Use certain methods and techniques to make single crystals grow from liquid or gaseous crystals
In principle, single crystals can be made of solidliquid state(melt or solution) or gaseous growth.In fact, most of the artificial crystals reach a certain level from the meltSupercoolingOr the solution reaches a certain supersaturation.Crystal growth is to use certain methods and techniques to make single crystals grow from liquid or gaseous crystals.Liquid crystallization can be divided into two categories: melt growth and solution growth.[1]
This method is one of the most important methods for single crystal growth from melt, and is suitable for batch production of large size perfect crystals.The heated crucible is filled with molten material, and the seed crystal rod with seed crystal is inserted into the melt from top to bottom. Since the melt near the solid-liquid interface maintains a certain degree of supercooling, the melt crystallizes along the seed crystal and grows into a rod shape with the gradual rise of the seed crystalsingle 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 garnetlithium niobateAre grown by this method.The main factors to control the crystal quality are the temperature gradient at the solid-liquid interface, the growth rate, the crystal rotation rate and the fluid effect of the melt.[1]
Crucible descending method
Place the crucible full of materials in a vertical furnace. The furnace is divided into two parts, the upper part and the lower part. The middle part is separated by a baffle. The upper part has a higher temperature, which can keep the materials in the crucible in melting state, while the lower part has a lower temperature. When the crucible slowly drops from the top to the lower part of the furnace, the material melt begins to crystallize.The bottom of the crucible is mostly tapered or has a thin neck, which is convenient for selecting seed crystals, and also has a hemispherical shape for seed crystal growth.The shape of the crystal is consistent with that of the crucible. Large alkali halide compounds and fluoridesOptical crystalIt grows in this way.[1]
Zone melting method
Put aPolycrystalline materialThe bar, through a narrow high temperature zone, makes the material form a narrow melting zone, moves the material bar or heating body, makes the melting zone move and crystallize, and finally the material bar forms a single crystal bar.This method can makeSingle crystal During crystallizationpurityIt is raised very high, and can also make the dopant mixed evenly.Zone melting technology includes horizontal method and floating zone melting depending on surface tension.[1]
Flame fusion
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 and melted through the flame and falls on the head of a crystal rod or seed crystal.Because the flame forms certaintemperature gradientThe powder melt can crystallize when it falls on a crystallization rod.Flame fusionGrowth principle: small hammer strikes the cylinder to vibrate the powder, which falls through the screen and hopper. Oxygen and hydrogen are mixed and burned at the nozzle through the inlet respectively. Seed crystals are inserted at the upper end of the crystal rod, which falls through the crystal rod, so that the fallen powder melt can keep the same high temperature level and crystallize.[1]
This method is the most mature for growing corundum and ruby, and produces many tons of corundum and ruby every year worldwide.The advantage of this method is that no crucible is needed, so the material is not polluted by the container, and crystals with melting point up to 2500 ℃ can be grown;The disadvantage is that the grown crystalinternal stressVery big.[1]
Solution growth method
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This method can be determined according to the solvent.Extensive solution growth includes aqueous solutions, organic and other inorganic solutions, molten salts and solutions under hydrothermal conditions.The most common isaqueous solutionGrow crystals.The main principle of crystal growth from solution is to make the solution reach supersaturated state and crystallize.The most common methods are as follows: ① According to theSolubility curveIncrease or decrease its temperature; ②Remove the solvent by evaporation to increase the concentration of the solution.Of course, there are other ways, such as using the solubility difference between the stable phase and the metastable phase of some substances to control a certain temperature, so that the metastable phase can be dissolved continuously and the stable phase can grow continuously.[1]
Aqueous solution method
Generally, crystal growth from aqueous solution requires a water bath crystallization device, which includes a crystal rod that can ensure both sealing and rotation to keep the composition of solution around the crystallization interface uniform. A solution is installed in the crystal incubator, and its temperature is strictly controlled by the temperature of water in the water bath to achieve crystallization.Master the proper cooling rate to keep the solution inMetastable stateIt is necessary to maintain proper supersaturation.[1]
