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CHAPTER 1

INTRODUCTION TO CRYSTAL GROWTH

AND NONLINEAR OPTICS

1.1 INTRODUCTION

Crystals have fascinated mankind for thousands of years. For at

least 50 years, crystals have attracted engineers as the key materials of

modern electronics, optoelectronics and other technical fields of application.

The formation of crystals in nature, like snowflakes and minerals, as well as

the preparation of crystals in laboratories and factories for technical

applications is called "crystal growth". Due to the fact that many of today's

technological systems in the fields of information, communication, energy,

transportation, medical and safety technologies depend critically on the

availability of suitable crystals with tailored properties and their fabrication,

crystal growth has become an important technology (Georg Muller et al

2004).

Fifty years before Steno's publication (1638-1686), Kepler (1611)

who was fascinated by the elaborately varied dendritic forms of snow flakes,

considered that snow crystals, although they exhibit thousands of different

dendritic forms, are all composed of equal-sized spheres in a closely packed

arrangement. This was the root of the concept of crystal structure.

Interestingly, both structural crystallography and the science of crystal growth

started from the curiosity as to why the same crystal species take elaborately

varied forms, not only polyhedral but also dendritic forms.

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This curiosity led to the development of studies along two different

lines: one to understand and analyze how a crystal is constructed by elemental

units and the other to understand how a crystal nucleates or grows and its

morphology is determined. The concepts of unit cell, crystal lattice, 14 lattice

types, 7 crystal systems, symmetry elements, 32 point groups, and 230 space

groups were established by the end of the 19th

century, and crystal structures

were experimentally verified and analyzed by X-ray diffraction in 1912. The

crystal structures of most solid materials were analyzed and structural

crystallography, crystal chemistry, and crystal physics developed rapidly. The

most essential problem underlying in these studies was to know in detail the

atomic arrangements, chemical bonding, and symmetry-property and

structure-property relations. The process by which such a regular arrangement

can be realized and how the process influences physical perfection and

chemical homogeneity of the crystal were not the main concerns for structural

crystallographers. Crystals were regarded as thermodynamically minimum

energy states that have ideally regular atomic arrangements (Sato et al 2001).

1.2 HISTORY OF CRYSTAL GROWTH

The art of crystallization extends far back in the past and antedates

considerably the written history of man. The crystallization of salt from sea

water by evaporation was already practised at many places in prehistoric time

and can be considered one of the oldest technical methods of transforming

materials. Crystallization procedures were recorded in written documents well

before the Christian era. The Roman Plinius in his “Naturalis historia”

mentioned the crystallization of a number of salts, for instance of vitriols. The

alchemist Geber, whose papers are dated in the 12th or 13th century,

described the preparation and purification of various materials by

recrystallization as well as by sublimation and distillation. Indeed, a historical

review reveals that the “modern” scientific development of crystallography

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started in the 17th century. In about 1600 Caesalpinus (1602) had already

observed that crystals of specific materials, like sugar, saltpeter, alum, vitriols

and so on, grown from solutions, exhibit typical forms, characteristic of each

material.

During the 18th century significant progress was achieved both in

the systematic description of crystals, especially minerals, as well as in crystal

growth experiments. Fahrenheit (1724) discovered the supercooling of water

and noted the release of heat when ice formation occurred. Towards the end

of the century Lowitz (1795) in his extensive work reaffirmed the earlier

implied requirement of supersaturation or supercooling for the initiation of

crystal growth and described the now well-known features of supersaturated

solutions. The supersaturation of a solution can be achieved both by

evaporation or supercooling; the degree of supersaturation that can be attained

depends on the particular salt and on the pretreatment of the solution. He also

used seeding and recognized a specificity of different nucleating agents. From

a mixed supersaturated solution, the separate salt that is used for seeding will

be deposited. The identity of the crystallizing salt and the nucleating agent is

not required in all cases (Feigelson 2004).

Fundamental aspects of crystal growth had been derived from early

crystallization experiments in the 18th and the 19th century (Elwell and

Scheel 1975, Scheel 1993). Theoretical understanding started with the

development of thermodynamics in the late 19th century (Gibbs, Arrhenius,

Van’t Hoff) and with the development of nucleation and crystal growth

theories and the increasing understanding of the role of transport phenomena

in the 20th century. The phenomena of undercooling and supersaturation and

the heat of crystallization were already recognized in the 18th century by

Fahrenheit and by Lowitz. The corresponding metastable region, the existence

range of undercooled melts and solutions, was measured and defined in

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1893/1897 by Ostwald and in 1906 by Miers, whereas the effect of friction on

the width of this Ostwald–Miers region was described in 1911 and 1913 by

Young. Although the impact of stirring on this metastable region is important

in mass crystallization of salt, sugar and many chemicals, it is not yet

theoretically understood (Hans Scheel and Tsuguo Fukuda 2003).

