36

Chapter – 2

INTRODUCTION TO CRYSTAL GROWTH METHODS WITH EMPHASIS ON DIRECT

VAPOUR TRANSPORT TECHNIQUE

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2.1 Introduction

Crystals are the unacknowledged pillars of modern technology. Without crystals,

there would be no electronic industry, no photonic industry, no fiber optic

communications, (which depend on materials/crystals such as semiconductors,

superconductors, polarizers, transducers, radiation detectors, ultrasonic amplifiers,

ferrites, magnetic garnets, solid state lasers, non-linear optics, piezo-electric, electro-

optic, acousto-optic, photosensitive, refractory of different grades, crystalline films for

microelectronics and computer industries). Crystal growth is an interdisciplinary subject

covering physics, chemistry, material science, chemical engineering, metallurgy,

crystallography, mineralogy, etc. In past few decades, there has been a growing interest

on crystal growth processes, particularly in view of the increasing demand of materials

for technological applications [1]. Atomic arrays that are periodic in three dimensions,

with repeated distances are called single crystals. It is clearly more difficult to prepare

single crystal than poly-crystalline material and extra effort is justified because of the

outstanding advantages of single crystals [2]. The reason for growing single crystals is;

many physical properties of solids are obscured or complicated by the effect of grain

boundaries. The chief advantages are the anisotropy, uniformity of composition and the

absence of boundaries between individual grains, which are inevitably present in

polycrystalline materials. The strong influence of single crystals in the present day

technology is evident from the recent advancements in the above mentioned fields.

Hence, in order to achieve high performance from the device, good quality single crystals

are needed. Growth of single crystals and their characterization towards device

fabrication have assumed great impetus due to their importance for both academic as well

as applied research.

Therefore, enormous amount of toil and treasure has been lavished on the

development of crystal growth techniques. The in dept explanation of various techniques

can be obtained in the literature [3-6]. There are three major stages involved in this

research. The first is the production of pure materials and improved equipments

associated with the preparation of these materials. The second is the production of single

crystals first in the laboratory and then extending it to commercial level. The third is the

38

characterization and utilization of these crystals in devices. In this section, various

methods of crystal growth with emphasis on high temperature growth techniques are

described.

2.2 Methods of crystal growth

Growth of crystals ranges from a small inexpensive technique to a complex

sophisticated expensive process wherein crystallization time ranges from minutes, hours,

days and to months. Single crystals may be produced by the transport of crystal

constituents in the solid, liquid or vapour phase. On the basis of this, crystal growth may

be classified into three categories as follows,

(i) Solid Growth: This involves solid-to-solid phase transformation

(ii) Liquid Growth: This involves liquid to solid phase transformation

(iii) Vapour Growth: This involves vapour to solid phase transformation

Large number of the references are available with detailed description the growth

techniques, only a brief description of different techniques is covered here.

The conversion of a polycrystalline piece of material into single crystal by

causing the grain boundaries to be swept through and pushed out of the crystal takes

place in the solid-growth of crystals [7-10]. These methods have been discussed in detail

by several authors [11-14]. Different techniques of each category are found in reviews

and books by Faktor and Garret (1974) on vapour growth, Brice (1973) on melt, Henisch

(1988) on gel growth, Buckley (1951) on solution growth and Elwell and Scheel (1975)

on high temperature solution growth.

An efficient process is the one, which produces crystals adequate for their use at

minimum cost. Better choice of the growth method is essential because it suggests the

possible impurity and other defect concentrations. Choosing the best method to grow a

given material depends on material characteristics.

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The liquid growth includes both melt and solution growth. A survey of the

methods of growth suggests that almost 80% of single crystals are grown from the melt

whereas roughly 5% from vapour, 5% from low temperature solution, 5% from high

temperature solution, and 3% from the solid and only 2% by hydrothermal methods

[15,16].

In contrast to the historical work, it seems that the essential task for the crystal

growers is to gain basic knowledge about the correlation of crystal properties and the

growth conditions defined to be special parameters. This basic understanding of the

deposition of atoms onto a suitable substrate surface – crystal growth – the generation of

faults in the atomic structure during growth and subsequent cooling to room temperature

– crystal defect structure, are the input for the design of crystal growth systems and

control of growth parameters. Though the fundamentals are relatively simple, the

complexities of the interactions involved and the individualities of different materials,

system and growth process have ensured that experimentally verifiable predictions from

scientific principles have met with limited success. As a result, crystal growth has long

had the image of alchemy. This is clearly expressed by the title of one of the first text

books on crystal growth ‘The Art (!) and Science of Growing crystal’ (Gilman 1963).

