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CHAPTER 3
EXPERIMENTAL TECHNIQUES
3.1 INTRODUCTION
There are so many deposition methods available for preparing thin
films. However, every method has its own specific limitations and involves
compromises with respect to process specifications, expected film properties,
substrate material limitations, cost, etc; these limitations redistrict us to select
the deposition technique. A detailed study has been made about the various
methods available for thin film deposition in this chapter.
3.2 PHYSICAL VAPOUR DEPOSITION
The physical vapour deposition (PVD) process are atomistic
deposition process in which material is vaporized from a solid or liquid source
in the form of atom or molecules and transported in the form of a vapour
through a vaccum or low pressure to the substrate where it condenses.
Typically PVD processes are used to deposit films with thickness in range of
few nanometres to thousand nanometers. Deposition rates are 10-100 A° (1-10
nanometres per second). The important physical vapour deposition processes
are vaccum evaporation, sputtering, arc evaporation and ion planting. In
vaccum evaporation, atoms are removed from the source by thermal means,
where as in sputtering, they are dislodged from target surface through impact
of gaseous ions. If the evaporated material is transported through a reactive
gas, the technique is called reactive evaporation. Flash evaporation technique
29
is used when we have to deposit a multi- component material which cannot be
heated to the evaporation point together. Ion plating refers to a process in
which the substrate and film are exposed to a flux of high-energy ions during
deposition this can provide dense coatings at relatively high gas pressures
where gas scattering can enhance surface coverage. Though these methods
appear simple they require advance sophisticated arrangements to control the
film properties. Maissel & Glang (1970).
3.3 CHEMICAL METHODS
There are verities of other important methods available for
preparation of thin films. Methods are chemical vapour deposition, chemical
bath deposition, electro less deposition, electrolytic deposition and
anodization.
3.3.1 Chemical Vapour Deposition (CVD)
Chemical vapour deposition is the deposition of atoms or molecules
by the high temperature reduction or decomposition of chemical vapour
precursor species which contain the material to be deposited between a carrier
gas and an Organo-metallic precursor. It includes hydride chemical vapor
deposition method, trichloride chemical vapor deposition method and metal-
organic chemical vapor deposition method. Chemical vapour deposition is the
condensation of compound or compounds from the gas phase on to a substrate
where reaction occurs to produce a solid deposit .The deposited material may
react with other gaseous species in the system to give compounds. Chemical
vapour deposition processing is generally accompanied by volatile reaction by
products and unused precursor species.
The chemical reaction is initiated at or near the substrate surface,
which produces the desired material in the form of a deposit on the substrate.
30
In some process, the chemical reaction may be activated through an external
agency, such as, application of heat, RF field, light or X-rays, an electric or
glow discharge and electron bombardment. The microstructure and adhesion
of the deposit is strongly influenced by the nature of the chemical reaction and
the activation process.
3.3.2 Chemical bath Deposition (CBD)
The Chemical bath deposition (CBD) method is one of the cheapest
methods to deposit thin films and nanomaterials, as it does not depend on
expensive equipment and is a scalable technique that can be employed for
large area batch processing or continuous. Chemical deposition techniques are
relatively low cost processes and can be easily scaled up for industrial
applications.
Most of the chemical bath consists of one or more metal salts Mn+,
a source for the chalcogenide X (X = S, Se, Te) and typically a complexing
agent, in an aqueous solution. The deposition of metal chalcogenide occurs via
the following four steps.
1. Equilibrium between the complexing agent and water,
2. Formation/dissociation of ionic metal-ligand complexes[ M(L)i]n-ik
3. Hydrolysis of the chalcogenide source; and
4. Formation of the solid.
During the step 3, the metal cations are pulled out of the solution by
the desired non-metal species provided through the hydrolysis of the
chalcogenide source, to form the solid film. The kinetics of the step 3 is highly
sensitive to the solution pH and temperature, as well as to the catalytic effects
of certain solid species that may be present, which in turn decides the rate of
31
formation of thin film on the surface of the substrate or bulk precipitation. The
basic principle involved behind the formation of desired solid film/bulk MmXn
(step 4) is the rising concentration of Xm- from step 3 causes the ionic product
[Mn+]m [Xm-] n to exceed the solubility product. During step 2, the formation of
complexed metal ions allows control over the rate of formation of solid metal
hydroxides, which competes with step 4 and which would otherwise occur
immediately in the normal alkaline solutions. These steps together determine
the composition, growth rate, microstructure and surface topography of the
resulting thin films.
