Melt growth method
This type of method is the most commonly used, mainly including Czochralski method (also known as Chukraski method), crucible descending method, zone melting method, flame melting Law (also known as Werner Ye method) and so on.
Czochralski method
This method is one of the most important methods for growing single crystals from melts, and is suitable for mass production of large-scale perfect crystals. The heated crucible contains the molten material, and the seed rod carries the seed crystal into the melt from top to bottom. Because the melt near the solid-liquid interface maintains a certain degree of supercooling, the melt crystallizes along the seed crystal and follows The seed crystal gradually rises and grows into a rod-shaped single crystal. The crucible can be heated by high frequency induction or resistance. Semiconductor germanium, silicon, oxide single crystals such as yttrium aluminum garnet, gadolinium gallium garnet, lithium niobate, etc. are all grown by this method. The main factors that control the quality of crystals when applying this method are the temperature gradient of the solid-liquid interface, the growth rate, the crystal transformation rate, and the fluid effect of the melt.
Crucible descending method
Place the crucible full of materials in a vertical furnace. The furnace is divided into two parts, separated by a baffle. The material in the crucible is maintained in a molten state, and the temperature at the lower part is lower. When the crucible is slowly lowered in the furnace from the top to the lower part of the furnace, the material melt begins to crystallize. The shape of the bottom of the crucible is mostly tapered or with a narrow neck to facilitate seed crystal selection, and there are also hemispherical shapes to facilitate seed crystal growth. The shape of the crystal is consistent with the shape of the crucible, and large optical crystals such as alkali halogen compounds and fluorides are grown in this way.
Zone melting method
A polycrystalline material rod is passed through a narrow high temperature zone to form a narrow melting zone, and the material rod or heating body is moved to make the melting zone It moves and crystallizes, and finally the material rod forms a single crystal rod. This method can improve the purity of the single crystal material during the crystallization process, and can also make the dopant doped very uniformly. There are two types of zone melting technology: horizontal method and floating zone smelting relying on surface tension.
Flame melting method
The principle of this method is to use the flame of hydrogen and oxygen combustion to generate high temperature, so that the material powder is scattered through the flame and melted, and falls on a crystallization rod or seed crystal Head. Because the flame forms a certain temperature gradient in the furnace, the powder melt can be crystallized when it falls on a crystallization rod. The growth principle of the flame melting method: the small hammer hits the barrel to vibrate the powder, and the powder falls through the screen and the hopper. The oxygen and hydrogen are respectively mixed and burned at the nozzle through the inlet. The seed crystal is inserted at the upper end of the crystallization rod, and it descends through the crystallization rod. The falling powder melt can maintain the same high temperature level and crystallize.
This method is the most mature method for growing corundum and rubies, and produces many tons every year worldwide. The advantage of this method is that no crucible is used, so the material is not contaminated by the container, and crystals with a melting point of up to 2500 ℃ can be grown; the disadvantage is that the internal stress of the grown crystal is very large.
Solution growth method
This method can be determined according to the solvent. A wide range of solution growth includes aqueous solutions, organic and other inorganic solutions, molten salts, and solutions under hydrothermal conditions. The most common is to grow crystals from aqueous solutions. The main principle of growing crystals from a solution is to make the solution reach a supersaturated state and crystallize. The most common methods are as follows: ① Increase or decrease the temperature according to the characteristics of the solubility curve of the solution; ② Use evaporation and other methods to remove the solvent to increase the concentration of the solution. Of course, there are also other ways, such as using the difference in solubility between the stable phase and the metastable phase of certain substances, controlling a certain temperature, so that the metastable phase is continuously dissolved and the stable phase is continuously growing.
Aqueous solution method
Generally, a water bath crystal growth device is required to grow crystals from an aqueous solution, which includes a crystal holding rod that can ensure sealing and rotation so that the composition of the solution around the crystal interface can be To maintain uniformity, there is a solution in the crystal incubator, which is strictly controlled by the temperature of the water in the water bath to achieve crystallization. It is very necessary to master the appropriate cooling rate to make the solution in a metastable state and maintain the appropriate supersaturation.
For materials with a negative temperature coefficient or a small solubility temperature coefficient, the solution can be kept at a constant temperature, and the solvent can be continuously removed from the crystal incubator to make the crystal grow. This method is used to crystallize It is called the evaporation method. Many functional crystals, such as potassium dihydrogen phosphate, β lithium iodate, etc., are grown by the aqueous solution method.