The recent advances which include reduction of growth temperature, the reduction or

elimination of reactant transport variables and the use of better controlled energy sources

to promote specific reactions, coupled with increased development and application of in-

situ diagnostic techniques to monitor and perhaps the ultimate control lead to simplified

growth systems. The crystal growth process has transferred and the field from an art to

science, technique and to technology.

2.3 Growth from solution

Materials, which have high solubility and have variation in solubility with

temperature can be grown easily by solution method. There are two methods in solution

growth depending on the solvents and the solubility of the solute. They are

1. High temperature solution growth

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2. Low temperature solution growth

2.3.1 High temperature solution growth

In high-temperature solution growth, the constituents of the material to be

crystallized are dissolved in a suitable solvent and crystallization occurs as the solution

becomes critically supersaturated. The supersaturation may be promoted by evaporation

of the solvent, by cooling the solution or by a transport process in which the solute is

made to flow from a hotter to a cooler region. The high temperature crystal growth can be

divided into two major categories:

1. Growth from single component system.

2. Growth from multi component system.

There methods are widely used for the growth of oxide crystals. The procedure is

to heat the container having flux and the solute to a temperature so that all the solute

materials dissolve. This temperature is maintained for a ‘soak’ period of several hours

and then the temperature is lowered down very slowly [17].

2.3.1.1 Hydrothermal growth

Hydrothermal growth implies conditions of high pressure as well as high

temperature [16]. Substances like calcite, quartz is considered to be insoluble in water but

at high temperature and pressure, these substances are soluble. Temperatures involved in

these techniques are typically in the range of 400° C to 600° C and the pressure involved

is large (hundreds or thousands of atmospheres).

Growth is usually carried out in steel autoclaves with gold or silver linings.

Depending on the pressure, the autoclaves are grouped into low, medium and high-

pressure autoclaves. The concentration gradient required to produce growth is provided

by a temperature difference between the nutrient and growth areas. The requirement of

high pressure presents practical difficulties and there are only a few crystals of good

quality and large dimensions which are grown by this technique. Quartz is the

outstanding example of industrial hydrothermal crystallization. One serious disadvantage

41

of this technique is the frequent incorporation of OH- ions into the crystal, which makes

them unsuitable for many applications.

2.4 Gel growth

It is an alternative technique to solution growth with controlled diffusion wherein

the growth process is free from convection. Gel is a two-component system of a

semisolid which is rich in liquid and inert in nature. The material, which decomposes

before melting, can be grown in this medium by counter diffusing two suitable reactants.

Crystals with dimensions of several mm can be grown in a period of 3 to 4 weeks. The

crystals grown by this technique have high degree of perfection and fewer defects since

the growth takes place at room temperature [18-20].

2.5 Growth from melt

Many materials can be grown in crystal form which involves melting congruently

without decomposition at the melting point and do not undergo any phase transformation

between the melting point and room temperature. Depending on the thermal

characteristics, the following techniques are employed.

1. Bridgman technique

2. Czochralski technique

3. Kyropoulos technique

4. Zone melting technique

5. Verneuil technique

In Bridgman technique the material is melted in a vertical cylindrical

container, tapered conically with a point bottom. The temperature of the container is

lowered slowly from the hot zone of the furnace in to the cold zone. Crystallization

begins at the tip and continues usually by growth from the first formed nucleus. This

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technique cannot be used for materials, which decompose before melting. This technique

is best suited for materials with low melting point [21,22].

In Czochralski method, the material to be grown is melted by induction or

resistance heating under a controlled atmosphere in a suitable non-reacting container. By

controlling the furnace temperature, the material is melted. A seed crystal is lowered to

touch the molten charge. When the temperature of the seed is maintained very low

compared to the temperature of the melt, by suitable water cooling arrangement, the

molten charge in contact with the seed will solidify on the seed. Then the seed is pulled

with simultaneous rotation of the seed rod and the crucible in order to grow perfect single

crystals.

Liquid encapsulated Czochralski abbreviated as LEC technique makes it possible

to grow crystals of materials, which consist of components that produce high vapour

pressure at the melting point. This refined method of Czochralski technique is widely

adopted to grow III-V compound semiconductors.

