2. METHODS OF CRYSTAL GROWTH

Transcription

1 2. METHODS OF CRYSTAL GROWTH The ideal crystal is an infinite lattice of atoms arranged in patterns, which repeat in all three dimensions with repeated distances (lattice spacing). In general, a single crystal is a periodic array of atoms arranged in three dimensional structure with equally repeated distance in a given direction. Natural crystals have often been formed at relatively low temperatures by crystallisation from solutions, sometimes in the course of hundreds and thousands of years. Now a days, crystals are produced artificially to satisfy the needs of science and technology. Crystal growth is rather an art than a science [28]. Many attempts have been made for a long time to produce good crystals of desired material. Presently, crystal growth specialists have moved from the periphery to the center of the materials-based technology. This Chapter briefly describes the different methods of crystal growth and various experimental techniques which are employed to obtain good quality crystals. Crystal growth methods are generally classified into four categories: i) growth from solid, ii) growth from melt, iii) growth from vapour and iv) growth from solution. 2.1 Growth From Solid The job of the crystal grower is to prepare large specimens of crystalline material such that there is a complete crystallographic continuity across a given specimen in all directions is achieved. There are two principal reasons for the deliberate growth of single crystals. 23

2 i) Many physical properties of solids are obscured or complicated by the effect of grain boundaries. ii) The full range of tensor relationships between applied physical causes and observed effect can be obtained only if the full internal symmetry of the crystal structure is maintained throughout the specimen. Solid state growth technique can be considered as the conversion of a polycrystalline material into a single crystal by causing the grain boundaries to be swept through and pushed out of the material due to atomic diffusion. But this is very slow at ordinary temperatures and is only rarely used. 2.2 Growth From Melt Melt growth can be achieved by a variety of techniques (e.g. free melt surface of confined configurations) depending on the specific properties of the material (e.g. contraction or expansion during solidification) and requirements. The growth from melt can be subgrouped into various techniques. The main techniques are: i) Czochralski technique ii) Bridgman-Stockbarger technique iii) Vernueil technique iv) Zone melting technique v) Skull melting process vi) Shaped crystal growth technique The major factor to be considered during the growth of crystals from the melt is volatility or dissociability, the chemical reactivity and the melting point. Czochralski method is the most commonly used technique to grow good quality crystals from melt. 24

3 2.2.1 Czochralski method In this method, the charge material is contained in a crucible which is heated to a temperature above the melting point of the charge. A pull rod with a chuck containing a seed crystal at its lower end is positioned above the crucible. The seed crystal is dipped into the melt and the melt temperature is adjusted until a meniscus can be supported by the seed crystal. The pull rod is then slowly rotated and lifted and by carefully adjusting the power supplied to the melt, a crystal of the desired diameter can be grown. The whole assembly is maintained in an envelope which permits control of the ambient gas and enables the crystal to be observed visually. The technique has been applied to an extremely wide range of materials from elemental metals and semiconductors to complex refractory high melting point oxides. Crystal pullers have revolutionized in the semiconductor industry with the development of the liquid encapsulation techniques. The important semiconducting compounds like GaAs, InP and GaP are grown by this method [29] Bridgman-Stockbarger technique 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. The furnace consists of two halves. The upper half maintains the little above the melting point and lower half keeps just below the melting point. The crucible is made of platinum quartz and has pointed lower end. The crucible is filled with the material and it is lowered slowly. The temperature gradient between halves is made as steep as possible. When the crucible crosses the zone corresponding to the freezing point of material, single crystal forms at the lower end of the crucible. The main advantage is to grow single crystal of any desired shape and size which can be 25

