high

accuracy. Subsequently, "C" spacings of these films vere also measured by26)

the same workers using microfocus X-ray technique and an excellent agreement

was found between C spacing values and those obtained by interferometry.17)Recently, Scott and Sirhata have used the above technique with some

17)modifications. They employed a protective collodion layer in measuring

thicknesses which improved the quality of fringes and hence the precision of

measurements. Use of the protective layer thus avoided the damage of the

monolayer in vacuum which is expected to have a marked influence on the27) 28)

overcoating deposited. In fact, it was indicated * that over 60% of

stearic acid film is removed by exposure to vacuum for 30 minutes.

As far as the uniformity of these films is concerned, it has already

been discussed that the selection of substrate must be made carefully. How-29)

ever, it has teen shown by Holt that increasing the number of monolayers

increases the degree of uniformity of the film,and many subsequent workers

have also noticed this fact. To achieve the required uniformity of the film,

the deposition of the first monolayer is very important since any voids or

imperfections in it, may lead to major disruption of the subsequent layers.

Therefore, one must make sure that the slide emerging from the solution is

completely dry before being re-immersed.

Whereas the films could be built containing as many as 3000 layersin the case of barium copper stearate simply by repeated dipping and with-drawal processes , some problems vere encountered when thick multi-

acid 6)layers of other fatty^salts vere deposited. Blodgett and Langmuir noticed

"fogged" appearance of thicker barium stearate films (300 layers) and also

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observed the "cracking" tendency of these films which increased with increasing

thickness. They then indicated that addition of copper ion concentration

(ft 2 x lcf M) in the solution checked both these difficulties and facilitated

film deposition. Other factors which seem to crop up sometimes during

deposition are the gradual crumpling or collapse of the stearate monolayers.

Henke avoided them simply by spreading a fresh monolayer over the solution

after every forty or fifty dippings,which has already been recommended in the

film deposition technique for obtaining a large number of monolayers. However,

in the opinion of the author, the collapse behaviour of stearate monolayers

is very unlikely to occur. Otherwise their use in the commercially available(c.f. •Ref,7j

step gauges, whose functioning is based on the known and controllable

thicknesses of the monolayer, would have been impossible.

3. Film structure

The obvious choice for investigating the detailed film structure is the31)well-known electron diffraction technique. Germer and Storks , particularly,

investigated the molecular arrangement of deposited mono- and multilayer

films of fatty acids and their soaps using this powerful technique. In their

studies in transmission as well as in reflection by depositing films on thin

supporting organic foils of Resoglaz and on a clean metal surface, the

molecules were found to stand almost perpendicularly to the solid surface in

the first monolayer. Barium stearate molecules had a more precise normal

alignment than the stearic acid molecules. The hydrocarbon chains of

stearate molecules in the first monolayer were close packed but irregularly

arranged on a clean metal surface whereas regular arrangement waE observed

in layers on top of the first one in case of films on Resoglaz foils. Thus,

the layered structure of these stearate films was shown to form simple

hexagonal crystals with the symmetry axis perpendicular to the film. The

stearic acid molecules were found to form monodinic crystals with their

chain axis nearly in the plane containing the surface normal and the dipping

direction. In earlier studies ' , it has been shown that these films

consist of superposed sheets of oriented molecules and form positive uniaxial

birefringent crystals with the optic axis perpendicular to the plane of the

film. However, some structural changes due to thermal disorientation of the

molecules or to evaporation were noticed at relatively high temperatures vhen

the temperature was raised in electron diffraction studies . This

might be possible because the soap multilayer films are relatively soft and29)

have,low melting points. On the other hand, in recent investigations of Holt

remarkable thermal stability of such films has been reported. In fact, these

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films could be heated at 90 C in damp air over prolonged periods (the melting

point of stearic acid is 69°C) without any damage. Further, Holt observed

no ehraga in th« electrical prop«JrtlM «vari though tna film* v«r« r«p*»t*aiy

and rapidly cycled from liquid nitrogen temperature to 50 C.in air.

Based on the above structural information obtained by electron

diffraction technique, these films were known to be monocrystalline in nature

and thus regarded as a special case of a layer-by-layer mechanical growth

forming almost "two-dimensional" crystals. There is, however, evidence that

barium behenate multilayers do, in fact,show an absence of crystallizationg

which has been demonstrated by electron micrographic studies . It has37)

also been reported by Khott et. al. that the formation of crystallities

may require some time after multilayer formation and no crystallities were

observed until 30-60 minutes passed. Mare and Messier have attributed

the two absorption peaks,observed at -100°C and -30°C, to dipole moments in

the amorphous and crystalline regions of the behenate layers, respectively. In

fact, they had no clear cut evidence for amorphous zones in the monolayers

and the qualitative interpretation offered by them was "based merely on the

assumption of disorganized structures. Therefore, the author would like to

stress here that in future investigations one must be careful in the inter-

pretation of the electrical data in terms of the film structure. Renewed

structural investigations would also be desirable to make sure of the presence

of amorphous regions in these films.

k. Defect nature

Many of the earlier workers have reported that organic filma obtained

by the classical process of monolayer transfer always contained holes, cracks39)

or such like imperfections. For instance, Epstein demonstrated in the

electron microscopic studies that the deposited films are discontinuous and

consist of clumps of molecules, when the transfer was accomplished at low1*0)

surface pressures. Similarly, Ries and Kimball observed discontinuous

circular "islands" or aggregates in the stearic acid monolayer transferred

at 10 dynes/cm. These observations were not surprising because the higher

the surface pressure, the more compact will be the film formed. In fact,

the monolayers have been shown to be continuous and homogeneous at higherhi)

pressures . Nevertheless, it would be an oversimplification to regard

the film transferred at high surface pressure as perfectly uniform, coherent

and defect free. Artifacts are introduced if proper care has not been tafcen

either during transfer processor the subsequent thermal evaporation of metal

electrodes over the film may disturb the film structure. In some radio-

autographic studies, the presence of spots, striations and other irregularities

like "folded over" regions in the deposited "barium stearate monolayer was2T)demonstrated in the earlier work of Roberts and Gains . On tha other hand,

1 )Handy and Scala also carried out radioautographic studies and have shown

that these layers are generally uniform with no apparent gross defects. One

of the reasons for the presence of one or the other type of defect in the

films demonstrated by these early workers is that the original technique

of B-L could not be perfected until then. Many of the recent modifications

in this technique have made it possible to obtain films largely free from

gross defects or imperfections -

The one work giving quantitative proof of the absence of even a small

fraction of holes and conducting imperfections in such monolsyers is that of Mann

and Kuhn • They had employed a modified B-L technique and took other

precautions. In their studies on "breakdown phenomenon of Langmuir films

(described later) Agarwal and Srivastava have also indirectly shown the

absence of "weak spots" and thus they inferred that the films are free from

gross defects. The development of striations in the deposited film was

ascribed to the irregularity of dipping and withdrawal motion of the sub-

strate . Recently, Nathoo has introduced a novel technique for this

and succeeded to obtain acceptable and reproducible devices for electrical

measurements with the use of this technique and other precautions that were ,

discussed above.

In spite of the fact that structural perfection may be achieved by

using Langmuir mono- and multilayers unlike in the evaporated film systems,

much attention must s t i l l be paid to identify the defect

structure of the films. Reliability and interpretation of electrical data

very much depends on the nature of defects and therefore special care must

be taken. As has been pointed out earlier,-tie deposition technique requires

one's experimental skill and enginery to obtain a structurally stable and

uniform film.

5. Skeletonization process

It is an established fact that the films of metal salts of fatty

acids or such like may contain free acid molecules, apparently in the solid

solution if the metallic ion concentration is low; their proportion depending

upon the pH of the solution ' . These free acid molecules can be

dissolved out of the multilayers by soaking the film for a short time in

benzene to which 1% of 90% ethyl alcohol is added . This process is

termed "skeletonization" and the film thus obtained is said to be

-15-

"skeletonized °} containing voids or holes in the places previously occupied

by the acid molecules. Optical studies have shown that the films remain

optically clear even after this treatment if the amount of acid molecules

contained is not more than 6o^but a striking change is observed in the inter-

ference colour reflected by the film. Further, it haa been shown that this

change in colour is due to a change of refractive index and not to a change

of thickness. If half of the acid content is dissolved, there is found to

be less than 1% of change in the film thickness. The stability of such a

skeleton film is not affected as indicated by its extraordinary rigidity even

if it contains as much as ^0$ of voids. The proof that skeletonized film did

not collapse was checked by Race and Reynolds who could restore the

original optical thickness by refilling the free spaces with some non-polar

oil. In fact, these holes and voids in the skeleton films may be refilled

by substances like petrolatum or other hydrocarbons; subsequently, if re-

guired, these substances can be dissolved in benzene giving again the same

skeletonized films. This method has already been used for getting the films?n \ much

with various refractive indices between 1.18 and 1.51. However not; attention

is paid to the study of electrical behaviour of such skeleton films carrying

foreign impurities like hydrocarbons. The author feels that the

method is very attractive for obtaining doped-like organic dielectrics as

is well known in the case of silicon, etc., and further exploration of their

electrical properties would "be worthwhile. Long back, Race and Reynolds 'the

made an attempt to study/electrical behaviour of such skeletonized films and

found the expected lowering of dielectric strength by skeletonization. The

concept of the physical structure of skeleton films, i.e. the remaining soap

and air (or any hydrocarbon substituted) are electrically in parallel, was

found to hold true in all the electrical measurements.

As discussed above, the removal of free acid from the deposited films

may also take place during their treatment in vacuum for electrode evaporation.

This process is known as "vacuum skeltonization". In fact, Roberts and Gains2*^' '

had noticed such removal of stearic acid from monolayers deposited on

various substrates. Evidently, the presence of voids or holes as reported

by many workers may be an effect of vacuum skeletonization. Many workers

failed to obtain useful devices for electrical measurements and it is presumably

because the spaces previously occupied by the free acid molecules might have

been filled with the metal during thermal treatment in vacuum and thus

shorting the device. Because of this fact, it is often stressed in the

literature that Langmuir films must be treated carefully in vacuum, particularly

when • electrical measurements have to be made. However, great caution must be

taken to hold the required pH of the solution because the proportion of con-

version to soap very much depends on it. Since the soap or metal salts of long

chain/acids are ionic substances forming stable monolayers (cf. Ref.7, p.193)n

there is a possibility that some ion content is present in the deposited mono-

layer which might get transferred during deposition. It has been shown

that with some cations, the ion composition of the deposited film is very

different from that on the water surface but much is not known about the

nature of such ions. The»presence of such ions and their motion under

applied electrical field has recently been pointed out by Barraud and co-1*5)

workers in their studies of thermally stimulated currents on MIM structures

with Langmuir films. A detailed study of the nature of ions and their

influence on electrical properties has to "be carried out in future works.

IV. DIELECTRIC PROPERTIES

Interest in the dielectric studies of Langmuir films of fatty acids

and their metal salts grew rapidly in the period just after the advent of their .

deposition technique. After that» such studies were taken up after a gap of

more than two decades when it was realized that Langmuir films can be uniform

in thicknesses and act as a good insulator. In this section the experimental

measurements on resistance, capacitance and dielectric constant of these film

systems are described. These measurements have been divided into two parts;

the first describes the work carried out in the late forties and the second

includes recent experimental work. Further* the dielectric loss measurements

and the theory for calculating the static dielectric constant are discussed,

and the readers are reminded to attach special importance to both of these

aspects in future works,

1, Earliermeasurements on capacitancet resistance and dielectric constant

The earliest work of Porter and Wyman (PW) reports the measurements

on impedance of films and related phenomena which they carried out on films

of Cu-Ba stearate and Ca stearate using both X and Y type of films. Initially,

mercury droplets were used for small area probe measurements and an a.c. bridge

was used for impedance measurements. The resistance of the films was found to

"be very low ( <£ 1 ohm) with high signal voltages, whereas it was of the order

of megohms with signals of 1 or 2 V. Ehe results were not satisfactory and

they then replaced the a.c. bridge by a radiofrequency bridge. The measurements

at frequencies of 1 MHz and 0.21+1* M H Z and for films containing

-17-

7 to lUl monolayers determined dielectric constant values ranging between 1.9

and 3.5 with an average value of 2.5. Considerable variation in the results

obtained on the same film vith different drops, or with the same drop in

different places, was due apparently to differences in the droplet size or in

the exact area of contact between the drop and film at different places. Ho

significant difference was found in the capacitance or resistance values between

X and Y films. In both types of film the capacity decreased with thickness,

as was expected from the relation used by them for capacity determinationj i.e.

in which 6 denotes the dielectric constant* TJ the number of layers, d

the layer thickness, and A the area of contact between drop and film. On

the other hand,the values of the resistance per layer showed a definite in-

crease with the thickness of the film. The specific resistance of films,

thus determined from their values of the resistance per layer, was about 10

ohms, having regard to the size of the drops and the thickness of the layers.

Some d.c. resistance measurements were also made by them using a drop of tap

water in place of a Hg drop,and a striking difference between the results was

found. With mercury drops the film ruptured permanently when a breakdown

voltage of 10 V cm was exceeded and the resistance was low. Whereas, no

such rupture was observed with water drops. Thus it was concluded that the

different affinities of the two liquids for the surface may be responsible

for the observed difference and probably the water molecules were able to

penetrate the films. Another conclusion arrived at by PW was the evidence

of a phenomena of power absorption in the films, increasing with frequency

over the range studied. This, in fact, was thought because the resistance

of the films measured at both frequencies (l and 0.2HU MHz) was far

less than^they determined from their d.c. measurements.

1*3)Race and Reynolds (RR) made some preliminary attempts to measure

the dielectric properties of barium stearate films in 1935 and found some

experimental limitations. For instance, the "bridge equipment used was not

sensitive; the area of contact of mercury droplets was not accurately

measured and the dust contamination could not be avoided, nevertheless, these

results encouraged them to develop a sophisticated experimental set-up and

necessary bridge equipment . Multilayer films were deposited on a clean

polished chromium slide and mercury droplets were used as the counter-electrode

All the measurements were made over a frequency range from kQ to 10 Hz '

using comparatively thicker multilayera ("both of X and Y type) of various

stearates and arachidates. The results obtained "by them are summarised below:

-18-

A) The dielectric constant of various multilayers was * 2.55 with standard

•variations of 3$. The variation was attributed more to the pH of the. solution

and therefore the proportion of acid to soap. .

ii)Dielectric constant was found independent of thickness as well as of

frequency in the range studied.

h3)Further, these workers measured the dielectric parameters of films

which were soaked for one to ten seconds in a solvent such as benzene. The

benzene dissolved the free acid leaving the soap film as a skeleton with air

filling the space previously occupied "by the acid. For calculating the

dielectric constant of such a skeletonized film, the remaining soap and the •

air spaces were considered in parallel. The relation thus used vas

• \

where € Q and e^ are the respective dielectric constants of unakeletonized

and skeletonized film; e = 1 is that of air and d,/drt- is the ratio ofa 1 0

densities of skeletonized and unskeltohized films. Their results on un-

skeletonized (0) and skeletonized (l) films of cadmium arachidate are given

in Table I. In the table, t and t. refer to the apparent optical thick-

nesses after and before skeleton!zation determined by Blodgett . To check

the collapse behaviour of the skeletonised films, the air space was refilled

by allowing a drop of oil to run over the film and the dielectric constant

was measured again and calculated usingEq. (k.2)by substituting the dielectric

constant of oil for e . They used non-polar oil as well as a polar liquid

(tetrachlorodiphenyl) to refill the air space in the skeleton, the corresponding

values of the dielectric constant have baen represented by € and € ,

respectively, in Table I.

As is evident from Table I, the agreement "between measured and cal-

culated values of e. is sufficiently close. Obviously, the dielectric

constant of the film (€_) increased, as is expected, whenthe air spaces in

the skeletonized film were refilled with a non-polar oil (columns 8 and 9)

having dielectric constant of 2.15. This was interpreted to constitute

a check on the structural conception of a skeletonized film, i.e. the

assumption that the remaining soap and air are electrically in parallel

(columns 6 and 7). However, considerably higher calculated values of £

than the measured one (columns 10 and '11} are found with a polar liquid

{dielectric constant 5.22), which were interpreted as indicating a reduction

of polar orientation of the liquid within the matrix of cadmium arachidate

molecules.

-19-

Buckwald, Zahl and co-workers -7'*' also became interested in the

measurements of the resistance and capacity of alkali stearates consisting of

varying numbers of layers and deposited on metal surfaces. Preliminary

measurements were made on barium stearate films " and,later, calcium stearate

films were chosen for studies. A special cell was designed by them consisting

of a cylindrical glass vessel partially filled with calcium chloride solution

or such like. Two metal electrodes were immersed in this solution and films

to be studied were deposited directly at the electrode surfaces. Using a

slightly modified conventional a.e. Wheatstone bridge network, a number of

readings for films (l-Ul layers) indicated the decrease in capacitance with

increasing thickness forone set of their measurements. However, in another

set of data, an increase to a maximum of three layers was observed after which

it started decreasing. ' The latter data has been attributed to the presence

of serious imperfections in the films and the earlier one is believed to be

the true. An average value of dielectric constant C^-3.7) for kl layers

was thus calculated from the observed capacitance values ( 1CT F). They

found two-fold variation in the apparent dielectric constant and several orders

of magnitude variation in the resistance in the thickness range studied. Further,

both the capacitance and resistance were found dependent on the nature and

concentration of the electrolyte. From the studies using copper sulphate as

an electrolyte of varying concentrations, they inferred that the significant

variation of the electrical (and presumably the structural) properties of

the film may be due to the presence of the copper ion because these values

differed widely from those taken vith Ca Cl as electrolyte. Finally, they

have emphasised that the stearate films may be electrically modified by water

content of various metallic salts used as electrolyte solution and the fact

ie that the mercury-droplet method of RR presents many advantages over

that used in their work.

2. Recent measurements on capacitance, resistance and dielectric constant

h2)

Handy and Scala after more than two decades realized that mono-

molecular layers obtained by the B-L technique satisfy all the conditions

required for reliable electrical measurements. For instance, such organic

layers are thin, homogeneous, uniform in thickness and an excellent insulator.

In their detailed investigations, they tried to obtain a more compact film to

minimize the presence of voids or holes in the organic multilayer and used

evaporated metal electrodes. Fig.3 shows the results of capacity measurements

on some 75 samples in which reciprocal capacity (l/c) is plotted as a function

of predicted number of layers (H ). The capacitance measurements thus

-20-

performed on stearate films (1-10 layers) led to e values between 2,1 to k,2

with a bulk value of £ = 2.5. These results agreed well with those obtained

by PW using liquid contacts. Tig.k ahows a typical graph of the capacity

variation as a function of frequency (upper curve) over the frequency range

0.1 - 20 KHz . On the basis of this curve, Handy and Scala predicted a slight

dependence of € with frequency {a drop of about 5% over the range studied).

The lower curve of Fig.l+ shows the variation of the imaginary part of the

complex dielectric constant (proportional to C X T , D being the dissipation

factor) with frequency. Evidently, there is a slight maximum near 700 Rz

which has been taken to be the indication of the presence of a weak polar

adsorption mechanism with a characteristic relaxation time (i irfrn) of

0.23 m sec. The measurement uncertainty in the dissipation factor D becomes

comparable with the observed values towards the high frequency region (> 10

KHz) (see the lower curve of Fig,!+).

The contribution of the oxide layer formed between the lower metal

electrode and the organic film was neglected in presenting capacity measurements

in view of its small resistivity compared with that of the organic layers

measured. In the ohmic conduction region «< 50 mV), a wide range of variation

of resistivity for the same film thickness was observed which supported the1+2)

contention of a porous structure of these films. They thus concluded that the

resistivity obtained for smaller thicknesses is characteristic of the oxide

film and its upper limit for a given film thickness corresponds to the film

possessing the smallest fraction of voids. The important conclusions arrived

at were that the poor reproducibility of the characteristics is due to voids

and inhomogeneities in the film and a reactive and hence oxidized surface is

necessary for the formation and retention of low porosity layers. Handy and

Scala also found that the physicochemical nature of the substrate influenced

the adhesion of more than the first monolayer and that the porosity alone was

considered unlikely for such long-range effects. Positive ion adsorption,31)

as described by Goranson and Zisman has been suggested as a possible

alternative and was shown to be consistent with some aspects of the electrical52)

measurements. Beck has reported measurements on capacitance for fatty

acid monolayers of different chain lengths sandwiched between Al electrodes1

and found reproducible capacitance within 3% unlike that of Handy and Scala

who found a variation of 50% in capacitance.