In Kyropoulos technique, the crystal is grown in a larger diameter. As in the

Czochralski method, here also the seed is brought in contact with the melt and is not

raised much during the growth, i.e. part of the seed is allowed to melt and a short narrow

neck is grown [23,24]. After this, the vertical motion of the seed is stopped and growth

proceeds by decreasing the power into the melt. The major use of this method is growth

of alkali halides to make optical components.

In the zone melting technique, the feed material is taken in the form of sintered

rod and the seed is attached to one end. A small molten zone is maintained by surface

tension between the seed and the feed. The zone is slowly moved towards the feed.

Crystal is obtained over the seed. This method is applied to materials having large surface

tension. The main reasons for the impact of zone refining process to modern electronic

industry are the simplicity of the process, the capability to produce a variety of organic

and inorganic materials of extreme high purity and production of dislocation free crystals

with a low defect density.

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In the case of vertical normal freezing, the solid-melt interface is moved upwards

from the cold bottom to the hot top so as to get better quality crystals [25]. The method is

more applicable in growing single crystals of materials with volatile constituents like

GaAs.

In the Verneuil technique, a fine dry powder having particle size 1-20 microns of

the material to be grown is shaken through the wire mesh and allowed to fall through the

oxy-hydrogen flame. The powder melts and a film of liquid is formed on the top of the

seed crystal. This freezes progressively as the seed crystal is slowly lowered. The art of

the method is to balance the rate of charge feed and the rate of lowering of the seed to

maintain a constant growth rate and diameter. By this method, ruby crystals are grown up

to 90 mm in diameter for use in jeweled bearings and lasers. This technique is widely

used for the growth of synthetic gems and variety of high melting oxides.

2.5.1 Electrocrystallisation

Electrolysis of fused salts is normally used for the commercial production of

metals such as aluminum and has great technological importance. The process of crystal

growth from fused salts is analogous in many respects, except for the requirement of

electron transfer in deposition of the metal. Fused salt electrolysis has been used to grow

crystals of oxides in reduced valence states [13].2.6 Low temperature solution growth.

2.6 Low temperature solution growth

Growth of crystals from aqueous solution is one of the ancient methods of crystal

growth. The method of crystal growth from low temperature aqueous solutions is

extremely popular in the production of many technologically important crystals. It is the

most widely used method for the growth of single crystals, when the starting materials

are unstable at high temperatures [11] and also undergo phase transformations below

melting point. The growth of crystals by low temperature solution involves weeks,

months and sometimes years. Though the technology of growth of crystals from solution

has been well perfected, it involves meticulous work, much patience and even a little

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amount of luck. A power failure or a contaminated batch of raw material can destroy

months of work.

Materials having moderately high solubility in temperature range, ambient to 100

°C at atmospheric pressure can be grown by low-temperature solution method [26]. The

mechanism of crystallization from solutions is governed, in addition to other factors, by

the interaction of ions or molecules of the solute and the solvent which is based on the

solubility of substance on the thermodynamical parameters of the process viz.

temperature, pressure and solvent concentration. The advantages of crystal growth from

low temperature solution near the ambient temperature results in the simple and straight

forward equipment design which gives a good degree of control of accuracy of ±0.01 ºC.

Due to the precise temperature control, supersaturation can be very accurately controlled.

Also efficient stirring of solutions reduces fluctuations to a minimum. The low

temperature solution growth technique is well suited to those materials which suffer from

decomposition in the melt or in the solid at high temperatures and which undergo

structural transformations while cooling from the melting point and as a matter of fact,

numerous organic and inorganic materials which fall in this category can be crystallized

using this technique. The low temperature solution growth technique also allows the

growth of variety of different morphologies and polymorphic forms of the same

substance by variations of growth conditions or of solvent. The proximity to ambient

temperature reduces the possibility of major thermal shock to the crystal both during

growth and removal from the apparatus.

The main disadvantages of the low temperature solution growth are the slow

growth rate in many cases and the ease of solvent inclusion into the growing crystal.

Under the controlled conditions of growth, the solvent inclusion can be minimized and

the high quality of the grown crystal can compensate the disadvantage of much longer

growth periods. After many modifications and refinements, the process of solution

growth now yields good quality crystals for a variety of applications. Growth of crystals

from solution at room temperature has many advantages over other growth methods

though the rate of crystallization is slow. Since growth is carried out at room temperature,

the structural imperfections in solution grown crystals are relatively low [13].

