CHAPTER - II METHODS OF CRYSTAL GROWTH

Transcription

1 2.1. INTRODUCTION CHAPTER - II METHODS OF CRYSTAL GROWTH Crystal growth is an interdisciplinary subject covering physics, chemistry, material science, chemical engineering, metallurgy, crystallography, mineralogy etc. In the past few decades, there has been a growing interest on crystal growth processes, particularly in view of the increasing demand of materials for technological applications (Binay Kumar et al., 2007). Natural crystals have often been formed at relatively low temperatures by crystallisation from solutions, sometimes in the course of hundreds and thousands of years. Nowadays, crystals are produced artificially to satisfy the needs of science and technology. Crystal growth is rather an art than a science (Brian R. Pamplin 1980). Many attempts have been made for a long time to produce good crystals of desired material. Presently, crystal growth specialists have been moved from the periphery to the center of the materials-based technology (Alan Holden et al., 1982). This chapter reviews the different methods of crystal growth and various experimental techniques which are employed to obtain good quality crystals. The growth aspects differ from crystal depending on their physical and chemical properties such as solubility, melting point, decomposition, phase change etc. This chapter gives a brief account of the methods to grow crystals. The classification of various crystal growth techniques is given in Table 2.1. Table 2.1. Classification of various crystal growth techniques Sl. No 1. Melt growth 2. Types Classification Techniques Vapour growth 3. Solution growth liquid to solid phase transition Gas to solid phase transition Solution-to-Solution phase transition Bridgman- stockborges method Czochralski Method Verneuil Method Zone Melting Method Strain Annealing Method Chemical transport method Physical transport method. Low temperature solution growth High temperature solution growth Hydrothermal growth Gel growth 40

2 2.2. CRYSTAL GROWTH FROM MELT Bridgman method In Bridgman technique the material is melted in a vertical cylindrical container, tapered conically with a point bottom. The material which is to be grown as a crystal is taken in a suitable container and heated in a furnace above its melting point. The melt is contained in a crucible and is progressively frozen from one end. Moving the crucible down a temperature gradient or moving the furnace over the crucible (Hurle 1993). The rates of movement for such processes range from about 1-30 mm/hr. Crystallization begins at the tip and continues usually by growth from the first formed nucleus. The latent heat of solidification is removed by conduction through the crystal and the crucible. The container restricts the shape of the crystal. This method is applied in growing a wide range of substance like metallic, organic and a number of dielectric single crystals such as oxides, fluorides, sulphides and halides. Single crystal of sapphire (Al 2 O 3 ) is grown by this method. This method is technically simple. Selecting the appropriate container can produce crystal of reassigned diameter Czochralski or Pulling method In Czochralski method, the material to be grown is melted by induction resistance heating under a controlled atmosphere in a suitable non-reacting container. This method was developed in 1916 as a result of an accident and through Czochralski's careful observation. This refined method of Czochralski technique is widely adopted to grow III-V compound semiconductors. The shape of the crystal is free from the constraint due to the shape of the crucible. The main advantage of this crystal pulling method is that the size and diameter of the crystal can be controlled, while it is growing. In this method the charge is in melted state and maintained at a temperature slightly above the melting point. The pulling rod is lowered to just touch the melt. Since the rod is at lower temperature condensation of melt occurs at the point tip of the pulling rod. The crystal is pulled slowly. The rate of pulling depends 41

3 upon various factors like thermal conductivity, latent heat of fusion of charge and rate of cooling of the pulling rod. The seed is rotated to keep the growing crystal uniform. To obtain good crystals the pull and rotation rate should be smooth and temperature of melt should be accurately controlled. The method lends to make it convenient in chemical composition control (doping) using the appropriate melt composition. Liquid encapsulated Czochralski abbreviated as LEC technique makes it possible to grow single crystals of materials, which consists of components that produce high vapour pressure at the melting point. The difficulties in this method are associated with engineering problems in accommodating rotation and pulling of the crystal and with the thermal configuration requirements for maintaining thermodynamic equilibrium between the vapour and the melt. The Czochralski method has a number of advantages, of which the following are worth monitoring. There is no direct contact between the crucible walls and the crystal, which helps to produce unstressed single crystal. The crystal can be extracted from the melt at any stage of growth, which implifies investigation on the study of growing conditions. Varying the temperature can change the geometrical shape of the crystal, require and rates of growth. Due to these advantages, Czochralski method gained popularity, particularly in growing single crystal of silicon, germanium, gallium phosphide, gallium arsenide, etc Vernuil method 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. Then it is melted in an oxy-hydrogen flame and a single crystal grown around a seed crystal. A film of liquid formed in the top of the seed is slowly lowered. To maintain symmetry the seed is rotated. The flame is made to impinge on 42