4 obtained by choosing the appropriate crucible. This method is more suitable for growing single crystals like GaAs, silver halides, etc [30] Vernueil technique In this method, chemically pure fine powder of 1-20 microns emerges through an oxy-hydrogen flame and falls onto the fused end of an oriented single crystal seed fixed to a lowering mechanism. The powder charge is fed from a bunker by means of a special tapping mechanism. Coordinating the consumption of the charge, hydrogen and oxygen with the rate of decent of the seed ensures crystallization at a prescribed level of the apparatus Zone melting technique Zone melting is a generic title given to a large family of techniques (floatzone, traveling solvent zone, zone-pass, etc) which have in common the following feature: A liquid zone is created by melting a small amount of material in relatively large or long solid charge or ingot. It is then made to traverse through a part or the whole of the charge. A seed crystal can be introduced at the starting end to grow single crystals Skull melting process The skull melting process is used for the growth of high melting point materials. This process is currently widely used for the growth of zirconium oxide. Zirconium oxide is a material with a melting point of about 2750 C. The high melting point and extreme chemical reactivity of the melt make it impossible to melt and crystallize zirconium oxide in conventional metallic or graphite crucibles. In the early 26

5 1970 s the Russians (Aleksandrov Osiko, Tatarinstev) devised an ingenious method whereby zirconium oxide is fused in a container or skull of its own substance. Zirconium oxide, cubic stabilized with yttrium oxide is an interesting material for an application as diamond imitation because of its high refractive index (2.15), dispersion (0.060) combined with its hardness [31]. This method is used to produce zirconium up to 10 cm long Shaped crystal growth technique Shaped growth of crystals from the melt has been practised for over a half century. In this method, the crystal is grown from a thin film of liquid on the top of a suitable die surface. The shape of the film and therefore of the crystal is determined by the external shape of the film and therefore of the crystal is determined by the external shape of the die. Unlike the more conventional crystal pulling techniques, growth rates are extremely high being in the range 1-5 cm/min as compared to cm/min for conventional crystal pulling. 2.3 Growth From Vapour Single crystals of high purity can be grown from the vapour by sublimation and chemical vapour deposition. In these processes, the source material which is a solid or one or two components of the phase to be crystallized is provided from the vapour phase. The ampoule must be translated through the temperature gradient at a rate equal to the linear growth rate of the crystal. This ensures that the supercooling conditions remain constant so that spurious nucleation does not occur. The most widely known sublimation method is the so called Piper-Polich technique for the 27

6 preparation of cadmium sulphide. Small size crystals of better quality can be grown like CdS, Al 2 O 3 and Hgl 2 [32]. 2.4 Growth From Solution The method of growing crystals from solutions may be used for substances fairly soluble in a solvent and not reactive with it. Moreover, growth of crystals from solutions is the only method for the crystallization of substances which undergo decomposition before melting Criteria for growth Crystals intended for practical and technical applications should have a well developed morphology and contain a low density of defects (such as inclusions, dislocations, etc). These requirements may be predicted from a consideration of thermodynamic (e.g. crystal-medium interface) and kinetic (e.g. equilibrium solute concentration, supersaturation, growth temperature and stirring rate) parameters which characterize the overall growth conditions. Thermodynamic parameters determine the growth mechanism while kinetic parameters determine the growth kinetics and generation of defects Metastable zone width Crystal growth takes place in the metastable supersaturated zone without the occurrence of three dimensional nucleation. The metastable zone width is an experimentally measurable quantity, although it is well known that a number of factors (such as stirring rate, cooling rate of the solution, presence of additional crystals or impurities) affect its value. 28

7 2.4.3 Crystal-medium interface The development of a crystal involves the incorporation of growth units on its surface. A crystal with well developed polyhedral morphology is obtained, when the crystal medium interface is smooth, so that the surface grown by the lateral displacement of layers (i.e. layers growth). When the surface is rough, integration of grown species into the crystal is continuous (continuous growth), and this results in the growth of dendrites and hopper crystals without technical applications. Whether an interface is rough or smooth may be known from the value of the surface entropy factor. The growth rate of crystal depends on the values of kinetic parameters and increases with solution supersaturation, growth temperature and crystal solubility. Higher the values of equilibrium solute concentration (solubility) and supersaturation lower the value of the surface entropy factor, which consequently lead to the generation of dislocations and capture of inclusions. Higher temperatures not only enhance growth rates but also lead to decrease in the generation of defects. Thus to ensure good crystal growth it is useful to have a sufficiently high value of the surface entropy factor of the system, medium supersaturation, elevated temperatures and nonturbulent stirring Impurities Growth aids which modify the properties of a growth system may be taken as impurities. Additives are present in the solution as ions (metal-ions, oxy-ions and dyeions). Small amounts of these ions are known either to produce improvements in crystal growth which is otherwise difficult from solutions or to change the growth 29