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In the low temperature solution growth, crystals can be grown from solution if the

solution is supersaturated i.e., it contains more solute than it can be in equilibrium with

the solid. Three principal methods are used to produce the required supersaturation:

(i). Slow cooling of the solution

(ii). Slow evaporation of the solvent

(iii). The temperature gradient method.

Low temperature solution growth is a well-established technique due to its

versatility and simplicity. It is possible to grow large crystals of high perfections as the

growth occurs close to equilibrium conditions (McArdle and Sherwood 1987). It also

permits the preparation of different morphologies of the same materials by varying the

growth conditions.

2.6.1 Slow cooling technique

It is the best method to grow crystals by solution technique. The Main limitation

of this method is the need to use a range of temperature. The possible range of

temperature is usually small so that much of the solute remains in the solution at the end

of the run. To compensate this effect, large volumes of solution are required. The use of a

range of temperatures may not be desirable because the properties of the grown material

may vary with temperature. Eventhough the method has technical difficulty of requiring a

programmable temperature control, it is widely used with great success. The temperature

at which such crystallization can begin is usually within the range 45 - 75 °C and the

lower limit of cooling is the room temperature.

2.6.2 Slow evaporation method

This method is similar to the slow cooling method in view of the apparatus

requirements. The temperature is kept constant and provision is made for evaporation.

With non-toxic solvents like water, it is permissible to allow evaporation into the

atmosphere. Typical growth conditions involve temperature stabilization to about ±

0.005°C and rates of evaporation of a few ml /hr. The evaporation techniques of crystal

46

growth have the advantage that the crystals grow at a fixed temperature. But inadequacies

of the temperature control system still have a major effect on the growth rate. This

method is the only one, which can be used with materials, which have very small

temperature coefficient of stability [27].

2.6.3 Temperature gradient method

This method involves the transport of the materials from a hot region containing

the source material to be grown to a cooler region where the solution is supersaturated

and the crystal grows. The main advantages of this method are that

(a) Crystal grows at a fixed temperature.

(b)This method is insensitive to changes in temperature provided both the

source and the growing crystal undergo the same change.

(c) Economy of solvent and solute.

On the other hand, changes in the small temperature differences between the

source and the crystal zones have a large effect on the growth rate [16].

Excellent quality crystals of ferroelectric and piezo-electric materials such as

Ammonium dihydrogen phosphate (ADP), Potassium dihydrogen phosphate (KDP) and

Triglycine sulphate (TGS) are commercially grown for use in devices by the low

temperature solution growth method [11].

2.7 Growth from vapour phase

The growth of single crystals from the vapour phase is probably the most versatile

technique among all crystal growth processes. Crystals of high purity can be grown from

vapour phase by sublimation, condensation and sputtering of elemental materials. To

obtain single crystals materials with high melting point, this method is used. Molecular

beam techniques have also been applied recently in crystal growth. The most frequently

used method for the growth of bulk crystals utilizes chemical transport reaction in which

a reversible reaction is used to transport the source material as a volatile species to the

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crystallization region. Finding a suitable transporting agent is a formidable, problem in

this technique. It is rarely possible to grow large crystals using this technique because of

multi-nucleation.

The commercial importance of vapour growth is the production of thin layers by

chemical vapour deposition (CVD), where usually irreversible reactions e.g.

decomposition of silicon halides or of organic compounds are used to deposit materials

epitaxially on a substrate. Doping can be achieved by introducing volatile compounds of

dopant elements into the reaction region. The thickness of the doped layer can be

controlled.

This class of technique can be broadly classified in to three categories.

(i) Sublimation technique.

(ii) Chemical vapour transport technique (CVT)

(iii) Direct vapour transport technique (DVT)

2.7.1 Sublimation

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

system, the material is sealed in a tube and placed in a furnace with thermal gradient.

The sublimation takes place in hotter portion of the furnace and the crystal growth in the

colder portion. In a float system, an inert gas is passed through the tube over the material

in the hot zone, carrying the gaseous species into the colder zone where it deposits.

