Kalai Project 1

CHAPTER- 1 INTRODUCTION TO CRYSTAL GROWTH

1.1

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 materials/crystals such as semiconductors, super conductors, polarizer, transducers, radiation detectors, ultrasonic amplifiers, ferrites, magnetic garnets, solid state lasers, non-linear optics, piezo-electric, electro-optic, acoustic-optic, photosensitive, refractory of different grades, crystalline films for microelectronics and computer industries. There are three major stages involved in this research. The first is the production of pure materials and improved equipment 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 production. The third is the characterization and utilization of these crystals in devices. 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 of the outstanding advantages of single crystals. 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.

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1.2

TYPES OF SEVEN CRYSTALS: The 7 Crystal systems (From least to most symmetric)

The 14 Bravais Lattices

1. triclinic (none)

simple

base-centered

simple

base-centered

simple

body-centered

2. monoclinic (1 diad)

body-centered

face-centered

3. orthorhombic (3 perpendicular diads)

4. rhombohedral (aka, trigonal) (1 triad)

5. tetragonal (1 tetrad)

6. hexagonal (1 hexad)

simple (SC)

body-centered (BCC) face-centered (FCC)

7. cubic (4 triads)

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1.2.1

CLASSIFICATION OF CRYSTALS:

There are four main categories of crystals, as grouped by their chemical and physical properties:

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1. COVALENT CRYSTALS

A covalent crystal has true covalent bonds between all of the atoms in the crystal. You can think of a covalent crystal as one big molecule. Many covalent crystals have extremely high melting points. Examples of covalent crystals include diamond and zinc sulfide crystals. 2. METALLIC CRYSTALS Individual metal atoms of metallic crystals sit on lattice sites. This leaves the outer electrons of these atoms free to float around the lattice. Metallic crystals tend to be very dense and have high melting points. 3. IONIC CRYSTALS The atoms of ionic crystals are held together by electrostatic forces (ionic bonds). Ionic crystals are hard and have relatively high melting points. Table salt (NaCl) is an example of this type of crystal. 4. MOLECULAR CRYSTALS These crystals contain recognizable molecules within their structures. A molecular crystal is held together by non-covalent interactions, like van der Waals forces or hydrogen bonding. Molecular crystals tend to be soft with relatively low melting points. Rock candy, the crystalline form of table sugar or sucrose, is an example of a molecular crystal. 1.3

METHODS OF CRYSTAL GROWTH 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. Single crystals may be produced by the transport of crystal constituents in the solid, liquid or vapor phases. On the basis of this, crystal growth may be classified into three categories as follows:

(1) Solid Growth (2)

Solid-to-Solid phase transformation

Liquid Growth

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-

Liquid to solid phase transformation

(2) Vapour Growth -

Vapour to solid phase transformation

In the above mentioned categories liquid growth includes both melt and solution growth. A survey of the methods of growth suggests that almost 80% of the single crystals are grown from the melt compared with roughly 5% from high temperature solution, and 3% from the solid and only 2% by hydrothermal methods. 1.3.1

GROWTH FROM MELT All materials can be grown in single crystal form the melt provided they melt

congruently without decomposition at the melting pint 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. Kryopoulos technique 4. Verneuil technique In Bridgman technique the material is melted in vertical cylindrical container, tapered conically with a point bottom. The container is lowered slowly form the hot zone of the furnace in to the cold zone. The rates of movement for such processes range from about 1 – 30 mm/hr. Crystallization begins at the tip and continues usually by growth form the first formed nucleus. This technique cannot be used for materials, which decompose before melting. This technique is best suited for materials with low melting point.

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Fig 1.3.2 Normal freezing also called directional freezing

Fig. 1.3.3 Techniques for nucleating single crystals from the melt (a) Conical bottom (b) Capillary (c) Necking In Czochralski method, the material to be grown is method 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 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 single crystals of materials, which consists of components that

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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 into 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. 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 wend. A small molten zone is maintained by surface tension between the seed and the feed. The zone is slowly moved towards the feed. Single crystals are 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 to produce dislocation free crystal with a low defect density.

Fig. 1.3.4 Zone refining (zone melting) 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. The method is more applicable in growing single crystals of materials with volatile constituents like GaAs.

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In the Verneuil technique, a fine dry powder of 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 growth up to 90mm 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. 1.4

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 solute. They are 1. Low temperature solution growth 2. High temperature solution growth In the solution growth, crystals can be grown from solution if the solution is supersaturated i.e., it contains more than it can be in equilibrium with the solid. Three principal methods are used to produce the required supersaturtion: i.

Slow cooling of the solution

ii.

Slow evaporation of the solvent

iii. The temperature gradient method

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.