3.3.3 Sol- Gel Method
Out of the different methods available for the preparation of
nanoparticles and nanocrystalline TiO2 thin films, the sol gel method is
simple, inexpensive, non-vacuum and low temperature technique. This sol-gel
process offers many advantages like, excellent control of the stoichiometry of
precursor solutions, eases of compositional modifications, customizable
microstructure, and eases of introducing various functional groups, requires
relatively low annealing temperature and has the possibility of coating over
large area substrates.
The unique property of sol-gel process is the ability to go all the
way from the molecular precursor level to the product level, allowing a better
control of the whole process and the synthesis of tailor made materials for
different applications. Sol-gel method is more suitable to prepare materials
because it permits molecular-level mixing and processing of the raw materials
and precursors at relatively lower temperature and produces nano-structured
bulk, powders and thin films. Hence the sol-gel technique is the very attractive
32
method to produce films for photovoltaic applications for which large-area
films are required at low cost, Pierre et al (2002).
In a typical sol-gel process, independent solid colloidal particles
ranging from 1 nm to 1 m are formed from the hydrolysis and condensation
of the precursors, which are usually inorganic metal salts or metal organic
compounds such as metal alkoxides. It is usually easy to maintain such
particles in a dispersed state in the solvent.
In the next stage, these colloidal particles can be made to link with
each other by further sol condensation, while they are still in the solvent, so as
to build a three-dimensional open grid, termed gel Pierre et al (2002) The
transformation of a sol to a gel constitutes the gelation process (figure 3.1).
Figure 3.1 Sol-Gel formations
33
3.3.4 Sol-Gel Chemistry
The sol and gel formation is based on the hydrolysis and
condensation of precursors. Most work in the sol-gel field has been performed
by the use of alkoxides as precursors. Alkoxides provide a convenient source
solvents, especially alcohol. Alcohols enable a convenient addition of water to
start the reaction. Another advantage of the alkoxide route is the possibility to
control rates by controlling hydrolysis and condensation by chemical means
(Schmidt 1988) with an alkoxides as a precursor, sol-gel chemistry can be
simplified in terms of the following reaction equations (3.1) Livage et al
(1992).
(1) Hydrolysis (hydroxylation) of metal alkoxides:
-M-OR + H2O -M-OH + ROH (3.1)
(3.2)
As shown in the equation 3.2, the mechanism involves
nucleophilic attack of a negatively charged HO - group onto a positively
charged metal M + and transfer of a proton from the water to a negatively
charged OR group of the metal and release of the resulting ROH molecule. As
soon as reactive hydroxyl groups are obtained, the formation of branched
34
oligomers and polymers with a metal oxo based skeleton and reactive residual
hydroxo and alkoxy groups occurs through a polycondensation process.
2) Condensation:
(3.3)
In this case X means either an H or R (an alkyl group). Oxolation is
also a three step nucleophilic substitution reaction which occurs through the
elimination of H2O or ROH. Generally, under a stoichiometric hydrolysis ratio
(H2O/M < 2), the alcohol producing condensation is favored, whereas the
water forming condensation is favoured for large hydrolysis ratios (H2O/M
>>2). The hydrolysis and condensation are responsible in the transformation
of metal alkoxide precursors to a metal oxo macromolecular network. The
recombination of these metal oxo polymers leads to the production of well
dispersed structures which occupy the whole volume. When these oxo
polymers reach macroscopic sizes, the reaction bath becomes a gel, inside
which, the solvent, reaction by products and free polymer are trapped. If the
polymerized structures do not reach macroscopic sizes, sols are produced.
Precipitates are formed if the reactions produce dense rather well dispersed
structures (equation 3.3) (Brinker et al 1990), (Sanchez et al 1994).
Applications for sol-gel process derive from the various special
shapes obtained directly from the gel state (monoliths, films, fibers, and mono
sized powders) combined with compositional and microstructural control and
low processing temperatures. Compared with other methods, such as the solid-
state method, the advantages of using sol-gel process include
35
(1) The use of synthetic chemicals rather than minerals enables high purity
materials to be synthesized.
