Funksional Kristal Materiallar

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The growth of crystals from melt is one of the most common and important methods for preparing large single crystals and single crystals of specific shapes.

Most of the single crystal materials needed in modern technical applications such as electronics and optics are prepared by melt growth methods, such as single crystal silicon, GaAs (gallium nitride), LiNbO3 (lithium niobate), Nd:YAG (neodymium-doped ytterbium aluminum garnet), Al2O3 (white gemstone) and certain alkaline earth metals and halogenated compounds of alkaline earth metals, etc.

Compared with other methods, melt growth usually has the advantages of fast growth and high purity and integrity of crystals. The simple principle of crystal growth by the melt method is to melt the raw material for crystal growth and solidify it into a single crystal under certain conditions. The melting of the raw material and the solidification of the melt are the two major steps.

The melt must be solidified in a directional manner under controlled conditions, and the growth process is accomplished by the movement of the solid-liquid interface. To grow crystals in the melt, the temperature of the system must be below the equilibrium temperature. The state in which the system temperature is below the equilibrium temperature becomes subcooling.

The absolute value of subcooling is the degree of subcooling, which indicates the magnitude of the subcooling of the system. The degree of subcooling is the driving force for crystal growth in the melt method. For a certain crystalline substance, the main factor that determines the crystal growth rate at a certain degree of subcooling is the relative size of the temperature gradient between the crystal and the melt.

Normal Temperature Solution Method

The growth of crystals from solution has the longest history and is widely used. The basic principle of this method is to dissolve the raw material solute in a solvent and take appropriate measures to cause a supersaturated state of the solution in which the crystals are grown. The solution method has the following advantages:

1. Crystals can be grown at temperatures well below their melting point. There are many crystals that decompose below their melting point or undergo undesired crystallographic transformations, and some have high vapor pressure when melting. The solution allows these crystals to grow at a lower temperature, thus avoiding the above problems. In addition, the heat source and growth vessel for making crystals grow at low temperatures are easier to choose.

2. Reduced viscosity. Some crystals are very viscous in the molten state and cannot form crystals and become glassy when they are cooled.

3. it is easy to grow into large, uniform crystals with a complete shape.

4. in most cases, the crystal growth process can be directly observed, which facilitates the study of crystal growth kinetics. The disadvantages of the solution method are the many components, the complexity of the factors affecting crystal growth, the slow growth rate, and the long period (usually it takes tens of days or even more than a year).

In addition, the solution method requires high precision in temperature control for crystal growth. The necessary condition for crystal growth by the solution method: the concentration of the solution is greater than the equilibrium concentration at that temperature, i.e. the degree of supersaturation. The driving force is the degree of supersaturation.

High-temperature solution method

The high-temperature solution method is an important method for growing crystals and was one of the means used in early alchemy. Growing crystals from a solution or a molten salt solvent at high temperatures allows the solute phase to grow at temperatures well below its melting point. This method has the following advantages over other methods:

1. strong applicability, as long as you can find the appropriate flux or combination of fluxes, you can grow single crystals.

2. many refractory compounds and in the melting point is very volatile or high temperature when the change in value or phase change materials, as well as non-identical composition of molten compounds, can not be directly from the melt to grow or can not grow a complete high-quality single crystals, flux method due to the growth of a low temperature, showing the The flux method shows a unique ability due to the low growth temperature.

Disadvantages of crystal preparation by the molten salt method:

slow crystal growth; not easy to observe; fluxes are often toxic; small crystal size; mutual contamination by multi-component fluxes.

This method is suitable for the preparation of the following materials:

(1) materials with high melting point;

(2) materials with phase transition at low temperature;

(3) components with high vapour pressure in the components. Basic principle: High-temperature solution method is a crystalline material dissolved in a suitable flux under high temperature conditions to form a solution, and its basic principle is the same as that of room temperature solution method. However, the choice of flux and the determination of the phase relationship of the solution is a prerequisite for the growth of crystals in the high temperature solution method.

Physical-chemical vapor phase method

The so-called gas-phase method for crystal growth is to convert the crystal material to be grown into gas phase through the process of sublimation, evaporation and decomposition, and then make it become saturated vapor through appropriate conditions and grow into crystal by condensation and crystallization. The characteristics of crystal growth by the gas phase method are:

1. high purity of the grown crystals;

2. good integrity of the grown crystals;

3. slow growth rate of the crystals;

4. a series of factors that are difficult to control, such as temperature gradient, supersaturation ratio, flow rate of the carrying gas, etc. Currently, the gas phase method is mainly used for whisker growth and the growth of epitaxial films (homogeneous and heterogeneous epitaxy), while the growth of large size bulk crystals has its disadvantages.

The vapor phase method can be divided into two main types: Physical

Vapor Deposition (PVD): the transformation of polycrystalline materials into single crystals by physical coalescence, such as sublimation-condensation, molecular beam epitaxy and cathodic sputtering;

Chemical Vapor Deposition (CVD): Transformation of polycrystalline raw materials into single crystals through the gas phase by chemical processes, such as chemical transport method, gas decomposition method, gas synthesis method and MOCVD method.