Material science Paper No. : Crystallography & crystal growth Module : Growth from melt II Development Team Prof. Vinay Gupta, Department of Physics and Astrophysics, Principal Investigator University of Delhi, Delhi Prof. P. N. Kotru ,Department of Physics, University of Jammu, Paper Coordinator Jammu-180006 Content Writer Prof. P. N. Kotru ,Department of Physics, University of Jammu, Jammu-180006 Prof Mahavir Singh Department of Physics, Himachal Pradesh Content Reviewer University, Shimla 1 Crystallography & crystal growth Material science Growth from melt II Description of Module Subject Name Physics Paper Name Crystallography & crystal growth Module Name/Title Growth from melt II Module Id 31 2 Crystallography & crystal growth Material science Growth from melt II 31 Bridgman-Stockbarger Growth Technique. 31.1 Introduction The techniques were originated by Bridgman (1925) and Stockbarger (1938) and so are named after them. In these techniques a crucible containing the material to be grown is lowered through a furnace in such a way that the lowest point in the crucible and the solidification surface rises slowly up the crucible. It means that the melt contained in the crucible is progressively frozen to yield a single crystal. The rate of lowering the crucible may range from about 0.1 to 200 mmh─1 but in most of the cases it may range somewhere in between 1-30 mmh─1. There are situations where the movement of the crucible is reversed. In other words, the crucible is raised up through the furnace and so is advantageously applicable for materials which are volatile; the interface with the vapour being the coolest part of the charge. In this case, it is important that the material does not have different densities when in the solid and liquid phases. Suppose the solid material has lower density than its melt, the crucible containing the charge is likely to crack. However, if the solid has a greater density than its melt, it will lead to differential dimensions of parts of the growing crystal with respect to the crucible. These are the problems with such materials and can be overcome by some specialized techniques which may, however, take the technique far from being a simple one. There is one important requirement of any crucible based technique and so applies to Bridgeman technique also. It is that neither the liquid nor its vapour attacks the crucible. If there is any possibility of crucible getting attacked, its slow dissolution will contaminate the growing crystal. Also, it is important that the crystal and crucible have the same thermal expansion coefficients. If it is not so, the adhering crystal will get strained which will adversely affect its utility. There have been several modifications in the basic technique of growth right from its very origin e.g., Stober (1925); Kapitza (1928); Pfann (1958) and several others. We shall, however, discuss here only the Bridgman technique. 31.2 Basic Principle. The basic principle behind this technique is normal freezing , also known as directional freezing, in which the ingot of the concerned composition is gradually frozen from one end to the other as shown in a schematic diagram of figure 31.1 3 Crystallography & crystal growth Material science Growth from melt II Figure 31.1: Schematic diagram revealing the principle of directional freezing or normal freezing. In this technique of growth a crucible is used and the directional solidification is carried out in order to achieve single crystal growth from melt. There are a number of ways in which this method is applied and are known as Bridgman technique, Stockbarger technique, Tammann technique, Stober technique and Obreimov and Schubnikov technique. The basic principle used in all these techniques is however, the same as applied by Bridgman for the growth of fluorite crystals. 31.3 Apparatus Used. The apparatus used in Bridgman technique is shown in figure 31.2 4 Crystallography & crystal growth Material science Growth from melt II Figure 31.2: Schematic diagram showing the experimental arrangement of Bridgman technique of crystal growth The furnace used here consists of two halves, the lower half and the upper half. The temperature maintained at the lower half is just below the melting point of fluorite and the temperature of the upper half is above the melting point of fluorite. The crucible is made up of platinum and has a pointed lower end as is shown in the diagram. The crucible is filled with high quality natural fluorite, or purified powder if required, to which desired impurities may also be added. A platinum wire hangs the crucible in the upper furnace until the contents are completely melted. This crucible is then lowered from the upper into the lower part of furnace over a period of several days. The lowering is done with the help of an electric motor and reduction gearing system. On lowering, since the tip of the