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In the high temperature solution growth a solid is used as the solvent instead of liquid and the growth takes place well below the melting temperature and is know as flux method. The high temperature crystal growth can be divided into two major categories. They are 1. Growth from single component system. 2. Growth from multi component system. This method is 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 fro a ‘soak’ period of several hours and then the temperature is lowered very slowly. 1.4.1 GROWTH FROM VAPOUR The growth of single crystal material from the vapour phase is probably the most versatile of 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 of high melting point materials this method is used. Molecular beam techniques have also been applied recently to crystal growth problems. The most frequently used method for the growth of bulk crystal utilizes chemical transport reaction in which a reversible reaction is used to transport the source materials as a volatile species to the crystallization region. Finding a suitable transporting agent is formidable, problem in this technique. It is rarely possible to grow large crystals 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

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

Fig .1.4.1 Growth by sublimation method

1.4.2

HYDROTHERMAL GROWTH Hydrothermal implies conditions of high pressure as well as high

temperature. Substances like calcite, quartz is considered to be insoluble in water but at high temperature plus pressure, these substances are soluble. This method of crystal growth at high temperature and pressure is known as hydrothermal method. Temperatures are typically in the range 400oC to 600oC and the pressure involved is large(hundreds of thousands of atmospheres). Growth is usually carries out in the 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.

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The requirement of high pressure presents practical difficulties and there are only a few crystals of good quality and large dimensions are grown by this technique. Quartz is the outstanding example of industrial hydrothermal crystallization. One serious disadvantage of this technique is the frequent incorporation of OH ions into the crystal, which makes them unsuitable for many applications. 1.4.3 GEL GROWTH It is an alternative technique to solution growth with controlled diffusion and the growth process is free from convection. Gel is a two-component system of a semisolid 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 since the growth takes place at room temperature. 1.4.4 ELECTROCRYSTALLISATION Electrolysis of fused salt is normally used fro the commercial production of metals such as aluminium and has great technological importance. The process of crystal growth from fused slats 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.

CHAPTER – 2 REVIEW ON NON-LINEAR OPTICS 2.1 NON-LINEAR OPTICS

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Non-linear optical (NLO) effects are analyzed by considering the response of the dielectric material at the atomic level to the electric fields of an intense light beam. The propagation of a wave through a material produces changes in the spatial and temporal distribution of electrical charges as the electrons and atoms interact by the filed on the charged particles is displacement of the valence electrons from their normal orbits. This perturbation creates electric dipoles whose macroscopic manifestation is the polarization (Narisimhamurty 1981). In linear materials, the response is always proportional to the stimulus. The induced polarization is proportional to the field and the susceptibility is independent of the field. In practice, this is always the case at the low fields. However, at high fields, the polarization is proportional to the fields and hence the susceptibility starts depending on the field. It is called Non-linear optics (NLO) because, at high intensity, the graph representing the dependence of optical polarization on the light field amplitude has curvature and deviates from straight line. When a string is bowed with much force or a wind instrument is blown hard, many overtones may be generated; similar thing happen to the electrons in matter when they are violently excited by high intensity light; overtones of light are created. This has the dramatic effect that a red light beam may be changed to a UV beam with twice or thrice the frequency or one half or one third of the wavelength. Coherent radiation at a few discrete frequencies can be produced by laser devices as in solid-state lasers or with marrow range of tenability as in dye lasers. Many applications require frequencies that are not readily available from such laser sources. The most effective way of converting a fundamental laser frequency to other frequencies, either to higher or lower frequencies, harmonic generation or parametric oscillation in a noncentro-symmetric crystalline medium (Bhawalker 1965). Now, after 40 years of research with NLO materials, it is possible to cover almost continuously the range from 170nm to 18nm. As a result, further extension

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of application to the limitations are significantly slowing the development of require optical devices. One of the obvious requirements for a non-linear crystal is that it should have excellent optical quality. This means that for new materials, for which single crystal specimens are not available. It is necessary to grow single crystal specimens of optical quality. Thus in many cases the search for new and better non-linear materials is very largely a crystal growing effort. It is realized that the requirements on optical quality for a useful non-linear optical material are more stringent than even the most exciting requirements on optical quality for materials used in linear optics. For a device to succeed it is vital that it meets a number of other criteria and these other criteria should receive greater emphasis. The relevant issues include reliable crystal growth techniques, ready availability, optical non-linearity, birefringence, moderate to high transparency and optical homogeneity for high conversion efficiency, mechanical strength, chemical stability, polishing and coating technology for case of fabrication, low absorption properties for high average power, damage threshold, non-linear absorption and brittleness index for lifetime and system capability. 2.2

THEORETICAL EXPLANATION OF NON-LINEAR OPTICS The explanation of non-linear effects lies in the way in which a beam of light

propagates through a solid. The nuclei and associated electrons of the atoms in the solid from an electric dipole. The electromagnetic radiation interacts with these diploles causing them to oscillate which, by the classical laws of electromagnetism, results in the dipoles themselves acting sources of electromagnetic radiation. If the amplitude of vibration is small, the intensity of the incident radiation increases the relationship between irradiance and amplitude of vibration becomes

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non-linear resulting in the generation of harmonic in the frequency of radiation emitted by the oscillating dipoles. Thus frequency doubling or second harmonic generation (SHG) and indeed higher order frequency effect occurs as the incident intensity is increased. In a nonlinear medium the induced polarization is non-linear function of the applied field. A medium exhibiting SHG is a crystal composed of molecules with asymmetric charge distributions arranged in the crystal in such a way that polar orientation is maintained throughout the crystal. At very low fields, the induced polarization is directly proportional to the electric field (Nalwa and Miyata). P=

ε

0

χ

(1)

E

(2.1)

Where,

χ(1) is the linear susceptibility of the material, E

is the electric vector,

ε0

is the permittivity of free space.