(2) It involves the use of liquid solutions as mixtures of raw materials. Since
the mixing is with low viscosity liquids, homogenization can be achieved
at a molecular level in a short time.
(3) Since the precursors are well mixed in the solutions, they are likely to be
equally well-mixed at the molecular level when the gel is formed thus on
heating the gel, chemical reaction will be easy and at a low temperature.
3.3.5 Successive Ionic Layer Adsorption (SILAR)
Among the di
simplicity of the successive ionic layer adsorption and reaction (SILAR)
method and its potential application for large area deposition make it very
substrates are immersed into separately placed cationic and anionic precursors
and precipitate formation in the solution, i.e. wastage of the material was thus
avoided. Also, SILAR can be used to deposit compound materials on a variety
of substrates such as insulators, semiconductors, metals.
Adsorption is the basic building block of the successive Ionic layer
adsorption and reaction method. Adsorption is the collection of a substance on
the surface of another one, and is possible due to attractive force between ions
in the solution and surface of the substrate. It is also a surface phenomenon
between ions and surface of the substrate. The forces may be cohesive force or
Van der Waals force or chemical attractive force. Atoms or molecules of
substrate surface possess unbalanced or residual force and that holds the
substrate particles. The successive ionic layer adsorption and reaction is based
36
on sequential reaction at the substrate surface. The substrate is rinsed in
solvent after each reaction, which enables heterogeneous reaction between the
solid phase and the solvated ions in the solution. The successive ionic layer
adsorption and reaction process is intended to grow thin films of water
insoluble ionic or ion covalent compounds by heterogeneous chemical
reaction at the solid solution interface between adsorbed cations and anions.
Figure 3.2 represents the deposition process of thin films using
successive ionic layer adsorption and reaction method. It consists of four
different steps such as adsorption, rinsing, reaction and rinsing. 1) Adsorption:
In the first Step, the cations present in the precursor solution are adsorbed on
the surface of the substrate and form the Helmholtz electric double layer. This
layer is composed of two layers: the inner (positively charged) and outer
(negatively charged) layers.
Figure 3.2 Schematic representation of SILAR method
The positive layer consists of the cations and the negative forms the
counter ions. 2) Rinsing: In this step, excess adsorbed ions are rinsed away
from the diffusion layer. 3) Reaction: In this reaction step, the anions from the
37
anionic precursor solutions are introduced to the system. Due to the low
stability of the material a solid substance is formed on the interface. This
process involves the reaction of cation surface species with the anionic
precursor. 4) Rinsing: In the last step, the excess and un-reacted species and
the reaction by product from the diffusion layer are removed.
After five minutes of drying these steps are repeated again and this
results in the formation of a thin layer of the material. During one complete
cycle the maximum increase in film thickness is theoretically one monolayer.
If the measured growth rate exceeds the lattice constant of the material, a
homogeneous precipitation in the solution should take place. The factors
affecting the growth phenomena are the quality of the precursor solution, pH
value, counter ions, complexing agent, pre-treatment of the substrate,
individual rinsing and dipping times.
3.3.6 Dip coating method
Dip coating technique can be described as a process in which the
substrate is immersed in the solution for a desired period. After that the
substrate is withdrawn with a uniform withdrawal speed. The dip coating is
done under controlled temperature and atmospheric conditions. During sol-gel
thin film formation via dipping, polymeric or particulate inorganic precursors
are concentrated on the substrate surface by a complex process involving
gravitational draining with concurrent drying and continued condensation
reactions. The structure of films deposited from polymeric precursors depends
on such factors as size and structure of the precursors, relative rates of
condensation and evaporation, capillary pressure, and substrate withdrawal
speed. In our present work TiO2 nanocrystalline this films were prepared by
Sol- gel dip drive method.
38
3.4 CHARACTERIZATION TECHNIQUES
3.4.1 X-ray Diffraction (XRD)
The X-ray diffractometer consists of three parts, a basic diffraction
unit, a counter goniometer and an electronic circuit panel with an automatic
recorder as shown in figure 3.3. The diffraction angles and intensity of lines
which describes the conditions for constructive interference of X-rays
scattered from atomic planes of a crystal. The condition for constructive
-rays, d is the
-
rays. The factor d is the related to the (h k l) indices of the planes and the
dimension of the unit cells. It is therefore seen that
Figure 3.3 Schematic of XRD measurement
the diffraction direction is solely determined by the structure and size of the
unit cell.
