23

CHAPTER 2

CRYSTAL GROWTH OF ZnX (X = S, Se, Te)

2.1. INTRODUCTION

Crystal growth involves a variety of research fields ranging from surface physics,

crystallography, material science and condensed matter physics. Crystal growth plays an

important role in both experimental and theoretical research fields. Fundamental aspect of

crystal growth had been derived from early crystallization experiments in the 18th and 19th

century. Theoretical understanding started with the development of thermodynamics in the

late 19th century and with the development of nucleation and crystal growth theories and

increasing understanding of the role of transport phenomenon in the 20th century.

Crystal growth technology and epitaxial technology had developed along with the

technological development in the 20th century. As the development of scientific instruments

and analytical methods such as X-rays, electron microscopy, NMR and Scanning tunneling

microscopy advanced, research on crystal growth and structure characterization has entered

in an atomic level, which makes it possible for further understanding of physical, chemical

and other properties related to structure and nature of various crystals. Also the rapid

advances in microelectronics, communication technologies, medical instrumentation, and

energy and space technology were only possible after the remarkable progress on growth of

large, rather perfect crystals and of large diameter epitaxial layers. Further progress in crystal

growth technology is required for the significant contribution to the energy crises. High

efficiency white light emitting diodes for energy saving illumination and photovoltaic/thermo

photovoltaic devices for transforming the solar and other radiation energies in to electric

power with high efficiency depend on significant advances in crystal growth and epitaxy

technology. Also the dream of laser fusion energy and other novel technologies can only be

realized after appropriate progress in the technology of crystal and epilayer fabrication.

The world crystal production is estimated at more than 20000 tons per year, of which

largest fraction of about 60% are semiconductors such as Silicon, GaAs, InP, GaP, CdTe and

24

G a s t o S o l idV a p o r G r o w t h

L i q u i d t o S o l idM e l t G r o w t h

S o l id t o S o l idS o l id G r o w t h

P h a s e T r a n s i t i o nP r o c e s s

G a s t o S o l idV a p o r G r o w t h

L i q u i d t o S o l idM e l t G r o w t h

S o l id t o S o l idS o l id G r o w t h

P h a s e T r a n s i t i o nP r o c e s s

its alloys. Application wise, the major production of the crystals is related to the optical,

scintillator and acoustic-optics type [1-2].

Large number of publications in the form of research papers, review articles and

books [3-7] are available that describe the crystal growth process, various techniques, their

advantages and disadvantages and the latest developments in this field. The survey leads to

the conclusion that the artificial crystals of most of the materials can be grown in the

laboratory. Single crystals find their own importance in fabrication of modern devices like

transistors, rectifiers, polarizer, lasers, scintillators, modulators, transducers, memory devices

for computers, etc. [8-17].

It is observed by earlier studies of Ronelle (1745) and Frankenteim (1835) that, heat

and mass transport phenomenon play a significant role during the growth of crystals from

fluid medium (i.e. melt, solution and vapor phase). The diffusion boundary layer defined by

Noyes and Whitney (1897) was used in the growth rate equation of Nernst (1904) and it was

confirmed by interferomatric measurements for concentration profiles around growing

crystals by Berg (1938) and others.

There has been remarkable development with respect to size and perfection of

crystals, with silicon, sapphire, alkali and earth alkali halides reaching diameters up to 0.5m

and weights of nearly 500 kg. [2].

In general, we may define three different categories of crystal growth processes

depending upon the phase from which the solid phase transition is occurred as shown in table

2.1.

Table -2.1 Process and phase transition.

25

These techniques are further divided in to three categories as-

(i) Growth from melt

(ii) Growth from solution

(iii) Growth from vapor.