Drexhage and Kuhn have measured the capacitance of cadmium

arachidate and cadmium stearate films sandwiched between aluminium electrodes.

Astonishingly, they found this method suitable for the determination of film

thickness even for a single monolayer, because the capacitance C is given by

-21-

Eq..{*i.l) . As is expected, a linear dependence of l/C with the number of

monolayers (M) vas found. Evidently this leads to an independent nature of e

with thiclcness. The slopes of the straight lines in a plot l/C vs U thus

yielded a value of d= 26.6 A for cadmium arachidate and d = 2k.6 A for

cadmium steaxate "by using € = £. 1+0 (obtained simply by the square of the value

of the refractive index of barium stearate determined earlier ). Holt

with a possibility of using Langmuir mono- and multilayer films as thin film

dielectrics carried out preliminary measurements on stearic acid and

stearate films sandwiched between metal electrodes. The standard deviation

of C/A on each device was as low as 3$ which he suggested to be of the order

of accuracy of producing and measuring the areas of the base and counter-

electrodes. No significantvariation of C and hence £ with frequency were

found. The interesting properties thus predicted by Holt, i.e. thermal

stability and uniformity of organic films, have already been discussed. Another

important suggestion made by him is that the dielectric constant can be varied

by altering the metal stearate proportions in the multi-monolayers.

In many of the measurements discussed above, the organic film was

sandwiched between evaporated aluminium electrodes. The fact that natural

oxide layer grows on the base aluminium electrode was realized but its effect1+2)

on the capacitance values of the device was neglected considering that the

resistivity of the oxide film is small compared, with the resistivity of the

organic layers. In the work of Mann and Kuhn , the presence of such thin

oxide layer, between metal electrodes and fatty acids is considered; the

relation used by them is

where d and e are the thickness and dielectric constant of the oxideox ox • the

layer, C is a dielectric constant of air and all^other symbols have the

usual meaning. For calculating € values, they plotted l/C as a function

of superimposed layers (N) and,from the slope of straight lines thus obtained,

the d/e values were determined. A plot of l/C vs Hi is given in Fig.5 for

stearate and behenate films which shows the same y intercept in both cases.

The capacitance and hence the e values were determined by MK for a series of

fatty acid monolayers of Cd salts. For instance, the values for stearic

acid £ « 2.71 ± 0.17 and for arachidic acid £ = 2,52 ±. 0.07 determined by

them were found to be comparable with respective values «" = 2.59 ± 0.08 and

e, a 2.1*9 ± 0.06 determined earlier by RR . However, their 1 5 calculations

show an increasing € with decreasing chain length which is qualitatively

-22-

explainable due to the influence of the highly polarizable cadmium carboxylate

groups which increases with decreasing fatty acid chain length.

Recent measurements of capacitance by Leger et al. " ' have also

shown a linear dependence if l/C with .respect to the number of transferred

monalayers . They made use of this simple method for knowing the thicknesses

of organic films and found it to yield satisfactory results. In an attempt1Q)

by Uathoo , a change (of less than 10?) in the capacitance of stearic acid

films, increasing slightly towards the low-frequency region, has 'been reported

in the frequency region 10~ to 100 Hz .

More systematic investigations have been made by Khanna and Srivastava

recently for measuring the capacitance and hence dielectric constant of a few

fatty acid salts. In .their measurements» sandwich structures of the type Al-

film-Al were employed and the capacity values were determined initially without

considering the effect of the oxide layer . For all the film systems,

measurements were made on 8 and 110 layers using a universal bridge. The

capacitance as well as the dielectric constant values were found to be

distinctly different in the two cases. However, all these results were in

good agreement with their theoretically calculated values of e for monolayer

and multilayer of all the substances." The values obtained by Khanna and

Srivastava, experimentally as well as theoretically are tabulated in Table II.

A slight variation in these values arises because their theoretical

formulation ?'' >? is based on an ideal structure of the film which is59) 'difficult to achieve in practice. Subsequently, Khanna calculated the s

values considering the effect of the oxide layer because this can be electri-

cally insulating and can contribute significantly to the capacitance of the

sandwich structure. For this, the expression already employed by Mann and

Kuhn (Eq,(^.3)) was used and the values for d = 30 A and £ = 8 wereox ox

taken. The e values thus obtained have also been given in Table II.

Evidently, the effect of the oxide layer is more significant in the case of

a monolayer as compared with that for multilayers.To justify the significant difference of e values for monolayers and

59)multilayers obtained theoretically as well as experimentally, Khanna carried

out a systematic and detailed study of thickness dependence of the dielectric

constant from 1 to 80 monolayers. It. vas observed that the dielectric

constant increased slowly with thickness initially but saturated at a

particular thickness ( 21 1000 A ) for all types of Langmuir films. A typical

graph is shown in Fig.6 in which the dielectric constant c is plotted as a

function of film thickness d for barium stearate films. The upper carve (B)

represents the variation of e when oxide layer effects were taken into

-23-

account and the lower curve (A) shows when no such effects were considered.

This unusual behaviour has not "been reported in the earlier works and the

constancy of the dielectric constant with respect to thickness has been

shown, * * * However, Khanna has explained her results qualitatively

as given below. Mainly, the observed dependence has been attributed to the

porosity of films in the lower thickness range which decreases with the in-

creasing thickness. In fact, an exactly similar thickness dependence of

dielectric constant has been observed in evaporated ZnS films , which has been

interpreted in terms of decreasing porosity or increasing continuity of films

with increasing thickness. The saturation of dielectric constant at higher

thickness range has also been observed for evaporated ZnS films. To explain

why the other workers failed to obtain such dependence, it has been suggested

that the films obtained by them were more compact. For example, in the present

wort, the film transfer was accomplished at a surface pressure ^ 30 dynes/cm^

whereas in the work of Handy and Scala , a high surface pressure of 39 dynes/cm

was used. Ries et al. * have already shown that voids in the monolayer

are inevitable if the transfer is accomplished at a surface pressure significantly

below the collapse pressure {1+2 dynes/cm in the case of stearic acid films).

Further, the thickness of about 1000 A at vhich the a value saturates has

been predicted as a rough estimate of the range of electrostatic interaction

in Langmuir fatty acid soaps.

The author feels, however, that the explanation offered above is in

no way sufficient for this unusual behaviour of dielectric constant. Presumably

the influence of highly polarizable metal carboxylate groups which increases

with increasing volume ratio would be a sound explanation for increasing €

with decreasing chain length for a given number of layers . For the

part that £ saturates after increasing with thickness, the qualitative

explanation may be that dielectric lossess are significantly less at higher

film thicknesses and therefore the dielectric constant does not change. The

magnitude of dielectric losses has been related to the compactness of theog \

monolayers in recent measurements of Marc and Messier and it has been shown

that it decreases with increasing compactness. nevertheless, rigorous

experimental work is required to establish such unusual behaviour and to in-

vestigate the role of structural defects like voids, holes or inhomogeneities

on the dielectric constant of Langmuir films. One more suggestion the author

would like to make is that the observed increase of £ may be understandable

in terms of the increasing polarization with increasing number of monolayers.

This, in fact, has already been shown by Tanguy and Hesto experimentally,it

but/requires some structural changes inside the film,vhich,in itself,is aproblem yet to be studied. This complex situation thus requires the specific

attention of the experimentalists.

-2k-

3. Theoretical work

Unfortunately, not much, attention has "been paid to developing the

theoretical formulation of dielectric parameters of Langmuir films. The only-

attempt in this direction has been due to Khanna and co-workers ->TJ»5o) ^ o

developed a theory for calculating the static dielectric constant of Langmuir

films. Their theory is "based on the layered structure of Langmuir films

constituting a simple hexagonal array with the molecules oriented normal to

the supporting surface and separated "by a distance 1+.85 A. ' Muller's

assumption for calculating the Lorentz field for simple hexagonal lattice

could not be applied here because the molecular size ( e 25 A) is large

compared wibh the lateral intermolecular distance {k.85 A ) . The local field

has been calculated by summing up the externally applied field and the field

due to all other dipoles within the specimen. A reasonable assumption has

been made for this purpose that each .molecule is divided into the small CH

groups separated by 1.27 A along the chain axis (androf course,the end carboxy-

late group). Further, all the dipoles are assumed to be point dipoles, parallel

to each other, with their axis taken to be the external field direction (Z axis).

a) Monomolecular films

Following Salem's assumption that D, the mutual distance between

two parallel linear chains, can be chosen as small as about k A if the basic

units (in the present calculations, GHp groups are the basic units) are bonds

of average length 1.5 A , an expression for the contribution due to inter-

molecular interaction between two molecular hydrocarbon chains was obtained

- p U-x)

where p is the moment of the basic CH9 unit (p - «E ) , oi is the electronic—2li 3 10c

polarizability of the CH group (1,81 x 10~ cm , cf.Ref.66), \ is the distance

between the two CH_ groups along the chain axis (1.27 A) and N is the number

of CH groups along the molecular chain. The electronic polarizability here

is assumed to be isotropic and the two chains of fatty acid soap molecule are

considered to be coincident. For calculating the effect of the end carboxylate

group (unit), the relation derived was

El(end) = pend

2 2 -i2X7 - D n2X2 -

-25-

J-"U

where p , is the moment of the end unit and X. is the distance of the i,end 1CH unit from the end unit along the Z direction.

For calculating the intramolecular contribution to the local field the

interaction between the units of the same molecule was calculated using the

expression^—' 2p,

4 = > -^ * (U.6)i Zl

where p. is the moment of the i unit and Z. is the distance "between the

reference and the different unit along the Z direction.

b) Multimolecular films

In case of multimalecular films, the molecules are considered to be

continuously distributed in all the layers except in the one in which the6k)

reference molecule is situated. Thus, following Muller's- treatment for

layer lattices, the contribution of the continuous distribution to the local

field is inrP (P is the polarization)and that of the reference layer is the one

calculated for a monolayer by the above method. The density of soap films is

taken here equal to that of the bulk which is not a very good approximation.

For calculating the dipole moment, the total polarizability has been

taken as the sum of electronic and atomic polarizabilities of all the groups

and the orientational polarizabilities of the polar end group. However, the

atomic polarizabilities, being small,are neglected. The contribution due to

metallic ion presence in the end group have also been neglected in the cal-

culation of orientational polarizabilities and the end group is assumed

equivalent to the - COOH polar group, its dipole moment being 1.5 debye

(cf. Ref.67). Finally, the static dielectric constant e. along the field

direction (the symmetry hexagonal axis) which is one of the principal axes of

the dielectric tensor, is calculated using

€ = 1 + UTP/E • (1.7)s

Using this theoretical approach, they first calculated e for barium5

stearate mono- and multilayers and found these values to be 1.6 and 3.0,

respectively. Later they J reported corrected values of € for bariums

stearate as 1,7 and 3.1, respectively, for a monolayer and multilayer. Here

another assumption was introduced; that a barium stearate molecule is equivalent

to the two "effective molecules" each consisting of one hydrocarbon chain and

the half of the end carboxylate group. These "effective molecules", were

regarded to form a hexagonal array with their axes normal to the supporting-26-

57)surface. Their previous assumption, that two hydrocarbon chains are

coincident* was hot consictent with the film structure reported by Germer31)

and Storks . Further calculations of s: for a few other fatty acid soaps

were carried out ° and these are given in Table II. All these calculations

are done only up to seven nearest neighbours,and short-range interactions being

small have been neglected. The surface effects have already been considered

in their calculations on monomolecular films and these are assumed to be in-

significant for multimolecular {sufficiently thick) films.

It is evident from the above review of dielectric properties that our

knowledge about them has advanced somewhat through experimental measurements

but theoretical understanding is particularly lacking- It is therefore

necessary to attach special attention to more rigorous theoretical work.

The only theoretical formulation of Khanna et a'l. ' described above has

been based on a simple classical approach^and a number of assumptions made

do not hold in true experimental conditions. For example, Miiller's treatment

for layer lattices,which has been followed for multimolecular films, no longer

holds if the macroscopic volume of the sphere is large enough to permit the

molecules to remain embedded in a continuous medium with the same dielectric

constant. The interaction of the sphere with surroundings thus requires

electrostatic calculation which would be worthwhile to account for in future

works. Metal ions such as Ca have been shown to reduce the mobility of the

polar groups by linking one molecule with another . The contribution of

such ions in the calculation of electronic polarizability of the end carboxylate

group [such as (C00)p Ba] has "been neglected in the above calculations. Many-

more assumptions in their approach require due attention. More interesting

would be the extension of their work to time-dependent fields which obviously

would involve the complex dielectric constant of the system and € would

become a special case of such a formulation. Both the real and imaginary parts

of complex dielectric constant» would be frequency-dependent and the imaginary

part would "be of more relevance to account for the dielectric losses. More

problems would crop up if one enteredinto the quantum-mechanical approach but

their description is beyond the scope of the present paper. For interested

workers, reference should be made to the monograph by Frohlich - which

is a comprehensive collection . on the theory of dielectrics.

k. Dielectric loss measurements

Some preliminary dielectric loss measurements were carried out in the

late Forties by RR . They found that the dielectric loss neither depends

upon the frequency, in the range from ko to 10 Hz, nor is it affected by the

-27-

film constitution. The order of magnitude determined vaa low, usually lessI40) 29)

than 0.001 times that determined by Buckwald et al. • Similarly, Holt

also obtained some information that the tangent loss angle (tan 6 ) drops from

0.02 at 225 A to 0.006 at 3700 A . The variation of tan5 with frequency was

predicted to follow normal thin film behaviour. However, these measurements

did not furnish any useful information except the order of magnitude of tsn<5 ,

since the standard deviation was 25%.

A systematic study of dielectric losses in Langmuir films of calcium

stearate and behenate has been reported recently by Marc and Messier

The device used is Al-film-Al and the measurements were performed in the

temperature range (-170° C to +50 C) and over the frequency region (100 Hz -

50 KHz). All measurements were made after a 2U hour degassing under vacuum,

since the dielectric losses decreased rapidly by a factor of 2 during the

first hours of degassing. The results thus obtained are analysed by means

of the equivalent circuit shown in Fig.7, where C and S refer to themm

capacitance/area and resistance of the organic monolayer and C» and RQ to

that of the aluminium oxide layer. R is the series resistance of several

ohms due to the metal electrodes,which introduces series losses of the form

2irRf; f denoting the frequency. Since the loss angle of the device (tan<$ )

varied as f at high frequencies, they have subtracted the series losses

arR C co systematically and considered the following quantity:

s

0 m

Evidently this

tan 6 - RC

•

quantity

C u

P

where ( n )u m

is proportional to the losses per unit

area in the alumina and the monolayers. Further» it was assumed that all the

N monolayers are equivalent and therefore can be represented by H impedances

in series. Calling d the thickness, e the relative dielectric constantm m

and p the resistivity of each of the monolayers, P may be given by^n s '

P

It thus shows P to depend linearly on H . - Fig.8 shows the variation ofs

loss angle of the behenate film (tan S ) measured at 1 KHz as a function ofm

temperature. The curves have been plotted for films consisting of 7> 13

and 25 monolayers which vere transferred at a surface pressure of UU dynes/cm.

-28-

Evidently, two absorption peaks are observed around -100 C and -30° C . The

peak, around -100° C is present in all • . three films- studied,whereas the other

appears only in thick structures (M = 13 and 25) which are known to crystallize

more easily. In a separate measurement, they found that the latter peak

disappeared when the structure of the film is disorganized (see curve b of

Fig.9). Curves(a)and(b)ln Fig.9 represent dielectric loss against temperature

measured in two places of the same film. Curve(a)corresponds to a point in

the middle of the film where the molecules are well organized and

curve(b)to a point near the edge of the substrate where the layer might get

disturbed due to dipping or withdrawal of the latter.

All the results thus obtained in their work are summarized below:

i) Tho-losses per unit area increase linearly with the increasing

film thickness, thus suggesting the movement of dipoles linked to the organic

molecules. This is in accordance with the expression showing the relation of

P and H (Eq. (1+.9)).5

ii) The losses due to the oxide layer are much higher than due to

the organic films and particularly for monolayer, the losses due to oxide are

dominant.

iii)The tan &m do not change much with frequency. The rise in the

measurement frequency merely causes a slight displacement of the tan 6"m

curve towards high temperature,

iv) The1 dielectric losses are much greater when the structure of the

monolayer is disorganized (see Fig.9). However, the order of magnitude of-3

tan 5 is 10 for a compact monolayer which they believe to be relatively

smaller than expected. For its explanation, their suggestion is that the Ca

ions considerably reduce the mobility of the polar groups by linking the

molecules,and the rotation of a dipole around the molecular axis introduces

fev losses.

v) The two absorption peaks observed around -100° C and -30° C have

been related to the degree of organization of the monolayers. Finally, based

on some experimental evidence of getting similar peaks in several other types

of organic substances {in polyethylene a peak similar to that at -100° C ;

in stearon and nonapentacontanone a peak similar to that at -30° C) these

peaks have been interpreted in terms of the dipole movements. The respective

peaks are thus explained to arise due to the movement of the dipoles in the

amorphous region of the monolayers and due to the molecular rotation around

the long axes in the crystalline parts of the layers.

-29-

The interpretation in terms of the movement of dipoles in these layers

does not seem to be convincing because the measurements are carried out under

homogeneous applied fields- In fact, motion of dipoles is possible only when

non-homogeneous fields are applied. Since the soaps or metal salts of long

chain fatty acids are known to be ionic substances forming stable monolayers

(cf. Ref.T, p.193) there may be some ion content present in the deposited mono-

layers. Therefore, it appears to the author that the observed peaks may be

attributed to the motion of these ions rather than the dipoles. The existence

of amorphous region in such organic monolayers, which these workers have thought

of, has not yet been verified in any other experimental work. Nevertheless,

this is the only systematic work on loss angle measurements which distinguishes

between the various kinds of losses and thus led to estimate, in particular,

those arising due to Langmuir monolayers themselves. In presenting their

measurements, they were able to exclude the dielectric losses due to the natural

19)

oxide layer whose capacity was determined to be 920 nF/cm

It is interesting to point out the recent studies on tan5 by Nathoo

for a 23 layer film of stearic acid at room temperature over the frequency-U

region 10 to 100 Hz . Under these experimental conditions, no peaks were

observed and the dielectric losses were found to decrease by an order of

magnitude with increasing frequency in the range studied.

V. ELECTRICAL CONDUCTION PROPERTIES

Greater emphasis was laid on studying electrical conduction through thin

dielectric films, particularly in the period following the second World War.

It was realized that the conductive properties of films should play an important

role in the development of devices. Most of the earlier work on conduction

behaviour has, however, "been done using either vacuum deposited films or

thermally grown oxide films on base metal electrodes. The reason which com-

plicated the interpretation of experimental data, has been the inherent defect

nature of such films. No doubt, highly sophisticated techniques have become

available now for depositing thin films, nevertheless, it is hard to accomplish

a judicious study of electrical conduction in thin dielectric films because of

their "structure sensitive*1 properties. The electrical conduction in films

has already been widely treated in the literature in recent years. The readers

are advised to refer to the recent 1

comprehensive picture of the subject.

69)are advised to refer to the recent book by O'Dwyer who has given a

-30-

Because of the defect nature of the films, it is not possible to

provide interpretation of experimental data using only one of the earlier

concepts of field emiiBion from a freo-electron metal by Fowler and Wordheim,71)

or metal-vacuum-metal junctions "by Frenkel , or metal-insulator-metal72)

junctions by Sommerfeld and Bethe , etc. In fact, the conduction process

has been recognized as an involved process and many co-operative phenomena

such as space charge modulation (due to trapped carriers) or,say, the Poole-

Frenkel effect or Schottky effect have been found to occur. In the oontext of

this paper, basic theories of conduction phenomena in thin insulating films

are discussed briefly just to remind the reader/the underlying basic concept.