The cr
cos0.94D .
39
relation tancosD
as the length of
the dislocation lines per unit volume of the crystal can be evaluated from the
relation 2D1 . The intensities of the diffracted beams depend on the possible
diffraction directions and the lattice parameters.
3.4.2 Optical properties
3.4.2.1 Introduction
Optical properties of thin films generally deals with absorption,
transmission, reflection, refractive index (n), extinction coefficient (k) and
absorption coefficient . Total energy incident on a thin film is conserved
through various processes namely, reflection, transmission and absorption. In
some cases scattering of light also occurs.
Optical properties of a solid emanate from its interaction with
electromagnetic waves and are manifested in optical frequencies. These
interactions may cause several transitions in its band structure such as band to
band, between sub bands or impurity levels and a band, transitions of free
carrier within a band and also resonance due to lattice vibrations. Hence a
detailed study of the absorption band spectra is likely to provide very good
information about the electronic band structure of thin films (Goswami 1996).
Thus the knowledge of optical properties of solid films has widely contributed
to the phenomenal growth of their applications in scientific technological and
industrial applications. Thin films are being used in optical devices, such as,
mirror coatings, interference filters, antireflection coatings, absorption filters,
optical and thermal detectors etc., Nath & Chopra (1973). Moreover, optical
characteristics of films are strongly influenced by the process parameters and
40
the deposition method. Optical films are primarily characterized by
absorption/transmittance and refractive index. The absorption/transmittance
versus wavelength graph can be divided basically into three regions (1) UV,
(2) Visible and NIR and (3) IR and far-IR. The desired region of high
absorption/transmittance is located in the second region and it strongly
depends on the material purity and stoichiometry. In region 1 the absorption
depends on electronic structure of the material and in region 3 it depends on
lattice vibrations or in the case of semiconductors, on free carrier absorption.
In general, mixtures of various compounds evaporate non
uniformly. Films with perfect optical homogeneity are rare in homogenities
with gradual increase or decreases of refractive index with film thickness are
more compared to in homogenities with abrupt changes and it is due to
accidental changes in the deposition parameters, like pressure rate,
temperature etc.,
3.4.2.2 Experimental details
The electromagnetic wave fields vary periodically with an angular
-direction with a complex
= j ( )1/2 = + i (3.1)
where is the magnetic permeability, the permittivity the
attenuation factor and the phase factor. The phase velocity of the wave is
given by
)1/2 (3.2)
41
In vacuum this wave travels with a phase velocity equal to that of
light (c) while it moves with a lesser velocity in any other material medium.
The refractive index of the medium is given by the ratio of these two velocities
as
)1/2 (3.3)
For a no 3.3) becomes
n = ( ) 1/2 or n2 = (3.4)
being a complex quantity it can be replaced by *,
where * = '- j " and by similar reasoning should be replaced by
*. Then the refractive index n can also be expressed as complex quantity n*
that is
n* = (n+jk) = ( *)1/2 = ( '- j ")1/2 (3.5)
where n and k are respectively the real and imaginary components
of n* and are known as the refractive and absorption indices.
From equation (3.5)
' = n2 k2 and " = 2nk (3.6)
where ' and " are termed as the optical dielectric constants of the
material. These equations reveal that both the dielectric constants ( ' and ")
and optical constants (n and k) are interrelated.
42
Further n and k can be evaluated from
21
21
1]tan21[21n (3.7)
21
21
21
1tan21
1tan21k ( 3.8)
Another important parameter, the absorption coefficient is
calculated using the transmittance (T) value measured for a particular
wavelength and the film thickness (t) using the relation
tTln
(3.9)
0eII (3.10)
where x is the distance through which the electromagnetic wave
travels to change its intensity from I0 to I. The absorption index or the
extinction coefficient k which is the attenuation per unit radian may be written
as
4
k (3.11)
The absorption of radiation that gives rise to transition of electrons between
the valence band and conduction band is of two types.
a) Direct Transition: The necessary condition for a direct transition to take
43
place is that in the excitation process no change in the k-values of the
electron should occur. The following dependencies are observed during
this transition
(Ev - Ei) ½ for allowed transitions
(Ev - Ei) 3/2 for forbidden transitions
v is the energy of the top of
the valence band and Ei is the energy of the initial state from which the
transition is made.
a) Indirect Transition: The transition involving a change in the crystal
momentum is termed as indirect transition. In this case absorption of
both a photon and a phonon or the absorption of a photon and the
emission of a phonon take place. The following dependencies are
observed during this indirect excitation.