1.3 GROWTH OF SINGLE CRYSTALS

Attempts to grow single crystals with high values, such as

gemstones, in laboratories using less expensive and easily available raw

materials may be regarded as an extension of alchemy. The first success in

growing gemstones was achieved in the middle of the 19th

century. Carat-

sized emerald crystals were grown from high-temperature solutions. Growth

of large single crystals of ruby from high-temperature solutions was

attempted, but it was not possible to grow cuttable-sized ruby crystals until

Verneuil made a breakthrough that enabled large ruby crystals to be grown

from the melt phase.

On the other hand, there were many scientific and industrial fields

that required the production of large single crystals. In turn, those industrial

fields greatly developed partly due to the contribution of crystal growth

science. Metallurgy is a good example. For metallurgists, it was necessary to

grow single crystals of metals to investigate their physical properties, which

must be examined in a single crystalline state. Other examples are optic and

piezoelectric crystals. Very large single crystals (at least several cm in

diameter) must be grown so that these crystals are employed in optic and

electric communication technology. For this purpose, the growth of large

single crystals, for example, quartz, ADP and KDP for the piezoelectric

application, was first achieved by using solution growth techniques (Sato et al

2001).

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1.4 REASONS FOR GROWING SINGLE CRYSTALS

Stated simply, the job of the crystal grower is to prepare large

specimens of crystalline material such that there is complete crystallographic

continuity across a given specimen in all directions. There are two principal

reasons for the deliberate growth of single crystals.

(i) Many physical properties of solids are obscured or

complicated by the effects of grain boundaries.

(ii) The full range of tensor relationships between applied physical

cause and observed effects can be obtained only if the full

internal symmetry of the crystal structure is maintained

throughout the specimen (SanthanaRaghavan and Ramasamy

2000).

1.5 IMPORTANCE OF CRYSTAL GROWTH

Today, crystals are the pillars of modern technology. Without

crystals, there would be no electronics industry, no photonic industry, no fiber

optic communications, very little modern optical equipment and some very

important gaps in conventional production engineering. Progress in crystal

growth and epitaxy technology is highly demanded in view of its essential

role for the development of several important areas such as production of high

efficiency photovoltaic cells and detection for alternative energy and

medicine, and the fabrication of bright long lifetime light emitting diodes, for

saving energy by wide use in illumination and traffic lights (SanthanaRaghavan

and Ramasamy 2000).

Single crystal has been used in a new era in an efficient manner in

various fields. Symmetry, purity and structural simplicity of single crystals

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have caused major transformation of the electronics industry and the systems

based on it. Modern technology is based largely on materials such as

semiconductors, ferrites, magnets, solid state lasers, piezoelectric, ultraviolet

and infrared sensitive crystals and crystalline films for microelectronics and

computer industries. The needs of growing single crystals and doped crystals

are increasing day by day. Single crystals look like single without grain

boundaries. They possess large uniformity and are capable of producing

specific properties relevant to the applications.

1.6 THE DEVELOPMENT OF CRYSTAL GROWTH

TECHNOLOGY

New materials are the basis of solid state research and device

technology. They are mostly discovered by crystal growers. The industrial

production of crystals started with A. Verneuil with his flame-fusion growth

method (1902). He can be regarded as the father of crystal growth technology

as his principles of nucleation control and crystal-diameter control are adapted

in most later growth methods from the melt, like Tammann, Stober,

Bridgman, Czochralski, Kyropoulos, Stockbarger, etc. The important crystal

pulling from melts named after Czochralski was effectively developed by

Teal, Little and Dash. Crystal-growth technology (CGT) and epitaxy

technology had developed along with the technological development in the

20th century. On the other hand, the rapid advances in microelectronics, in

communication technologies, in medical instrumentation, in energy and space

technology were only possible after the remarkable progress in fabrication of

large, rather perfect crystals and of large-diameter epitaxial layers (epilayers).

Further progress in CGT and education of CGT engineers is required for

significant contributions to the energy crisis. High efficiency white light

emitting diodes for energy saving illumination and photovoltaic/thermo-

photovoltaic devices for transforming solar and other radiation energy into

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electric power with high yield 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 (Hans Scheel and Tsuguo

Fukuda 2003).