4 a pedestal where a small pile of partly fused powder quickly builds up. As the pile rises, it reaches the hotter part of the flame so that the tip becomes completely molten. The molten region increases in size and starts to solidify at the lower ends. As more and more power arises, the solidifying region broadens. The pedestal is lowered to grow lengthy crystals called boule. By this method ruby crystals are grown up to 90 mm in diameter for use in jeweled bearings and lasers. This technique is widely used for the growth of synthetic gems and variety of high melting oxides. This is no container, which eliminates the problem of physio-chemical interaction between the melt and the container material. It is technically simple and the growth of crystal can be observed. Single Crystal can be grown by this method and various shapes like plates, discs, hemi-spheres and cones can be grown by this method Zone melting method In the zone melting technique, the feed material is taken in the form of sintered rod and the seed is attached to one end. A small molten zone is maintained by surface tension between the seed and the feed. The zone is slowly moved towards the feed. Single crystal is obtained over the seed. To grow single crystals the zone is first moved up to a seed crystal and is then moved slowly away at the required rate of the growth. 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. Multiple recrystallization of the substance is possible, which permits chemical purification of the substances. When growing thermally unstable substance one can consider reducing the disturbance in their stoichiometric by keeping the width of the melt zone minimum and at regular intervals. 43

5 Strain -Annealing method In this method, the metal rod in fine grain structure is subjected to a strain at an elevated temperature, some grains are observed to grow. This technique is useful for metals. Working and annealing at low temperature produce a fine grain structure. In materials with solid-state transitions, the strain induced by the phase change can be used to device crystallization the specimen is annealed alternatively above and below the transition temperature (James Coble Brice et al., 1973) GROWTH FROM VAPOUR GROWTH 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 crystals utilizes chemical transport reaction in which a reversible reaction is used to transport the source material as a volatile species to the crystallization region. Finding a suitable transporting agent is a formidable, problem in this technique. It is rarely possible to grow large crystals because of multi-nucleation (Faktor, et al., 1974). Crystallization from vapour is widely adopted to grow bulk crystals, epitaxial films and thin coatings. Techniques for growing crystals from vapour is divided into two types they are 1. Chemical transport method 2. Physical transport method Chemical vapour transport method This method involves a chemical transport in which material is transported as a chemical compound (halide), which decomposes in the growth area. 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 on a substrate. In this case depending on the nature of the reaction involved, the growth region may be either hotter or cooler than the source. 44

6 Physical transport method This method involves direct transport of materials by evaporation or sublimation from a hot source zone to a cool region II-VI compounds (ZnS, CdS) are widely grown by this method either in vacuum or with a moving gas stream. In the both cases the growth can be a suitable seed crystal, which can either be of the material being grown or some other material with similar lattice spacing. In this case the substance evaporates and diffuses from hot end to a cooler growth end. It deposits in the form of single crystals. Bulk crystals can be obtained in this method. Films can be obtained by the close-spaced transport method and decomposition of compounds. Crystals of silicon, diamond, gas, semiconductor compounds can be grown by this method CRYSTAL GROWTH FROM SOLUTION Materials, which have high solubility and have variation in solubility with temperature, can be grown easily by solution method. In this method, crystals are grown from aqueous solution. This method is widely used for producing bulk crystals (Brice 1986). The four major types are 1. Low temperature solution growth 2. High temperature solution growth 3. Hydrothermal growth 4. Gel growth Low temperature solution growth The method of crystal growth from low temperature aqueous solutions is extremely popular in the production of many technologically important crystals. The low temperature solution growth technique is well suited to those materials which suffer from decomposition in the melt or in the solid at high temperatures and which undergo structural transformations while cooling from the melting point and as a matter of fact numerous organic and inorganic materials which fall in this category can be crystallized using this technique (Nye 1985). The main disadvantages of the 45

7 low temperature solution growth are slow growth rate in many cases and the ease of solvent inclusion into the growing crystal. This is a widely practiced method. The techniques used here are 1. Slow cooling method 2. Solvent evaporation method 3. Temperature gradient method The solvent used here are water, ethyl alcohol, acetone, etc. The temperature is maintained under control throughout the process and hence the whole growth must be placed in a temperature controlled room (or) thermostat Slow cooling method A saturated solution above the room temperature is poured in a crystallizer and thermally sealed. A seed crystal is suspended in the solution and the crystallizer is kept in a water thermostat, whose temperature is reduced according to a preassigned plan, which results in the formation of large single crystals. The need to use a range of temperatures is the origin of disadvantages. The possible range is usually small so that much of the solute remains in the solution at the end of run. To compensate for this effect, large volumes of solution are needed. Among other techniques slow cooling is the technique used to grow bulk single crystals from solution. In this technique, supersaturation is achieved by a change in temperature usually throughout the crystallizer. The crystallization 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 limited, the supersaturation requires systematic cooling. It is achieved by using thermostated crystallizer, which is selected based on the desired size of the crystals and the temperature dependence of the solubility of the substance Solvent evaporation method This method used the similar apparatus as that of the above but in this method super saturation is achieved by evaporating the solvent at a fixed temperature. In this method, an excess of a given solute is established by utilizing the difference 46