Crystals grown in this manner are of extremely high purity. The method may be

applicable to a material that has reasonably high vapour pressure at temperature upto

1000 °C [28,29]

2.7.2 Chemical vapour transport technique (CVT)

In this technique, the process occurs through chemical reactions in which solid

phase reacts with a transporting agent like iodine, bromine, NH4Cl etc. at the source zone

to form vapour phase products. In this technique, the temperature gradient is maintained

in a multiple zone furnace for the transport of material from source zone to growth zone

of the encasing tube. Proper growth conditions at the growth zone would lead to the

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growth of the crystals. A large number of transition metal dichalcogenide crystals have

been grown with this technique [30-36]. There are various theoretical attempts available

in literature by Lever [37,38], Mandel[39,40] and Arizumin and Nishinaga [41], Kaldis

[42,43] which have shown that it is possible to grow crystals upto several centimeters in

size by this technique under well-controlled nucleation condition at smaller

supersaturation rate. Schafer [44] explained transport reactions systematically for the

preparation of high purity refractory metals.

This technique mainly depends upon a chemical reaction between the source

material to be crystallized and a transporting agent. The reaction product is volatile and

can be transported in the vapour form (phase) at temperatures well below the melting

point of the compound. Transport of the material occurs between two zones having

different temperatures. Usually the starting reaction occurs at higher temperature and is

reversed at the lower temperature to deposit molecules of compound at the most

favourable crystalline sites. Initially, a random deposition takes place and seed crystals

are formed. Thus, the initial seed crystals need not be placed prior to the start of the

process. The transport of the reaction products in the vapour phase can be obtained by

continuous gas flow from external supplies (as in the case of open tube chemical vapour

transport technique) or by its recirculation within a tubular ampoule (as in case of the

close tube chemical vapour transport technique). As a result, one can transport unlimited

amount of material (just in ideal condition) with only a small amount of transporting

agent. Schafer [44] has shown that optimum transport occurs when the reaction is not far

from the equilibrium condition between the solid phase and the vapour phase. For

chalcogenides, the halogens are most commonly used transporting reagents. It has been

observed that bromine proves to be a better transporting reagent for MoSe2 compound

[45]. Nitsche [46] has shown that the concentration of the transporting agent which is

most suitable for optimum transport of the material to be grown is around 5mg/cc.

Different mechanisms through which the transport of material takes place

(a) Diffusion mechanism when the pressure inside the closed ampoule is low and the

dimensions of the ampoule are small.

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(b) Convection currents set up due to the thermal gradient when the pressures as well

as the dimensions of the ampoule are large.

(c) Laminar flow of the reaction product takes place due to pressure gradient along

the ampoule if there is equal number of molecules in the vapour phase on each

side of the ampoule. This results into a reaction like

Compound + Transporting agent = Reaction product

After detailed analysis of the transport reaction, Nitsch [47] has given the

following points for successful growth of material in crystal form using a vapour

transport technique.

(1) The rate of transport of the material must be lower than the rate of the growth of the

seed.

(2) The empirical evaluation of the optimum crystallization should be done for each

system keeping in view the possibility of polytypism.

(3) The dimensions of the tube should be large enough to avoid intergrowth between

adjacent seed. This is sometimes enhanced by asymmetric heating.

(4) The distribution of temperature in the tube should be uniform to avoid partial re-

evaporation of already grown crystals.

(5) The temperature gradient between two zones must be small when the diameter of

the tubes is more. This ensures the even distribution of growth products along the

tube. This is because the vapour flow is the rate determining factor.

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

incorporated as impurities in the crystals during the growth process. As a result, the

thermal and electrical properties of the grown crystals get affected up to a certain extent.

2.7.3 Direct vapour transport technique (DVT)

A better method to obtain pure crystals is the direct vapour transport technique,

where in the transport of material takes place directly from the source zone to the growth

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zone due to the temperature gradient set across the encasing tube without using any

transporting agent.

A.A. Al. Hilli et al. [48] have shown the utility of this method by successfully

growing good quality single crystals of some transition metal dichalcogenides. Detailed

study of transport reactions occurring during the growth process have been carried out by

Shafer [44]. The reaction taking place to form AB compound can be given as

( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )gT

gsgT

gsT

ss ABBAABBABA →++→+→+

One of the elements has lower melting point. So it goes in to vapour form at lower

temperatures. This vapour reacts with the other elements at high temperature forming the

compound. The same principle has been used to grow the transition metal dichalcogenides

using a direct vapour transport technique. In present investigations, MoSe2 crystals have

been grown using this technique.

2.8 Choice of the growth technique

The majority of compounds of the transition metal dichalcogenides belonging to

MX2 group are insoluble in water and decompose before their melting points are reached.

Therefore, the growth of such crystals from the melt and aqueous solution is not possible

and hence the growth of single crystals of these compounds, using vapour phase technique

was found to be most suitable.