The simplest expressions used for the interpretation of experimental data are

also given.

1, DC conduction theories in thin filmR

All the theories discussed here will apply to metal-insulator-metal

systems and the insulator in question may contain a significant proportion of

impurities. In general the electron transport at a metal-insulator interface

depends upon the nature of contacts and the conduction processes, in turn, are

distinguished as follows:

i) If the contacts are ohmic (if <• . ) , the conduction in the systemm 1

is limited by the rate of electrons flowing through the bulk of the insulator

rather than the rate at which the electrons are supplied by the electrode.

The process is then termed "bulk limited".

ii) In the case of blocking contacts (ip > ty±) , the rate of electrons

flowing through the system is limited by the rate of electrons flowing over the

interfacial barrier and the process is referred to as "electrode limited".

iii) Tf the contacts are neutral, i.e. ^ = ^ , there is no necessity

of charge transfer between the electrode and insulator. This situation

corresponds to a transition between an ohmic and blocking contact. Under

special circumstances, neutral contacts also occur if 1J1 < ij and the trap

level is positioned more than 1 eV above the Fermi level. Traps in insulating

films arise because of the grain boundary defects or because of the large

stresses, particularly, in vacuum deposited films. These traps are an

"extrinsic" property of the amorphous insulator and their concentration may beig _^

as high as 10 y cm J .

However, if the system is subjected to sufficiently high voltages,

transition from bulic limited to electrode limited processes,or vice

versa,may occur again depending upon the nature of contacts. We shall be con-

cerned here with several distinct conduction mechanisms only and the discussion

-31-

of theories will be limited to three voltage ranges: low, intermediate and

high.The process is electrode limited for low and intermediate voltages and is

bulk limited in the higher voltage region. In practice, however, many more

co-operative phenomena might occur, because no clear cut criteria may "be assigned

to distinguish these three voltage regions. For greater details, reference73)should be made to a recent monograph on the subject.

a) Fowler-Nordheim type tunnelling

This process requires that the system be subjected to high voltage

biases so as to lower the barrier thickness at the negatively biased Fermi

level to less than 50 A . The well-known expression of Fovler-Hordheim type70)

of tunnelling is then given by

T 3.33 x 10 1 0 F 2 f 0.69 <t>3/2

where J is the current density, F is the applied field and $ is the mean

potential barrier height above the Fermi level of the cathode. This expression

is true only if the ratio of the insulator effective mass to the free-electron

mass (m ) is unity. Otherwise the exponent of equation would become

y \m f) ^ This process is electrode limited.

b) Bicliardson-Sihottky effect

The current flow through the insulator in this process is governed by

the rate at which the electrons are thermally excited over the interfacial

potential barrier into the insulator conduction band. Thus, the current

flowing in the system is determined by Richardson's thermionic equation,

= AT2 axpf- j|j , (5.2)

where A is the well-known Richardson constant ( — m | — 120 if J is

expressed in A cm~ ) and <j>n is the interfacial barrier height. However,

when the applied field interacts with the attendant electrode image potential,

it causes attenutation of the barrier height from <& and permits more current

to flow over the barrier with increasing applied voltage. This effect is7k)

known as the Schottky effect and the attenuation thus caused is given by

iiji * 8 F* . The current is then determined bys s

-32-

J =

vhich is the Richardson-S eh ottky equation.

The additional exponential term accounts for the attenuation of the

barrier height caused due to the interaction of the applied field and the image

f e3 )1/2force. 8 is a constant = y—-5 , e* being the high frequency

Q

dielectric constant of the material, For sufficiently ligii fields across

the interface, tunnelling may be the dominant conduction mechanism and it is

then governed by a suitably modified form of Eq.{5.l) in which the image force

has been included,

c) Space charge limited currents

In both of the processes discussed above, space charge effects were not

considered. However, the traps are bound to exist in most of the film systems

used and thus it is necessary to account for their contribution. First, the

situation will be discussed for a trap-free insulator. In the case of ohmic

contacts, a negative space charge region exists inside the surface of the

insulator and an equal and opposite (positive) surface charge on the

electrodes. When the field is applied, the space charge in the insulator

increases and its redistribution takes place depending upon the nature of

voltage bias. After this redistribution, if the positive charge exists on

the cathode, i.e. the cathode region acts as a charge reservoir to supply the

required charge to the anode, the process is called*space charge limited"(SCL).

For trap-free insulators, the expression derived by Mott and Gurney to

calculate the current density is given by

where d is the insulator thickness. However, in practice, much lower

currents have been observed and are found to be temperature sensitive,vhich

could not be explained with the above equation. Rose then pointed out

that when the insulator contains traps, a large fraction of the injected space

charge condenses into traps. Thus, it was suggested to multiply the right-hand

fclBide of Eq..(5.M'by a quantity 6 = N p exp[-Et/kT),where Nt is the trap

density. E is the depth of the trap "below the bottom of the conduction

bandrand Nc is the effective density of states in the conduction band. Thus

Eq..(5.J+) may be expressed as:-33-

L 8 <T I2

Since 6 is independent of V , the proportionality J ^ Y , as in

the case of trap-free insulator^still holds.

d) Poole-Frenkel effect

When the applied field is high,the coulambic potential "barrier of a77)donor centre or trap is lowered by an amount

which is twice the attenuation of the barrier height due to the attendant

image forces in the case of the Schottky effect. This occurs because the

potential energy of an electron in a coulombic field is four times that

due to the image force effects. fJpp is thus

• " • £ € <(5.7)

This, in turn, amounts to the lowering of the ionization potential, Eg, ofthen

the donors in a solid. The bulk current density is,( determined by the well-

known Foole-Frenkel equation

where J = ey N F exp(-Eg/2XT) and is the low field current density. How-

ever, in actual practice with thin film insulators* the traps having a

coulombic type of barrier would experience the PF effect at high fields and

thus result in the increase of escape probability of immobile electrons therein

The expression for insulators with shallow traps is then given by:

T . To o+[fljLfJ-] (5.9)

The reader is reminded that the coulombic "barrier exists not only

between a donor centre and its electron but also between a charged trap and

an electron. The charged trap means a trap which is charged when empty and73)neutral when full. Furthermore, Simmons has discussed another situation

-3k-

when the insulator contains "both donor centres (.lying "below the Fermi level)

and shallov neutral traps. The expression for the bulk I-V characteristic

of the film ia then given by

-J:•where

(5,11)

The suffixes d and ta here refer to the donor and shallow traps,

respectively. This equation holds even if there exists a range of trap and

donor levels and one of the trap levels is more dominant than the others.

Final remarks should be made that although the functional dependence

of the current density is the same for the Schottky effect (Eq.(5.3)) and the

Poole-Frenkel mechanism, a clear-cut distinction may be made by a plot of1/2

log J versus F /kT which is a straight line of slopes gQ or 8 _ ,b irF

respectively. Therefore,close scrutiny must be made in the interpretation

of experimental data to avoid any mistake arising due to this similar functional

form. Since the scope of this paper does not allow one to mention many other

co-operative conduction processes occurring in thin films, it would be advisable

to refer to the earlier cited Refs.69 and 73 again.

2. Conduction mechanism in Lanfimuir films

Of all the conduction mechanisms that are discussed above, the dominant

one in Langmuir films seems to be tunnelling. Obviously -t for measuring

tunnelling currents, one needs to separate two metal electrodes by a barrier

of 20 A - 100 A in thickness which must be uniform, homogeneous and an excellent

insulator. Thin oxide films grown on "base metal electrodes were employed for

such studies by Giaver and Nicol et al. because of their compactness

and uniformity. The fact that Langmuir films of several organic materials

may fulfill all these requirements for studying tunnelling was recognized byfli \ ftT \

Miles and McMahon , These workers employed a monolayer of bariuma

Btearate ( - 25 A) as an insulating barrier between tin-lead and tin-indium

evaporated film pairs. The experiments, however, did not prove useful

because ah irreproducible and strongly time-dependent resistance of the device

was observed in these studies. This is not surprising,because the monolayer

transfer technique vas in its infancy and only a handful of electrical

• . - 3 5 -

measurements were carried out "before that. It was also not known whether voids,

defects or inhomogeheities in these films could be reduced to such a limit

as to permit the construction of tunnel sandwiches. Hw«ver» these studiesfin)

of Miles and McMahon provoked interest Of many others in the conduction

behaviour of Langmuir films.

Handy and Scala subsequently tried to obtain new information on

the conduction behaviour of Langmuir multilayers and considerable efforts were

made by them to deposit compact films for their studies. In their extensive

studies on Ba or Ca ste&rate films {1-10 monolayers) with counter-evaporated

electrodes of Sn, Cu, Ag and Pb (and also using Hg as the counter-electrode in

a few cases), the conduction characteristics were found to "be highly non-linear

and strongly temperature dependent. Pig.10 shows the typical behaviour of a

symmetric Sn-3 layer calcium stearate-Sn sandwich ( ^ 1 mm area) at 0.1 Hz

and at room and liquid nitrogen temperatures. They further studied thethick

behaviour of a 3-layer and a 5-layer^film at constant voltages for two junctions

which is shown in Fig.11. Detailed analysis of temperature dependence of

conduction for a 3-layer sandwich led to believe that the electron tunnelling

is the operative conduction mechanism at low temperatures (< -100° C) and

small thickness, and thermionic emission at high temperatures {y hQ° C) and

larger thicknesses. A superposition of both of these mechanisms has, however,

been assumed by them at an intermediate temperature range. Handy and Scala

also determined the zero voltage thermal activation energy 6, as 0.25 < $> < 0.30 eVrb b

when the thermionic contribution to the current was plotted on a Schottky plotf I-I' 1of log • • •• — vs . 1/T as shown in Fig,12. In an attempt t o analyse t h e i rI T J

Qp") fi^}

results using theoretical expressions derived by Stratton and Simmons ,

they failed to obtain a satisfactory fit of the observed data. Consequently

no satisfactory explanation could be given for the observed temperature;

dependence. In fact, they have shown that the fatty acid films are promising

for electron tunnelling and low values of $. could be due to the defects and

impurities within the film. One of the important conclusions arrived at is

the necessity of a reactive and hence oxidized surface for the fornatioh and

retention of low porosity layers. In an attempt with the use of noble metals

(e.g. Ag or Au) as the first electrode, the Junction resistance vas found to

be less than 1 ohm and none of the gold based samples were successful for

electrical studies.8MIt appeared also to Horiuchi et al. that Langmuir films should

prove suitable as an insulating barrier between1metal electrodes,because of

their known/thicknesses. The inspiration came largely through the pioneering

and extensive studies made by Handy and Scala . who had shown promising

-36-

behaviour of such films for studying electron tunnelling phenomena. Calcium

stearate films sandwiched bet-ween asymmetric electrodes of tin and aluminium

were employed "by them to study the voltage and temperature dependence of the

current. As has "been shown ^n Fig.13a the observed current I, was found to

be strongly temperature dependent above 200° K and,nearly independent below

this temperature. This was the situation when the lover tin electrode, was

negatively biased. Conversely, when the upper aluminium electrode was

negatively biased, the current Ip was observed to be temperature independent in

the temperature range studied. This type of behaviour of MIM junctions

suggested that more than one mechanism of electron transfer is operative

{see Fig.13b). Then they analysed their V-I characteristics using the trape-

zoidal energy barrier model of Simmons a 3^» 8 5' and Pollack et al. '' T' and

thus determined barrier heights <£_ at the two interfaces; 6, =1.3 eV (for• D ' b

stearate-aluminium) and <fi m 0.8 eV (stearate-tin). This indicated that the

work function of tin is lower by 0,5 eV than aluminium, which is in contra-

diction with the known difference of 1.5 eV, This discrepancy, however, could

not be resolved even when they studied symmetric junctions of tin electrodes

and took the natural oxide layer into consideration.

From their detailed analysis it was shown that the temperature and

voltage dependence of the current satisfies the Schottky emission mechanism

whereas tunnelling has been predicted from the voltage dependence of the low

temperature current. Alternatively, the two mechanisms have been found to

be operative for negatively biased lower Sn electrode and upper Al electrode,

respectively. To resolve the difference of barrier height determined from

the two operative mechanisms* they have assumed the presence of some conduction

patches ir. the films which would let the tunnelling current pass through them

readily. This seems reasonable,as tunnelling emission was found to be more

strongly dependent on the barrier height than the Schottky emission. From

their studies of I-V characteristics,which were symmetrical with respect to the

polarity of applied voltage, the dominating conduction mechanism has been

suggeeted to be barrier limited because the characteristic was dependent on

electrode material. To avoid the formation of oxide layers, efforts were made

to study gold based devices,but measurements could not be carried out because

the samples prepared showed ohmic characteristics with very small resistance.

1+2) 81) 8MUnlike in the measurements of previous workers * * , , Mann and

IS) 881 •

co-workers succeeded in giving a quantitative proof of the tunnelling

theory in their measurements of d.c. conductance through Langmuir films of a few

fatty acid salts. The success of their measurements lies in the fact that33)film deposited by using a modified B-L technique did not possess even a

-37-

small fraction of holes and conducting imperfections. Analysis of the

experimental data wus made in accordance with the concept of quantum-mechanical

tunnelling proposed by Sommerfeld and Bethe

72)To determine the conductivity, Bethe's relation for small values

of applied voltages was used, which is expressed as:

) 1CTt

where ^ is the differenc "between the work function of the metal and the

electron affinity of the insulator. All other symbols have their usual

significance. This equation refers to equal metals and?if the metal electrodes

are different, ^ should be replaced by

respectivewhere ^ and jL are the <p values for the (metal electrodes. Since each

insulator is expected to have some impurity-dependent conductivity o*. in-

dependent of d , besides the tunnelling conductivity a , they have

analysed the results "by separating out these two contributions from the

experimental total conductivity a(- a. + a ) .

Mann and Kuhn investigated many samples in the thickness range

containing 5 to 21 layers with lower Al electrode "biased both positively and

negatively. The measurements of d.c. conductance at 20 C and -35° C , with

different upper electrodes, indicated that the logarithm of the current

density j is proportional to V and inversely proportional to d, as is

expected. Fig.lU is a typical plot of Nj versus V of cadmium arachidate

film,where H refers to the number of layers. The dotted line represents

the current due to impurities j. and the other curve for H = i gives the

total current j which is a sum of y and j. (tunnelling current). In

Fig.15, log Cf. Co* * a - a. ) is plotted against thickness d for four types

of cadmium salts of the fatty acids CH (ClO „ COOH with n = 18 - 21,

Evidently, the experimental points for both 20 and -35 C are on the same

straight line in accordance with Bethe's equation. This exponential decrease

of conductivity with increasing1thickness of the insulating barrier d

verifies the tunnelling theory. The vacuum work functions of Al and Hg

determined in their work theoretically were found in good agreement vith the

known values. '•

-38-

Further experiments were performed for measuring the photocurrent in

the sandwich assemblies between similar and dissimilar electrodes. From these

measurements, the vacuum work functions of Al and Hg were determined, in-

dependently .and an excellent agreement "between theoretical and experimental

values led to the "belief of the lack of larger imperfections in the monolayer

structures. Absence of even small fractions of holes and conducting im-

perfections in the devices which strongly influence the resistance have been

verified quantitatively. If is thus remarked that the earlier attempts on

similar measurements failed to yield satisfactory, results because of the high88)

sensitivity to imperfections in.;the organic films. These workers ' had

also verified the tunnelling theory in a previous work on photographic

sensitization. Fatty acid layers were used as "distance keepers" in the

sensitizers to distinguish between energy transfer mechanism and electron

injection mechanism. The dye was still found to be sensitized which led to

the conclusion that the sensitization is due to the energy transfer mechanism

since the electron tunnelling through fatty acid monolayers was extremely89) :

improbable .

A strong evidence for tunnelling at low voltage has been presented by

•Leger et al . • who studied electronic transport through organic films of

cyanine with two grafted stearic chains (St2 Cy). A typical I-V characteristic

at 1.5° K is shown in Fig.l6 for a sample Al-cyanine-Pb for different voltage

ranges. The curve(B)is quadratic for voltages less than 0.1 V and then exponential.

Such behaviour is clearly non-compatible vith a conductance dominated by pin-

holes in the,organic film and tunnelling through the natural oxide layer. In

another curve (A), the energy gap of superconducting lead i^Q) i s clearly

visible. Another striking feature of the characteristic is i ts non-linear

behaviour for voltages >50 mV which,together with the above observations,

led to conclude that tunnelling is the dominant conduction mechanism at

low voltages. However, contradictory results have been obtained in a similar19)work on Langmuir films of stearic acid containing 5 layers . Fig.IT shows

a plot of log I versus V1'2 at room temperature which fits veil to an ex-

pression of the form I °c exp 0 V1^2 with. 8. <• 1.8 x.105 (Vm)1'2 .. Predicted

theoretical valueB of B are ^ = M * 1° (Vm) for Poole-Frenkel con-

duction and .&_ = 2.3 x 10~7 ;(Vm) ' for Schottky conduction. From such a

S !comparison, i t was concluded that the latter is the dominant mechanism instearic acid.films. Dominance of Schottky conduction mechanism in Langmuir

•• ' 90)filmB in the; range of 1-10 monolayers has also been proposed by Snvastava

based on experimental studies of thickness dependence of breakdown field

(described later in detail)*

: . .. -39- ' :

All the conduction studies, described above, have been carried out in

the low thickness range of Langmuir films tl-10 inonolayers) and different

conduction procesaea have been found operative under different conditions.

However, it appears from careful examination that the existing data predict

tunnelling mechanism more decisively even though there are indications of .

Schottky conduction. In fact, tunnelling has been found to be the dominant

conduction mechanism in many other types of film systems in the comparable

thickness range and tunnel diodes have been developed. On the other hand,73) ;

it has been discussed that Schottky "barriers in thin films are much more

easily investigated using the a.c. technique. Nevertheless, to make sure that

tunnelling is the operative conduction mechanism in Langmuir films, more

experiments are required.

Since, tunnelling currents are known to have a-slight temperature

dependence and the Schottky currents depend strongly on temperature,•such

temperature-dependent studies may prove useful for the choice of the conduction

mechanism in these films. Unfortunately, most of the data have "been inter-

preted without considering the nature of contacts vhich have a lot of influence

on the underlying conduction process. it is essential to examine the

nature of contacts in the future studies on these films. The presence of a

space charge region in KIM structures has also not been encountered in any

of the measurements up to now, and this is perhaps the reason that some workers

have indicated for more than one conduction process to be operative but failed

to determine what exactly it is. Due to the lack of any judicious study which

accounts veil for all the factors influencing the conduction process and in

the light of the existing contradictory experimental evidence, it is premature

to conclude anything about the conduction process in Langmuir films. In

general, renewed investigations on conduction mechanisms under different

conditions are desirable to resolve the discrepancy and to solve the complexity

arising because of the earlier results.

3. High field conduction characteristics '

Multimoleciilar films of stearic acid were chosen for high field con-

duction studies because of their ordered structure and because such films had

been used by many workers in the study of tunnelling phenomena. The interest

of Nathoo and Jonscher was to study the behaviour of these films under

high applied (a.c. and d.c.) fields and to get more information about their

dielectric properties. The sandwich structure of the type Al-film-Al was

used and the measurements were made by placing the device in vacuum as the

exposure to air-caused reversible increase of d.c. conductivity "by an order

of magnitude. The thickness range 6-UO monolayers was chosen which they

thought beyond the range in which tunnelling would be the dominant conduction

mechanism.

The response of the metal film device to a step voltage WAS characterized

"by an initial surge followed by gradual decay which lasted for hours and some-

times for several days in thicker films. At room temperature the current (i)0 7was time dependent following the relation I oC t . On removal of a steady

bias and shorting of the sample, a corresponding discharge current was observed.