1.7 THERMODYNAMIC FUNDAMENTALS TO CRYSTAL

GROWTH

The nature of a crystallization process is governed by both

thermodynamic and kinetic factors, which can make it highly variable and

difficult to control. Factors such as impurity level, mixing regime, vessel

design, and cooling profile can have a major impact on the size, number, and

shape of crystals produced. Thermodynamics is an important practical tool for

crystal growers. It helps to derive the most effective phase transition, i.e.

growth method, and the value of the driving force of crystallization. From

thermodynamic principles, one can estimate the nucleation and existence

conditions of a given crystalline phase, the width of compound homogeneity

regions, and optimize the in-situ control of the crystal composition during the

growth. In a word, no technological optimum can be found without

considering thermodynamic relationships. In general, crystal growth involves

first-order phase transitions. This means there is the coexistence of two

distinct uniform phases that are stable at the equilibrium point and separated

by a phase boundary, i.e. an interface. Close to the equilibrium point the

phases can still exist, one as thermodynamically stable, the other as the

thermodynamically metastable phase, whereas the metastable phase is

supersaturated (supercooled) with respect to the stable (equilibrium) phase.

As a result, a thermodynamic driving force of crystallization will appear

leading at a critical value of supersaturation to spontaneous nucleation of the

crystalline phase within the metastable fluid phase. A controlled propagation

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of the solid/fluid phase boundary, however, takes place by providing a seed

crystal or a substrate in contact with the fluid phase. Classic thermodynamics

is concerned with macroscopic equilibrium states of quasiclosed systems.

Such an approach for crystal growth is allowed due to the slow time scale of

macroscopic processes compared with the kinetics of atoms, and due to the

relatively small deviations from equilibrium. In order to describe

nonequilibrium processes of quasiopen crystallization systems, characterized

by continuous flows of heat and matter (i.e. entropy production), one uses

linear nonequilibrium thermodynamics (Peter Rudolph 2003).

1.8 CRYSTAL GROWTH METHODS

Growth of crystal ranges from a small inexpensive technique to a

complex sophisticated expensive process and crystallization time ranges from

minutes, hours, days and to months. The starting point is the historical works

of the inventors of several important crystal growth techniques and their

original aim. Accordingly the basic growth methods are:

(i) Growth from melt

(ii) Growth from solution

(iii) Growth from vapour

The growth of crystals from liquid and gaseous solutions, pure

liquids and pure gases can only occur if some degree of supersaturation or

supercooling has first been achieved in the system. The attainment of the

supersaturated state is essential for any crystallization operation, and the

degree of supersaturation, or deviation from the equilibrium saturated

condition, is the prime factor controlling the deposition process. Growth of

crystals can be considered to comprise three basic steps:

(i) achievement of supersaturation or supercooling.

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(ii) formation of crystal nuclei of microscopic size and

(iii) successive growth of crystals to yield distinct faces.

Crystal growth from liquid falls into two categories, namely, melt

growth and solution growth. In approaching the task of selecting a method,

one is confronted by two orthogonal sets of constraints, those set by the nature

of the method, and those set by the properties of the material. In many cases

all the three categories can be employed, on a thermodynamic basis, for

growing single crystals of a given material. In such cases the selection of

method must be based on growth kinetics (rate of growth) and the

requirements of size, shape, purity and economics (SanthanaRaghavan and

Ramasamy 2000).

1.8.1 Growth from the Melt

Crystals will grow from a melt much more rapidly than they will

grow from the vapor phase or from a solution. This is simply because the

density of material in the melt is comparable to that in a crystal, so the atoms

or molecules are essentially there already to grow the crystal. For both vapor

and solution growth, the density of atoms or molecules in the mother phase is

much lower and the growth rate depends on the rate at which they arrive at

the surface of the crystal. For the commercial growth of crystals, the faster

those crystals of acceptable quality can be grown, the better. This is also true

for the non-commercial growth of experimental crystals. So melt growth is

the preferred method. There are various reasons why many crystals cannot be

grown from the melt, but if a crystal can be grown from its melt, it will be

good (Kenneth Jackson 2004). In principle, all materials can be grown as

single crystals from the melt, provided they melt congruently, they do not

decompose before melting, and they do not undergo a phase transformation

between the melting point and room temperature.