The layered crystals grown by the CVT method usually contain small amount of

the transporting agent (e.g. Iodine, Bromine), which may remain as an active impurity

and affect the measured properties up to a certain extent. It has been reported that the

better method to obtain pure crystals is the direct vapour transport technique and it is

possible to grow fairly large crystals of TMDCs by DVT [46] where the transport of

materials takes place directly due to the temperature gradient set up across the charge

containing ampoule without any transporting agents. Since the direct vapour transport

technique seems to be a better, easier and comparatively an economical method for the

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growth of pure crystals, this method has been used for the growth of MoSe2 crystals in

the present case.

2.9 Basics for the direct vapour transport growth

In this technique, the materials in their elemental form from which the crystals are

to be grown are transported from the source zone, which is kept at comparatively high

temperature than the growth zone, kept at lower temperature of an enclosed system.

During the growth process, the compound in the volatile form reacts or decomposes. This

can be accomplished only if certain requirements of the growth mechanism, concerning

the encasing tube as well as the furnace constructions, are satisfied. The requirements of

the growth process by the direct vapour transport method are:

•••• Since a temperature profile with gradient is necessary for the crystal growth for 200

to 300 hrs, the furnace should essentially have two zones with separate temperature

controllers.

•••• The transportation of the volatile compounds should be done in an impurity free,

enclosed system / environment to ensure the purity of the grown crystal.

•••• The material used to make the encasing assembly should sustain at higher

temperature compared to the melting / boiling point of the material to be grown. At

the same time, the material of the encasing tube should be non-reactive at these

temperatures in order to avoid contamination of the grown crystals.

Considering the above requirements, a two-zone furnace has to be constructed

using windings of a material which can withstand temperatures up to 12000C. Also, the

encasing tube should be of the material which does not melt at high temperatures and it

should be non-reactive at these temperatures in order to avoid contamination of the

grown crystals. Under these circumstances, quartz is considered as the best choice for

making the encasing tube.

2.10 Nucleation

Nucleation is an important phenomenon in crystal growth and is the precursor of

the overall crystallization process. Nucleation is the process of generating the initial

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fragments of a new and more stable phase within a metastable motherphase, capable of

developing spontaneaously gross fragments of the stable phase. Nucleation is

consequently a study of the initial stages of the kinetics of such transformations

(Santhanaraghavan and Ramasamy 2000). Nucleation may occur spontaneously or it may

be induced artificially. There are cases referred to as homogeneous and heterogeneous

nucleations respectively. Both these nucleations lead to primary nuclei and occur in

systems that do not contain crystalline matter. On the other hand, nuclei are often

generated in the vicinity of crystals present in the supersaturated system. This

phenomenon is referred to as secondary nucleation.

Growth of crystals from solutions can occur if some degree of supersaturation

supercooling has been achieved first in the system. There are three steps involved in the

crystallization process.

(i) achievement of supersaturation or supercooling

(ii) formation of crystal nuclei

(iii) successive growth of crystals to get distinct faces

All the above steps may occur simultaneously at different regions of a

crystallization unit.

2.11 The chemical physics of crystal growth

If the crystal is in dynamic equilibrium with its parent phase, the free energy is at

a minimum and no growth will occur. For growth to occur, this equilibrium must be

disturbed by a change of the correct sign, in temperature, in pressure, chemical potential

(e.g. saturation) electrochemical potential (e.g. electrolysis), or strain (solid state growth).

The system may then release energy to its surrounding to compensate for the decrease in

entropy occasioned by the ordering of atoms in the crystal and the evolution of heat of

crystallization. In a well – designed growth process, only one of these parameters is held

minimally away from its equilibrium value to provide a driving force for growth.

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Crystal growth, then, is a non-equilibrium process and thought must be given to

the temperature, concentration and other gradients and the fact that heat of crystallization

is evolved and must be removed to the surroundings. At the same time, the crystal growth

process must be kept as near equilibrium and as near to a steady state process as possible.

This is why, the control of the crystal growth environment and a consideration of growth

kinetics both at the macroscopic and the atomic levels are of vital importance to the

success of a crystal growth experiment. It is particularly important to avoid constitutional

supercooling and the breakdown of the crystal-liquid interface that this can cause.