8 habit. In general highly polarizable metal-ions and oxy-ions are the most effective impurities, and the habit of crystals of ammonium and alkali metal compounds is readily modified in the presence of these impurities Stirring For the successful and relatively fast growth of a substance from solution containing a reasonably high soluble concentration and having a high viscosity, effective stirring is an important operation. The rate of stirring is to sweep off the depleted solution at the crystal surface, providing it with fresh supersaturated solution which would otherwise have been supplied by diffusion. Stirring may be achieved by various types of stirrers, at a rate greater than the optimum, induces turbulence at a point in the system, which favours trapping of inclusions on the crystal surface. The simplest method of stirring is unidirectional rotation of the crystal fixed at the holder of a stirrer. This type of stirring leads to the formation of cavities in the central regions of a crystal face because of malnutrition of the solute there is comparison with edges and corners which receive more solute supply. Periodic rotation of the crystal in opposite directions suppresses eddy formation but does not eliminate the formation of the central cavity. Consequently, eccentric reversive rotation is often used Growth temperature In order to grow the crystal of a substance in a given phase and / or composition at a resonable rate, the choice of an optimum temperature interval is important. As in the use of other processes, growth at elevated temperatures takes place faster. However, at elevated temperatures, smooth growth necessitates better 30

9 temperature control while increased vapour pressure creates problems of control of supersaturation and spurious nucleation. These difficulties may be overcome during crystal growth from boiling solutions Solubility and supersaturation Solubility corresponding to saturation, is the equilibrium between a solid and its solution at a given temperature and pressure. Thermodynamically this means that the chemical potential of the pure solid A is equal to the chemical potential of the same solute in saturated solution. Solubility changes with temperature and pressure. A solvent in which the solute has solubility between 10-60% may be considered suitable for crystal growth. Crystals grow only if solution is out of equilibrium, i.e. if it is supersaturated. Supersaturation can be achieved by solvent evaporation, solution cooling (or heating, in the case of reverse solubility), change of ph, adding of a common ion, mixing of soluble reactants. Supersaturation can be expressed in different ways. For soluble compounds, if C S and C e are the actual and equilibrium concentrations, we have: C = C S C e (absolute supersaturation), β σ = C S / C e (supersaturation ratio), and = (C S C e ) / C e = β 1 (relative supersaturation). When β = 1, the system is saturated; when β > 1, it is supersaturated and the crystal can grow; when β < 1, it is undersaturated, and the crystals dissolve. Figure 4 shows the solubility diagram showing different levels of saturation. 31

10 C Metastable Labile Concentration III C B B I II Stable D C B A Temperature BB - Solubility curve CC - Super solubility curve AB C - Evaporation and cooling D - Crystallization point Figure 4: Solubility diagram showing different levels of saturation Choice of solvent Once the method and high purity starting material are ensured, the next requirement is that a solvent should be chosen which allows prismatic growth and in which the solute has high solubility. The ideal solvent should yield a prismatic habit in the crystal and also have the following characteristics: i) high solute solubility, ii) high positive temperature coefficient of solubility, iii) low viscosity, iv) low volatility, v) density less than that of the bulk solute, vi) low toxicity, vii) low vapour pressure at the growth temperature, viii) cheap in the pure state and readily available, etc. 32