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Melt growth is the process of crystallization by fusion and

resolidification of the pure material, crystallization from a melt on cooling the

liquid below its freezing point. In this technique apart from possible

contamination from crucible materials and surrounding atmosphere, no

impurities are introduced in the growth process and the rate of growth is

normally much higher than that possible by other methods. Mainly for latter

reason, melt growth is commercially the most important method of crystal

growth (SanthanaRaghavan and Ramasamy 2000). The growth from melt can

further be sub-grouped into various techniques. The main techniques are:

(i) Bridgman Technique

(ii) Czochralski Technique

(iii) Verneuil Tecnique

(iv) Zone Melting Technique

(v) Heat Exchanger Method

(vi) Skull Melting

(vii) Shaped Crystal Growth

1.8.1.1 Bridgman Technique

This technique was named after its inventor (Bridgman 1925:

Stockbarger 1938). In this process the material to be grown is taken in a

vertical cylindrical container, tapered conically with a point bottom and made

to melt using a suitable furnace. Initially the sample is heated in the furnace,

and melted completely. It is then lowered slowly from the hot zone of the

furnace to the cold zone. The rates of movement for such processes range

from about 0.1 to 200 mm/hour, but are mostly in the range 1–30 mm/hour.

Crystallization begins in the tip and continues usually by growth from the first

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formed nucleus. The bottom end of the container is usually tapered to a point

to minimize the probability of forming many nuclei. In principle, a seed

crystal can be used in the bottom end of the container, but in practice, it is

difficult to see the interface in the seed in order to determine when it is partly

melted. If the starting material, which is usually polycrystalline, is not all

melted, or if the seed melts completely, then the seeding process fails. And

usually the time interval between these two events is short. So Bridgman

growth is usually unseeded, and the orientation of the resultant crystal is

random. In more sophisticated setups, baffles or multi-zone heaters are used

to control the temperature of the sample. Special precautions are usually

necessary to remove the crystal from the tube after growth (Kenneth Jackson

2004).

1.8.1.1.1 Advantages

The advantages of this technique are low cost, with the added

advantage that the melt temperature increases with distance from the solid-

liquid interface and the system is therefore density stable and less prone to

convection effects than the Czochralski technique (SanthanaRaghavan and

Ramasamy 2000). Compared to capillary - based techniques, these processes

have advantages that the shape of the crystal is a direct result of the shape of

the crucible; there is no concern with control of the crystal shape. As both

diameters are equal, the furnace is generally smaller than for other techniques.

In the other crystal growth methods studied, it is necessary to apply relatively

high thermal gradients in order to control the crystal shape (Thierry Duffar

2010). This is detrimental to crystal quality because high thermal gradients

generate thermal stresses in the hot crystal, above the elasticity limit, and

dislocations are then generated in the crystal.

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1.8.1.1.2 Disadvantages

A disadvantage is that low growth rate used, typically 1-30 mm/hr,

means that the melt is in contact with the container for long periods,

increasing the risk of impurity pick-up. Adhesion of the solid materials to the

ampoule wall or compression of the solid by the contracting container during

cooling can lead to the development of stresses high enough to nucleate

dislocations in the material. A further drawback of the technique is that the

container wall acts as a preferential, spurious nucleation site, resulting in

polycrystalline rather than single growth unless temperature gradient and the

liquid–solid interface shape are well controlled. This technique cannot be

used for the materials which decompose before melting or which undergo

solid state phase transformation between their melting points and the

temperature to which they will be cooled (SanthanaRaghavan and Ramasamy

2000).

1.8.1.2 Czochralski Technique

This method was originally developed by Jan Czochralski in 1918.

It is often referred to as a crystal pulling. Most of them involve relative

motion of a seed and the melt so that the crystal is literally pulled from the

melt. For a given material the process first described by Czochralski (1918) is

the fastest melt growth method and is therefore almost always the method

which produces crystals most rapidly. Crystal pulling is also the method

which is likely to produce the highest quality in the sense of the most perfect

and homogeneous. This method is applicable only to materials which melt

congruently or nearly congruently, i.e. the compositions of the solid and the

melt in equilibrium with it cannot differ much. Crystal pulling is usually

faster than (Brice 1986) Bridgman growth so that less contamination occurs.

Crystal pulling makes more demands on the equipment than Bridgman

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growth. Until reliable automation systems were developed, pulling also

required more man hours of labour. However, while the labour costs for the

two groups of methods are now comparable, the capital cost of pulling is

higher than for Bridgman technique. Thus for a given material, pulled crystals

are more expensive than Bridgman grown ones. Pulling is therefore used

when the greatest perfection attainable is needed.

1.8.1.2.1 Advantages

The main advantage of the Czochralski method is the possibility of

fast growth of good quality large single crystals. Moreover, since the crystals

are grown using oriented seeds they adopt required orientation (Berkowski

et al 2000). Using conventional pulling equipment, one can grow a range of

crystal types whose properties vary from low melting point metals and

organics to very high melting point oxides such as spinal. The basic reason for

this arises from the ease and ability to manipulate the thermal conditions by

means of heat shields, crucible shapes etc. By suitably programming the

growth rate and the rotation rate, it is possible to tailor or control doping

profile in a crystal. Additionally the rotation imparts very good cross-

sectional doping uniformity in crystals. It is possible to grow dislocation free

seed thereby grow a dislocation free crystal by a combination of necking and

temperature gradient control (SanthanaRaghavan and Ramasamy 2000).