Melt growth can be further sub divided in to –

(i) Growth with crucible

(ii) Growth without crucible

Growth with crucible can be further divided in to three more groups as-

(i) Verneuil flame fusion method

(ii) Float zone method

(iii) Chemical dissolution and zone movement method

Some typical growth techniques are known by their inventor’s names. Some of them are-

(i) Czocharalski technique

(ii) Kapitza technique

(iii) Bridgmann – Stockbarger technique

Some other techniques are known by the specific methodology used in that particular

techniques are –

(i) Float zone

(ii) Vertical gradient freeze

(iii) Directional solidification

(iv) Growth under micro and hyper gradient

26

Different types of techniques are suitable for different types of materials or

compounds for their crystal growth. Therefore a detailed survey of available literature is

required to choose the particular growth technique for specific material. Most of the basic

techniques of the crystal growth required modifications as per the requirements [18-23]. The

basic techniques are –

(1) Gel growth technique

(2) Melt growth technique

(3) Solution growth technique

(4) Flux growth technique

(5) Vapor growth technique.

2.2 GEL GROWTH TECHNIQUE

This is a simple and popular technique, because it does not require any sophisticated

instrumentation. Growth of crystal in gel is an intermediate process between growth in solids

and solution. Crystal of reasonable size, incorporating nucleation control mechanism, at near

ambient temperature can be grown by this method [22, 23]. Gels are two phase systems

comprising of a porous solid with liquid filled pores. The solid separating the pores is thin

while the pore dimensions depend upon the concentration of gel material.

During the growth process the supersaturation of desired product is created by

diffusion of one or more components to the growth site. The aqueous solution of soluble salts

is allowed to come close to the gel. The gel provides the medium controlled diffusively for

salt solution. Sometimes seed crystals are also introduced to enhance the growth process.

Grown crystals are held in the gel itself without damage. This gel growth process is a slow

process and it takes about a week to grow the crystal of 2 mm to 4 mm in one direction of a

crystal. The time taken for the growth of the crystal is proportional to the square of the length

of a crystal. This technique is not suitable for the materials having high melting points.

27

2.3 MELT GROWTH TECHNIQUE

In this technique, the melt contained in a crucible is progressively cooled to yield

single crystals of the material. The essential condition required for the growth of crystals in

this method is that the material to be grown should melt congruently i.e., the melt and the

crystals should have same composition. The crucible material should not lead to the

contamination of the melt. Some times seed crystals are placed at the top surface of the melt

and very slowly pulled upwards with the temperature of the surface maintained at the melting

point of the crystals using a computer controlled crystal puller. Provision is also kept for the

rotation of the seed crystals as to enable the growth of crystals of uniform composition. Such

technique is known as Czochralski growth technique. The material gets crystallized at the

point of contact of seed crystal on the surface of the melt. The grown crystals are very slowly

pulled upwards, approximately at the rate of 1mm /hour. The Czochralski and Bridgman

growth technique are the modification of the basic melt growth technique.

2.3.1 CZOCHRALSKI CRYSTAL PULLING TECHNIQUE

This method was developed by Czozharalski in 1981, which is basically a

modification of the methode developed by Kyropolus [24-26]. The basic condition here is

that the melt and the crystal should have the same composition.

In this technique, the seed crystal is placed at the top surface of the melt and slowly

pulled upward with the temperature of surface maintained at the melting point of the crystal.

The seed and melt are now slowly rotated and the temperature of melt is slowly reduced. The

growth rate in this technique is relatively fast and with the suitable precautions, the

cylindrical crystal can be grown.

28

2.3.2 BRIDGMANN-STOCKBARGER TECHNIQUE

This technique was originated by Bridgmann in 1925 and was modified by

Stockbarger [27, 28]. Here the material to be crystallized is placed in a cylindrical conical

shaped crucible. The substance is placed in a two zone vertical furnace where, the

temperatures of upper and lower zones are above and below the melting points respectively

to the eventual material to be crystallized. This method is useful in preparation of crystals of

metals, semiconductors, alkali, alkaline earth halides complex ternary fluorides of alkali and

transition metals. Though, it is not suitable for the materials which expand on solidification.

2.3.3 VERNEUIL FLAME FUSION TECHNIQUE

This technique was developed by Verneuli [29]. The largest use of this technique has

been for the growth of gem-quality ruby and emeralds and others with high melting point and

for which no suitable crucible is found. An oxy-hydrogen or oxy-acetylene flame is

established and used for heating process. The powder of the material to be crystallized is

shaken mechanically from the hooper through sieve using small vibrator. The flame is made

to be impinging on a pedestal, where a small pile of partly fused alumina, quickly build up.