Hydrothermal method
Under high temperature and high pressure, the material is dissolved by various alkaline or acidic aqueous solutions to reach supersaturation and then crystallize. The method of growing crystals is called hydrothermal growth. . This method is mainly used to synthesize crystals. Other crystals such as corundum, calcite, blue asbestos, and many oxide single crystals can be generated by this method. The key equipment for hydrothermal growth is the autoclave, which is made of high temperature and high pressure resistant steel. It uses a self-tightening or non-self-tightening sealing structure to keep the hydrothermal growth at a high temperature of 200-1000°C and a high pressure of 1000-10000 atmospheres. The raw materials needed for cultivating crystals are placed in the lower part of the autoclave at a higher temperature, while the seed crystals are hung on the upper part at a lower temperature. Because the autoclave contains a certain degree of fullness of the solution, and because of the temperature difference between the upper and lower parts of the solution, the saturated solution at the lower part is brought to the upper part by convection, and then supersaturated and crystallized on the seed crystal due to the low temperature. The solution of the precipitated solute flows to the lower high temperature zone to dissolve the culture material. Hydrothermal synthesis is to grow crystals through such cycles.
Flux method
This method refers to dissolving the crystal raw materials in a salt solvent that can melt at a lower temperature at a high temperature to form a uniform saturated solution, so it is also called molten salt Method. By slow cooling or other methods, a supersaturated solution is formed and crystals are precipitated. It is similar to general solution growth crystals. For many high melting point oxides or materials with high vapor pressure, this method can be used to grow crystals. The advantage of this method is that the temperature required for growth is lower. In addition, this method can be used to grow crystals for some materials that have non-identical melting (peritectic reaction) or phase change when cooled from high temperature. The successful growth of BaTiO3 crystal and Y3Fe5O12 crystal are representative examples of this method. When using this method, attention should be paid to the phase balance between the solute and the flux.
Vapor growth method
Generally, sublimation, chemical vapor transport and other processes can be used to grow crystals.
Sublimation method
This means that the solid becomes the gas phase directly after the temperature is increased, and the gas phase reaches the low temperature zone and directly condenses into crystals. The whole process does not go through the liquid crystal growth method . Some elements such as arsenic, phosphorus and compounds such as ZnS and CdS can be obtained by sublimation method.
Chemical vapor transport
This technique of growing crystals means that solid materials generate volatile compounds through the chemical reaction of the transport agent:
Solid + transport agent is a volatile compound
If the produced compound is used as a material source, through the reversible process of volatilization and deposition, and control, the crystal can grow in a certain area or on the substrate Come out. This technology is called chemical vapor transport. The typical nickel purification process is the chemical transport process.
Epitaxy
Also known as epitaxial growth, it refers to the growth of a single crystal thin layer on a single wafer. This thin layer must be structurally similar to the original crystal ( Called the substrate) to match. Epitaxy can be divided into homogeneous epitaxy and heterogeneous epitaxy. The epitaxy of a layer of silicon on a silicon wafer like a semiconductor material is homoepitaxial; if silicon is epitaxial on a white sapphire substrate, it is heteroepitaxial.
The methods of epitaxial growth mainly include vapor phase epitaxy and liquid phase epitaxy, as well as molecular beam epitaxy. Epitaxial growth is widely used in the development of semiconductor materials, and the development of magnetic bubble materials has also applied epitaxial methods.
Vapor phase epitaxy materials are deposited on a single crystal substrate in a vapor phase. This method of growing a single crystal film is called vapor phase epitaxy. Vapor phase epitaxy has two methods: open tube and closed tube. Silicon epitaxy and gallium arsenide epitaxy are mostly open-tube epitaxy.
Liquid phase epitaxy dissolves the material used for epitaxy in a solution to make it saturated, then soaks the single crystal substrate in the solution, and then makes the solution supersaturate, which causes the material to continue to Crystals precipitated on the substrate. Control the thickness of the crystalline layer to obtain a new single crystal thin film. Such a process is called liquid phase epitaxy. The advantages of this method are simple operation, lower growth temperature, and faster speed, but it is difficult to control the gradient of impurity concentration during the growth process. The epitaxial layer of semiconductor material gallium arsenide, the growth of magnetic bubble material garnet film, mostly use this method.