At high fields, polarization becomes independent of the field and the susceptibility becomes field dependant. Therefore, this non-linear response is expressed by writing the induced polarization as a power series in the field.

P=

ε0 ( χ(1) E + χ(2) . E + χ(3) Ε. +.........)

(2.2)

In non-linear terms, product of two or more oscillating fields gives oscillation at combination of frequencies and therefore the above equation can be expressed in terms of frequency as:

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P(ω0)=

ε0 { χ(1) ( −ω0 ; ω1). E ( ω0) + χ(2) (ω0; ω1, ω2). E ω1.ω2 + χ(3) ( −ω0; ω1,ω2ω3). E ω1. ω2.ω3 +........}

(2.3)

Where,

χ(2), χ(3) ......... χ(1)

are the non-linear susceptibilities of the medium.

is the linear term responsible for material’s linear optical properties like,

refractive index, dispersion, birefringence and absorption.

χ(2) is the quadratic term which describes second harmonic generation in noncentro-symmetric materials.

χ(3) is the cubic term responsible for third harmonic generation, stimulated Raman scatting, phase conjugation and optical bi-stability. Hence the induced polarization is capable of multiplying the fundamental frequency to second, third and even higher harmonics. The coefficients of

χ(1), χ(2) and χ(3) give rise to certain

optical effects. These are listed in Table 2.1.

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TABLE 2.1 OPTICAL EFFECTS OF NON-LINEAR MATERIALS

ORDER

1.

CRYSTAL

EFFECTS

APPLICATION

χ(1)

Refraction

Optical fibers Frequency

SGH (ω

+ ω=2ω)

doubling

Frequency mixing

Optical parametric

(ω1 + ω2 = ω3)

Oscillators

Pockels effects

χ

(2)

2. Electro optical

(ω + 0 = ω)

modulators Raman Coherent spectroscopy 4 wave mixing phase Real time

gratings

holography Ultra high speed optical gates

χ(3) 3.

Kerr effect Optical Amplitude

If the molecule or crystal is centro-symmetric then

Amplifiers, Choppers etc.,

χ(2) = 0.

If a field + E is

applied to the molecule (or medium), equation 2.3 predicts that the polarization

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induced by the first non-linear term is predicted to be + E2 . This contradiction can only be resolved if

χ(2) = 0 in centro-symmetric media.

If the same argument is used for the next higher order term, + E producers polarization + E3 and – E and produces – E3 , so that

χ(3) is the first non-zero non-

linear term in centro-symmetric media. In second harmonic generation, the two input wavelengths are the same

2ω1 = ω2 (or) (χ1 = χ2).

(2.4)

During this process, a polarization wave with the second harmonic frequency 2ω1 is produced. The refractive index, n1 is defined by the phase velocity and wavelength of the medium. The energy of the polarization wave is transferred to the electromagnetic wave at a frequency

ω2. The phase velocity and wavelength of this

electromagnetic wave are determined by n2, the refractive index of the doubled frequency. To obtain high conversion efficiency, the vectors of input beams and generated are to be matched.

2π ∆Κ = .................................. λ(n1 – n2)

(2.5)

Where,

∆Κ

represents the phase – mismatching.

The phase-mismatching can be obtained by angle tilting, temperature tilting or other methods. Hence, to select a non-linear optical crystal, for a frequency conversion process, the necessary criterion is to obtain high conversion efficiency. The conversion efficiency , η where deff is the effective non-linear coefficient, L is the crystal length, P is the input power density and

∆Κ

is the phase-mismatching.

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In general, higher power density, longer crystal, large non-linear coefficients and smaller phase mismatching will result in higher conversion efficiency. Also, the input power density is to be lower than the damage threshold of the crystal. Table 2.2 lists the laser and crystal parameters for selecting a NLO crystal. TABLE 2.2 PARAMETERS FOR SELECTING A NLO CRYSTAL LASER PARAMETERS

CRYSTAL PARAMETERS

NLO process

Type of phase matching

Power, Repetition rate

Damage threshold

Divergence

Acceptance

Bandwidth

Spectral acceptance

Beam size

Crystal size, walk-off angle

Pulse width

Group velocity mismatching

Environment

Moisture, temperature acceptance

2.3.