As the pile rises, it reaches in to hotter part of the flame so that tip becomes completely

molten. The molten region increases in size and start to solidify at the lower end. As more

and more powder arrives, the solidifying region broadens in to a crystal growing in length.

Such a crystal is called boule.

2.3.4 ZONE-MELTING TECHNIQUE

This technique was developed by Pfann in 1952 [30]. Zone refining technique is the

most important technique where numbers of molten zones are passed along the charge in one

direction, either horizontally or vertically. Each zone carries a fraction of impurity away to

settle to the end of the charge thereby purifying the remainder. The product is usually a large

29

pure single crystal. This process is used in growing crystals as well as purifying several

metals and compounds.

2.4 SOLUTION GROWTH TECHNIQUE

This is the simplest and one of the oldest methods of growing crystals by dissolving

the material in the solvent to the desired degree of super saturation [31]. With the proper

available solvents, one can grow a crystal without furnace. The crystals grown with this

method are generally water soluble. Crystals of organic and inorganic materials can be

obtaining from growth from water solution. Alkali halides crystals, several nonlinear optics,

like potassium and ammonium dihydrogen phosphates have been grown from water solutions

[32, 33]. Now a day this technique at higher temperatures is also used in some cases.

2.5 FLUX GROWTH TECHNIQUE

In this modified solution growth technique, the crystals are grown at high temperature

from the solutions. The growth of the crystals from solutions mainly relies on the availability

of the suitable solvents. Solutions of oxide and halide solvents, which are often called as

fluxed melts, are used for the growth of ionic materials. The choice of solvent should

facilitate minimum contamination.

The material to be crystallized is dissolved in a suitable solvent at high temperature

and the crystals are grown as solution becomes critically supersaturated. The principal

advantage of this technique is that, crystal growth occurs at lower temperature than that

required for growth from the pure melt.

2.6 VAPOR TRANSPORT TECHNIQUE

From the vapor phase, good quality crystals can be obtained. The vapor transport

technique is used to grow thin crystals. In this technique, the material from which the crystals

are to be grown is transported to the growth zone from the source zone. The temperature of

30

the growth zone is kept sufficiently lower than that of the source zone. This technique can be

basically classified in to three categories

(1) Sublimation

(2) Chemical Vapor Transport Technique (CVT)

(3) Direct Vapor Transport Technique (DVT)

2.6.1 SUBLIMATION

This method is carried out either in a static or floating gas system. In a static

system, the material is sealed in a tube in a furnace with thermal gradient. The sublimation

take place in hotter portion of the furnace and the crystal growth take place in a colder

portion of the furnace.

In a float system an inert gas is passed through the tube over the material in a hot

zone, carrying the gaseous species towards the colder zone, where it deposits. Using this

technique, high purity crystals can be grown. This method is useful for the materials having

high vapor pressure at temperature up to 1000C.

2.6.2 CHEMICAL VAPOR TRANSPORT TECHNIQUE

Several compounds which are not accessible by usual crystal growing

methods such as modified Czochralski or Bridgmann - Stockbarger techniques can be

prepared by this method. It is particularly suited for high melting point compounds or for

those which decompose without melting. Application of this technique stems on the growth

of metal single crystals in halogen atmosphere.

In this technique, the chemical reaction take place in which, a solid phase reacts with

a transporting agent like iodine, bromine, NH4Cl etc. at the source zone. The temperature

gradient is maintained in a dual zone furnace so that the material from the source zone can be

transported to the growth zone. It is necessary to maintain a proper temperature gradient

between source and growth zones for the good quality crystals to be grown.

31

Using this technique a large number of crystals have been grown [34-40]. The

crystals of the size of several centimeters in length can be grown using this method [41, 42].

The reaction product is volatile and can be transported in a vapor phase at temperature well

below the melting point of the material. Usually a starting reaction occurs at higher

temperature and the reverse process at the lower temperature, which deposits the molecules

of the compound at the growth zone. In the initial stage very small crystals are formed. The

transport of the reaction products can be obtained by continuous gas flow or by its

recirculation within a tubular ampoule. For chalcogenides, halogens are most commonly used

transporting reagents.