(Ev - Ei)2 for allowed transitions
(Ev - Ei)3 for forbidden transitions
The refractive index of thin films often differs from that of bulk
material. The refractive index of a material at optical frequencies is mainly
determined by polarizability of the valence electrons. In compounds the type
of bonding also influences the index. The analysis of the wavelength
dependence of the optical constants n and k is of considerable interest due its
optoelectronic applications. The optical parameters can be estimated by means
of the equations corresponding to the propagation of electromagnetic waves
through the layers of plane-parallel faces consisting of an absorbing thin
semiconductor film and a transparent glass substrate. The value of k can be
evaluated from the formula.
44
tk4 (3.12)
and the refractive index.
cos22)(1n)(1)(1nT
1122
210
22
212 (3.13)
Where 210
21
20
1 )n(nnn , 2
21
22
21
2 )n(nnn and tn 2 1 .
0, n1 and n2 are
the refractive index of air, film and glass respectively. Substituting the
experimental values for T, A, t n0 and n2 in the above equations can lead to
solving the values of n1 and k1 using the method with different iteration till the
desired convergence can be achieved.
3.4.3 Thickness Measurement
Thickness is one of the most important parameter, which influences
various properties of the films. Hence a very accurate measurement of
thickness is vital. Various techniques are adopted for the measurement of
thickness up to a fair accuracy. In the present study the following two methods
are adopted to determine the thickness and they are cross-verified for
accuracy. The multiple beam interferometry is adopted in the present work
because of its simplicity and accuracy. When a wedge of small angle is formed
between un-silvered glass plate illuminated by monochromatic light as shown
in figure 3.4, broad fringes are seen arising from the glass on the two sides of
45
are measured and the thickness (t) is deduced from the equation 2
t ,
matic light.
Figure 3.4 Multiple beam interferometer and Fizeau fringes
Films are deposited on to glass substrate and a silvered thread
having fine edge (flat type) is tied to the substrate to form a channel. An over
coating of silver is given to produce a lower reflecting plane, which is forming
the step profile in multiple beam interferometer. A parallel beam of
monochromatic light is used at normal incidence to form Fizeau fringes. The
fringe spacing and the displacement measurement are found out and the
thickness of the film is calculated by using traveling microscope of least count
0.001 cm. The average of the calculated thickness values at different points is
taken to be the thickness of the film. It is checked with gravimetric method.
46
3.4.4 Spectrophotometer
The essential components of a spectrophotometer (as shown in
Figure 3.5) include: (1) a stable source of radiant energy, (2) a monochromator
to resolve the radiation into component wavelengths or bands of wavelengths,
(3) sample compartment and (4) a radiation detector.
3.4.4.1 Sources of radiation
A tungsten filament lamp is the most satisfactory and inexpensive
source of visible and infrared radiation. The filament is heated by a stabilized
D.C. power supply, or by a storage battery. The tungsten filament emits
continuous radiation in the region between 350 nm and 2500 nm. For source
of ultraviolet radiation, the hydrogen lamp and deuterium lamp are used. They
consist of a pair of electrodes, which are enclosed, in a glass tube provided
with a quartz window and filled with hydrogen or deuterium gas at low
pressure. When a stabilized high voltage is applied to the electrodes, an
electron discharge occurs which excites other electrons in the gas molecules to
high-energy states. As the electrons return to their ground state they emit
radiation, which is continuous in the region roughly between 190 nm and 350
nm. Similarly, a xenon discharge lamp is used as a source of ultraviolet ration.
The xenon lamp produces higher intensity radiation, but it is not as stable as
the hydrogen lamp. It also emits visible radiation, which may appear as stray
radiation in ultraviolet applications.