11 A simple rule of thumb in the proper selection of solvent is the chemical similarity between the solvent and the material to be grown. For example, crystals of nonpolar organic compounds easily grow from nonpolar organic solvents. The chemical similarity also determines crystal solubility in the solvent. Consequently, because of the interaction of the surface of growing crystals and the solvent molecules, the solvent also provides a control over the crystal habit Methods of crystal growth methods: Low temperature solution growth can be subdivided into the following i) Slow cooling method, ii) Slow evaporation method, and iii) Temperature gradient method Slow cooling method This is the best method to grow bulk single crystals from solution. In this method, supersaturation is created by a change in temperature usually throughout the whole crystallizer. The crystallalization process is carried out in such a way that the point on the temperature dependence of the concentration moves into the metastable region along the saturation curve in the direction of lower solubility. Since the volume of the crystallizer is finite and the amount of substance placed in it is limited, the supersaturation requires systematic cooling. It is achieved by using a thermostated crystallizer. The temperature at which such crystallization can begin is usually within the range C and the lower limit of cooling is the room temperature. The apparatus used for the growth of single crystals by this method is shown in Figure 5. 33

12 L - Heater, B - Constant temperature bath F - Flask S - Stirrer, T - Thermometer SG - Stirring gland Figure 5: Schematic diagram of the apparatus for the slow cooling method Slow (free) evaporation method In this method the solution loses particles which are weakly bound to other components and, therefore, the volume of the solution decreases. An excess of a given solute is established by utilizing the difference between the rates of evaporation of the solvent and the solute. Normally, the vapour pressure of the solvent above the solution is higher than the vapour pressure of the solute and, therefore, the solvent evaporates more rapidly and the solution becomes supersaturated. It is sufficient to allow the vapour formed above the solution to escape freely into the atmosphere. This method of crystal growth is the oldest and technically it is very simple. For nontoxic solvents such as water evaporation is permissible into the atmosphere but for toxic and inflammable solvents precautions are taken to avoid the leakage of solvent vapour in the atmosphere. 34

13 Figure 6: Schematic diagram of a simple apparatus for the slow (free) evaporation method The simplest apparatus used for the growth of single crystals by this method is the one shown in Figure 6 with a few holes in the lid to allow solvent evaporation. The rate of crystallization depends on the rate of solvent evaporation which may be governed by changing the total area of the holes. In sophisticated crystallizers evaporation is controlled by passing air or an inert gas at a controlled rate over the solution. Good control of evaporation rate can also be obtained by using some sort of condenser to allow the removal of condensed solvent at a controlled rate Temperature gradient method In this method, the materials are transported from a hot region containing the source material to be grown to a cooler region where the solution is supersaturated and the crystal grows. The main advantages of this method are: 35

14 i) Crystal grows at fixed temperature; ii) This method is insensitive to change in temperature provided both the source and the growing crystal undergo the same change; iii) Economical use of solvent and solute; etc. On the other hand, changes in the small temperature difference between the source and the crystal zones have a large effect on the growth rate. In crystal growth systems, the cool growth zone is separated from the hot saturator and the solution is pumped from one vessel to the other. Supersaturated solutions tend to nucleate when pumped. If he solution saturated at T + T pumped directly to growth vessel, undissolved particles are transferred to the growth region. To overcome such problems crystallizers having three-vessel growth system is normally used. The temperature in the saturator vessel will be 10 C above the crystallizer and the solution temperature in the super heater vessel will be much higher than the saturator. During the crystal growth run the solution flows from super heater vessels to the crystallizer and then to the saturator and returns to the super heater vessel. The solution pumps fitted in the saturator and super heater vessels are fitted with filters of size 100 µm respectively. The apparatus used for the growth of single crystals by this method is shown in Figure 7. Thermostat for dissolution at a temperature T 1 Nutrient Thermostat for growth at a Temperature T 2, T 2 < T 1 Vane type agitator Growing crystal Figure 7: Schematic diagram of the apparatus for the temperature gradient method 36