In this technique the disadvantage is that, the transporting agent may get incorporated

as impurities in the crystals during the growth process. This may affect the properties of the

grown crystals.

2.6.3 DIRECT VAPOR TRANSPORT TECHNIQUE

The main disadvantage of the chemical vapor transport technique is the high

level of unintentional doping of transporting agent on the crystal. To overcome this, the

direct vapor transport technique [43-46] can be used. Here transport of material take place

directly without any transporting agent, only due to the proper temperature gradient settled

across the closed ampoule.

The reaction taking place to form AB compound from A and B materials, can be

symbolically represented as

gT

gSgT

gST

SS ABBAABBABA

Here it can be seen that one of the element (B) has a lower melting point and it goes

in to vapor form earlier than the other one (A). This vapor reacts with the other element at

high temperature and form the compound AB. In the present investigations ZnS, ZnSe and

ZnTe crystals have been grown using the direct vapor technique.

32

2.7 CHOICE OF THE GROWTH TECHNIQUE

All wide band gap II-VI compounds are insoluble in water and are having

comparably higher melting points. In this case crystallization from the vapor phase has

various advantages over other growth techniques [47]. These advantages results mostly from

(i) The lower processing temperature involved-as the melting temperature of II-VI

compounds are higher, melt growth process is very difficult to be handled.

(ii) Physical vapor transport act as a purification process [48] because of difference in

vapor pressure of native elements and impurities.

(iii) Most solid-vapor interface exhibit higher interfacial morphological stability [49-

51] during the growth process because of their low atomic roughness [52] and

consequently the pronounced growth rate anisotropy.

To increase the transfer rate and consequently reduce the growth temperature,

transport agent such as I2 is widely employed for ZnS [53-55], ZnSe [54-61] and ZnTe

[49]. But the disadvantage of this chemical vapor transport technique is the high level of

unintentional doping of transporting agent [55, 56, 59]. Thus in our case for the growth of

the II-VI compounds crystals – ZnS, ZnSe and ZnTe, we found the Physical Vapor

Transport technique is suitable, which is also experimentally simpler and having minimal

complex process control in comparison with the other techniques.

2.8 REQUISITES FOR THE DIRECT VAPOR TRANSPORTGROWTH

In this technique the dual zone furnace is used. One of the zones is kept at

higher temperature compared with the other zone. The material travel from the source zone

to the growth zone in vapor form and if the temperature gradient between two zones is

properly maintained throughout the whole process, crystal growth takes place at the growth

zone of the closed ampoule. Thus the crystal will grow only if the certain requirements of the

33

growth mechanism through this technique are satisfied. Some important requirements to be

satisfied are listed below.

1. Maintaining the proper temperature gradient between two zones of the furnace is one

of the most important factors through out the whole growth process, because it can

affect both the size and the quality of the grown crystal.

2. A sealing of the ampoule to sufficient vacuum level is also an important factor.

Proper vacuum sealing improves the quality of the grown crystals and minimize the

risk of ampoule blast particularly when the high vapor pressure materials like sulfur is

used.

3. The material used to make the encasing assembly (ampoule) should be capable of

sustaining the higher temperatures compared to the melting points of the materials

from which the crystals are to be grown.

4. Controlling of the temperatures of each zone of the dual zone furnace is very

important through out the whole growth cycle, as it affect very effectively on the size

and the quality of the grown crystals.

Considering all above requirements it is found that a dual zone furnace is required to

work with the temperature in excess of 1000C. Also the material of the encasing tube

(ampoule) has to be selected such that it does not react with the compound and can withstand

the required high temperatures. Here the quartz tube has been used to make ampoules for the

growth of ZnSe and ZnTe crystals using DVT technique to grow the crystals within it.

2.9 CONSTRUCTION OF DUAL ZONE FURNACE

The well designed furnace is an important apparatus to grow the crystals of zinc

monochzlcogenides. Two-zone furnace provides an appropriate temperature gradient over the

entire ampoule length. Normally the temperature employed is fairly high. The temperature

gradient within the furnace is required over a length of about 25 cm. Stability of the

temperature plays an important role, therefore, for this purpose electronic temperature

controllers were used.