NON-LINEAR OPTICAL MATERIALS Advances in the development of NLO materials can be divided into three

different areas. (i)

Discovery of new NLO materials

(ii)

Growth of promising NLO crystals

(iii)

Improving the characteristics of NLO crystals

Many organic and inorganic materials are highly polarizable and thus ae good candidates for study. However, the net polarization of a material depends on its symmetry properties, with respect to the orientation of the impinging fields. It can be shown that the odd order terms in equation (2.2) are orientation independent, but the even terms vanish in a Centro-symmetric environment. Thus materials for second order NLO must be orientationally noncentric to be functional. No such restriction applies to third order materials.

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Non-linear 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. The search for new and efficient materials in which to carry out non-linear optical process has been very active since SHG was first observed in single crystal quartz by Franken and co-workers in 1961. In the beginning, studies were concentrated on inorganic materials such as quartz, potassium dihydrogen phosphate (KDP), lithium niobate ( LiNbO3), and semiconductors such as cadmium sulfide, selenium, and tellurium. The first observation of SHG in organic material (benzypyrene) was made in 1965 by Rentzepis and Pao. 2.4.

APPLICATION OF THE NON-LINEAR OPTICAL MATERIALS Non-linear optical materials have important applications in the following areas 

Optical signal processing



Information technology



High density optical data storage



Printing



Spectroscopy



Telecommunications



Biomedicine, etc……

CHAPTER-3

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LOW TEMPERATURE SOLUTION GROWTH 3.1

INTRODUCTION The method of crystal growth from low temperature aqueous solution is

extremely popular in the production of many technologically important crystals. Material having moderate to high solubility in temperature range, ambient to 100C at atmosphere pressure can be grown by low temperature solution growth method. The mechanism of crystallization from solutions is governed by the interaction of ions or molecules of the solute and the solvent that is based on the solubility of substance on the thermo dynamical parameters of the process, temperature, pressure and solvent concentration. 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 temperature and which undergo structural transformations while cooling from the melting point. At least 90% of the crystals produced by low temperature solution methods are soluble in water. Rates of growth from solution fall in the range of 0.1 to 1 mm/day. Solution is a homogeneous mixture of a solute in a solvent. Solute is the component, which is present in a smaller quantity. For a given solute, there may be different solvents. The solvent must be chosen taking into the account the following factors to grow crystals from solution. A solvent of choice is the one with i.

a good solubility for the given solute

ii.

a good solubility gradient

iii.

less viscosity

iv.

less volatility and

v.

less corrosion

vi.

non-toxic

If the solubility is too high, it is difficult to grow bulk single crystals and too small a solubility restrict the size and growth rate of the crystals. Solubility gradient

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is another parameter, which dicates the growth procedure. Neither a flat nor a steep solubility curve will enable the growth of bulk crystal from solution; while the level of supersaturation could not be varied by reducing the temperature in the former, even a small fluctuation in the temperature will affect the supersaturation to a large extent in the later disabling the growth of good quality bulk crystals in both cases. 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. Growth of crystals from solution is mainly a diffusion- controlled process: the medium must be less viscous to enable faster transport of the growth units from the bulk solution by diffusion. Hence a solvent with less viscosity is preferable (Ohara et 1973). Supersaturation is an important parameter for the solution growth process. Crystal grows by the accumulation of the solute in the solution as a degree of supersaturation is maintained. The solubility data at various temperatures are essential to determine the level of supersaturation. Hence, the solubility of the solute in the chosen solvent must be determined before starting the growth process. The solubility of the solute may be determined by dissolving the solute in the solvent maintained at a constant temperature with continuous stirring. On reaching saturation, the equilibrium concentration of the solute may be determined gravimetrically. A sample of the clear supernatant liquid is withdrawn by means of a warmed pipette and a weighed quantity of the sample is analyzed. The solubility curve can then be plotted in this way by repeating the above for different temperatures.

3.2

EXPRESSION OF SUPERSATURATION

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The supersaturation of a system may be expressed in a number of ways. The basic units of concentration as well as temperature must be specified. The concentration driving force (∆C), the supersaturation ratio (S) and relative supersaturation (σ) are related to each other as follows:

The concentration driving force ∆C = C – C* Where, C is the actual concentration of the solution at a given temperature, C* is the equilibrium concentration at a given temperature. Super saturation ratio S = C / C* Relative super saturation σ = (C – C*) / C* σ= S – 1

CT

C' Labile

C"

Metastable

B"

B

C BB' - Solubility curve AB"C" - Evaporation and cooling

B' A

Stable CC' - Super solubility curve CT- Concentration

Temperature Fig. 3.1. Solubility diagram showing different levels of saturation

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If the concentration of a solution can be measure at a given temperature and the corresponding equilibrium saturation concentration is known, then, it is easier to calculate the supersaturation. Meir carried out extensive research into the relationship between supersaturation and spontaneous crystallization (Meir et al 1987) and the results can be represented as shown diagrammatically in (figure 3.1). the lower continuous line is the normal solubility curve for the salt concerned. Temperature and concentration at which spontaneous crystallization occurs are represented by the upper broken curve, generally referred to as the supersolubility curve. This curve is not well defined as the solubility curve and its position in the diagram depends on the degree of agitation of the solution. The diagram is divided into three zones.