crucible enters the lower furnace first the fluorite will start to crystallize there. It is how the nucleation problem is overcome. With the continuous lowering of the crucible, the crystallization proceeds until 5 Crystallography & crystal growth Material science Growth from melt II all the contents get solidified. With the temperature kept constant, crucible shaped correctly and with the perfectly uniform lowering, the crucible contents will end up as single crystals of fluorite. In general, the material to be crystallized is contained in a cylindrical crucible which is lowered through a two zone vertical furnace (shown in figure 31.3). The temperature profile of the furnace is arranged to have the form as shown in figure 31.3.This method, however, cannot be used for materials which expand on solidification. 31.4 Examples of Some Crystals Grown By Bridgman Technique There are several materials which have been successfully grown by Bridgman technique. Only a few may be given here as examples, besides those which have been mentioned at different places in this discussion. These are: 1. Al2O3 crystals having melting point of 2037°C have been grown using Molybdenum crucible. 2. FeAl2O3 crystals having melting point of 1790°C have been grown using iridium crucible. 3. Ge crystals having melting point of 937°C have been grown using carbon coated silica. 4. Cu crystals with melting point1083°C have been grown using crucible made of graphite powder. 6 Crystallography & crystal growth Material science Growth from melt II Figure 31.3: Schematic diagram showing two zone vertical furnace alongwith temperature profile used in Bridgman technique of crystal growth The technique is best suited for low melting point materials. Many of the materials grown by this technique are NaCl (halite), KCl (sylvite), CaF2 (Fluorite), CaWO4 (Scheelite), AgCl (Cerargyrite), AgBr (Bromyrite) and several other metals and semiconductors. This was the method used by Bridgman and others for the growth of large metal single crystals and later by Stockbarger for the growth of optical quality alkali halide crystals for prisms and lenses. It was developed independently in Europe by Tammann and Obreimov and Schubnikov. 31.5 Crystal Growth by Zone Melting. 31.5.1 Introduction. 7 Crystallography & crystal growth Material science Growth from melt II This technique was developed by Pfann of Bell Telephone Laboratory in 1952. Initially this technique was applied for the purification of semiconductor materials. It can be classified as a solid-liquid-solid process and is used both as crystal growth and a purification technique. 31.5.2 Basic Apparatus. The apparatus used for the growth of crystals by this method is shown in figure 31.4.The material to be grown as crystal is placed in a long boat with dimensions of 1 foot in length and 1 inch in diameter, so that it is completely filled. Then a section of the material is melted with the help of narrow heating coils. The molten material is now traversed along the boat by moving either the boat or the heating coil. As heating coil proceeds the material melts leaving material behind it to solidify. The process of travelling of the heating coil continues with the melting of new material in front and solidifying to form crystalline material at the back. After several hours of this process and under suitable conditions the contents of the boat end up as one single crystal. For purification many more runs are performed repeatedly till the desired results are obtained. Figure 31.4: Schematic diagram showing arrangement for zone melting technique of crystal growth 31.5.3 Floating-Zone Process. A modification of the above said technique was given by Keck and Golay in 1953. The modified version is known as “floating zone technique”. In this modified technique the material to be 8 Crystallography & crystal growth Material science Growth from melt II purified or grown as a single crystal is arranged in a vertical compact rod as shown in figure 31.5. Figure 31.5: Schematic diagram showing arrangement of floating zone technique using vertical ingot This technique does not require any crucible. The molten zone floats below two solid parts of the rod held in place by surface tension. Since it does not require retaining crucible, the technique has the advantage that possible contamination from the container is avoided. It is used for the growth of silicon which is the only material grown on a very large scale around 1000 tonnes per year. It is also used for the growth of very pure crystals, mostly metals but on a small scale just for research purposes. Because of radio-frequency heating, the surface of the molten zone is heated and the liquid near to the melt- vapour surface is hotter than elsewhere. As a result, fluid flow is produced. The shapes of the solid-liquid surfaces are controlled by these flows and the rate movement of the zone.