Fig.18 shows a typical recorder tracing of the decay current after switching

on a steady field for 8 layer thick film vhich follows I u t n "behaviour with

t = -0.7 obtained by the slope of log I vs.log t . The d.c. characteristic

shown here was obtained by subtracting the discharge current from the charging

current. Further, the current was slightly non-linear at fields < 10^ V cm*"1

varying according to I &- E . At higher fields in excess of 10 V cm~ ,

the characteristic, however, was found to obey Foole's law of the form

93)where 2S is the distance between donor-lilte Poole centres , i . e . neutral

when occupied by the charge in question. To determine the value of 2S, Nathoo

and Jonscher Obtained I-V characteristics for a film containing 11 layers at

four different temperatures and found that each of the characteristics obeys

Poole's law. At temperatures between 198OK - 295°K, the values of 2S varied

between 33 A andUOA . It is expected that the donor-like centres should be

separated by an integral multiple of the molecular length of the fatty acid91)

film under investigation. In the present case, they expected that the donor-

like centres are very likely to fall at the more reactive COOH end group of

the molecules which separate them by twice the molecular length (^50 K) .

Since the observed values of 2S were found to lie between one:and two

Massumingan effective carrier9k)temperature T in excess of the ambient lattice temperature . Subsequently,

these authors measured "low field" a.c. conductance c(w) at peak amplitudes

< 10 V cm and observed a relatively weak temperature dependence of the

absolute magnitude of cr{w) . ; Fig.19 is a plot of o(w) as a function of

the frequency for a stearic acid film (ll layers thick) at different temperatures.

Evidently* all the characteristics may be represented by the power law

dependence on frequency df the, form

"a(u) oc w n , . '.' (5.15) .

-in-

where n is temperature dependent and yaries "between 0,5<n<l at high and

lay temperatures, respectively. Similar behaviour of a(w) was observed in

many other hopping systems, but these were mostly disordered or amorphous.

From the fact that organic films, which are known to be largely ordered, have

also shown a similar "behaviour, they concluded that the frequency dependence

of a is a general feature of hopping systems and not restricted, to disordered

solids. Regarding the nature of the hopping charges, their suggestion is

that theEe could be electrons'or protons». much less likely heavier ions. Later,95)Jonscher has justified the observed behaviour of <r ir these ordered

systems. He explained that even a perfectly ordered system in which all

"hops" had equal probability distribution, the actual delay times would vary

over a certain rangefwhieh could be considerable,to give rise to the observed

dependence.

Further measurements vere made "by these workers in the "high

field" regime extending up to 2 x 10 V cm in a wide range of frequencies.

In particular, their interest was to get information on whether or not the time-

and frequency-dependence of the response was a function of the amplitude of

the electric field. All the measurements were made on a 5-layer thick stearic

acid film sandwiched between aluminium electrodes. Fig.20 shows, the frequency

domain measurements on two areas of the same film for a wide range of field

amplitudes. Curve (a) shows that the results are different only in the lower

frequency region (<0.1 Ho) which is due to the increasing level of d.c.

conduction and is strongly non-linear for fields over 10 V cm , Curve (b)

represents a similar characteristic taken at 1 x 10 V cm when the d.c.

level was subtracted from the total current. On extrapolation of the high

frequency line, the exponent of Eq.(5-15) n ^ 0.8 was found. Obviously,

the entire characteristic at a field of 2 x 10 V cm shifts upwards by a

factor of about 2.5 in the frequency region studied but its slope does not

change much. Thus it has been suggested that the onset of the frequency

dependence of the characteristic does not change significantly because of the

onset of a.c. non-linearity.

Time domain measurements were made on a different area of the same

film and the plots of charge currents tafter subtracting the steady (d.c.)

current) and discharge currents versus time are shown in Fig.21. The data

fits well to give a dependence of the type I ^ t n which,in the linear

regime,transforms to give the conductivity dependence of the type given by

Eq.(5.15). The slope of discharge currents increases from 0.71* at the "low

fields" (2.25 x 105 V cm"1) to 0.87 at the "high fields" (1.8 x 106 V cm"1).

The discrepancy between the field-dependent slopes in the time domain and in

-1*2-

the frequency domain is attributed more to the inapplicability of the Fouriertheoretical QT)

transformation in the non-linear regime. Even the^suggestion that the

effect of a high field on the step function response be represented to a first

approximation, by a change of the amplitude of response vithout a change of

time dependence could not "be strictly applicable in the present measurements.

However * it has been taken to explain the results as a reasonable first approx-

imation which then permits Fourier transformation in the non-linear regime.

Validity of such an approximation is further supported "by the fact that a small

but definite increase of capacitance with increasing field has "been observed

in both the time and frequency domain measurements. Such behaviour was also98) 97)

found in the measurements on silicon oxide and is also expected theoretically

The whole range of data has been considered as evidence for a small

but definite effect of high fields on thetime dependence of step: function1 response.which is more pronounced in the time domain than in the frequency

domain. Such a definite trend could not be determined in the frequency981

domain measurements on silicon oxide . Although the results have been9l) 96)

interpreted well qualitatively by the authors . , no commitment has

been made . regarding... the transient behaviour of such films under high

applied fields (d.c.) or about their a.c. conductivity. Nevertheless, they

have opened up a new series of measurements and further measurements on these

guide lines would yield many more facts about the high field conductionpoint

characteristics of Langmuir films. The most interestingj^would be to know

the nature of the hopping charges by studying the field assisted hopping

mechanism between closely spaced donor-like centres in several other organic

compounds*

VI. ELECTRICAL BREAKDOWN BEHAVIOUR

The problem of dielectric breakdown in solids has been the subject of

numerous investigations for a long time. Nevertheless, the basic mechanism

of dielectric breakdown has yet to be understood in all details. One of

the crucial points in the breakdown process is to establish whether the "break-

down results from a strictly local eventor,rather,is a consequence of

phenomena propagating with an increase of intensity. Thickness dependent

studies have, however, implied that a mechanism of the latter type, presumably

avalanche multiplication of charge carriers, is the prime agent of "breakdown

initiation. But the concept could not gain universal acceptance because

-U3-

some experimenters * attributed breakdown to thermal effects, i.e. the

current density, even without an avalanche multiplication, generated more heat

into the dielectric than the dielectric itself had dissipated. Thus» the

whole breakdown phenomenon seems to be based on two principal theories, via.,

electron avalanche mechanism and thermal effects only. The former,which

interpret the breakdown behaviour of Langmuir films,will only be discussed

briefly here. For details of these theories (both electronic and thermal)

as well as for wider knowledge about the experimental data on other types of

film systems reference should be made to reviews,, books and other papers

cited in the recent bibliographical survey on the subject .

On the basis of the nature of the initiating step and also of the

continuation process of breakdown in thin insulating films, Klein has

characterized the breakdown events into three categories:a) thermal; b) electronic;

and c) electronic, modified thermal. In the case of thermal event, the Joule

heat causes a significant temperature rise resulting in an exponential rise

in the electrical conductivity of the specimen. Instability arises in the

field range above 10 V cm when the temperature rise may be of the order of

a few tens to a few hundred deg. C. Such events have been observed in the3 7 - 1

range 10 - 10' V cm and might occur beyond this range. Electronic eventsh -1

occur at low fields (1-10 V cm ) and the temperature rise is unnoticeable,

in contrast to thermal events. The event is initiated by an electronic process

such as impact ionization, double injection or such like. Such events have

been found to occur in semi-insulating films and not in insulators. The last,i.e.

electronic modified thermal event, is that which has been found to occur

in a wide variety of insulating films. The event is necessarily initiated by

an electronic process such as avalanching at fields typically larger than

10 V cm (range extending from 10 V cm to 10 V cm ). In contrast to the

pure electronic event, the temperature rise may be sufficient to cause local

instability by a thermal process and current runaway occurs due either to the

temperature rise or to field distortion. The electrostatic energy dis-

charges at the instability point, leading to irreversible changes by melting

and evaporation. The electronic breakdown involves only a small volume of

the specimen,in contrast to thermal instability involving the whole uniform

specimen.

To make sure which of the breakdown events is taking place in thin

films, one must know the basic underlying mechanism. The breakdown in films

is particularly sensitive to the conditions, such as, specimen thickness,

nature of electrodes, ambient medium, temperature and the type of voltages,

etc, Nevertheless, thicltness dependent studies of breakdown phenomena carried

-kk-

out in the past few decades, led to a "better understanding of the complicated

"breakdown mechanism. Most of the experimental data could be satisfactorily

interpreted in terms of the breakdown theories assuming the electron avalanche

as the basic mechanism.

1. Electronic breakdown theories

In fact, the earliest concept of electron avalanches produced by impact

ionization was predicted by von Hippel J and FrShlieh ^ almost at the

same time in the thirties to explain the intrinsic nature of breakdown in

insulators. However, the whole concept could not be acceptable as such because

their theories failed to explain the observed data in thin films. In sub-

sequent development of theories, many refinements and new suggestions were

incorporated by different workers but none of them could be widely acceptable.

It was so until 196^, when Forlani and Minnaja made a successful break-

through and proposed a theory of electrical breakdown in thin dielectric films.

This theory could explain a wide range of existing data on thickness dependent

breakdown studies and also explains the data on Langmuir films. For the reason

that earlier theories have already been reviewed well in many of the publications

(see Ref.1) we shall limit our description to the recent developments in

the breakdown theories made after

a) Torlani-Minna.1a's (F-M) theory

The fact not considered in the earlier proposed models was the origin

of the charge carriers in an insulator; F-M in their theory assumed that these

are injected from the cathode into the insulator by field emission and are then

multiplied by impact ionization. Thus,considering the consequences of avalancheTO)

multiplication of Fowler-Hordheim emission , the injected current density

J j-i- c a n ^ e writtencatn

= To «+.[-' (6.1)

where

is the effective height of the potential barrier at the cathode-dielectric

interface. Other notations have the usual meaning. The validity of Eq.(6.l)

has been discussed at least in the range of applied fields for which break-

down is expected. Neglecting the effect of lattice vibrations in the dielectrics

in slowing down the electronE in a strong field, the collision-ionization per

unit length (d,) is approximately (cf.Ref.69, p.22*0

a - eP/I , (6.2)

vhere I is the difference between the mean energy of an electron when it is

able to ionize and mean energy of electrons emerging from an ionization event.

Further, considering the film thickness much lower than the recombination length,

the current arriving at the anode as a result of the collision-ionisation multi-

plication will "be given by

(6.3)

If it is assumed that the threshold current density above which melting or

evaporation of the dielectric material occurs is of the same order as Jn ,

the following expression for the critical field strength is obtained;

(6.1;)

theThis relation evidently shows the dependence of|breakdown field on

specimen thicltness and has "been shown to explain the experimental data for

various inorganic dielectrics by Budenstein and co-vorkers , and for

Langrauir films by Agarval and Srivastava , It has "been discussed

further by F-M that if the electron injection takes place at the negative

electrode and the effective height of the potential barrier is very low, the-1/1*thickness dependence of F is expressed as F_ 0^ d . They have thus

concluded that the real thickness dependence of F^ lies "between these two

idealized situations.

At a later stage, F-M have discussed another situation,in which the

electron injection is governed by Schottky mechanism rather than by the

tunnel effect. This is expected when the applied field is very large because

of the predominant role of the electron image force on the shape of the potential

barrier. Therefore, Eg., {6.1} should be replaced by an equation similar to

(5.3 in Sec.V )» giving the injected current density of the cathode. Following

the same arguments adopted in their earlier approach and by means of proper

evaluation of order of magnitudes, F-M obtained the following expression for F_:

- 0*tf- /JL.-L

Till a equation practically means the independent nature of the breakdown voltage

with respect to dielectric thickness. It is valid even if the voltage drops

in a non-linear way through the dielectric layer. This equation has "been

shown to interpret thickness-dependent data on Langmuir films in the lower

thickness range .

It must be pointed out that Eq.(6,1*) does not contain any temperature

term,because the basic conduction mechanism is taken to be the tunnel effect.

However, the dependence of the type F^ cc d" ' , based on the electron-

phonon interaction, can be shown to depend upon temperature. An increase

of dielectric strength with increasing temperature would be expected, consistently

with other theories based on electron-phonon scattering a02)>103),112} ^ A

reverse dependence is obtained if electron-electron scattering is properly taken

into account, '

b) O'Dwyer's theory

In spite of the fact that the F-M theory ' is based on many simplifying

assumptions, the important omission in it is that of space charges which are bound

to arise because the positive charges left behind by the ionizing electronsllM

were assumed to be immobile relative to the latter. O'Dwyer then proposed

a simple theory assuming that the mobility of conduction electrons is much

greater than the mobility of the holes. For considering the space charge

effects, Poisson's equation is added to the current continuity, and to the

heat transport equation (when necessary). To make life easy* it was further

assumed that the collision ionizations are achieved only by those electrons

which are accelerated from a low energy to the ionizing energy without suffering

any collision with the lattice during that period. With these assumptions,

the functional dependence of field strength with the dielectric thickness could

be expressed in terms of certain dimehsionless parameters, although no explicit

form was given for the function itself.

The fact, however, is that the data on thickness dependence of break-

down strength could not be explained by fitting it in terms of the dimensionless

parameters, i.e. dimensionless mean field strength and dimensionless dielectric

thickness. When F-M r L 1 ' pointed out the above difficulty* O'Dwyer 69),115)

made further progress and proposed a more realistic model. Now he assumed that

highly mobile electrons injected into the insulator at the cathode by field

or thermionic emission produce avalanching and that the current is totally

electronic. Again he obtained the set of equations and solved for the

steady state. The highest field, below vhich the instability and current

runaway might arise* was taken to be the d.c. breakdown field.

-1*7-

Whereas O'Dwyer's modified theory could explain data of various

experimental investigations on alkali halides, it did not prove to be sothe

successful to explain^ data quantitatively on thin films. For example,

it failed to interpret the destruction of the film at higher current densities

and lover fields because the model is based on the current instability criterion

rather than the condition for destruction. It has been pointed out by

Klein that the earlier theories as veil as the above theory assume uniform

charge injection and avalanching in the vhole specimen, whereas, in practice,

the situation is different. He has shown quantitatively that the continuum

theories, as that by O'Dwyer, do not explain the breakdown event adequately

if it is a consequence of localized-avalanches which are spaced widely from

each other.

c) Klein's localized electronic breakdown theory

The concept of localized breakdown was given in earlier theories of

Frohlich and Seitz but their theories based on single electron

avalanche do not include the effect of space charge and the occurrence of117)instability prior to destruction. Following the suggestion by Watson et al.

that a rapid succession of avalanches may produce the breakdown, Klein

has developed a model to show how a succession of avalanches can produce break-

down in thin films. This model, in fact, explains the dependence of break-

down phenomena on almost all the parameters and is supported by a wide

variety of experimental data. However, only essential points will be

discussed here, which are relevant in the context of the present paper to

explain the data on Langmuir films.

According to the proposed statistical model assuming localized

electronic breakdown, the breakdown is initiated by electron injection into

the conduction band of the insulator and develops in a sequence of stages.

The injected electron (by field emission or thermionic emission) at the cathode

produces an avalanche of free electrons by impact ionization and the positive

charges are left behind in the insulator. The latter, being nearly immobile,

drift slowly to the cathode and form a positive charge cluster which results

in the enhancement of the cathode field. The local electron injection rate

is thus increased and a finite probability reached for an electron to be

injected during the transit of the charge cluster through the insulator.

Consequently, there is further avalanching for the continuation of the break-

down process. Since the local cathode field during the transit of the charge

cluster increases with avalanche length, the formation of large avalanches

also depends upon the film thickness, in addition to many other parameters

involved.

T"-

In the present model, the ionization coefficient Ct has been assumed118)

to obey the relation a = F/V. which was proposed by Shockley for "high

fields" applicable to semiconductors; F is the applied field and V. is

the ionization voltage. This was found suitable in view of the fact that the

observed breakdown fields extended over a wide range in most insulators. How-

ever, it has been shown that the proposed theory also inteprets the breakdown

events at the "low fields" obeying the relation-proposed in O'Dwyer's model

for Ci . The mean avalanche length a has been shown to depend upon c£

by the following relation;

U v - " - p- > {6,6)

where K > 1 is a factor taking account of the average increase of the mean

free paths for ionization towards the anode,and i is the number of collisions

of the injected electron while travelling through the insulator. It has been

shown that the effective field acting on the electrons in the film is so low

beyond a that no more impact ionization occurs when a is not much

smaller than the insulator thickness d

How, if the film is too thin (d •£ a }, the full-size avalanches cannot

develop and the probability of "breakdown becomes very small. Therefore, for

the incidence of breakdown,even at relatively low rates, it is necessary to

raise the applied field to an extent that a £ d or from Eq. (6.6)

F >

Evidently, this shows that the breakdown field {at which breakdown

begins to occur) in very thin insulators is inversely proportional to the

thickness d . Further, it has been shown for d > a^ , that the rate of

decrease in the breakdown field with increasing d becomes smaller for all

values of mean time to instability. The observed experimental .data on Langmuir

films (discussed later In this section) support the predictions,of this theory.

For the discussion of temperature effects on the breakdown field, It

has been shown that it varies with the nature of the injection current process,

the hopping mobility and the ionization coefficient values. To predict the

temperature effects on electronic breakdown events, one must determine how

the above parameters change. In consequence, some of the temperature effects

promote and others oppose the development of "breakdown. Also experimentally

both types of behaviour of the breakdown field (increasing and decreasing with

temperature) have been observed in various thin film insulators.

This theory of localized electronic breakdown in insulating films

has been shown to• interpret the observed data of various types of film systems

over a wide rango including almost all the parameters, such as, temperature,

thickness, electrode materials space charge, etc., which are Itnown to affect

the breakdown phenomena. In addition, one of the most successful parts of

this model is that it also explains the completion of the "breakdown event

including the destruction of the insulating film. It has been discussed that

the breakdown follows in a sequence of stages, i.e. in the initiating staget

harmless avalanches occur in the whole specimen and the temperature rises

during the avalanching of electrons- Consequently, a significant enhancement

of the electrical conductivity takes place which causes voltage collapse

through a current runaway. Impact ionization may stop during the latter stage

but the "breakdown events continue until the destruction occurs because of the

thermally unstable state, due to temperature rise at the site. Thus, a complete

breakdown event mainly comprises the initiation of breakdown > instability due

to heating and,finally,the destruction of the capacitor with voltage collapse.

In the author's opinion, Klein!s proposed model is the most successful approach

among all the models discussed by various workers. Nevertheless, it does not

take into account explicitly the presence of defects in the insulating films

and future works might improve the theory further,

2. Experimental studies on Langmuir films

Langmuir films of fatty acid salts are known to have high breakdown

strength s 10 V cm for a long time. The earliest studies of the break-

down of the films under high d.c. voltages were carried out by Porter and

Wyman . The experimental arrangement employed for such measurements was

very simple,consisting of a galvanometer to read the current. Using a Hg

drop as the upper electrode, no current was detected until the voltage

applied across the specimen reached a certain critical value in the case of

30 layer thick filjns vhereas,with thinner films, appreciable currents were

present almost from the start. The breakdown per layer was found to rise

sharply at a thickness of about 20 layers and its order of magnitude was

£ 10 V cm" . Very high specific resistance 10 ohms, below the

breakdown voltages^was found. Later, Hg droplets were replaced by a drop

of tap water and the device behaved almost the same way as with Hg, However,

there was one striking difference,that the critical voltage at which there

vas a sudden rise of current was independent (nearly) of film thickness. It

was higher for thin films and lover for thick films than the corresponding ones

when mercury was used. No striking difference was observed in the breakdown

-50-

voltage of X and Y films and some difference in I-V characteristics is attributed

simply to electrical asymmetry in the film,probably associated with the initial

layer. Further, the film was disrupted permanently when the "breakdown voltage

vas exceeded and mercury drops vere used. On the other hand,with water drops

there was no change in the nature of I-V curves even after the film had been

subjected to a voltage beyond the breakdown point. The difference has been

ascribed to the different affinities of the two liquids for the surface and

presumably the water moleciiLes were able to penetrate the film.