1.8.1.2.2 Disadvantages

The need of crucibles seriously limits applicability of the

Czochralski method. The crucible should be compatible with the melt to

crystallize (higher melting point and chemical stability) and the surrounding

atmosphere. The crystal growth rate is limited by diffusion rate of its

component to the crystal and melt interface, intentionally introduced dopants

and unintentional admixtures in opposite direction from the melt interface.

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When the growth rate is too high, a large concentration gradient of the

admixtures is created in the melt, and consequently the melt neighboring to

the crystal growth region becomes supercooled. As a result, a tendency to

spontaneous nucleation appears in the melt in front of the growing crystal, which

causes formation of various defects in the crystal. The chemical composition of

crystals grown by the Czochralski method very often slightly differs from the

stoichiometric composition of the crystallized material. Segregation of the

dopants is yet another disadvantage of the Czochralski method. It leads to a

difference in composition between the crystal and the melt if the segregation

coefficient differs from one another (Berkowski et al 2000).

1.8.1.3 Verneuil Technique

The Verneuil technique is a commercial method for growing

gemstones. It was developed in 1902 by Verneuil. Flame fusion is the

principle of this method. The apparatus is basically an oxyhydrogen torch

through which powders of the material to be grown are to be passed. The

powders melt in the flame and form a small puddle on a seed crystal. The

crystal is grown on the seed crystal, which is lowered as the crystal builds up.

The form of the crystal grown by the Verneuil method is cylindrical.

The Verneuil method has a great advantage of growing crystal

without crucible. In addition, it can grow larger crystals of gemstones at

higher temperatures than any other technique. The upper temperature is

limited by the oxyhydrogen flame temperature. By regulating the hydrogen

and oxygen ratio in the flame, one can sometimes grow oxide crystals that are

difficult to prepare in other methods. Because the method is nonconservative,

solid solutions of uniform composition can be grown. A disadvantage is that

the growing crystal is exposed to a steep temperature gradient. Some of

grown crystals have consequently been broken into pieces. The synthetic

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ruby, sapphire, star ruby, and star sapphire crystals were grown by this

method (Byrappa et al 2002).

1.8.2 Growth from Solution

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. The growth of materials by low

temperature solution growth involves weeks, months and sometimes years.

Much attention has been made to understand the growth mechanism of the

process. Though the technology of growth of crystals from solution has been

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

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

destroy months of work.

Materials having moderate to high solubility in temperature range,

ambient to 100 °C at atmospheric pressure can be grown by low temperature

solution method. 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 such as temperature, pressure

and solvent concentration. The advantages of crystal growth from low

temperature solution nearer the ambient temperature results in the simple and

straight forward equipment design which gives a good degree of control to an

accuracy of ± 0.01°C. 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

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

using this technique. The low temperature solution growth technique also

allows variety of different morphologies and polymorphic forms of the same

substance can be grown 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 on 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. Among the

methods of growing single crystals, solution growth at low temperatures

occupies a prominent place owing to its versatility and simplicity. After

undergoing so many modifications and refinements, the process of solution

growth now yields good quality crystals for a variety of applications

(SanthanaRaghavan and Ramasamy 2000).

1.8.2.1 Solution, Solubility and Supersolubility

A solution is a homogeneous mixture of a solute in a solvent.

Solute is the component, which is present in a smaller quantity and that one

which gets dissolved in the solution. For a given solute, there may be different

solvents. The solvent must be chosen taking into account the following factors

to grow crystals from solution. The solvent must possess

(i) a good solubility for the given solute

(ii) a good temperature coefficient of solute solubility

(iii) less viscosity

(iv) less volatility

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(v) less corrosion

(vi) non toxicity

(vii) small vapor pressure

(viii) low cost

However solvent having all the above characteristics together do

not exist. Solvent controls the growth of crystal and this depends on the

interaction of the surface of the crystal during growth and solvent molecules.

The commonly used solvents are water, both light (H2O) and heavy (D2O),

ethanol, methanol, acetone, carbontetrochloride, hexane, xylene and many

others. Almost 90% of the crystals produced from low temperature solutions

are grown by using water as a solvent. High purified water is used for crystal

growth in order to avoid contamination.

