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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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