Similar attempts were made by Race and Reynolds in the early period

and data was collected on films of cadmium arachidate obtained from solutions of

varying pH values in the range 5.1 to 6.62. As is known, the proportion of

conversion to soap depends upon the pH of the solution, the highest dielectric

strength was found only at the higher pHs, which correspond to a higher

proportion of converted soap. Experiments on skeletonized films showed

lowering of dielectric strength, as expected. Further measurements on di-

electric strength as a function of frequency showed a wide scatter in the

observed data and thus no conclusion could be drawn. However, at 1000 Hz,

the highest valueE vere obtained. They had admitted that,in spite of their best

efforts, the inclusion of dust particles in the multilayer films could not be

checked and,therefore, the data, in general^were disappointing. Nevertheless,

they succeeded to deposit 201 layer thick films that could withstand from 150

to 190 volts corresponding to the dielectric strength of 2 x 10 V cm" ,

Because of the difficulty to obtain large areas of the films free from cracks,

holes or dust particles, the possibility of using such films as practical

capacitor dielectrics was ruled out in their work.

Evidently, these earlier measurements do not predict anything about

the breakdown mechanism and no efforts vere made by these workers to interpret

their data in terms of any known theory at that time. Some more attempts also

yield only the orders of magnitude of dielectric strength of Langmuir films.

For instance, Thiessen et al. vere able to apply electric fields up to

5 x 10 V cm on barium and calcium stearate film capacitors in the thicknesso e 29)

range 100 A - 1000 A . Similarly, Holt , while considering the use of

Langmuir films as thin film dielectrics, reported the dielectric strength

— 10 V cm for varying thickness range of barium stearate films. It was

not until the last few years that Agarwal and Srivastava ' *

have made detailed and systematic investigations on thickness-dependent behaviour

of breakdown strength (d.c. and a.c.)011 various fatty acid soap films.

Such studies were undertaken to illustrate the underlying breakdown mechanism

as well as to obtain information about the breakdown strength over a vide

-51-

range of film, thicknesses, which is of fundamental importance for the develop-

ment of devices. Similar measurements on temperature dependence behaviour123)

were also undertaken in the same school and some preliminary measurements on

a.c. breakdown strength of barium stearate films were carried out. In the

following section, all experimental investigations have been reviewed. '

a) Thickness-dependent studies

In studies of Agarwal and Srivastava 1 0 9 «110'» 121),122) t h g c a p a c i t o r

geometry of Al-film-Al has been Used and the thickness range covered is 1-80

layer, corresponding to 25 A - 2000 1 . In their earliest investigations only

on barium stearate films Hg drops were used as the upper electrode.

Three principal breakdown voltage thresholds ,vi2. onset breakdown, destructive

breakdown and maximum breakdown voltages have been encountered. These have

been distinguished by the initial abrupt rise in the current, by the commencement

of "visitle" destruction in the film and "by the destruction of the film capacitor

over large areas, respectively, when the applied d.c. electrical field was in-

creased linearly across the capacitor. For the sake of simplicity, we shall

describe here the data in two parts, one consisting of onset breakdown

voltages when there is no destruction of the film and the other consisting of both

the events of destructive type.

i) Onset breakdown voltage

Measurements of d.c. onset breakdown voltage are carried out * * '

on mono- and multilayer Langmuir films of barium paLaitate, margarate, stearate

and behenate. Fig.22 shows a log-log plot of breakdown field F, versus

film thickness d on barium stearate films. The dielectric strength is alwaysCL

found to be a power-dependent function of the film thickness d , i.e. Fn oc a

with ct varying in the different thickness ranges. The curve (A) in Fig.22

is a plot corresponding to 1-10 layers and the curve (B) corresponds to the

higher thickness range of 10-80 layers. Evidently a - 0.01 in the lower thickness

range (25-250 A) and & = 0.59 (approximately) in the higher range (250 A -

2000 A) . Similar values of a = 1.08, 1.08 and 1.02 (for 1-10 layers) and

<L = 0.52, 0.52 and 0.53 (for 10-80 layers) were obtained for barium palmitate,

margarate and behenate films, respectively. Agarwal and Srivastava y '

interpreted their results in the higher thickness range in terms of F-M theory

of breakdown for small thicknesses which predicted 0, to be 0.5 for the high

energy gap dielectrics and for not very large electron affinity (Eq,. {6.h)),However, for the results in the lower thickness range, it was proposed that these

may be explained in terms of the increased boundary scattering.

-52-

90)In a recent communication , Srivastava has explained the above results

in the lower thickness range in terms of the F-M theory which considers a

transition from tunnelling to the Sehottky effect "because of the predominant

role of the electron image force. Evidently, Eq..(6.5) thus explained these

results showing the independent nature of "breakdown voltage with respect to90)

film thickness. Based on this interpretation, Srivastava has proposed

that the breakdown data in the lower thickness range of Langmuir films constitute

evidence of Schottky dominated dielectric "breakdown in these films. It is also

pointed out that the breakdown field for 10 monolayers (thickness ^ 250 A)

may be regarded to be near the "transition field" on these fllmSjWhich is not

possible to calculate theoretically mainly because of the unknown band structural

parameters of the insulating films. The author , however, has discussed

recently that it is premature to agree fully with the evidence of Schottky90)

dominating mechanism in Langmuir films . It is because most of the data on

conduction mechanism in these films have shown that the electron tunnelling is

the operative conduction mechanism in the lower thickness range- Further,

Eq.{6.5)» also shows an explicit dependence of the breakdown field upon the

temperature. Unfortunately, no experimental work seems to have "been reported

showing the temperature effects in the breakdown field in the lower thickness

range and thus supporting the contention of Schottky-dominated breakdown in12U)

these films. Even, the work by Agarwal and Srivastava on barium stearate

films (thickness ^ 515 A) in the temperature range -^O C - Uo C neither

corresponds to the thickness range under consideration nor conforms to the F-M

theory based on electron-phonon interactions.

On the other hand, it has been shown that- Klein's statistical model

is much liiore realistic for the interpretation of the present thickness dependentthickness

data in the lower range. Obviously, Eq.{6.7) explains the observed^independent

nature of the breakdown field at which breakdown begins to occur. The

relevance of this theory in the present context is supported by the fact that

the whole breakdown phenomena from its inception till completion has been in-127)

terpreted by Klein's theory in a recent communication . One of the

limitations of the F-M theory is that it does not interpret the occurrence of

destruction of the film due to thermal instabilities developed at high applied

fields.

ii) Destructive breakdown

Destructive breakdown in Langmuir films consists of two events, i.e,

the destructive breakdown voltage and the maximum breakdown voltage. These

events have been determined in various fatty acid soaps in the thickness range

-53-

o o

temperature in the range -1+0 C to +40 C . The capacitor geometry used

was Al-film-Al and the two breakdown events were distinguished on the same

lines as discussed in the thickness-dependent studies. Curve A in Fig.26

shows the plot of temperature versus breakdown field F for a barium stearate

film (thickness 515A). Evidently, F decreases slowly with increasing

temperature which is similar to that observed by Budenstein and co-workers

in some evaporated thin film systems. Theoretically such behaviour Is

expected provided that the electron-electron scattering is properly taken into

account. However, these workers failed to interpret the results in terms102) 103) 112)

of any electronic breakdown theory based on electron-phonon scattering * *

or in terms of recent F-M theory, which showtan increase of dielectric strength

by increasing the temperature (see Eq.(6.5)).125)

Further, these workers obtained data on the temperature dependence of

the destructive breakdown field F__ jn barium stearate film capacitors andb max

found that the decrease of F, with increasing temperature is more rapidb max o r

than that of F . Curve B in Fig.26 is a typical plot of F with respect

to the temperature of a barium stearate film (515 A thick). These results

were also not explained by them because no theory is yet available to account

for the temperature effects on destructive breakdown field. In fact, the

temperature behaviour of electronic breakdown events is not yet well understood

both theoretically and experimentally.' The recent Klein model explains

both the possibilities that breakdown strength can either increase or decrease

with increasing temperature, depending upon the nature of electron injection

which effects the mean time to instability and consequently the breakdown1210,125)

strength. J-V characteristics plotted in the work of Agarwal and Srivastavathe

and^transmission photomicrograph of the capacitor at destructive breakdown

voltages do not exhibit any more information than that obtained in thickness-

dependent studies. The breakdown voltage has, however, been found to decrease

with increasing capacitor area in agreement with the observations of107}

Budenstein et al.

c) a.c. breakdown studies^ ' 123)

In these preliminary measurements of a.c. breakdown , the onset

breakdown strength has been studied as a function of thickness and frequency

in the thickness range 100 A - 1500 A and in the frequency region 10-200 KHz .

Barium stearate films were always sandwiched between two aluminium electrodes

and the occurrence of breakdown was inferred from an abrupt rise in the

current. Fig.27 shows the behaviour of a.c. onset breakdown voltage V^

with film thickness d at a fixed frequency of 30 KHz . Evidently, V ^

-56-

increases with increasing film thickness. When the corresponding breakdown

strength 7 . was plotted as. a function of thickness d on a log-log scale>

it was founa that F c*r a with & = 0.68. Incidentally, this is the same

behavioiir as these workers * had already obtained in their corresponding d.c

studies. Further, a graphical representation of V versus frequency t for

a film of thickness 500 A (Fig.28) shews that V increases with increasing129)frequency. Similar studies were carried out by Klein and Levanon on

SiQ films and three types of a*c. breakdown processes were distinguished as

were found in their d.c. investigations . A theory was also proposed by129)

Klein and Levanon which was based on the thermal nature of breakdown and

predicted a decrease of breakdown strength with increasing frequency for the

high-frequency range studied. In view of the observed contradictory123)

experimental data in the Langmuir films, Agarwal and Srivastava deduced

that the breakdown is non-thermal in nature. Fortunately, the electronic

nature of the first breakdown event has already been established by these

workers through their d.c. investigations,

The experimental data could not be interpreted because of the lack of

an a.c. electronic breakdown theory. It has, however, been pointed out that

in spite of the high structural perfection of these films, the breakdown is,

presumably, initiated as "single hole" breakdown. The scatter in the

experimental data has "been attributed to the inevitable voids and inhomogeneities

in the deposited film as well as to the effects of electrode thickness whicht

of course, the authors tried to control in their measurements. The onset a-c.thickness,

breakdown strength of 'barium stearate films, in order of increasing film / -jraafound to lie between 1.33 x 10 and 0.22 x 10 V cm" , which is smaller than

6 6 - 1the corresponding d.c. breakdown strength (1.75 x 10 - 0.27 x 10 V cm" ).

13This observation gets support from the observations of Race and Reynolds

who obtained higher values of d.c. breakdown strength than the a.c. values.

The situation regarding breakdown phenomena in Langmuir films can best

be summed up by saying that it is not yet well understood. Although thickness

dependent studies could be interpreted adequately in the light of existing

theories, these are not enough to make sure what exactly is the underlying

conduction mechanism. On the other hand, temperature dependent data is

just in its primitive stage and could not be interpreted. Many factors

may be pointed out in these studies which,if taken into consideration,

would lead to a better understanding of the whole phenomenon. To list a few;

the breakdown rate has not been investigated and the I-V characteristics are

recorded only up to the stage of destruction. Even the I-V characteristics

do not give sufficient information about the pre-breakdovn conduction. The

-57-

influence of oxide layer formed between the base aluminium electrode and the

film has also not been discussed.

Whereas these factors must be considered in future works, more attention

must also "be paid to the determination of breakdown strength as a function of

electrode material, polarity of applied voltage, ambient conditions and nature

of applied voltage wave form, etc. A few groups, e.g. Klein and co-workers,

Budenstein et al. and Qsburn's group have been actively engaged in the study

of breakdown behaviour of varied types of dielectric films,and each of their

papers "brings new information. The reader is therefore advised to refer to

their original papers and follow all the guidelines in the studies on Langmuir

films. The recent bibliographical survey on breakdown conduction in thin

films is a comprehensive and up-to-date collection of such literature.

One of the most significant contributions of Ag&rwal and Srivastava,

who have carried out experiments on Langmuir films, is that the breakdown

strength haa been determined over a range of three decades of thickness which

ia very useful from the device applications point of view. The reliability

of these data lies with the fact that the onset breakdown strength, which is

of prime importance in the device applications, has been found to fit the

theories very well over the whole thickness range for all types of

Langmuir films studied. The a.c. breakdown and temperature dependent

behaviour have only been done on barium stearate films and hence more

experimental work must be done before one can place too much reliance on

these data. Presumably, the recently proposed theories must also be modified

to explain the observations of the latter two types of experiments on Langmuir

films. necessarily, further development in the understanding of electrical

breakdown depends on the nature of experiments and one must identify the

physical mechanism rather than simply study the dependence of breakdown strength

on various parameters.

VII. FORMING PROCESS AHD DIFFERENTIAL NEGATIVE RESISTANCE

The .application of voltage greater than a minimum Vp across a metal-

thin film insulator-metal device nay cause a radical and essential change-in

i ts electrical properties, such as a large permanent increase in the con-

ductivity. This process is designated "forming" process and V? is known as

the "forming voltage". This process has been found to occur most readily in

many thin film,,insulators with reactive anions, such as oxides (SiCL, Ta2<> ,

AlrtO_) and fluorides(caF > MgFp. MnF ) and was thought to be a characteristic

1 -58-

feature of non-stoichiometric insulators. After the sample was "formed", the

device also showed a pronounced differential negative resistance thenceforth

DNB) in its I-V characteristics. Until a few years ago, DNR behaviour was

established only for amorphous or polycrystalline insulators. Recent invest-

igations on Langmuir films * have shown that LKR also occurs when

organic monolayers are sandwiched between metal electrodes and formed. There-

fore the phenomena under consideration are to a large extent independent of

specific properties of the insulating material. However, some distinct

differences vere observed in the case of organic monolayers, which will be1 described in the course of the discussion. Before we proceed to dicuss the DUE

in Langnruir films, some of the characteristic features of the "forming" process

will "be outlined and. the models to explain the process will be discussed

"briefly. For greater details on the subject the readers are referred, to a132)recent review paper

1, Factors influencing forming

The major factors which influence the degree of forming are the applied

voltage and its nature, temperature, atmospheric conditions in vacuum, in-

sulator thickness and the electrode material. Application of a voltage pulse

( V ) of a few seconds duration or a sinusoidal voltage of amplitudes greater

than V at room temperature "forms" the sample to a degree reflected "by the

maximum current flowing through the sample. The new I-V characteristics of

a formed sample exhibit currents larger by a factor a 10 than the unformed

If, however, the voltage exceeds VF, the degree of forming may increase further,

providing V does not exceed a certain final forming voltage to cause catastrophic

breakdown of the insulator . The rate of the forming process is very

much sensitive to the temperature. In general, if the temperature is high

enough, the sample will form to completion at a faster rate than it does at

lower temperature. At a particular value of Vp , the device may continue

to form if the temperature is increased without any change in V^ . However,

the degree of forming is constant at given Tp (forming temperature) and Vp

(final) providing the latter is not increased any more. It has been shown that132)

the forming voltage is independent of the insulator thickness . Neverthe-

less, the degree of forming does depend on the thickness d and the peak current133)

of a formed device is approximately given by :

Imax

-59-

The degree of forming being sensitive to the peak current , should alsoincreasing

depend on thickness. In fact, the degree of forming decreases with/thickness, the

thickness range of the insulators suitable for the forming process is regarded

to lie between 100 k - 3000 A . As already indicated, for a thin insulator,-

V p may exceed the breakdown field and also the probability of electrode-electrode

tunnelling becomes significant. The decrease in peak current is observed

with increasing insulator thickness ^^^.

The forming process is found to occur under varying vacuum conditions,

for instance, pressure of 1 torr or lower was found to be adequate by Hickmottj jl ^

10 to 10 torr was found necessary in the investigations by Barriac et al. 1 -

who also reported that no forming occurs at 10 or above. The widely accepted

pressure, however, is 10 torr or better. This process is not affected too

much if gases like He, Ke, H , Ar are used instead. However, an oxygen

atmosphere completely inhibits the forming and may have permanent effects. The

forming process is investigated using a wide variety of electrode materials such

as Al, An, Ta, Zr, Ag, etc.. with different combinations of the counter-

electrodes. Different behaviour of the device has been exhibited with different

polarity of the electrodes. The dependence on electrode material is different

for the anode to which forming is sensitive and the cathode, vhich does not1^2)

appear to affect forming. Dearaley et al. ' have tabulated different

electrode materials based on the experimental observations and have shown which

of these exhibit forming or not in different conditions.

2. Models of the forming process

Since, Hickmott pronounced the occurrence of "forming" in M M

devices and proposed a model to explain the observed behaviour of the device,

many more models have been developed. Each of these explained the "forming"

and consequent changes in the device characteristics in s somewhat different

manner. However, none of these models accounts fully for the numerous

observations demonstrated in different devices. To remind the reader of the

very basic concepts of these various models, we reproduce a concise summary of1S2 i

all five models in a tabular form (Table III), after Deamley et al.

More discussion will follow when the experimental observations on Langmuir

films are described later in this section.

Once the device is "formed" many electrical phenomena have been observed,

which an unformed device does not exhibit. The most commonly observed is the

differential negative resistance together with switching and memory phenomena.

In some structures, however, electroluminescence and electron emission might

also occur. Since Langmuir films exhibit only DNR in both the experiments on

diode structure (MIM) and triode structure (MIMIM) we shall discuss

-60-

briefly how the DNR can be explained In terms of various models. The other132)

observed phenomena* described in an earlier review , vill not be included

in the present paper. Further, ve shall limit our discussion to the voltage-

controlled (H-type) type DNR which has been observed in Langmuir films.

The earliest model proposed by Hickmott 135J»136J states that the

ini t ia l part of the I-V characteristic is due to space-charge limited con-

duction in the impurity band of the insulator. When a high field is applied,

some process reduces the number of impurity centres which in turn reduces the con-

ductivity and an N-type characteristic results. For instance, neutralization

of the impurity centres by field induced inter-impurity tunnelling may be one13U)possible process to cause the reduction of conductivity. In another model ,

the electron transport mechanism has been assumed to be hopping between traps

of very similar energies when the applied voltage exceeds a particular, limit ty t

below which electron tunnelling from one electrode to another is probable.

At high voltages (> C ), the insulator can be crossed only by those electrons which'

are entering the insulator with energies close to the electrode Fermi energy and

onlyif they are making transition to traps of lower energy. Since the top of

the impurity band in the centre of the insulator has been assumed to fall below

the anode Fermi level, charge trapping would occur in the depletion layer

near the anode. The density of states of electrons within the electrode

decreases resulting in a rapid fall of the number of electrons contributing to

conduction and thus giving rise to the negative resistance in the characteristic.

In a model by Barriac and co-workers non-ohmic behaviour at low-

voltage has been attributed to space-charge limited ionic currents. They

have thus introduced an additional feature of ionic motion unlike in the

previous models of gickmott ' and Simmons-Verderber . At higher

voltages, the dominant transport mechanism has been suggested to be electron

tunnelling,which neutralizes the positive ionic space charge by being trapped.

Consequently, the two current-carrying components are largely cancelled and an

N-type characteristic results.

A model based on an altogether different approach involving some

conducting filament formation in the device is due to Dearnley

Basically, the model assumes ohmic conduction along the filaments which are

physically different from the host matrix. The appearance of negative

resistance has been attributed to the rupture of these filaments because of

the .Joule heating to a point exceeding the melting point of the matrix. On

fracture, the filament becomes non-conducting,resulting in the fall of

conductivity- An earlier model having features in common with

Dearnley's model postulates only one filament and explains the occurrence of

-61-

switching in the device but no negative resistance. One of the recent models1^2) 132)

by Sutherland is similar to that of Dearaley et al. and this also

assumes that the DNR arises from local breaking of thin conducting filaments

by Joule heating. Thus, al l these models discussed above propose diverse

mechanisms for DIIH and i t will be shown later in this section which of these

offer interpretation to the observations in Langmuir films.