In some growth techniques, there is no crystal initially present. Here, the

nucleation problem is met which, in essence, is due to the fact that the surface-to-volume

ratio of small particle is much higher than that for a large crystal. Surfaces lose energy

because of discontinuities in atomic bonding. Thus, the nucleation of a new phase is a

discontinuities, not a quasi equilibrium process. This is the reason why pure melts

supercool and solutions become supersaturated. Thus, the growth system departs

considerably from equilibrium before a crystal nucleates, and when it comes the new

born crystal grows very rapidly at first which is full of defect. Some of these defects

propagate into the later stages of near equilibrium growth. Crystal growers, thus, seek to

use methods where a seed crystal can be introduced into the system to avoid the

unwanted nucleation. In the last three decades, great strides have been made toward

achieving crystal perfection motivated by the needs of the electronics and optics

industries. While thermodynamics excludes the possibility of growing a perfect crystal,

gross defect like grain boundaries, voids, and even dislocations can be eliminated with

care and point defects, like impurities, vacancies, intertitials, and antistructure disorder

can be minimized by attention to growth environment and purity of reagents and

apparatus.

2.12 Dual zone horizontal furnace

Taking the above requisites into consideration, a two-zone horizontal furnace was

found to be convenient to produce an appropriate temperature gradient over the entire

length of quartz ampoule. The temperature of both the zones should be highly stabile and

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the required temperature gradient has to be maintained over the whole length of the

ampoule. As the growth of sizable crystals with good quality requires longer time, good

control of the temperatures of both the zones in the furnace has to be facilitated. Stability

of the temperature plays an important role; therefore, for this purpose temperature profile

controllers were used.

A two-zone furnace was fabricated in the University Science and Instrumentation

Centre, Sardar Patel University, Vallabh Vidyanagar by using a special sillimanite muffle

tube of grade KR 80 GA HG (fig. 2.1). This muffle tube is closed at one end and having

450 mm length, 70 mm outer diameter and 56 mm inner diameter, with a threaded pitch

of 3 mm, imported from Koppers Fabriken Feuerfester, Germany. A super Kanthal A-1

wire of 17 SWG was used for winding. This wire can withstand temperatures up to

14000C and was wound directly on to the furnace muffle in two different regions. The

muffle was enclosed in glass wool jacket, brick powder and refractory bricks for proper

insulation. The complete arrangement was fully encased in the metal sheets and the entire

assembly was supported in a steel framework.

It has been observed that the temperature profile and its stability over a long

period is one of the important factors for the growth of large size crystals of good

stoichiometry. In order to accomplish this, temperature controllers (Make: Select, Model:

PR502) with profile programming facility based power unit were used for the two zones

of the furnace.

With the help of temperature programmers, a required temperature gradient could

be established across the length of the working tube in the required temperature range.

Thermocouples used were Pt (13 %), Rh - Pt. 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 furnace tube temperature. The complete structure of the furnace

with the temperature controlling system is shown in figure 2.2.

Figure 2.1 The dual zone horizontal furnace with axially loaded ampoule.

Figure 2.2 The complete furnace structure with temperature controlling system.

Quartz tube

The dual zone horizontal furnace with axially loaded ampoule.

The complete furnace structure with temperature controlling system.

Quartz tube Quartz ampoule

Steel body

55

The dual zone horizontal furnace with axially loaded ampoule.

The complete furnace structure with temperature controlling system.

Muffle

Steel body

56

2.13 Ampoule preparation

As discussed in section 2.9, quartz was considered as the best choice for making

the encasing tube. High quality fused quartz tubes having a melting point of about 1500 oC were used for the growth experiments. The length of the ampoule was chosen to be

around 24 cm as per the requirement of the furnace construction and temperature

distribution within it. The inner and outer diameters of the ampoules were 23 mm and 25

mm respectively. One end of the ampoule tube was sealed and the other end was drawn

into a neck, which was then joined to another quartz tube with dimensions of 8 mm of

inner diameter, 2 mm thickness and 30 cm in length. This arrangement was required to

connect it to the vacuum system for evacuation after introducing the source material

2.14 Cleaning process of ampoule

The ampoules were cleaned prior to filling them with the stoichiometric mixture

of required materials. The following cleaning steps have been monitored.

• They were first washed with boiling water along with a suitable detergent.

• They were rinsed with concentrated H2SO4 and washed with double-distilled

water once again.

• A further rinsing was done with the mixture of concentrated HNO3 and

concentrated HCl taken in equal proportion

• They were washed with double distilled water two to three times.