(i)

The stable (under saturated) zone, where crystallization is not possible.

(ii)

The second region is a metastable zone, between the solubility and super solubility curves, where spontaneous crystallization is improbable. However, if a crystal seed is placed in such a metastable solution, growth would occur on it.

(iii)

Third region is the unstable or labile (supersaturation) zone, where spontaneous crystallization is more probable.

If the solution whose concentration and temperatures represented by point A in (figure 3.1) , is cooled without loss of solvent (line ABC) spontaneous crystallization cannot occur until conditions represented by point C are reached. At this pint, crystallization may be spontaneous or seeding, agitation or mechanical shock may induce it. Further cooling to some pint D may be necessary before crystallization can be induced, especially with very soluble substances such as sodium thiosulphate.

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The evaporation of solvent from the solution may also result in supersaturation. Line AB’C’ represents an operation carried out at constant temperature. Penetration beyond the super solubility curve into the labile Zone rarely happens, as the surface from which evaporation takes place is usually supersaturated to a greater degree than the bulk of the solution. Crystals, which appear on this surface eventually fall into the solution and seed it. In practice, a combination of cooling and evaporation as represented by the line AB” C” in

(figure 3.1) is adopted.

Low temperature solution growth can be subdivided into the following methods,

(i)

(i)

Slow cooling method

(ii)

Slow evaporation method

(iii)

Temperature gradient method

Slow Cooling Method It is the best way to grow single crystals by solution technique. The main

limitation 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, larger volumes of solution are required. The use of a range of temperature may not be desirable because the properties of the grown material may vary with temperature. Even though 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 began is usually within the range 45 - 75O C and the lower limit of cooling is the room temperature.

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(ii)

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

requirements. The temperature is fixed constant and provision is made for evaporation. With non – toxic solvents like water, it is permissible to allow evaporation into atmosphere. Typical growth conditions involve temperature stabilization to about + 0.005O C and rate of evaporation of a few ml / hr. the evaporation techniques of crystal 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. (iii)

Temperature gradient method This method involves the transport of the material from a hot region

containing the source material to b 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.

Chapter – 4

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GROWTH AND CHARACTERISATION OF POTASSIUM DIHYDROGEN PHOSPHATE (KDP) CRYSTAL 4.1. INTRODUCTION Ferroelectric potassium dihydrogen phosphate (KDP) crystals are widely used in modern short wavelength laser techniques, non-linear and integrated optics. These crystals are applied as three-dimensional electro-optical devices, solid optical media for the frequency transformation of coherent radiation (generators of harmonics, generators of sum and difference frequencies for high power laser radiation, optical parametric oscillators for the infra-red spectral range) and integrated optical waveguides. They are also used as Q – switches and shutters for high – speed Photograph. KDP is characterized by high non-linear performance, wide optical transparency range and well – developed technology pf growth. The point group of the crystal in 42m and the space group is I4 2D (122). The structure belongs to the scalenohedral (twelve face polyhedron) class of tetragonal system. Organic crystals [1-3] possess high efficiency of frequency conversion, high damage, threshold, wide range of transparency. In spite of having all positive factors, the organic crystals could not be employed satisfactorily, because of their poor mechanical and thermal stability. So, semi organic crystals which have combined properties of both inorganic and organic species are expected to have good optical, thermal and mechanical properties. KDP crystals have been investigated through powder XRD, FTIR, FTRaman, UV and thermal analyses along with dielectric properties in the microwave region.

4.2

EXPERIMENTAL

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4.2.1 GROWTH OF THE CRYSTAL The pure KDP crystals were grown using solution growth technique. Recrystallised salts of KDP (99% purity) and triple-distilled water were used in the present crystal growth experiment. Saturated solutions of KDP (each 250ml) are separately mixed with 0.1 M solutions and stirred well using a stirrer. The well – mixed solutions are allowed to evaporate at room temperature. Good sized crystals were grown within two weeks and the photographs of the grown crystals are presented in Figure 4.1. 4.2.2 SCOPE OF THE PROJECT KDP is a non-linear optical crystal. The crystals are synthesized from potassium dihydrogen phosphate. The grown crystals are

i)

Powder X-Ray diffraction was taken for the confirmation of single crystals.

ii)

FTIR spectrum was taken to confirm the different functional groups in the grown crystals.

iii)

UV- visible spectrum was taken to find the lower cut off frequency.

The grown crystals are used for non-linear optical application.