34

Quartz tubeQuartz ampoule

Muffle

Steel body

Quartz tubeQuartz ampoule

Muffle

Steel body

The furnace was constructed in University Science and Instrumentation Center

(USIC) by using a special sillimanite threaded tube (grade KR 80 GA HG) closed at one end,

50cm in length, 7cm outer diameter, 5.6cm inner diameter with threaded pitch of 3mm. Super

Kanthal A1 wire of 17 SWG was wound directly on the furnace tube into two different zones

or regions. The tube was enclosed in the insulating brick slabs constructed locally and the

brick shell was fully enclosed in thick asbestos sheets, and the entire assembly was supported

by a steel framework. This arrangement is shown in Figure 2.1. The power supplied to the

furnace windings was regulated by the control circuit shown in Figure 2.2. The two regions

of windings were provided with independent power supplies and temperature controllers.

Transformers with 70, 80 and 100 V taps with 20 A current capacities in secondary windings

were used to supply sufficient power in order to achieve the required higher temperature.

Figure-2.1 A dual zone furnace with axially loaded ampoule.

35

INDOTHERM401

INDOTHERM401

o

Co

C

Muffle

Temperature Controller

Temperature Controller

Thermocouple

Thermocouple

Transformer

220 V AC

INDOTHERM401

INDOTHERM401

o

Co

C

Muffle

Temperature Controller

Temperature Controller

Thermocouple

Thermocouple

Transformer

220 V AC

2.10 TEMPERATURE CONTROL IN THE DUAL ZONEHORIZONITAL FURNACE

Temperature control during the whole process of crystal growth is extremely

important factor, which is directly related with the size and quality of the grown crystals. In

order to accomplish a stable temperature profile, temperature controllers (Indotherm make)

have been used. A schematic of the controllers with the muffle windings connections is

shown in figure 2.2.

The fluctuations in electrical supply were controlled by AC voltage stabilizer

with 180-260 V input and 230 1% output volts of capacity 3 kVA. The output of stabilizer

was fed to the primary windings of the transformer, which heated the furnace windings and

helped to maintain the stability of growth conditions. With the help of temperature

programmers, a required temperature gradient could be established across the length of the

working tube in the appropriate temperature range. Cr–Al thermocouples were used and

temperature programmers were calibrated using these thermocouples. It was found that the

thermocouples were stable over the prolonged use in the furnace, and they were supported

within the furnace tube itself showing the temperature of furnace tube.

Figure 2.2 Temperature controllers connections.

36

2.11 AMPOULE PREPARATION

A high quality quartz tube having melting point more than 1500C, have been

used in the present growth process. A length of the ampoule is 250mm with inner and outer

diameter of 23mm and 25mm respectively. One end of the ampoule was sealed and the other

end was drawn in to the neck. At the neck end, a tube of 8mm inner diameter was joined

having 300mm length for evacuation purpose.

2.12 CLEANING PROCESS OF AMPOULE

Before using the prepared ampoule, it is necessary to make it properly

cleaned. Following steps were followed to clean the ampoule.

1. Washed with a boiling water using suitable detergent

2. Rinsed with H2SO4 and then with double distilled water.

3. Further rinsing with HCl and HNO3 and with double distilled water.

After cleaning process is over, the ampoule was filled with about 10ml of

concentrated HF and was heated to make the inner surface rough, so that proper

preferential nucleation can take place at that surface during growth process. This ampoule

was once again washed with distilled water and then was heated at about 100C to dry it

properly.

2.13 CHARGE PREPARATION AND CRYSTAL GROWTH

The material for the crystal growth was loaded in to the cleaned ampoule.

Both the compounds (Zn and S/Se/Te) were taken in their stoichiometric proportion. A total

charge of 10 gram was used in each case. The details of the materials that have been used for

present work are given in Table 2.2.

37

Aldrich Corp.99.99Tellurium (Te)

Chiti Chem, Vadodatra99.99Selenium (Se)

Fluca Chemic. GmbH99.99Sulfur (S)

Fluca Chemic. GmbH99.99Zinc (Zn)

SupplierPurity%

Material

Aldrich Corp.99.99Tellurium (Te)

Chiti Chem, Vadodatra99.99Selenium (Se)

Fluca Chemic. GmbH99.99Sulfur (S)

Fluca Chemic. GmbH99.99Zinc (Zn)

SupplierPurity%

Material

Table 2.2 Selected materials for crystal growth.