• Then, the ampoules were filled with concentrated HF and heated for some time so

that the inner surface of the ampoules became rough due to etching. These rough

surfaces are absolutely essential to enhance growth sites for preferential

nucleation during the crystal growth.

• A thorough and final washing was done again with double-distilled water to

remove any residue of these chemicals.

• The so-cleaned ampoule was then kept in a SlCO constant temperature oven at

100oC for 24 hours to make it moisture free.

57

2.15 Sealing of ampoules

The required source materials were loaded in a transparent quartz ampoule duly

prepared. The stoichiometric proportion of the materials from which the crystals are to be

grown is given in table 2.1. A total charge of about 8 gms was used in each experiment.

The ampoule containing the source material was evacuated to a pressure of 10-5 Torr to

minimize the reaction of the elements with atmosphere at elevated temperatures and also

to create an inside pressure very low so that the vapour pressure developed at high

temperature within the ampoule does not lead to its blasting. Proper care was taken while

evacuating the ampoule so that material from ampoule does not enter into the vacuum

system. Once the required vacuum was reached, the loaded ampoule was sealed off at the

neck. Proper mixing of the contents was ensured by shaking thoroughly the sealed

ampoule.

Table 2.1 Selected materials for crystal growth.

Material Purity Supplier

Molybdenum (Mo) 99.9% Pure Alpha Aesar, Lancs – U.K

Selenium (Se) 99.9% Pure Alpha Aesar, Lancs – U.K

2.16 Crystal growth

The sealed ampoule containing stoichiometric mixture of required materials was

loaded in the two zone horizontal furnace and the temperatures of both the zones were

raised slowly for reaction between elements. The temperatures and the period for which

the ampoule was kept in the furnace depended upon the material, which was being

grown. A slow heating was necessary to avoid any possibility of explosion due to the

strong exothermic reaction between the elements. After some stipulated time of keeping

the ampoule at elevated temperatures, a slow cooling process was carried. The rate of

cooling should be as slow as possible for enhancing the nucleation and growth process.

The typical temperature profile used in present investigations is shown in figure 2.3. The

growth parameters such as temperature distribution, growths period etc used for the growth

of MoSe2 have been given in table 2.2.

58

Figure 2.3 The temperature Profile used for the growth of MoSe2 crystals

Figure 2.4 Temperature → Time cycle of grown MoSe2 crystals.

Table 2.2 Used parameters for the growth of MoSe2 crystals.

Source Zone

Temp

(C°)

Growth Zone

Temp

(C°)

Growth period

(hrs)

Average

dimensions

1085 1070 197 7mm×4mm×5µm

The Source zone of the furnace was maintained at 10850C while the growth zone

was kept at 10700C for the growth of above mentioned single crystals. The growth of any

compound in crystal form depends upon various parameters such as the length of the

900

950

1000

1050

1100

0 10 20 30 40 50 60 70

Tem

per

atu

re (°C

)

Distance (cm)

0

200

400

600

800

1000

1200

0 50 100 150 200

Tem

para

ure

(°C)

Time (hours)

SourceGrowth

59

ampoule, purity of the source materials used, quality of the quartz tube, level of vacuum,

amount and type of transporting agent, temperature distribution of the furnace,

appropriate proportion of constituent element, time duration for growth cycle, the rate of

increase and decrease of temperature etc.

The temperature was increased for the growth of all the crystals at the rate of 50 0C/hr, till it attained the required temperature in both the zones. The ampoule was left in

the furnace for two days to ensure the proper mixing and reaction of the source materials.

Later on, the temperature of the growth zone was lowered down to 10000C at the rate of

20C per hour. The furnace was left with the difference of 500C temperature between both

the zones for around 2 days. After that, the temperatures of both the zones were

decreased at the rate of 50C per hour up to 9000C. Afterwards, the cooling of both the

zones was carried out by lowering the temperatures simultaneously at a rate of 50 0C /

hour till the growth zone reached the room temperature.

Once the temperature of both the zones came down to the room temperature, the

furnace was switched off. The ampoule was carefully taken out of the furnace. It was

seen that at the growth zone, the ampoules contained gray, shiny irregular shaped

platelets of crystals. The grown crystals were carefully taken out by breaking the

ampoule. The crystal dimensions of MoSe2 grown in this manner were of (4 X 11 ) mm2

and a 4-10 micrometers thick. Figure 2.5 shows some of the MoSe2 grown crystals.

Figure 2.5 Grown crystals of MoSe2.

60

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