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Figure 4.1 Photographs of pure KDP crystals

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CHAPTER – 5 CHARACTERIZATIONS METHOD 5.1. INTRODUTION Researches in material science and devices engineers want to know the degree of purity and perfection of crystal to interrupt structure dependent properties in order to determine whether the material can be successfully employed in the experiments or device fabrication process. It is also important to know the nature and distribution of the imperfections present in crystals. Detailed studies of the crystal can provide information to deduce how the growth techniques should be modified so that the perfection of the crystal may be increased. Characterization of the crystal consists of its chemical compositions, structure. Defects and the study; of their electrical, mechanical and optical properties. The measurement of optical properties includes the study of optical transmission and absorption of the crystal and SHG conversation efficiency. Characterization of NLO crystal can be divided in to following topics. 1. Structural analysis of crystal. 2. Measurement of optical properties. 3. Measurement of thermal and electrical properties.

5.1.1

BRAGG LAW

29

Fig.5.1.1 Schematic diagram of Bragg’s law W.L. Bragg presented a simple explanation of the different bear from a crystal. Suppose that the incident waves are reflected from specular parallel planes of atoms in the crystal, with each plane reflector only a very small fraction of the radian, like a lightly silvered mired. The diffracted beams are found when the reflections from parallel plan of atoms interfere constructively, as in Fig.5.1.1 We treat elastic scattering, which the energy of the x-ray is not changed on reflection. Inelastic scattering, with the excitation of elastic waves, is discussed at the end of the chapter. Consider parallel lattice planes spaced apart.

The radiation was incident in

the plane of the paper. The path difference for rays reflective from adjacent interference of the radiation from successive planes occurs when the path difference is an integral number n of wavelengths λ, i.e. 2d sinθ = nλ. This is the Bragg law. Although the reflection from each plane is specular for only certain values of θ will the reflections from all parallel planes as up in phase to give a strong reflected beam.

30

In each perfectly reflecting plane, only the first plane of a parallel set would see the radiation and wavelength would be reflected. But each plane reflected 10-3 to 10-5 of the incident radiation. The Bragg law is a consequence of the periodicity of the lattice. The law does not refer to the arrangement of atoms in the basis associated with each lattice point. The composition of the basis determines the relative intensity of the various orders n of diffraction from a given set of parallel planes. Bragg reflection can occur only wavelength λ ≤ 2d. This is why we cannot use visible light. 5.1.2

EXPERIMENTAL DIFFRACTION METHODS The Bragg law requires that θ and λ is matched; monochromatic x-rays of

wavelength λ striking a three-dimensional crystal at an arbitrary angle of incidence will not in general be reflected. To satisfy the Bragg law requires an accident, and to create the accident it is necessary to scan in either wavelength or angle. The standard methods of diffraction used in crystal structure analysis are designed expressly to accomplish this. We describe three simple, order methods, still used by physicist; but for professional crystallography these techniques have been replaced by complicated precession camera methods. 5.1.3 LAUE METHOD In the Laue method (Fig.5.1.3), a single crystal is stationary in a beam of xray or neutron radiation of continuous wavelength. The crystal selects and diffracts the discrete values of λ for which planes exist of spacing d and incidence angle θ satisfying the Bragg law. A source is used that produces a beam of x-rays over a wide range of wavelengths, perhaps from 0.2 Ǻ to 2Ǻ. A pinhole arrangement produces a well-collimated beam. The dimensions of the single-crystal specimen need not be greater than 1 mm. Flat film receives the diffracted beams. The diffraction pattern consists of a series of sports. The pattern will show the symmetry of the crystal; if a crystal has a fourfold axis of symmetry parallel to the beam, the Laue pattern will show fourfold symmetry. The Laue method is widely used to orient crystals for solid state experiments [Banwel, C.N., 1996].

31

Fig.5.1.3 Crystal structures are determined experimentally by X-Ray diffraction 5.1.4 ROTATING-CRYSTAL METHOD In the rotating-crystal method, a single crystal is rotated about a fixed axis in a beam of monoenergetic x-rays or neutrons. The variation in the angle θ brings different atomic planes into position for reflection. The film is mounted in a cylindrical holder concentric with a rotating spindle crystal mount. The incident x-ray beam is monochromatized by a filter or by reflection from an earlier crystal. The beam is diffracted from a given crystal plane when in the course of rotation the value of θ satisfies the Bragg equation. Beams from all planes parallel to the vertical rotation axis will lie in the horizontal plane. Planes with other orientations will reflect in layers above and below the horizontal plane. The intensity distribution of the radiation from a 30 keV x-ray tube with a molybdenum target and the distribution of neutrons are emerging from a nuclear reactor. If we reflect the beam from a monochromatic crystal, we get the crosshatched distribution.