2.13.1 CHARGE PREPARATION

A 10 gram mixture of Zn and S /Se/ Te were taken in stoichiometric proportion in

three different quartz ampoules for a charge preparation of ZnS, ZnSe and ZnTe respectively.

These ampoules were evacuated at the pressure of 10-5 Torr and than sealed. These sealed

ampoules were placed in a dual zone furnace of constant reaction temperature to obtain a

charge of the materials. During the synthesis of the charge, temperature was slowly increased

up to 1023 K at the rate of 10 K/hr. The ampoules were kept at this final temperature for 4

days. Then the furnace was slowly cooled at the rate of 20 K/hr and brought to room

temperature. The resulting whitish, yellowish and dark reddish charges were obtained in

three ampoules for ZnS, ZnSe and ZnTe respectively.

38

1123

1193

1238

GrowthZone (K)

SourceZone (K)

121851173ZnTe

81851233ZnSe

91851283ZnS

Averagedimension of

grown crystals(mm)2

GrowthPeriod(hrs)

TemperaturedistributionSample

1123

1193

1238

GrowthZone (K)

SourceZone (K)

121851173ZnTe

81851233ZnSe

91851283ZnS

Averagedimension of

grown crystals(mm)2

GrowthPeriod(hrs)

TemperaturedistributionSample

2.13.2 CRYSTAL GROWTH

These charges of ZnS, ZnSe and ZnTe compounds were transferred to other three

different quartz ampoules cleaned by a process as discussed above and then sealed at a

pressure of 10-5 Torr. They were then placed in the furnace for 5 days with different suitable

temperature gradients between the source zone and growth zone for all three ampoules. After

that, furnaces were cooled down to room temperature at a rate of 10 K/hr. Thus the materials

have been found to be converted into the form of crystals at the cooler end of the ampoules.

The colors of the grown crystals of ZnS, ZnSe and ZnTe have found whitish, yellowish and

dark reddish respectively. The average sizes of the crystals were from 8 mm2 to 12 mm2.

These crystals were collected carefully after breaking the ampoules. The growth parameters

and temperature profiles of ZnS, ZnSe and ZnTe crystal growth are shown in the table-2.3

and figure 2.3 respectively and the as grown crystals of ZnS, ZnSe and ZnTe are shown in

figure 2.4 (a), (b) and (c) respectively.

Table 2.3 Growth parameters of grown crystals using DVT technique.

39

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

9 0 0

1 0 0 0

1 1 0 0

1 2 0 0

1 3 0 0

1 4 0 0

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0

H o u r s

Te

mp

era

ture

(K

)

G r o w th Z o n e

S o u r c e Z o n eZ n T eZ n S e

Z n S

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

9 0 0

1 0 0 0

1 1 0 0

1 2 0 0

1 3 0 0

1 4 0 0

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0

H o u r s

Te

mp

era

ture

(K

)

G r o w th Z o n e

S o u r c e Z o n e

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

9 0 0

1 0 0 0

1 1 0 0

1 2 0 0

1 3 0 0

1 4 0 0

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0

H o u r s

Te

mp

era

ture

(K

)

G r o w th Z o n e

S o u r c e Z o n e

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

9 0 0

1 0 0 0

1 1 0 0

1 2 0 0

1 3 0 0

1 4 0 0

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0

H o u r s

Te

mp

era

ture

(K

)

G r o w th Z o n e

S o u r c e Z o n eZ n T eZ n S e

Z n S

ZnSZnS

Figure-2.3. Temperature profiles used for the growth of ZnS, ZnSe & ZnTe crystals.

Figure 2.4 (a) As grown crystals of ZnS using DVT technique.

40

ZnSeZnSe

ZnTeZnTe

Figure 2.4 (b) As grown crystals of ZnSe using DVT technique.

Figure 2.4 (c) As grown crystals of ZnTe using DVT technique.

41

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