3. Experimental observations on Langmuir films

Experiments were performed both on diode structures ,(metal-organic

film-metal) and triode structures (metal-film-metal-film-metal) using Al

electrodes and organic films of cadmium arachidate. The unformed samples~3were placed in a sealed brass box evacuated to 10 torr whose temperature

could be varied between TT°K and room temperature. Then a unipolar sawtooth

voltage, which increased at 0.3 V sec and decreased to zero within 0.02 sec,

was applied across the devices. The J-U characteristics do not manifest DNR132) \3h)in the return trace if the voltage is removed so quickly * . The

measurements on diodes have used 11-layer thick . films and on triodes 9-layer

thick films. In the case of diodes, characteristics are also obtained with

Au as the upper electrode to explore the influence of electrode material on

the characteristics. Some devices with Au as the base electrode were preparedU2)

which invariably showed short circuit as was reported by Handy and Scala

a) DNR on diode structures

Once the sample is "formed" at TT°K with top Al electrode positive,

typical J-U characteristics are obtained, similar to that shown in Pig.29

for a sample consisting of 11 organic monolayers. As a rule, DNB is not ob-

served in the first voltage cycle but appears in the second one. However, it

disappears in the successive two to four further cycles and finally the

characteristic becomes smooth. Fig.29 at T7°K corresponds to this smooth

characteristic. On warming up of the device at about 5 deg/min, the J-U

characteristic changes at about 150°K and behaves at different temperatures

as shown in Fig.29, Evidently, the most pronounced DNR (peak to valley

ratio of the current is maximum) is observed at about 190 K and it goes on

diminishing beyond this voltage, disappearing above about 210°K . Almost

similar results were obtained with Au top electrode but the device always

exhibited short circuit above about 170°K and in this case appreciable DNR

occurred at about 150°K.

To investigate the temperature effects, the unformed sample was

always cooled to 100°K and sawtooth voltage applied until the device exhibited

smooth characteristics. In Fig.30, curves 1, 2 and 3 represent the respective

-62-

characteristics of an unformed sample in the first voltage cycle; formed

sample and second voltage cycle exhibiting DNB; smooth, characteristic after

a few voltage cycles. The device vas now treated at a particular temperature

in the range(ll0OK - 300°K) and every time the voltage was removed while

raising the temperature. The DHR was observed only for the first few voltage

cycles in the systematic studies of different temperature regions. The

typical curves so obtained are shown by the curves h to 7 in Fig,30. The

observations made in different temperature ranges are:

i) 110°K - 150°K exhibited DNR but not well pronounced

ii) 150°K - 190°K better DNR than above

iii) 190 K - 225 K pronounced DNR (see curve k Og Fig.30 )

iv) 225°K - 300°K increase in conductivity; little or no DNE.

It was also observed that the current maxima shifted towards lower

voltages with increasing number of voltage cycles until the DUE disappeared.

However, it could be re-obtained by increasing the temperature in the range

100 to 225°K . Thus, it was found that the larger the temperature increase*

the better the DNR is developed. Curve 8 in Fig.30 illustrates a character-

istic at 190°K>vhich the device exhibited in the first voltage cycle after

interrupting the voltage bias for about 3 hours. The DNE is more pronounced

for longer resting times. Curve 9 shows a smooth characteristic obtained

after a few more voltage cycles at 190°K after interruption as in curve 8. •

Further experimental observations concerning DNR in diode structures by

Gundlach and Kadlec may be summarized as below,

i) The device could be formed by applying 5 to 6 V or more when

the top electrode (Al or Au) was biased positivv.-ly. The temperature at which

the device "formed" was T7°K unlike in the case of many oxides and insulators

(T = 200°K for Al-SiO-Au 133' and T a 250°K for Al-Al-oxide-Au *). This

has been explained in terms of the very weak binding forces between continuous

hydrocarbon chains oriented perpendicular to the film plane of the insulator.

In contrast to oxides which do not have such an anisotropy in binding forces,

the mobility of impurity ions (here metal ions) along the long chains of the

molecules is expected to be higher than in oxides. For the fact that the

device cannot be formed (at 77°K) when the base electrode is positive, a

qualitative suggestion has been made that the thin Al-oxide film (formed

between the lower Al electrode and the Langmuir film) inhibits the metal ion

injection from the base electrode into the organic layers.

-63-

: ; ) Once the device is formed, the DNR is observed for both polarities

of voltage bias. The threshold voltage for the appearance of DNR was found"

to l ie between 2 and 3 volts.

i i i ) The investigations of DNR with respect to the multilayer thickness

showed that the former becomes mote pronounced with the increase of the la t te r .

The average values determined in Ref. 130 for the peak-to-valley ratios from the

observations of 35 samples, were U.2, 2.1 and l.li5 for 11, 9 and 7 monolayers,

respectively. No DNR was observed for the devices having less than 5 mono-

layers, Gundlach and Kadlec were not able to explain this behaviour

of the characteristics.

To explain the existence of DNB, no single theory was found adequate.

However, the. appearance of DNR below 225°K (see Fig.29) has been related to

the so-called "dead time" which is a measure of the time between filament132) 132)

fracture and reaching a state from which i t may reform . In this model

the transition between two states is interpreted as relaxation of space charge

polarization. On the other hand, the dead time is related to electron13M ll|3)diffusion through the insulator " . However, both of these models

predict strong increase of dead time with decreasing temperature. It has been

thus suggested that these devices, presumably, show a dead time of the order

of hours because DNR appears below 225°K, But life was not so simple because

the dead time has been reported to increase irreversibly from seconds to hourso lUU)

by warming the sample up to 500 K for about an hour . This observation

was made on Al-Al oxide-Au samples. Finally, the results have been left with-

out an adequate explanation by the existing theories.• - ' within experimental error,

The peak voltage U was found to agree,/with an empirical relation1?61 •P ' r • ^ '

given by Hickmott , U = k - O.36 v£y. , when the device was warmedsteadi ly (see Fig.29) . For cadmium arachidate, the d i e l ec t r i c constant

CO \€.= 2.1+ was considered to obtain U a 3.1+5. However, the values ofr • p

U measured at constant temperature (Fig.30) are much .higher. ThereforeGundlach and Kadlec 130^ suggested that U cannot be related simply to band

Vmodel parameters which assumes the constancy of U for the devices of thesame material. Confirmation of this suggestion appears from the measurements

on Au-ZnS-Au devices , It has been shown that U can be increasedP

drastically by applying voltage in the region of current minimum of the

characteristic for a period extending to few minutes. In the model proposed

U is related to the distribution of filament resistance which in turn dependsPon spacing of positive centres in the conducting filament. To some extent.

the observations of the present workers seem to be supported by the filamentary

models which imply that the dead time and the voltage for current peak (U^)

-6k-

can be changed for a given sample, hut no satisfactory explanation could "be

possible. The disappearance of DWR above a certain temper attire has also been

left unexplained. Nevertheless* these observations hava shown that low

frequency DNS does occur in Langmuir films and the phenomena under consideration

are to a large extent independent of specific properties of the insulator

material.

b) DHR on triode devices

Hickmott made the first attempt at studying this phenomenon in

triode structures Al-(cathode)~SiO-Al(grid)-SiO~Au(plate). Similar studies

were also performed on aluminium oxide triodes . In both the formed

devices, the potential distribution was highly non-linear between the plate-

grid ( V ) and grid-cathode (V ) regions, V was proportional to V and

was rather small whereas V was nearly equal to V (pc refers to plate-cathode

region). This was the situation when the DNH became fully pronounced. When

the polarity of V was reversed, the voltage distribution did not change,pcHowever, the main voltage drop shifted to plate-grid and then V vas nearly

egproportional to J . [in the discussion V , J , V and V refer to

cp pc pc pg gc

plate positive and V , J , V and V to plate negative.] Hickmott

could not satisfactorily explain these phenomena in triodes.

Gund]ech and Kadlec after a successful attempt to study the DNR

on diode structures (discussed above), extended their studies on triodes by

malting use of organic monolayers and following the arrangement and circuitry,136)

etc., of Hickmott . However, all three electrodes used were of aluminiumo

to avoid unnecessary asymmetry. The grid,of thickness * 50 A, contained a

significant number of pinholes and the other two electrodes (plate and cathode)

were several thousand angstroms thick. The device was formed at T7°K as

in the case of diodes; DNR was found to occur in the second voltage cycle

and to disappear in the next few cycles. At about 1TO°K , the DNR was more

pronounced and V and V changed little as compared with that at 77°K (VpC

was equally divided between V and V at 7T°K). In Fig.31, the J -Vpg gc pc pc

characteristic at 170°K and the change of V and V with respect to Vare shown. Evidently, both V and V increase nearly proportionally to

pg 6CV but these are independent of the J - V characteristics. Thepc PC pc

explanation offered for the latter observation is in terms of the filamentary

model of Dearnley et al. according to which DNR occurs by filament

breaking because the fractured filament does not contribute to the current.

After the so-called dead time, the filament may reform , which presumably

occurs at about 2,8 V in the present case (see Fig.31). It has been assumed

-65-

that the current J flows along the conducting filaments which cross thepc

grid at pin:holes.of

Wanning up/the trio&es further results in a drastic change in the

J - V characteristic as veil as in the voltage distributions V and Vpc pc pg gc

At about 230 K, the DNE disappears (see Fig.32) which is typical for organic

multilayer triodes. A slight non-linearity in the characteristic is observed

in the plate-grid region; the spike 'a1, however, originates in the grid-

cathode region. The dotted curve in Fig.32 is a J - V c characteristic at

205°K,vhich has been considered to represent the sum of two types of currents

below 5.5 V. The first is slightly non-linear without shoving JMR and the

other shoving DNR vhich ceases at 230°K. Non-occurrence of DNR above a certain

temperature and hence the disappearance of the current associated with DHR

has been interpreted by assuming that either the filament does not reform or

i t needs rather a long time. The presence of spike 'a1 and the slightly

higher current above 5-5 V haY.e been attributed merely to some new events

occurring at 230°K. Nothing in particular has been stated about these new

events.

After a sufficient number of voltage cycles, the voltage V or VrO g

was found to be generally proportional to J ; unlike that shown by the dotted

curve in Fig.32. Reversing the junction bias at 270°K (plate negative) led

to an increase in V from 7.5 V to 9-5 "V, which resulted in higher currentcp

J (than J ) and , the re fo re . a changed J - V c h a r a c t e r i s t i c . On the o thercp pc ' cp cp

hand, V decreased considerably but i t was s t i l l p ropor t iona l t o J . OneC£ 131) c p

remarkable fea ture i s t h a t Gundlach and Kadlec a l so succeeded t o developthe main vol tage drop in the grid-cathode region when V was p ropor t iona l t oJ . This was not repor ted in t he e a r l i e r experiments on oxide t r i o d e s

pc

Thus, i t has been concluded t h a t t he re i s no region in t he monolayer t r i o d e s

which e x h i b i t s p r e f e r e n t i a l DNR and i t might be due t o t he e lec t rode symmetry

of these t r i o d e s . Resistances R and R of a formed device (measured a tpg gc

0.3 V) were found to be 1 to 2 orders of magnitude higher than R .which

resembled the observations on SiO triodes

In both the experiments on diodes and triodes, Gundlach and Kadlec

have , explained the observations qualitatively in terms of the filamentary

models; nevertheless, they have remarked that the model itself is still

speculative. With the use of Langmuir films as the insulator, some typical

observations, e.g. the device formations at 77°K, the dependence of DNR on the

film thickness and disappearance of DNR above a certain temperature, are yet

to be interpreted. Therefore, the immediate requirement is the development

of a new model essentially based on filamentary models which could partly-66-

explain the DKR phenomena in these films. One suggestion of Gundlach

and Kadlec, that the thin oxide film (between the "base electrode and the

organic multilayer) inhibits the injection of metal ions from the "base electrode-

into the organic layers does not appear to "be convincing.

In the recent work on thermally stimulated currents in sueh organic

layers» the device used was Al-film-Al ana the ion displacement from one

electrode to the other has "been shown to form ionic space charge vhich is

responsible for the polarization. Particularly in future works, the

possibility of the occurrence of several other phenomena,like switching and

memory, which have already been observed in a wide variety of oxides, must

be ascertained in Langmuir films. This might seem to be a speculation at the .

first instance,but the reader must remember that even the WB. was never thought

to occur in these monolayers before Gundlach and Kadlec made a successful

attempt. At least, the studies of DNB have shown that such a phenomenon is

not restricted to insulators of a particular chemical composition and crystal

structure. More investigations on DKR occurrence must also be carried out

using different types of organic films sandwiched between varying metal

electrodes to ascertain the Influence of the electrode material on "forming"

process and to strengthen the proof of DNE in these insulating materials.

-67-

VIII. IONIC TRANSPORT PHENOMENA IN LANGMJIR FILMS

The study of ionic polarization and transport in -many insulating

materials has been pursued *by workers engaged in this film investigations.

Such studies were reported by Curie as early as 1886 on quartz crystals,

and subsequently Warburg and Togetmeier demonstrated the steady-state

transport of ions like Na , Li and K through quartz at temperatures below

25O°C. Since then, ionic motion has been shown to occur in a vide range of

insulating or semiconducting materials and there has been a many-fold

development of the subject.

In recent years, some problems of commercial importance, for instance,

the surface stability of MQS structures for reliable operation, have been

associated with the motion of ions within the oxide layer on the semiconductor

surface and,therefore,the necessity of such studies was realized. The change

in the capacitance-voltage characteristics of MIS structures has been

suggested to provide a powerful tool for the observation of ionic motion in thinIkf)

insulating films. Similarly, the method of measuring ionic thermocurrents

is known to be a more sensitive method to investigate polarization effects due to

ionic motion in insulators. It has been shown that this method reveals finer

details of the phenomena than the earlier two different methods in use,i.e. by

considering the change in time of the electrical current in a device subjected

to d.c. external field * and by measuring the dielectric losses

Fortunately, in recent years some workers have made successful attempts

to study Langmuir films in the form of MIM and MIS structures to determine

the polarization effects and C-V characteristics, with respective structures.

Here we shall mainly be concerned with the discussion of such studies on Langmuir

films. Preliminary data obtained on change of capacitance with applied voltage

on these films will also be given in this section.

1. Properties of MIS structures

The study of capacitance-voltage characteristics in MOS structures has

proved to be of potential importance in developing a certain type of devices. To152}

investigate the usefulness of Langmuir films for such devices, Tanguy

carried out studies on MIS structures using the organic films of Ca-behenate

and orthophenanthroline with three stearate chains. The p-type silicon

surfaces, after etching in CPlt, cleaning and drying;were used to deposit organic

multilayers and were then covered "by aluminium in the form of two circles

-68-

connected lay a channel wnich served as the top contact. The back contacts

were of evaporated gold or of an Au-Si eutectic. To know the thickness for onemonolayer of ortliophenanthrollne, measurements of the device capacitance C were

themade with respect to/number of monolayers IT deposited. The linear relation

between l/C and H thus gave a monolayer thickness of about 15 A, All the

capacitance measurements were performed on a General Radio 1620 A bridge under low

pressure for good reproducibility of the results- Fig. 33 shows the C-V

curves obtained with 3» 5 and 9 monolayers of orthophenanthroline in which both

accumulation and inversion regions are visible. Evidently, even a small

variation in bias voltage results in a large inodulation of C, which has

been thought to arise from the small thickness of the insulator and from the high

resistivity (- 600 ft cm) of the p type silicon. The curves shown in Fig, 33

represent the situation a little time after etching. In the case when silicon

was left for a few hours in the ambient conditions before roonolayer deposition,

the flat band voltages shifted towards the positve charge and the slope of the

curve was found to become less steep than shown in Fig. 33. When the aluminium

top electrode was replaced by gold, the flat band voltage shifted to zero and

the distance between the flat band voltages of both curves corresponded to the

difference between the respective work functions.

In several studiesJ-53j-15oJ o n M o s structures, the two types of

hysteresis, normal and anormal, are attributed to the ion displacements in the

insulator and to the trapping at interface states. Tanguy ^ ' also obtained

both typesof hysteresis (shown in Fig. 3i+}- Fig. 31+ shows C-V curves of

5-layer thick orthophenanthroline (OP,) device at various temperatures. The

normal hysteresis is related to V (flat band voltage) shift opposite to the

voltage bias.whereas the anormal one is characterized by V__ displacement towards

the bias voltage. In accordance with the earlier workers, Tanguy has also tried

to correlate the existence of these two hystereses with ionic motion in the

insulator and trapping,respectively. Presence of ions in the fatty acid soaps is

a well known feature as these are ionic substances forming stable monolayers

{•Ref. 7.p,i93) and there is a possibility that some ion content might get

transferred during monolayer deposition. Many times, ions can be adsorbed later

during electrode evaporation. Tanguy has, however, made use of the novel

technique of studying thermally stimulated currents (TSC) in these organic films

to make sure of such ionic motion. TSC measurements were performed on MIM

structures with one layer of 0P_. These experiments gave a clear-cut indication

that the short circuit currents arising on warming up of the structure are a

consequence of ion displacements because the amplitude of the current peaks varied

greatly with the bias temperature. At lower temperatures, such peaks were not

' ; -69-

observed. Further, it was estimated from TSC peaks at 20°C that about

2 x 10 charges are displaced or trapped, which corresponds to a C-V curve

shift of the order of 100 mV for 3 OP- monolayers. To make sure of this

statement, C-V curves were obtained for 3 OP. monolayers with different

temperature M a s conditions which yielded the V,™ shifts agreeing quite well

with the above estimation. This estimation assumes that the mobile ions can

move from one electrode to another. In a separate measurement of TSC in

calcium behenate monolayers, it was found that the number of free ions is less

important, which has been attributed to the fact that stearic acid chains are

more compact in behenate than in OP .

Anormal hysteresis, which appears below 20°C in the case of 0P_

(see Fig. 3*0, has been found to become very important for calcium behenate.

In the latter case, a continuous Vp shift extending to about I V in an hour

has been observed. To explain the existence of anormal hysteresis, Tanguy

assumed that the traps occur in the natural oxide layer. However, it could not

be proved in these preliminary investigations that no trapping exists in '

molecular layers. In one of the models155) proposed to explain the anormal

hysteresis in MOS structures, it has been assumed that the holes emitted from

the valence band of the silicon are trapped in the oxide or at the interface and

these positive charges cause V_ shift towards negative voltage. Some workersI'D

found a correlation between the number of charges in the oxide layer and

the number of surface states in the silicon. This has been regarded as an

explanation of the observed instabilities with the calcium salt in the presentinvestigations.

1*5)In an earlier communication, Tanguy and co-workers reported I-V

characteristics on MIM structures using OP as an insulator. When similar |

experiments were repeated on MIS structures by Tanguy, the currents observed :

were in good agreement with those obtained from MIM structures. It has .

been found however, that the currents are weak compared with those obtained from

MOS structures j5°) with the oxide of comparable thickness. Fig# 3c *

shows the I-V curves for MIS structures obtained on 1, 5 and 9 monolayers of I?

OP . Evidently, there is a shift of I-V curves near zero volts which reveals ;

the existence of an electromotive force in the monolayers when there is no f

external bias. It has been suggested "by Tanguy that the e.m.f.'s arise, ;

presumably, from chemical reactions in the layer at the interface. In an earlier \

work "by Leger et al. , the existence of e.m.f. in MIM structures was •

reported. It was found that the Junctions act as batteries because a voltage I;

of about 200 mV was obtained when a high impedance voltmeter was connected !•

across the junctions. The existence of e.m.f. has been suggested to arise !

due to the motion of charged ions across the insulator to attain the equilibrium \

- 7 0 - • .• I

state. These workers could not cement "what type of ions actually cross themolecules "but some kinetics- of these tons nave "been etudied in the range

20°-T0°C. For instanca, th« Internal reflistunoe R i n t * ftxp(E0/KT} wheraEQC- O.36 eV)is the activation energy, vhich is comparaTale with the activation

energy {0.6 eV) required for water molecules to cross a fatty acid monolayer1 en 1

spread on water surface to prevent its evaporation Jyj, It has also been-

noticed that the initial e.m.f, of the battery depends on the nature of the

sample under investigation but EQ remains the same. To realize the practical2

importance of observed e.m.f. in these junctions, 226 Junctions of 0.25 ram

each were prepared and connected in series. The values obtained for total

voltage was 50V, R. , - 10 fi and the total volume of MIM structureslilt*2

- 0.1 imn . . It has been thus suggested that such a high impedance battery with

so small a volume might prove of special significance in a few practical applic-

ations. The capacity of these cells {- 1 uc) has "been found to correspond to

the oxidation of about one atomic layer.152)

One of the most significant contributions of Tanguy has been in

obtaining the reproducible MIS structures which behaved almost as MOS

structures. This has opened up a new field of investigations for studying semi-,

conductor surfaces using organic molecules which might be exposed to high electric

fields. In these "basic investigations, occurrence of anormal hysteresis in

MIS structures could not be explained very well and,therefore,further studies

with other types of organic molecules are needed for a better understanding of

the phenomenon. Technological applications of MIS devices, like those of MOS

devices, must also be worked out after successful studies on MIS structures.