Solubility of the solute in a solvent decides the amount of the

material, which is available for the growth and hence defines the total size of

the limit. It is difficult to grow bulk single crystals if the solubility is too high

and it restricts the size and growth rate of the crystals if the solubility is too

small. Growth of bulk crystal from solution depends on the solubility curve.

Flat and steep solubility curve will not enable the growth of bulk crystal.

Supersaturation plays a vital role in the growth of good quality crystal. A

small fluctuation in the temperature will affect the supersaturation to grow

good quality bulk crystals. If the solubility gradient is very small, slow

evaporation of the solvent is the other option for crystal growth to maintain

the supersaturation in the solution.

The solubility of the solute may be determined by dissolving the

solute in the solvent maintained at a constant temperature with continuous

stirring. The equilibrium concentration of the solute may be determined

gravimetrically on reaching saturation. A sample of the clear supernatant

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liquid is withdrawn by means of a warmed pipette and a weighed quantity of

the sample is analyzed. By repeating the above procedure for different

temperature, the solubility curve can then be plotted (SanthanaRaghavan and

Ramasamy 2000).

A solubility diagram is shown in Figure 1.1. The whole

concentration temperature field is separated by the saturated solution line

(solubility curve) into two regions unsaturated and supersaturated solutions.

Saturated solutions are those mixtures, which can retain their equilibrium

indefinitely in contact with the solid phase with respect to which they are

saturated. The solubility of most substances increases with temperature (the

temperature coefficient of the solubility is positive). Crystals can be grown

only from supersaturated solutions which contain an excess of the solute

above the equilibrium value. The region of supersaturated solutions can be

divided into two sub-regions; metastable (stable) and labile (Unstable) zones.

Nucleation will occur spontaneously in the labile zone. Metastable zone refers

to the level of supersaturation where spontaneous nucleation cannot occur and

a seed crystal is essential to facilitate growth.

Labile Metastable

Stable AB

B’B”

C’

C”

C

III II I

BB’ – Solubility curve

AB”C” – Evaporation and cooling

CC’ – Super solubility curve

Co

ncen

trat

ion

Temperature

Labile Metastable

Stable AB

B’B”

C’

C”

C

III II I

BB’ – Solubility curve

AB”C” – Evaporation and cooling

CC’ – Super solubility curve

Co

ncen

trat

ion

Temperature

Labile Metastable

Stable AB

B’B”

C’

C”

C

III II I

BB’ – Solubility curve

AB”C” – Evaporation and cooling

CC’ – Super solubility curve

Co

ncen

trat

ion

Temperature

Labile Metastable

Stable AB

B’B”

C’

C”

C

III II I

BB’ – Solubility curve

AB”C” – Evaporation and cooling

CC’ – Super solubility curve

Co

ncen

trat

ion

Temperature

Figure 1.1 Solubility diagram

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1.8.2.2 Solvents and Solutes

While some materials can be crystallized from the vapour phase,

the vast majority are crystallized from solution. To grow crystals from a

solution one must first obtain a saturated solution of the material and then

make it supersaturated. During the supersaturation phase, crystals will begin

to grow. Thus the choice of solvent is one of the most important parameters in

the crystallization process. For crystals that do not crystallize easily, the most

productive approach is to simply screen a large number of solvents. If a

limited amount of material is available this may require that the screening

process be carried out on a microscale. The choice of solvent is important,

because the solvent influences the mechanism of crystal growth. Some points

are to be noted while preparing solution.

(i) Using pure solvent.

(ii) Avoiding solvents like hexanes and petroleum ether.

(iii) Make sure the compound to be grown as crystal is pure

before attempting to crystallize.

(iv) For highly soluble materials crystal growth tends to be very

fast because of high degree of supersaturation.

(v) Highly soluble materials yield imperfect crystals. In such

cases make dilute to moderately concentrated solution or

choose solvent where material is not highly soluble.

(vi) Better crystal can be produced by slowing down the growth

step.

(vii) Avoid highly volatile solvents like diethyl ether or methanol

because they may quickly ruin the crystals.

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(viii) Polar solvent is used for materials which are ionic.

(ix) Choice of counter ion is essential.

(x) Some counter ions should be avoided as they are likely to be

disordered.

(xi) Do not try to separate or dry the crystals. It is better to bring

the crystals to the Crystallography laboratory in the mother

liquid.

1.9 NONLINEAR OPTICS

The theory of nonlinear optics (NLO) builds on the well understood

theory of linear optics, particularly that part known as the interaction of light

and matter. Ordinary matter consists of a collection of positively charged

cores (of atoms or molecules) and surrounding negatively charged electrons.