For withNegative temperature coefficientOr the material with smaller solubility temperature coefficient can keep the solution constant temperature, and constantly remove the solvent from the crystallizer to make the crystal grow. This method of crystallization is called evaporation method.Many functional crystals such asPotassium dihydrogen phosphate、βLithium iodateAre all created byaqueous solutionIt is obtained by growing.[1]
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, which is called hydrothermal growth method.This method is mainly used toSynthetic crystalOther crystals such as corundum, calcite, blue asbestos and many oxidessingle crystalCan be generated in this way.The key equipment of hydrothermal growth is autoclave, which is made of high temperature and pressure resistant steel.It maintains the hydrothermal growth at the high temperature of 200~1000 ° C and the high pressure of 1000~10000 atmospheric pressure through the self tightening or non self tightening sealing structure.The raw materials needed for crystal culture are placed at the bottom of the autoclave with slightly higher temperature, while the seed crystal is suspended at the upper part with slightly lower temperature.Because the autoclave is filled with a certain degree of solution, and because of the temperature difference between the upper and lower parts of the solutionSaturated solutionIt is brought to the upper part by convection, and then forms supersaturation due to low temperatureCrystallizationOn the seed crystal.The solution of precipitated solute flows to the lower high temperature zone to dissolve the culture material.Hydrothermal synthesis is to grow crystals through such a cycle.[1]
Flux method
This method refers to dissolving crystal raw materials in a salt solvent that can be melted at a lower temperature under high temperature to form a uniform saturated solution, so it is also called molten salt method.By slow cooling or other methodsSupersaturated solutionAnd precipitate crystals.It is similar to the general solution growth crystal.This method can be used to grow crystals for many oxides with high melting point or materials with high evaporation pressure.The advantage of this method is that the temperature required for growth is low.In addition, melting of some materials with different compositions（Peritectic reaction）Or when cooled by high temperaturephase transitionAll materials can grow crystals in this way.The successful growth of BaTiO3 crystal and Y3Fe5O12 crystal are representative examples of this method. When using this method, pay attention to the phase balance between solute and flux.[1]
Vapor phase growth method
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Generally, crystals can be grown by sublimation, chemical vapor transport and other processes.[1]
Sublimation method
This means that the solid will directly become the gas phase after rising the temperature, and the gas phase will directly coagulate into crystals when it reaches the low temperature zone. The whole process will not go through the liquid crystal growth mode.Some elements such as arsenic, phosphorus and compounds such as ZnS and CdS can be obtained by sublimationsingle crystal。[1]
Chemical vapor transport
This technology of crystal growth means that the solid material generates volatile compounds through the chemical reaction of the transport agent:
Solid+Transport Agent Volatile Compounds
If the generated compound is used as the material source, through the reversible process of volatilization and deposition and control, the crystal can grow in a certain area or on the substrate.This technology is called chemistryGas phaseTransportation.A typical nickel purification process is a chemical transport process.[1]
extension
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It is also called orientation epiphyte, which refers to the growth of a single crystal thin layer on a single crystal chip. This thin layer should match the original crystal (called the substrate) in structure.Epitaxy can be divided into homoepitaxy and heteroepitaxy.Like the silicon wafer of semiconductor material, another layer of silicon is homogeneous epitaxy;If silicon is epitaxial on a sapphire substrate, it is heteroepitaxy.
The methods of epitaxial growth mainly include vapor phase epitaxy, liquid phase epitaxy andmolecular beam epitaxy Etc.epitaxial growth It is widely used in the development of semiconductor materials,Magnetic bubble materialThe extension method is also applied in the development of.
Vapor phase epitaxy materials are deposited on single crystal substrates in the vapor phase. This method of growing single crystal films is called vapor phase epitaxy. Vapor phase epitaxy has two ways: open tube and closed tube. Silicon epitaxy and gallium arsenide epitaxy in semiconductor preparation mostly use open tube epitaxy.
Liquid phase epitaxy dissolves the material used for epitaxy in the solution to make it saturated, and then immerses the single crystal substrate in this solution to make the solution supersaturated, which causes the material to continuously precipitate crystals on the substrate.Control the thickness of crystalline layer to obtain newsingle crystalfilm.This process is called liquid phase epitaxy.The advantages of this method are simple operation, low growth temperature and fast growth rate, but it is difficult to control the gradient of impurity concentration during the growth process.Epitaxial layer of semiconductor material GaAs,Magnetic bubble materialMost garnet films are grown in this way.[1]