8 between rates of evaporation of the solvent and the solute. In contrast to the cooling method, in which the total mass of the system remains constant, in the solvent evaporation method, the solvent evaporation method enables the total mass of the system as constant and losing the particles which are weakly bound to other compounds cause reduction in volume of the solution. In almost all cases, the vapour pressure of the solvent above the solution is higher than the vapour pressure of the solute. Thus the solvent evaporates more rapidly and the solution becomes supersaturated. This technique is similar to the slow cooling method in terms of apparatus requirements. The temperature is fixed and provision is made for evaporation. With non-toxic solvents like water, it is permissible to allow evaporation into the atmosphere. Typical growth conditions involve a temperature stabilization of about 0.05 C and rates of evaporation of a few mm 3 /h. The evaporation technique has an advantage that the crystals grow at a fixed temperature. Changing the temperatures controls the rate of evaporation. But inadequacies of the temperature control system still have a major effect on the growth rate. This method can effectively be used for materials having moderate temperature coefficient of solubility. Substances that are moderately soluble at room temperature, and which are either not much more soluble or are unstable at higher temperatures are good candidates for crystal growth by slow evaporation Temperature gradient method Here, the transport of material from a hot region containing the source of the material to be grown, to a cooler region where solution is super saturated result in the formation of crystal growth. A smaller variation in the temperature between the source and the crystal has larger effects on growth rate. This method involves transport of materials from hot region containing the source material to be grown to a cooler region, where the supersaturation is achieved and the crystal grows. The main advantages of this method are that Crystals grows at fixed temperature. They are insensitive to changes in temperature provided both the source and growing crystal undergo the same change. 47

9 The economy of solvent and solute High temperature solution growth Hydrothermal implies conditions of high pressure as well as high temperature. Growth is usually carried out in steel autoclaves with gold or silver linings. Depending on the pressure the autoclaves are grouped into low, medium and highpressure autoclaves. The concentration gradient required to produce growth is provided by a temperature difference between the nutrient and growth areas. In hightemperature solutions, the constituents of the material to be crystallized are dissolved in a suitable solvent and crystallization occurs as the solution becomes critically supersaturated. The solvents are considered generally effective at temperatures above room temperature. Also the concepts of low temperature solution growth are applicable equally well (Elwell et al., 1975) Hydrothermal growth This is regarded as an intermediate case between growths from the vapour and solution. This is a type of growth from aqueous solution at high temperature and pressure. Here production of crystal is from supersaturated fluid. The liquid starts usually alkaline aqueous solutions. Temperatures are typically in the range ºC and the pressure involved is large ( atmosphere). Growth is usually carried out in steel autoclaves with gold or silver linings. The concentration gradient required to produce growth is provided by temperature difference (usually ºC) between the nutrient and growth areas. The materials like calcite, alumna, antimony, etc., which is insoluble in water at ambient conditions can be grown using this method. It occurs in air at a temperature much lower than the melting of the crystallizing substance. Films can be grown by this method. Single crystals of diamond, ironyttrium garnet and barium titanates are grown by using this method. 48

10 Disadvantages At elevated temperature of crystal growth there are some disadvantages. When the temperature of the crystal growth is reduced from higher temperature to lower temperature occur a breakage of crystal. Sometimes the crucible in which the substance is kept may react with the crystal Growth from Gel This is regarded as an intermediate case between growth in solid and in solution. When the growth of mono crystals by usual techniques have problems due to decomposition before melting or non-availability of suitable flux then they can be grown by the diffusing in gels. 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 having dimensions of several millimeters can be grown within two to three weeks and a simple apparatus is used as a tube containing the appropriate solution where the reactions. The crystals grown by this technique have high degree of perfection and fewer defects since the growth takes place at room temperature CRYSTAL PERFECTION The perfection of the final crystal is based on 1. The purity of the starting material 2. The quality of the seed crystal 3. Cooling rate employed 4. The efficiency of agitation Hence, high quality crystals can be grown from quality seeds in an efficiently stirred solution. aaaaa 49