2. Thermally stimulated currents

1VT).16O) a ^

The ionic thermoconductivity method , which involves the study of

the thermal activated release of dielectric polarization has already been used in

a wide variety of dielectrics. The essential steps of the method consist of;

(i) Polarization of the sample in a static field E at the temperature T for

a time t - The temperature should not be too high to develop heavy space-charge,

(ii) The sample is then cooled down to a temperature TQ•« T so that the

relaxation time X'fT ) is of the order of several hours or longer, (iii) The0 , ,

electric field E is now turned off (at T = Tn) and the dielectric is warmedp u .

up with a constant rate g = dT/dt -and'the "discharge current is- registered as a

function of temperature. The polarization current i(T) first increases

-71-

exponentially, reaches a maximum and then drops to aero. Thus the analysis of

the peak obtained allows- the determination of the activation energy of the

relaxation process and reveals the finer details of polarization.ICO \

Tanguy J ' initially used this method on Al-orthophenan-

throline-Al structures- to observe ion displacements in -monomolecular films. Such

studies were carried out to explain the normal hysteresis in MIS structures and

it was stated that the mobile ions in the mono-layers can move from one electrode

to another. In the structures studied, ion displacement was held responsible for

the large variation in the amplitude of the current peaks at higher temperatures.

Later on, Tanguy and Hesto have -made a systematic study of the polarization

phenomena in MIM structures by thermally stimulated currents (henceforth TSC)

method. We shall first discuss the theory used by these workers to explain TSC

in organic films and then the experimental observations will be described.

a) Theoretical model

Of all the models, the one found suitable to explain the TSC peaks

assumes that the electrical charges can move between two potential wells

separated by only a fev Angstroms. In this model, the charge contained ~by the

two potential wells changes when the device is subjected to an applied field and

gives rise to a volume polarization called orientational polarization in the

specimen. The position and shape of the TSC peaks thus obtained depend only

on the potential barrier height and not on the initial experimental conditions

(temperature,electric field, polarizing time) as in the other theories

Assuming N pairs of potential wells per unit volume which are randomly

distributed and if 2a is the separation between two centres, the total polar-162)

ization Pj is given by :A

P = eaN Z tanh (uZ)dZ , (8.1)

00

eaETwhere u a ^m

• P

If the electric field EL is relatively small, then u « 1 and Eq, (8.1)

reduces to „/ ,2 „B(ea) E

Pi = 3kTP

-E (8.2)

which shows that polarization is a linear function of the applied field K .

Further, the relaxation time T which is the average time needed by a charge to hop

from one position to another over a potential barrier £ » has been defined as

% ' (8.3)

where rR is a constant.

-72-

The initial polarization is a function of the polarization time t

during which the sample is held at the initial temperature T (room temperature)

according to

If a MIM device is then cooled rapidly to 77°K, it releases the stored

charge on reheating so that the discharge short current may be expressed as

"dP _ P

dt"~~r > (8.5)where P is the deviation of polarization from equilibrium.

If the reheating is a linear function of time, T = T- + gt, where 3 is0 160)

a constant, the discharge current as a function of temperature can be writtenas (using expressions (8.3) and (8.5)1

P. . . . _

(8.6)

This polarization current gives a maximum at T = T and its' derivativem

at T = T presents a relation between T , €. and tn expressed as;m m u

Clearly, from Eqs. (8.6) and (8.7), the shape of TSC depends only

on the parameter £ ancL the maximum peak amplitude depends only on P . In

their wort, these authors have fitted the experimental peaks on the theoretical

curves of Eq. (8.6) and determined the polarization parameters £ and Y o .

Separate values of N and a have also been calculated by fitting the experimental

curves of P as a function of the electric field in the theoretical curves

obtained from Eq. (8.1).

b) Experimental measurements

Tanguy and Hesto tried to obtain data under various experimental

conditions,i.e. with varying temperature (T )» voltage (V ), time (t ) and. P P P

specimen thickness (a). in the measurements with respect to T« on different

numbeisof monolayers of OP^, the TSC peaks were found independent of T .

Fig. 36 gives a graphical representation of TSC peaks on 23 monolayers thick

0P3 at various temperatures ranging between 28l°K to 315°K when all other

parameters V . t and 3 (rate of heating) were kept constant. Evidently, a

-73-

polarization maximum is observed at T = 301°K and there is a decrease beyond

this temperature due to thermal agitation. This is clearly compatible vith the

theoretical expression (8.1*) at high temperatures vhen *y "becomes small and

hence P. reaches a maximum. However, for T > 301°K, the peak tail has been

assumed to appear because of space charge formation. Since TSC peaks are veil

defined in position at room temperature, absence of space charge formation has

been concluded.

When MIM structures were biased at different voltages V for a fixed

time at fixed temperature, the TSC peaks obtained vere perfectly stable. According

to expression (8.1), PQ must show a saturation at increasing electric field for

u > 1, which these workers failed to obtain for 3-monolayer thick 0P_ structures6 -1

because the applied field exceeded the dielectric breakdown (~ 5 x 10 V cm" ).

However, on thick structures, such saturation was found to begin at E = 10 V cm"

for 7 layers and E = h x io v cm" for 21 layers. It thus appears that the

applied electric field sufficient to obtain maximum polarization depends upon the

thickness of the film which in turn increases a in the expression for u (see

Eq,.(8.l}). While studying P as a function of polarization time t for 3

monolayers at 8°C, keeping the bias under one volt, two distinct curves corres-

ponding to t, 60 seconds and f > 2000 seconds were obtained. The former has* 2

been related to the charging time of lower energy peaks and the latter to thecharging time of higher energy peaks. The time needed to form 80$ of the total

polarization was found to be ten minutes at 8°C and to reduce to one minute when

. the temperature was increased to 20°C.

Thickness-dependent studies of total polarization showed that the total

polarization charge increases almost linearly vith the number of monolayers Nb

when T , t and E are held constant. Clearly, increasing Kb increases the

value of a in Eq. (8,2) and thus the polarization charge increases because of

the Ma term. According to this expression.(8,2^ the total polarization must

not vary with increasing lib for a given electric field E because the numberP

of potential veils N in the dielectric does not vary vith che dielectric thickness.

Since, the basic structure of the monolayers changes with increasing dielectric

thickness, the respective distance between two potential wells 2a and the

activation energy c are modified resulting in the higher energy peaks of group d

with increasing Mb Fig. 37 shows the TSC peaks obtained at different dielectric

thicknesses vhen all other parameters were identical.

One of the important measurements carried out by these workers is the

study of influence of water molecules which, presumably, are adsorbed near the

electrodes (possibly in the alumina). Initially, when a MIM structure was placed

under low pressure and heated, a large space charge accumulation appeared and a

broad peak vith a maximum was obtainedt This "was the situation for temperatures

-Ik-

loeyond 6O°C and vlnen the dielectric thickness was relatively small, For

higher thicknesses CK 23 i&onqlayers-I, the' space charge disappeared under low

pressure at 3O°C during approximately one hour. On the contrary, th«

elimination of space charge needed about US hours for 3 monolayers and a few

days for 1 monolayer. Thia has teen qualitatively explained in terms of the

influence of the electrodes on the electrical properties of the MIM device

which decreased with increasing dielectric thickness. It has "been found that

the polarization due to residual water molecules became negligible after a short

stabilizing time of less than an hour at 30°C.

Tanguy and Hesto (63) further attempted to calculate the polarization

parameters by fitting the experimental peaks to the theoretical curve according

to Eq.. (S.6). In their experimental investigations, several groups of peaks

were identified, each characterized by a polarization temperature. In particular,

they concentrated on d peaks which are more pronounced in the case of multi-

layers and are composed of several mono-energetic peaks d. » d_ , d^ , d, and d

as shown in Fig. 37 • Since the polarization was found to depend greatly on

the temperature and polarization time, these workers fitted two peaks obtained

with 21 monolayers (see Fig, 38) at temperatures of 290°K and 297°K and

obtained the characterizing polarization parameters of all the d peaks. In

another attempt, a fit of theoretical values of Po(Ep) (according to Eq.(8.l))

with the experimental curve for 21 monolayers vas also obtained and the

parameters N and a were obtained for 3, T, 21 and 170 monolayer-thick film

structures. It has been reported that for depositing 170 monolayer thick film,

OP,. molecules dissolved in "benzene were directly laid on the substrate and the

solvent vas allowed to evaporate, From these theoretical values, it was found

that the polarization increases with increasing Kb , presumably due to an

increase in the average distance 2a between tvo ion equilibrium positions. In

fact, 2a has been found to increase from 1.2 A for 3 monolayers to 10 X for

27 monolayer thick structures. This phenomenon has thus been regarded aa

evidence for some basic structural changes in the multilayer with increasing

thickness. From the calculated high values of activation energies ranging

between 1.1 eV to 3.1* eV and because PQ varied rapidly with the

polarization temperature, the hopping charges have been thought to be ions

which move between two equilibrium positions with 2a distance apart.

It has been discussed earlier that Marc and Messier ' " attributed the

two absorption peaks to the dipolar movement in the amorphous and crystalline

regions in the behenate monolayers (Sec.IV) and thus suggested some structural

modification in these monolayers. Similarly, these workers have assumed

that as the monolayers grow thicker and thicker, the molecules tend to form

-75-

crystallities whose number and volume increase with increasing film thickness.

This assumption has "been further Justified "by some experimental evidence. Forthey cpuId *liu

instance,^ observe the structural modification optically "by a colour change

due to light reflection on the lower metallic electrode when the multilayers were

heated to 35°C. At this temperature, presumably the CH2 chain fuses and OP

molecules are reordered forming crystallities. These crystalline regions in

the multilayers have been assumed to he the trapping centres for the ions vhich

are thermally released and can move in disordered regions giving rise to the

observed polarization phenomenon. It has been remarked,further,that calcium salts

of stearic and behenic acid are relatively little polarizable especially when the

chains are long enough and compact. In these eases, the polarization has been

attributed more to an ionic space charge formation.

Whereas Tanguy and Hesto ; have carried out a systematic and detailed

study of polarization phenomenon in OF- molecules, a few points are not yet

made very clear in their analysis. To the knowledge of the author, there is no

other reported work in which the thick multilayers could have been obtained by

simply laying the molecules,dissolved in solvent,on the substrate. It is rather

surprising how these workers could form a uniform film in this simple way.

Nevertheless, this simple method seems to be fascinating and,in future works»

specific attention must be paid to the suitability and applicability of

this method to many other organic substances. Another remark must be made

concerning the degree of structural modifications in behenate or stearate molecules

and OF,, molecules. According to Tanguy and Hesto , the structural changes

in the more compact behenate or stearate molecules are less evident,whereas Marc

and Messier J \ have based all their experimental results on structural changes

in these molecules. Therefore, structural studies have to be renewed and

scruplous attention must be devoted to this point. The role of metal electrodes

and of the oxide layer formed, must also be considered in future investigations.

3. Capacitance studies with applied voltages

All the experimental investigations discussed in Sec.IV relate to the

capacitance studies of MIM structures as a function of monolayer numbers which

have also been employed by various workers as a simple tool to obtain monolayer

thicknesses in the case of several organic compounds. Similarly, Leger e.t al.a

obtained the • thickness of a stearate monolayer, about 22.A. These workers

further studied the variation of capacitance of MIM structures as a function

of applied voltage V and obtained some useful information. Fig.-3°A

represents plots of AC/C (where AC is the change in the capacitance)versus

\ -76-

applied voltage V on Al^iarium stear&te-Al samples with, ths capacitor area

- 0,25 ma. , for a varying number of layers. Clearly, the nature of the curves

varies with the number of deposited monolayers and all the data can "be fittedAC 2

to obey the universal linear dependence of the type ~rr - AE (shown in theArt £> p

corresponding Fig.39B ' vith A = (1.6 ± 0.15) * 10 V m . In thia

figure the applied electric field E has been plotted in terms of V/H, H

being the number of deposited monolayers. Obviously, the curves for AC/C

versus V are not linear and this has "been taken to be the evidence for an

internal field inside the organic layers. These curves, which have a minimum

for V = 0 t lOOmV, have been regarded to. set an upper limit for an internal2

field. The assumption that the linear dependence of capacitance on E is due to

the compression associated with the electrostatic pressure on the capacitance

plates,leads to a compressibility coefficient of the stearate molecules which

would be one order of magnitude larger than for bulk paraffins. Although, such

a value could be plausible, other phenomena,e.g. electrostriction action on the

dielectric constant of the insulator, should possibly be taken into account.

In the opinion of the author, it is hard to draw any conclusion from these

preliminary measurements. The physical mechanism suggested by these workers

does not appear to be conceivable until further measurements could be made.

However, the presence of an internal field inside the monolayers reported by

these workers have been verified further in the ionic transport studies of these15 2)

structures and some similar indications also come from the work of Tanguy ' .

As has been suggested "by Leger e^.al. themselves, the best course would

be to repeat similar measurements on various organic substances and to find out

whether the order of magnitude of the compressibility coefficient of organic

molecules is plausible.

IX. FUTURE TRENDS ABD PROPOSED APPLICATIONS

In fact, much has already been said regarding the future trends of

research on Langmuir films. Nevertheless, to sum up, a few more remarks are to

be made which concern us in any type of electrical studies of these films.

Essentially, one of the challenging problems is to solve the riddle about the

structure of Langmuir films. It has been regarded for a long time that the films

obtained by B-L technique form ''two-dimensional" crystals,whereas some recent

investigations completely rule out this possibility and indicate some structural

changes inside the organic layers. Renewed structure studies are particularly

important because all the electrical properties* by and large, are "structure

-77-

sensitive". Another equally important aspect ia to concentrate on many other

organic substances, e.g. proteins-, chlorophylls-, dyes, esters,etc., which are

eligible for monolayer formation and subsequent deposition "by using the

B-L technique. It is rather surprising that even in the recent investigations,

the workers have not paid -much attention to organic films other than those of

fatty acids or their salts. Presumably> only French workers have made a break-

through in this respect by studying orthophenanthroline molecules which were

deposited by E-L technique. It is interesting to point out specifically the

organic compotind "cholesterol" (C.-Hi -OK) which has been studied by some Russian

workers ' for "forming" and subsequent phenomena. In their studies the

monolayers or bilayers of cholesterol were obtained by a different technique

involving adsorption of polar molecules on a molecularly smooth surface of

liquid-mercury. However, these molecules, in principle, must also be obtainable

by B-L technique and efforts in this respect would be highly desirable. It has been

reported by the Russian workers (Ref.l61* and references therein) that electrical

studies on generally structured films of.surface active organic molecules are

of special significance to give information on the nature of phenomena occurring

in "biological membranes.

It has been noticed "by several workers that Langmuir films deposited on

gold base electrodes are shorting in nature and the electrical measurements are

not possible on such structures. On the other hand, with aluminium base

electrodes, the oxide layer formation does complicate the situation In the

interpretation of electrical data. There are some controversies in the existing

literature about the influence of oxide layer on the electrical properties of

Langmuir films. For instance, some workers have taken into account the oxide

layer whereas others have neglected it(considering its low resistance as compared

to that of the organic films. There are also few instances In which the

electrical data could be Interpreted in terms of the oxide layer presence in the

device. The reason for obtaining shorting devices with gold base electrodes

has been the poor adherence of molecules and hence the increasing porosity in

the organic films. Despite the fact that some alternative metals have to be

tried for the base electrodes to avoid oxide layer formation, the author

proposes to take advantage of the recently developed thermal evaporation

technique to obtain organic films on gold base electrodes. Comparison of

studies performed on Langmuir films and on evaporated organic films of the same

substances on Al and Au electrodes,respectively, might prove useful to

separate out the Influence of the oxide layer. The presence of water molecules

adsorbed during monolayer transfer in the B-L technique can also be avoided if

the renewed studies are performed on evaporated organic films. Fortunately*

-78-

Baker has pointed out that organic film could "be obtained "by thermal

evaporation down to one monolayer in thickness which, possessed more purity

than the original evaporant. Scrupulous attention however, must "be paid to

develop the thermal evaporation technique in future.

Despite the fact that Langmuir films have been reported to possess

structural perfection and could "be obtained free from even a small fraction of

holes and conducting impurities, it seems to "be an optimistic view to call

these films ideally suitable for electrical studies and device applications.

The problems which have hindered their use in the electronic device application

until now are the same as with many other films systems. For instance, problems

with respect to stability of the fatty acid films, which are soft, possessing

low melting points, and tend to collapse slowly with time. Although considerable .

efforts have already "been expended to improve the disposition technique and to

obtain stable films, in'actual practice, however, every new application

requires a lot of painstaking work before Langmuir films with reproducible and

stable electrical properties can be obtained. Nevertheless, some of the

possible applications are discussed here to throw light on the status and role

of Langmuir films mainly in the electronics industry.

One of the most attractive applications of Langmuir films may be in the

fabrication of "low loss capacitors" which find their use in active filters.

For the choice of suitable material to fabricate a capacitor, one needs to

consider many dielectric parameters,e.g. dielectric constant, dielectric strength,

dielectric loss, stability, temperature and voltage coefficients of capacitance,

surface smoothness and insulation resistance. In the case of fatty acid

films, several of these parameters are already known. For instance, the

dielectric constant of stearate is 2.1+ and films with capacitance densities

0.008 to 0.07 pF/cm have been obtained. These films could also have "been

deposited containing as many as 300 layers ( at 7500 A) and the capacitance

has been found to vary only slightly with the frequency. The dielectric

strength is a function of thickness and it is of the order of 10 Vcm ;

dielectric losses 2?0.02 to 0,006 have been reported. All these experimental -

values fulfill the optimum conditions to a greater extent for "low loss

capacitors" as has been reported by Harrop and Campbell . Fortunately,

many organic polymers and other organic materials have already been used for

this purpose.

Another possible application may be to develop tunnelling spacers between

superconductors, as was attempted by Miles and McMahon ; a long time ago.' These

workers failed to obtain useful tunnelling spacers simply because the device

obtained showed highly non-linear characteristics. In fact, the deposition

technique was not well developed in those days and one may hopefully proceed in the

light of recent advances. For the controversy regarding conduction behaviour

whether it is dominated by tunnelling or Schottky emission, the best test would

be to study inelastic electron tunnelling between two metal films separated

by organic insulating material. If it exists. This technique of studying

inelastic electron tunnelling was- introduced "By- Jaklevic and Lambe *

who distinguished it from the well-known tunnelling mechanism in which an electron

goes through the barrier without energy loss. Inelastic tunnelling results in

the energy transfer from electrons to the vibrational states of molecular species

contained within the barrier. It has been pointed out that for such studies

the insulating film between two metal films must be about 50 A thick which can

support a field of U x 10 vcm~ ., This may, however, introduce some difficult-o organic

ies because the dielectric strength of 50 A thick]film has been reported to be109), 121) h

little lower but these are likely to be overcome in future investigations

as the dielectric strength measurements vere made under atmospheric conditions

rather than under vacuum.