Light interacts primarily with matter via the valence electrons in the outer

shells of electron orbital. The fundamental parameter in this light matter

interaction theory is the electronic polarization of the material induced by

light (Richard Sutherland 2003).

1.9.1 Second Harmonic Generation

Nonlinear optics is the study of phenomena that occur as a

consequence of the modification of the optical properties of a material system

by the presence of light. Typically, only laser light is sufficiently intense to

modify the optical properties of a material system. The beginning of the field

of nonlinear optics is often taken to be the discovery of second harmonic

generation by Franken and coworkers in 1961, shortly after the demonstration

of the first working laser by Maiman in 1960. They observed ultraviolet light

at twice the frequency of a ruby laser light ( =6493 ), when the light was

used to traverse a quartz crystal. This experiment attracted widespread

21

attention and marked the beginning of the experimental and theoretical

investigation of nonlinear optical properties. A schematic diagram of the

experimental arrangement is shown in the Figure 1.2. A ruby laser beam

=6493 ) with average power of the order of 10 kW is focused on a quartz

slab. Then, the transmitted light was passed through a filter which cuts off the

red light and allows UV light to pass through it. The emerging light was

incident on a photocell. Radiation with wavelength =3471 and the power

of the order of 1 mW was observed in the transmitted light.

Second harmonic generation (SHG) was first realized successfully

in quartz. It was subsequently generated in many other crystals such as:

potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate,

barium titanate and lithium iodate etc. The importance of second harmonic

generation lies in the fact that it is one of the principal methods of effective

conversion of infrared radiation into visible and visible into ultraviolet (Laud

2008). In general, the nonlinear susceptibilities depend on the frequencies of

the applied elds. Second order nonlinear optical interactions can occur only

in noncentrosymmetric crystals that do not display inversion symmetry

(Robert Boyd 2007).

Figure 1.2 Experimental arrangements for the detection of second

harmonic generation of light

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1.9.2 Symmetries in Second-Order Nonlinear Optics

Hyperpolarizabilities and susceptibilities exhibit various types of

symmetry that are of fundamental importance in nonlinear optics: permutation

symmetry, time reversal symmetry, and symmetry in space. The time reversal

and permutation symmetries are fundamental properties of the susceptibilities

themselves, whereas the spatial symmetry of the susceptibility tensors reflects

the structural properties of the nonlinear medium. Hence, for centrosymmetric

materials, all tensor components ijk(2)

are null. As a consequence, second-

order nonlinear optical effects are not observed in centrosymmetric crystals

(Thierry Verbiest 2009).

1.9.3 Second-Order Nonlinear Optical Materials

Most of the materials developed initially for NLO applications were

based on inorganic systems. Ferroelectric materials lacking a centre of

symmetry were prime candidates. With inorganic materials, the optical and

acoustic phonons as well as the electronic polarization contribute to the NLO

effects. The NLO effects in inorganics can be interpreted only at a bulk level;

extension of the atomic or ionic polarizabilities to the bulk NLO properties is

complicated. There is growing interest in developing -conjugated organic

molecules for NLO applications. Push-pull organic molecules have very high

values of nonlinear hyperpolarizability and when they crystallize in a non-

centrosymmetric space group, they possess large nonlinear susceptibility. The

nonlinearity in these systems is dominated by electronic polarization effects.

In the solid state, these organic compounds form molecular crystals in which

the molecules interact through weak intermolecular forces and retain their

individual identity to a high degree (Ravi Mosurkal et al 2004).

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1.9.4 Classification of NLO Materials

Before the major research issues in the development of new materials

for nonlinear optics are discussed, several distinctions and features of

materials for application in the field are commented on. They are broadly divided

into two types, organic NLO materials and inorganic NLO materials. Many

organic and inorganic materials are highly polarizable and are good candidates for

nonlinear applications. However, the net polarization of a material depends on its

symmetry properties, with respect to the orientation of the impinging fields.

Nonlinear optical materials will be the key elements for future photonic

technologies based on the fact that photons are capable of processing information

with the speed of light.

1.9.4.1 Inorganic NLO Materials

The first major class of materials is bulk inorganic materials.

Nonlinearities in these materials are thought of as arising from electrons not

associated with individual nuclei, such as those in metals and

semiconductors. Inorganic crystals, such as KDP, ADP, KTP and beta BaB2O4

are the best nonlinear materials increasingly being used for the second

harmonic generation, and also in elctro-optical applications. However, in these

systems the nonlinear responses are undoubtedly related to individual bond

polarizabilities.

Merits

(i) high electro optic coefficient

(ii) high degree of chemical inertness

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Demerit

(i) difficulty of synthesis

Towards this direction, newer materials are currently being explored

and this has led to a new class of NLO materials.