32

Several variations are common use. In oscillating-crystal photographs the crystal is oscillated through a limited angular range, instead of being rotated through 3600. The limited range reduces the possibility of overlapping reflections. The precession camera developed by M. J. Buerger gives a picture of the various levels of the reciprocal lattice. Modern diffractometers use scintillation counters or proportional counter tubes to data, needed because complex structures may exhibit 10,000 diffracted rays. Nearly all crystals with simple structures were solved by x-ray analysis long ago. One present center of interest in x-ray structure analysis is in the determination of the configuration of enzymes with a molecular weight between 10,000 and 100,000. The crystallization of an enzyme and the subsequent x-ray analysis of the structure of the crystal is the most effective method for the determination of the shape of the molecule. The coordinate 500 to 5000 atoms in a cell are wanted, so at least this number of x-ray reflection lines is required. Computer programs have enormously simplified the problem of structure determination. 5.1.5

POWDER METHOD In the Powder method the incident monochromatic radiation strikes a finely-

powdered specimen or a fine-grained polycrystalline specimen contained in a thinwalled capillary tube (Fig. 5.1.5).

The distribution of crystallite orientations will

be nearly continuous.

Fig. 5.1.5. The Debye-Scherrer Camera

33

The powder method is convenient precisely because single crystals are not required. Diffracted rays go out from individual crystallites that happen to be oriented with planes making an incident angle θ with the beam satisfying the Bragg equation. Diffracted rays leave the specimen along the generators of cones concentric with the original beam. The generators make an angle of 2θ with the direction of the original beam, where θ is the Bragg angle. The cones intercept the film in a series of concentric rings. (Fig. 5.1.6).

Fig. 5.1.6 Photographic film after developing 5.1.7

FOURIER TRANSFORM INFRARED SPECTROSCOPY

Fig. 5.1.8 Schematic diagram of a Fourier transform infra-red spectrometer

34

Infra-red spectroscopy extends outside the limits we have discussed so far in this chapter, and in particular a good deal of useful molecular information is contained in spectra below 400cm-1, i.e. the far infra-red region, from about 400cm-1 to 20cm-1 or 10cm-1. Because sources are weak and detectors insensitive, this region is known as ‘energy-limited’ and difficulty is experienced in obtaining good signal-to-noise ratios by conventional means. The advent of Fourier transform spectroscopy has made the far infra-red much

more accessible,

and has

considerably speeded and improved spectroscopy in the infra-red region in general. In this region Fourier transform (FT) methods are used in absorption. The apparatus derives from the classical attempt by Michelson to measure the ‘ether wind’ by determining the velocity of light in two perpendicular directions. A parallel beam of radiation is directed from the source to the interferometer, consisting of a beam splitter B and two mirrors M1 and M2 (Fig. 5.1.7). The beam splitter is a plate of suitable transparent material (e.g. potassium bromide) coated so as to reflect just 50 per cent of the radiation falling on it. Thus half the radiation goes to M1 and half to M2, returns from both these mirrors along the same path, and is then recombined to a single beam at the beam splitter (clearly half the total radiation is sent back to the source, but this is immaterial). It is well known (and the essence of the Michelson experiment) that if monochromatic radiation is emitted by the source, the recombined beam leaving B shows constructive or destructive interference, depending on the relative path lengths B to M1 and B to M2. Thus if the path lengths are identical or differ by an integral number of wavelengths, constructive interference gives a bright beam leaving B, whereas if the difference is a half-integral number of wavelengths, the beams cancel at B. As the mirror M 2 is moved smoothly towards or away from B, therefore, a detector sees radiation alternating in intensity. It is fairly easy to imagine that if the source emits two separate monochromatic frequencies, v1 and v2, then the interference pattern (beat pattern) of v1 and v2 would overly the interface caused by M1 and M2; the detector would see a more complicated intensity fluctuation as M2 is moved, but computing the Fourier Transform of the resultant signal is a very rapid way of obtaining the original frequencies and intensities emitted by the source.

35

Taking the process further, even ‘white’ radiation emitted by the source produces an interference pattern, which can be transformed back to the original frequency distribution [Nakamoto, K., 1995]. A typical interference pattern or interferogram for a ‘white’ source, where the wide range of frequencies causes a rapid diminishing of signal away from the position at which both mirrors are at an equal distance from the beam splitter (the so-called zero retardation peak). No real source is truly white. The variation in total intensity caused by varying source output and beam splitter efficiency across the IR range for a typical FT spectrometer. Since FT infra-red spectroscopy is carried out as a single-beam technique, this background variation must be taken into account for each spectrum. If the beam from such a source is directed through a sample before reaching the detector, sample

absorptions

cause

gaps

in

the

frequency

distribution

which,

after

transformation, will appear as down-going peaks. The production of a spectrum, then, is a two-stage process, which may be thought of as follows. Firstly, without a sample in the beam, mirror M2 is moved smoothly over a period of time (e.g. one second) through a distance of about 1cm, while the detector signal-the interferogram is collected into a multi-channel computer (it may be, for instance, that the detector signal is monitored every thousandth of a second during the mirror traverse, and each piece of information put serially into one of a thousand different storage locations in the computer); the computer carries out the Fourier Transformation of the stored data to produce the background spectrum. Secondly, a sample interferogram is recorded in exactly the same way, Fourier transformed, and then ratioed against the background spectrum for plotting as a transmittance spectrum. Alternatively, the sample and background spectra may each be calculated in absorbance forms and the latter simply subtracted from the former to give an absorbance spectrum of the sample alone.