Since Langmuir films have been shown to undergo "forming" and to exhibit

voltage controlled negative resistance, some practical applications may be

discussed which make use of such characteristics. The best use may be in the

development of a thin film resistive memory arrays based upon voltage controlled

negative resistance in Langmuir films. Such a device was proposed by Hielson

and Eashara and vas described later by Simmons and Verderbei °* for

SiOx materials. Before making an attempt In this direction, one has to study

more carefully the memory switching of the diode and triode structures and

whether the information is stored in the form of permanent changes in the interior

of the film. Similarly, one can also think of some "hot"electron devices* In.

these devices, the basic requirement Is that of a metal-insulating film-metalso

structure in which the anode is sufficiently thin/that hot electrons traversing

the insulating film are able to escape into the vacuum above. It has been shown

by Gundlach and Kadi;c 13°J>131J tnat the phenomenon of DNB may be attributed

largely to the exists ice of filamentary conduction paths in these films. If the

proposed filamentary conduction does exist In these devices, this application

would be worthwhile to attempt. As has already been pointed out, if electro-

luminescence is also exhibited in these films many more applications might be

possible.

The application of Langmuir films in the form of metal-insulator-semi-152)

conductor devices is also possible,as Tanguy could successfully deposit

these films on silicon surfaces. In fact MOS capacitors and transistors are •

already in the development stage and therefore analogous MIS structures might

also be considered for such devices. Presumably, MIS capacitors and

transistors vould be more successful than the corresponding MOS ones because

the organic films may be obtained largely free from defects,which, in the case of •

oxides, IB rather difficult. The use of organic films in the form of MIS-80-

structures also opem up another practical application for poasiyated dielectric

films on silicon. Pt \yiner films hare already 136611 shown to possess favourable

properties for similar purposes °i. The only difficulty may "be due to the

hysteresis phenomenon, "but it must "be surmounta"ble "by appropriate treatment of the

silicon surface. It would also be advisable to explore the possibility of

achieving controlled doping of an organic dielectric in the way that was attained

with crystalline germanium and silicon. The'fact^that alternating monomolecular

layers of different organic materials may be deposited m e on top of the other,

offers an exciting possibility to fabricate some novel devices. One similar use

of fatty acid monolayers has "been in the form of distance keepers between dyeop \

molecules, while studying photographic sensitization mechanismsome uf the possible

Having discussed / applications of Langmuir films, it is essential to pointou

once again that although these films possessmany attractive features unlike those

of the evaporated film systems,nevertheless one must not be too optimistic and

expect all that has been said. In addition to these electronic device applications

one of the most successful uses of Langmuir films has been in the fabrication of a

fairly stable optical device called "step gauge" or thickness gauge which is

commercially available from General Electric Co., USA. Other optical applications

of these films are worth mentioning,e.g. antireflection gratings and interference

filters. From the purely scientific point of view, the resemblance of Langmuir

films to biological membranes suggests that a careful study of these films would

lead to the understanding of many biological or life processes. Some more159)industrial applications of these films are to achieve evaporation control and

to reduce friction , etc.

Since only a handful of electrical data is available on Langmuir films,

it would be unfair to make any committment regarding their practical applications.

A few proposals made above in this respect are in a way speculative until these

possibilities are explored further. Therefore, it seems to the author that

the potentiality of Langmuir films 3till lies beneath the hands of experimentalist!

and many-fold development of thin film technology would "be achieved if successive

efforts are continued. The application potential of these films is more likely

come up from a systematic study of many diversified electrical phenomena. Of

all the proposed applications, the development of capacitors is expected to

lead in the near future to electronic hardware for thin film microcircuitry.

-81-

ACKNOWLEDGMENTS

The author wishes to express his sincere gratitude to Professor V. Celli

for critical reading of the manuscript and for many helpful discussions. Thanks

are due to Professors A. Many, C,A. Neugebauer, N.H. March and G, Dearnley,

who were at the International Centre for Theoretical Physics during the Winter

College on Surface Science, for many "useful suggestions to improve the text

of the manuscript. The author is grateful to Dr. V.K. Srivastava, in partioular,

and his other colleagues, who (while the author was in India) initially inspired

him to write up the present manuscript. The preparation of the manuscript

vas initiated during the'author's participation in the Winter College on

Surface Sciences at the ICTP and completed during the following Workshop.

It is with great pleasure that the author expresses his thanks to the

ICTP staff for all the co-operation and help without which work on this

manuscript would have not progressed so smoothly. The author also wishes to

thank Professor Abdus Salam, the International Atomic Energy Agency and UNESCO

for hospitality at the International Centre for Theoretical Physics, Trieste.

The author would be failing in his duties without acknowledging the permission

granted by the respective authors and publishers of different research papers

{and journals) for the reproduction of the figures and tables in the present

review. The Swedish International Development Authority is gratefully

acknowledged for making possible his Associateship at the ICTP.

-82-

APPENDIX

NOTATIONS IN THE PAPER UNLESS OTHERWISE DEFBffiD IN THE TEXT

6 = dielectric constant

C a capacitance

d = monolayer thickness

M = no. of monolayers transferred

A = area of the film capacitor

n = number of carbon atoms in the molecular chain

I = current -

V » voltage

k « Boltzmann's constant -

T = absolute temperature

r a work functionf = frequency

J = current density

e = electronic charge

•R = Planck's constant

m = electronic mass

e Q B permittivity of free space

y - electron mobility

^ = potential barrier height

-83-

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-89-

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2:Tp=287#K

2W 240 270 300 330.

KA)

Id'

1

Tp.293°KVp= IVolt

. tp = 10 mn0 < 0.16°K/ite

I N b s a

4210 240 270 300 330

-107-

• +

Oo

- i nfl-

Pig.

Fie.

1

2

3

FIGUKE CAPTIONS

Diagrammatic representation of the apparatus for •building up the

Langnruir films [modified after Langmuir I-

The molecular orientation of built-up Langmuir films, (a.) X-type;

(b) Y-type; and (c) Z-type.

Reciprocal capacity l/C vs. predicted number of layers W . The

heavy dashed line corresponds to the bulk value (£ = 2.5) and the

dotted lines represent the experimental values of £ = 2.1 and

U.2. The cluster of points shown in the figure fall on top of

each other and are slightly misplotted to the left or right to show

the density distribution of data. [After Handy and Scala. ']

Fig, h Capacity C vs. frequency f (upper curve) and capacity x

dissipation factor (CXD) vs. frequency f {lower curve). [After Handy

and Scala. ^]

Fig.. 5 Reciprocal capacity l/C vs. number of layers N for cadmium

stearate (n = 20) and cadmium behenate {n = 22). [After Mann and

Fig. 6. Dielectric constant & vs. film thickness d of barium stearate

film. (A) without oxide layer effect, (B) with oxide layer effect.

[After Khanna. ]

Fig. 7 Equivalent electrical circuit of a metal-Langmuir film-metal

structures. [After Marc and Messier. ]

Fig. 8. Loss angle of hehenate layers tan <Sm vs. temperature & measured

at f ' 1 KHz and at a surface pressure T of kk dynes/cm.

[After Marc and Messier. ]

Fig. 9 Comparison of dielectric losses with respect to temperature (a)

corresponding to a point in the middle of the behenate film, veil

organized structure; and (b) at a point near the edgei disorganized

structure. Data reported for N *• 25» "^ - kk dynes/cm and

f - 1 KHz. The lowest curve represents losses due to oxide layer,r 38) 1[After Marc and Messier. J

110 Current-voltage charactersitics at 0.1 Hz of Sn-3 layer calcium2

stearate-Sn sandwich of «lmm area at room and liquid nitrogen

temperatures. Polarity is that of organic coated tin "base layer.

[After Hanay and Scalal*2^]

-109-

Fig. 11 Plot of current vs. temperature at constant voltage for ** 3 layer

thick film (uppermost curve) and for ~ 5 layer thick film tthree

lower curves). Polarity of test voltage is organic coated "base

electrode positve. [After Handy and Scala.

,t \ / _ *__ \ _ J \ I -* XFig. 12 Plot of log (i/T ) vs. (l/T) and log

T2'vs. (l/T) for 3 layer

sandwich, where I - total current, I = low temperature current in

amperes and T = absolute temperature. [After Handy and Scala, 3

Fig. 13 (a) Plot of I and I vs. V at fixed temperature for Sn - 3 layer Ca-

stearate-Al junctions;and (b) plot of I and I vs. — at fixed '

voltage for Sn-Ca stearate junctions. Black dots and white circles

represent I2 and I , respectively. [After Horiuchi et al. ']

Fig, lit' jN vs. V for Al-Cfi stearate-Hg devices when Al electrode positively

biased. The dashed curve N = 5 to 21; J = ,}. (impurity dependent

current), proportional to V and l/N. Full curve N = 1;

i ~ <L + J • (J4. tunnelling current). j. surplus over j. is given

by dashed curve* [After Mann and Kuhn * J,

fjtd 1Fig. 15 Logarithm of a = \—r- V •+ 0 (o tunnell ing conductivity) vs. d1 * I, v j z •

for Al-Cd salt of CH {CH ) COOHH d i t 20 d 35°C3 2 n

n = 18 - 21. [After Mann and

I, v jfor Al-Cd sa l t of CH {CH ) COOH-Hg devices at 20 and -35°C with

3 2 n-<>

2Fig. 16 Plot of current vs. voltage for Al-cyanine-Fb device of area "*lmm

at a temperature of 1+.2°K (curve A).

Both the curves (A) and (B) are plotted to show the behaviour in

different voltage ranges. The appearance of the superconducting

lead gap Ap^ is marked in the curve (A).

Fig. IT Current-voltage characteristic for a 5 layer thick stearic acid film at

room temperature plotted as log I vs. V . [After Nathoo. ]

Fifi. 18 Recorder tracing of the current I after switching on a steady field

for Al-8 layers thick stearic acid film-Al. The insert is a

corresponding log I - log t plot giving a slope of -0.7- [After

Kathoo and Jonscher. ]

Fig. 19 The a.c. conductivity o(u)) vs. frequency u for 11 layer thick film2

with the specimen area ~* Jprm . The conductance scale refers to the

lowest temperature,»higher temperature characteristics being successively

displaced by one decade for clarity. [After Nathoo and Jonacher. 3

-110-

Fig. 20 Frequency domain measurements on two areas la) and (hi of the same

device, Al-5 layer stearic acid film-Al, for a range of electric fields

at room temperature. The dashed line for sample (b) represents

the "true a.c." data after deduction of the direct current. Data for

sample (b) are displaced downwards by two orders of magnitude for

clarity. The curves represent log-log plots of (i/E) vs. f where

i is the current per cm and E is the field in V/cm. [After96),

Jonscher and Nathoo. J•

Fig. 21 Time domain measurements of the discharge currents {symbols indicated)

and charge currents (open circles) for a different area of the same

film as in Fig. 20 , for different charging fields. Charge currents

have d.c. subtracted. The curves are log-log plots of I vs. t

(time). [After Jonscher and Kathoo. J

Fig. 22 Plot of log F vs. log d for Al-barium stearate-Hg devices. Curves

A and B corespond to the thickness range 25-250A (1-10 layers)

and 25O-2OOOA (10-80 layers),respectively. The bars represent the

scatter in the experimental data at a particular thickness and the dots

correspond to the mean values of several observations made on different

samples of the same thickness. [After Agarval and Srivastava. ^' ]

Fig. 23 V-J characteristic in the non-destructive phase for a UO-layer barium

stearate sandwich with area = 0.5cm at room temperature when the

series resistor (- 5K) between the source and device: was connected.

Point A represents the"onset breakdown voltage" and B the

"destructive breakdown voltage" of the device. [After Agarwal and

Srivastava}22h

Fig. 2k Transmission photomicrograph'showing the destruction of the2

film capacitor (area ~0.^cm ) of thickness *v 16 layer at 8^ volts

when the series resistor (= 5K) was used. [After Agarwal and

Srivastava. ]

Fig. 2$ Log-log plot of maximum breakdown strength F vs. film thickness d

for barium stearate films in the thickness range (Uooi - 1000A)

at room temperature. The dots and bars represent the same relationship

as in Fig. 22. . [After Agarwal and Srivastava. *]

Fig. 26. (A) Plot showing breakdown field F,_ vs. temperature T for1 Al-20• — • * • — • • • * U

layer barium stearate-Al sandwich in the temperature range -40 to +40°C.

(B) F^ VS. T for a similar specimen as in (A). :The Tjars and dotsb max

indicate the came relationship as in Fig. 22. [After Agarwal and

-111-

Fig. 27 Plot of a.c. breakdown voltage V . vs. film thickness d of

"barium stearate at fixed frequency " 30KHfc in the thickness range

100A - 1500A . The relationship between dots and bars is the same

as in Fig. 22 . [After Agarval and Srivastava. 1

Fig, 28 a.c.breakdown voltage V , vs. frequency f for "barium stearate film

of thickness 500A in the frequency region 10-200KHz. The relation-

ship between bars and dots is similar to that indicated in Fig. 22 .123)

[After Agarwal and Srivastava. ]

Fig. 29 Current-voltage characteristics of an Al-(Aloxide)- 11 monolayers-Al

sandwich for increasing temperature top electrode "being positive. The

vertical scale is displaced for each curve for clarity. DNR completely

disappears above about 210°K. [After Gundlach and Kadlec. ]

Fig. 30 Current-voltage characteristic of the device as in Fig. 29 at different

voltage cycles. Curve 1: first voltage cycle of an unformed sample.

Curve 2: second voltage cycle, the sample is now formed and exhibits

DWR. Curve 3: smooth characteristic after a few voltage cyclesj

sample temperature was 100°K. Curves h to l'< first four voltage cycles

after temperature increases from 100°K to 190°K. Curve 8: first

voltage cycle after interrupting junction bias for 3 hours, at 190°K.

Curve 9- smooth curve after some more voltage cycles at l°0°K. [After130). .

Gundlach and Kadlec. J

Fig. 31 Current-voltage characteristic and voltage distribution of an Al-9

raonolayer-Al device at 170°K, the plate being positive. [After131),

Gundlach and Kadlec. ]

Fig. 32 Current-voltage characteristic (full curve) and voltage distribution

of the device similar to, that in Fig- 31 at 230°K. The dotted curve is

J-V characteristic at 2O5°K. The spike 'a1 in the curves is probably

just a consequence of new events at 230°K, [After Gundlach and

Kadlec.131 >]

2Fig. 33 C-V curves for MIS structures (area - 125mm ) with 3, 5 and 9

monolayers of orthophenanthroline at a frequency of 1KHz. [After

Tanguy. ]

Fig. 3^ Shift of C-V curves with 5 OF- monolayers deposited on p-type

silicon as a function of temperature. The area of MIS device was2

10mm and measurements taken at f = 1KHz. Normal and anormal type152)

of hystereses are indicated. [After Tanguy. ]

-112-

Fig. ,35 Current voltage characteristics of MIS deyices with different

number of OP, . monolayers deposited on Si, Notice the shift ofI-V curves near zero volts indicating the existence of an e.m. f.

without external "bias. [After Tanguy. ^ ]

Fig. r36 T.S.C. peaks of a M2M device containing 23 monolayers of OP

at fixed V =» 1 volt, t = lOmn.and 3 " O.l6°K/sec. The peaksp p

observed 1 to 6 correspond to various polarization temperatures

T = 281, 287, 291*, 301, 308 and 315°K. [After Tanguy and Hesto,

Fig. 3T. T.S.C. peaks of a MIM device as a function of number of OP

monolayers at fixed T = 293°K, V = 1 volt, t = lOmn. andP P P

8 = 0.16 K/sec. d peaks consisting of several monoenergetic peaks

&-,» <L* **•*» k BJl^ *«; a r e 8^°'WI11 [After Tanguy and Hesto. ]

Fig. 38 Typical curve fittings with 2 superimposed peaks for 21 monolayers of

OP at temperatures Tm = 29O°K and Tm - 297°K, The solid lines

in the figure represent theoretical fits and the circles are

experimental points. [After Tanguy and Hesto."']

Fig. 39, (A) Change of capacitance AC/C vs. applied voltage V for different

number of Ba-stearate monolayers at temperature 77°K and area of

the device 3 0.25mm . (B) Corresponding plot of AC/C vs. square c

applied electric field ( ~ V 2 / N 2 ) . [After Leger et al. ]

TABLE CAPTIONS

TABLE I Electrical data, on unskeletonized (.0) and skeletonized GU films of

cadmium arachidate.' [After' Race and Reynolds.^' i

TABLE II Calculated and measured values of dielectric constant of a few long

chain fatty acid Langmuir films. [After Khanna and Srivastava. >

TABLE III Summary of the models to explain the "forming" process in insulators

[After Dearnley etal. 132^

-113-

TABLE I

Filmlamples

A

B

C

D

(1)

Opticalthickness

75

61

61

59

(2)

53

hi

51

51

(3)

<V*0>0.707 '

J

0.705

0.837

0.86

0-571* :

0.571

0.761*

0.797

(5)

£ 0

Measd.2.1*8

2 ^T

p i Q

2.1*1*

2.1*3

2.1*7

2.50

2.50

2.50

2.1*9

(6)

Measd.

1.69

1.7^

1.71

I.87

1.73

1.62

1.68

I.69

1.96

1.9U

2.02

1.97

2.10

(7)

Calcd.

1.85

1.82

2.15

2.18

(8)Filled wittnon-polar

Measd.

2.29

2.20

2.19

2.25

2.32

2.38

(9)1

oil

Calcd.

2.3^

2.31

2.U2

(10) (11)

Filled with tetra-chloiodiphenyl

€ 3 "£3Measd. Calcd.

3.01*

3.25

1

; 3.63

TABLE I I

(l) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 )

s •

film substance

Ba-palmitate

Ba-margarate

Ba-stearate

Ba-behenate

monolayerthickness

23.25

2^.05

25.75

30.05

C : (from theory)cala.

for ,monolayers

1.65

1.63

1.67

1.50

multilayers

2.90

2.70

3-10

2.00

£ , (from experimental data whencalc. r

oxide layer effect was inclusive).

for monolayer

1.6T ± 0.12

1.65 ± 0.07

1.62 ± 0.08

1.60 + 0.10

far UO-layers

2.85 ± 0.20

2.59 ± 0.02

2.72 ± 0.07

2,25 ± 0.02

C , (when oxidelayei effect excluded).

for 1monolayer

2.29

2.09

2.10

2.01

formultilayers

2.88

2.77

2.63

2.27

•

t

Capacitance per unitarea ^ iF /cm 2

formonolayer

0.636

0.582

O.551*

0.1*70

formultilayer

0.00988

0,00866

0.0061*2

O.OO60I4

Author

HickmoH

Simmons-Verde rbcr

Dearnalev

Barriac tt al.

Greene el at.

Model proposed

Schottky ionization ofimpurities near middleof bandgap; formationof localized states forimpurity band and*pace charge TO aidinjection of electrons *

Injection of ions fromanode to form broadimpurity band with asharp top. Band bend-ing fr&m space charge

Propagation ofconductingfilament through

fusion of insulatorand electrode, ivithsubsequent ion injec-tion. Also traps formed"

High-field electrolysiswith injection ofvacancies from cathode

Locatorgeneralforming

Uncertain'

Not local

Local

Probably notlocal

Probably local

TABLE I I I

Dependenceon electrode

No detailed model;probably depends onwork function

Impurity levels depend onspecies; band bendingdepends also on workfunction

Forming initiated atanode and can beinhibited jf electrolytic-»lty released gas reactsreadily with anode

Not discussed

If the anode reacts readilywith gas releasedgrowth of insulator at.mode may inhibitforming

Dependenceon insulator

Kp=:i(energy gap);voltage-dependentsince ionization ofimpurities is so

Vr usually less than• (energy gap)'; dependson electrode, throughimpurity levels. Volt-age-dependent becauseimpurity spectrum andbandgap fix \'\-

Vf probably related tocrystal formationenergy or to breakdownstrength. Voltage-dependent becauseforming initiated in

' «mall region - •

Not discussed

free energy offormation of insulator

Dependenceon atmosphere

No detailed model;probably impurity-oxygen reaction

Apparently none since iheinjtcted ions do notreact strongly withoxygen

Oxygen causes competingreactions which preventfilament propagalion orinitiation

Oxygen neutralizes thetons which constitutethe space charge

Oxygen causes other com-peting reactions atcathode

. ft -i