1.9.4.2 Organic NLO Materials

Organic materials have emerged during 1980's as an important class

of nonlinear optical materials that offers unique opportunities for

fundamental research as well as for technological applications. There has been a

growing interest in fundamental and scientific research in the area of molecule

based nonlinear optical (NLO) materials with large second and third order

nonlinearities. The research was primarily motivated by applications of optical

and electro-optical devices based on these materials in the telecommunications

and optical data-processing industries. In particular, second order NLO

materials offer many attractions, such as large nonresonant ultrafast response

times, low dielectric constants, and intrinsic architectural tailorability. The

noncentrosymmetric organization of chromophores is an essential requirement

for efficient bulk second order nonlinear optical materials (Ravi Mosurkal et al

2004).

Organic molecules are built up primarily from atoms of the second

row of the periodic table and may give rise to two types of orbitals with

contrasted properties: orbitals and orbitals. They are distinguished by their

symmetries with respect to the internuclei axis. The orbitals exhibit an axial

symmetry around it while orbitals have a nodal plane containing the axis. The

orbitals overlap less than orbitals and form weaker and more polarizable

bonds (Chemla and Zyss 1987). The enhancement to nonlinearity in

comparison to inorganic materials arises due to the existence of electrons in

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the organic materials. Large second optical linearity originates from organic

conjugated molecules having an electron acceptor group at one end and a

donor group at the opposite end (Davydov et al 1970).

Merits

(i) optically more nonlinear

(ii) high optical damage threshold

(iii) intrinsic tolerability

(iv) low cost

(v) broad spectral range

(vi) birefringence used for phase matching

Demerits

(i) poor mechanical strength

(ii) poor thermal strength

(iii) highly volatile

1.9.4.3 Semiorganic NLO Materials

In general organic materials possess high second order nonlinear

optical efficiency. But most of the organic nonlinear optical materials have

poor mechanical and thermal stability. In order to increase their mechanical

strength and thermal stability, organic compounds are added with inorganic

compounds. Presently, inorganic and organic materials are replaced by

semiorganic materials. This is due to the development of the mechanical and

thermal properties along with good efficiency second harmonic generation.

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1.10 SCOPE OF THE THESIS

Nonlinear optical materials are attracting increasing attention due to

their wide application in the area of laser technology. Demand for efficient

nonlinear optical materials is increasing day by day. Harmonic generation

becomes one of the important processes in laser technology, particularly in

the production of laser sources with different wavelengths. The extension

wavelength range of a laser source by harmonic generation is elegant and

more economic when compared to the actual production of lasers through

laser production units for different wavelengths. Generally, amino acid

organic materials show a good efficiency of second harmonic generation. The

present thesis is focused at the growth and characterization of glycine based

organic and semiorganic nonlinear optical single crystals by slow evaporation

solution growth method. The introductory chapter discusses the crystal

growth methods and nonlinear optics.

Gamma glycine single crystals, a polymorphic form of glycine,

have been grown by many scientific researchers at low pH value and studied

the -phase transition temperature. The thesis deals with the growth of -

glycine single crystal by slow solvent evaporation technique from the aqueous

solution of glycine containing potassium nitrate and lithium nitrate

compounds at nearly neutral pH value. The grown crystals were analyzed by

various instrumental methods and particular attention was paid to the phase

transition temperature by the effect of nitrate compounds. In order to grow

crystals with higher thermal and mechanical stability semiorganic glycine

based halogenide nonlinear optical materials triglycine zinc chloride and

triglycine calcium dibromide crystals were grown successfully by slow

evaporation technique. They both crystallize from nonstoichiometric molar

ratio of glycine and zinc chloride, and glycine and calcium dibromide

compounds respectively. The growth mechanism and the reason how the

27

crystal forms were discussed in the consecutive chapters. Optically good

transparent organic glycinium trichloroacetate nonlinear optical single crystal

was grown under the process of multiple recrystallization using slow

evaporation technique at room temperature. This was done in order to remove

impurities away from the crystal. The grown crystals were subjected to various

studies to reveal their properties. Thermal behavior of the grown crystal was

elucidated from thermogravimetric and differential thermal analysis.

Amino acids with ionic salts have been investigated in the NLO

field and have been recognized as materials that have good nonlinear optical

properties. Semiorganic triglycinium calcium nitrate crystal was grown and

the optical, thermal and mechanical properties have been investigated.

Microstructural imperfection was analyzed by etching studies for glycinium

trichloroacetate and triglycinium calcium nitrate nonlinear optical crystals.

Finally the summary and suggestions for future work was discussed.