36

5.1.9 UV – visible spectroscopy UV – Visible Spectroscopy is defined as the measurement of the absorptions or emission of radiation associated with changes in the spatial distribution of electrons involved are usually the outer valence are bonding electrons, which can be excited by absorption of UV or visible radiation. Excitation of a bound electron from the highest occupied molecular orbital to the lowest unoccupied molecular orbital increases the spatial extent of the electron density larger and more diffuse, and often more polarisable. The probability for electronic transitions determines the intensity of spectral lines. There must be large overlap between the vibrational states in the initial and final electronic stated ti have a large absorption cross-section or high probability that the molecule will absorb emit UV visible light. The most probable position for which the transition can occur is from the equilibrium bond distance for diatomic molecule. Since, with only zero point vibration occurring, the vibrational wave function is a symmetric even function with no nodes. Electronic transitions are possible for wide range of vibrational levels with in the initial and final states. 5.2

RESULTS AND DISCUSSIONS:

1. Powder analysis XRD of KDP crystal: Powder pattern XRD of KDP crystal is shown in Fig. 6.1. The XRD pattern shows the crystalline nature of the grown KDP crystal. The table 5.2.1 gives the d and hKl values of grown KDP crystal. It is observed that the grown KDP crystal is monoclinic structure.

37

Table 5.2.1 INDEXED XRD DATA FOR PURE KDP CRYSTAL !

D

2Theta

I (rel)

I (abs)

FWHM

1.

5.056435

17.5252

68.1

281

.0800

2.

3.716402

23.9248

100.0

412

.0800

3.

3.006801

29.6877

24.4

101

.0800

4.

2.907638

30.7247

66.9

276

.0800

5.

2.636420

33.9766

28.4

117

.1200

6.

2.548941

35.1800

7.6

31

.1200

7.

2.344242

38.3667

8.1

33

.0400

8.

2.224214

40.5253

6.7

28

.1200

9.

1.985669

45.6514

10.3

43

.1200

10.

1.956959

46.3599

36.4

150

.0800

11.

1.911553

47.5280

14.7

60

.0800

12.

1.702547

53.8006

9.3

38

.0800

13.

1.671024

54.9000

14.3

59

.1200

14.

1.585015

58.1545

13.1

54

.0800

15.

1.574086

58.5974

75.7

312

.0800

16.

1.459828

63.6956

5.0

21

.0800

17.

1.353094

69.4010

7.6

31

.0800

2. FTIR analysis of KDP: The FTIR spectrum of KDP is shown in Fig. 6.2. The spectrum has shown the various absorption frequencies of the grown KDP crystal. Table 5.2.2 gives the absorption frequencies of various groups present in the KDP crystal and compared with the standard values. It is observed that the growth of KDP crystal consists of all functional groups present in the compound.

38

Table 5.2.2

FTIR and Frequency assignments of pure KDP crystal

KDP Assignment

FTIR (cm-1)

(po4)

3-

γ1

(X4)

909.4(S)

Sym.bend

γ2 (E) δd (4*4)

433.3

In plane bending

γ3 (F2)

1098.8

γd (X4) Asym.stretching

γ4 (F2) δd (4*4)

536.2

Bend(or) deformation O |

1696.1(S)

P - O – H stretch P = O stretch

1302.1(S)

P – O stretch

1098.8(S)

P - O – H stretch

909.4(S)

O – H stretch

2500-3800 2447.9(W)

O – H bending

2763.0(W) 1330-1420 650-769

Opp – O bending K - O stretching

300-650 320-450 2400(S)

3. UV – spectroscopy result:

39

The absorption spectrum of KDP crystal is recorded at the room temperature. The measurement range from 200 - 1100nm. The absorption spectrum of KDP crystal is shown in the fig. It can be seen that transparent of KDP crystal covers 234 – 1100nm and the UV cut off wavelength are at 234nm and very low percentage of absorption in the entire visible region which is very essential property for NLO crystal.

6.1

XRD ANALYSIS OF PURE KDP CRYSTAL

40

6.2

FTIR spectra of pure KDP crystal

41

.0800

6.3

UV – VISIBLE ANALYSIS OF PURE KDP CRYSTAL

42

5.3

CONCLUSION

43

The grown KDP crystals were subjected to different Characterisation techniques. Powder XRD shows the grown KDP compound is a crystalline material. FTIR shows the confirmation of all functional groups present in the crystal. UV – visible spectrum shows the grown crystal may be used as a detector in UV - visible and IR regions.

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44

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