3.4.4.2 Monochromator
The sources of radiation commonly used to emit continues radiation
over wide ranges of wavelengths. However, narrow band widths have many
advantages: (1) narrow band radiation will allow the resolution of absorption
47
bands which are quite close to each other, (2) with narrow band radiation a
peak may be measured at its absorption maximum, thus increasing the
sensitivity and (3) the absorption of narrow band radiation will tend to show
absorbed is measured. Monochromators resolve wide band polychromatic
radiation from, the source into narrow bands or monochromatic radiation with
monochromator include: (1) an entrance slit which admits polychromatic
radiation from source; (2) a collimating device, either a lens or a mirror; (3) a
dispersion device, either a prism or grating which resolves the radiation into
component wavelengths; (4) a focusing lens or mirror and (5) an exit slit. The
effective bandwidth of radiation emerging from the monochromator depends
on several factors, including the dispersing element and the slit widths of both
the entrance and exits slits. Narrow slit widths isolate narrow bands; however,
the slit width also limits the radiant power, which reaches the detector.
Therefore, the minimum bandwidth may be determined by the sensitivity of
the detector.
3.4.4.3 Detection devices
Any detector absorbs the energy of the photons, which strike it and
converts this energy to a measurable quantity such as the darkening of a
photographic plate and electric current or thermal changes. Most modern
detectors generate an electric signal. Any detector must generate a signal
which is quantitatively related to the radiant power striking it. The noise of a
detector refers to the background signal generated when no radiant power
from the sample reaches the detector. This noise may be caused either by
random changes within the detector itself or by electrical pick up of other
48
signals in the vicinity of the detector unit. Important requirements for detectors
include: (1) high sensitivity with a low noise level in order to allow the
detection of low levels of radiant power, (2) short response time, (3) long term
stability to insure quantitative response and (4) an electronic signal which is
easily amplified for typical readout apparatus and exits slits. Narrow slit
widths isolate narrow bands; however, the slit width also limits the radiant
power, which reaches the detector. Therefore, the minimum bandwidth may be
determined by the sensitivity. Ultraviolet and visible photons possess enough
energy to cause photo-ejection of electron when they strike surfaces, which
have been treated with specific types of compound. Their absorption may also
cause bound, non-conduction electrons to move into conduction bands in
certain semiconductors. Both processes generate an electric current, which is
directly proportional to the radiant power of the absorbed photons. Device,
which employs these systems, are called photoelectric detectors and are sub-
classified as phototubes and photovoltaic cells.
Figure 3.5 Block diagram of Spectrophotometer.
49
3.4.4.4 Refractive Index
The refractive index of thin films often differs from that of bulk material.
The refractive index of a material at optical frequencies is mainly determined by
polarizability of the valence electrons. In compounds the type of bonding also
influences the refractive index. The analysis of the wavelength dependence of the
optical constant refractive index (n) is of considerable interest due to its opto-
electronic applications.
The optical parameters can be estimated by means of the equations
corresponding to the propagation of electromagnetic waves through the layers of
plane-parallel faces consisting of an absorbing thin semiconductor film and a
transparent glass substrate. The refractive index can be calculated from the
transmission spectra using the interference fringe region by the method of Manifacier
et al. In this method the maximum transmittance (Tmax) and minimum transmittance
(Tmin) are considered to be continuous functions of wave length through the refractive
envelopes of the maxima Tmax minima Tmin
refractive indices of the films are calculated using the relations by (Manifacier et al
1976)
21
21
20
22 )nn(NNn (3.14)
minmax
minmax1021
20
TT)T(Tn2n
2nnN (3.15)
where no and n1 are the refractive indices of air and glass respectively.
50
3.4.5 MORPHOLOGY
Morphology gives the form and structure of a surface. Surface
roughness is one of the important parameters; it affects the light absorption
capacity. Substrate roughness affects the morphology and growth kinetics in
three primary ways namely, nucleation, impurity diffusion and sometimes by
acting as a metallurgical shunt. The following paragraphs discusses in details
about the techniques to study the morphology of deposited films.
3.4.5.1 Scanning Electron Microscopy (SEM)
SEM is a useful technique for the direct observation of surfaces,
employed to predict the growth mechanisms leading to reminiscent
structures. In a SEM analysis, the areas or micro-volumes to be examined
are irradiated with a fine electron beam produced by the electron gun and
focussed by electron lenses. Scanning coils deflect this beam and sweeps it
over the film surface. A cathode ray tube is scanned synchronously with the
electron beam Brightness of display tube is modulated by the signal which
arises from interactions of the beam with film surface. The strength of this
signal is thus translated into image contrast.
The types of signals produced when electron beam impinges on
specimen surface include secondary electrons, Auger electrons,
characteristic X-rays and photons of various energies. These signals are
obtained from specific emission volumes within the samples, which
ultimately determine surface topography, crystallography, composition, etc.
In the present work surface morphology of the deposited TiO2 films were
analyzed by scanning electron microscopy Hitachi S-3400 N SEM.
51
3.4.5.2 Energy Dispersive Analysis of X-rays (EDAX)
EDAX helps to determine the elemental contents in sample. In
this technique, an energetic beam of electrons is allowed to be incident on the
film. These incident electrons interact in elastically with both the inner shell
electrons and outer shell electrons of the atoms of thin sample material
generating X-rays. Outer shell electrons generate soft X-rays due to this
interaction whereas innermost shells generate characteristic X-rays, which
depend on energies of these shells and hence are characteristic of atoms
radiating these X-rays. Hence by analyzing the energy of these characteristic x-
rays, typical of which are K , K , L , L etc., information about the type of
atoms present in the sample and their concentration can be determined. In the
present study JEOL 840 SEM-EDX has been used to determine the
elemental contents present in the prepared, TiO2 thin films.
3.4.6 Fourier Transform Infra-Red Spectroscopy (FTIR)
Fourier transform infrared spectroscopy has become the standard
technique for chemical characterisation. Infrared spectroscopy is one of the
most powerful tools available for identifying organic and inorganic
compounds. Indeed most molecular species absorb infrared radiation. It is
based on the fact that the absorbed radiation stimulates molecular vibration.
These vibrations are characteristics for each organic functionality, such as
methyl or aldehyde groups for example. Each molecular species has a unique
infrared absorption spectrum. Unlike a dispersive instrument (grating
monochromator or spectrograph) an FTIR spectrometer collects all
wavelengths simultaneously. This feature is called the Multiplex or Felgett
advantage. Fourier transform infra-red spectrometer collects and digitizes the
interferogram, performs the FT function and displays the spectrum.
52
Fourier transform infrared spectrometer is typically based on a
Michelson interferometer. It consists of a beam splitter, a fixed mirror and a
mirror that transmits back and forth, very precisely. The beam splitter is made
of a special material that transmits half of the radiation striking it and reflects
the other half. Radiation from the source strikes the beam splitter and
separates into two beams. One beam is transmitted through the beam splitter to
the fixed mirror and the second is reflected off the beam splitter to the moving
mirror. The fixed and moving mirrors reflect the radiation back to the beam
splitter. Again, half of this reflected radiation is transmitted and half is
reflected back at the beam splitter, resulting in one beam passing to the
detector and the second back to the source. The quality of FTIR spectrum is
dependent on sample contact rather than sample thickness. In the present work
the samples were analysed using the instrument ABB Bommn, MB 3000,
Canada.
3.4.7 Atomic Force Microscopy (AFM)
The Atomic Force Microscope consists of a cantilever with a sharp
tip at its end and it is used to scan the specimen surface. The cantilever is
typically silicon or silicon nitride with a tip radius of curvature in the order of
nanometre. When the tip is brought into proximity of a sample surface, forces
between the tip and the sample lead to a deflection of the cantilever according
include mechanical contact force, vander waals forces, capillary forces, etc.
Along with force, additional quantities may simultaneously be measured
through the use of specialized types of probe. Typically, the deflection is
measured using a laser spot reflected from the top surface of the cantilever
into an array of photodiodes. Using a Wheatstone bridge, strain in the AFM
cantilever due to deflection can be measured.
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If the tip was scanned at a constant height, a risk would exist so as
the tip collides with the surface, causing damage. Hence, in most cases a
feedback mechanism is employed to adjust the tip-to-sample distance to
maintain a constant force between the tip and the sample. Traditionally, the
sample is mounted on a piezoelectric tube, which can move the sample in the
direction for maintaining a constant force, and the and y directions for
crystals may be employed, with each responsible for scanning in the x, y and z
directions. This eliminates some of the distortion effects seen with a tube
scanner. In newer designs, the tip is mounted on a vertical piezo scanner while
the sample is being scanned in X and Y using another piezo block. The
resulting map of the area z = f(x, y) represents the topography of the sample.
The AFM can be operated in a number of modes, depending on the
application. In general, called contact mode and a variety of dynamic (or non-
contact) modes where the cantilever is vibrated.