The particles inside the single crystal are regularly and periodically arranged in the three-dimensional space, or in other words, the whole crystal is composed of the same space lattice in the three-dimensional direction. Pure element single crystals are solid substances that consist of structural motifs (atoms, atom groups, ions) arranged in long-range order within a three-dimensional space. The unidirectionally ordered arrangement determines that it has the following characteristics: uniformity, anisotropy, self-limitation, symmetry, minimum internal energy and maximum stability. Monocrystalline material is a new material that has become increasingly widely used. It consists of a single crystal and its diffraction pattern is a regular lattice. The entire crystal lattice of the single crystal is continuous and has important industrial applications. This article mainly introduces several common methods for the preparation of single crystal materials.
2 The Preparation of Single Crystals
The preparation of a single crystals is also referred to as the growth of a crystal, and is a process of converting a material in amorphous or polycrystalline state, or a reactant capable of forming the substance, into a single crystal by certain chemical means. The crystallized material is first converted into a melt or a solution by melting or dissolving, then control its thermodynamic conditions to form a crystalline phase and let it grow. With the rapid development of the discipline theory and practice of crystal growth, the means of crystal growth are changing with each passing day. The preparation of single crystals can generally be divided into melt growth method, solution growth method and phase growth method.
2.1 Melt growth method
The method of growing crystals from the melt is the earliest research method, and it is also a widely used synthesis method. The methods for growing crystals from the melt include the flame fusion method, the Czochralski method, and the zone melting method.
2.1.1 Flame fusion method
The flame fusion method is a method of growing a single crystal substrate from the melt. The raw material powder is melted after passing through a high-temperature hydrogen-oxygen flame, and the droplets cool during the falling process and solidify on the seed crystals to gradually grow to form crystals. The vibrator causes the powder to pass through the high-temperature zone generated by the oxyhydrogen flame from top to bottom at a certain rate. The powder melts and falls on the seed crystal to form a liquid layer. The seed crystal moves downwards to crystallize the liquid layer. This method is mainly used to prepare crystals such as gemstones.
2.1.2 Czochralski method
The Czochralski method is a commonly used method for growing crystals from the melt. Using this method can pull out a variety of crystals, such as single crystal Si, Ge and most of the laser crystals. In the 1960s, Czochralski method further developed to a more advanced method of stereotyped crystal growth-EFG (edge-defined film-fed crystal growth method). It is a pulling method that controls the shape of a crystal, ie, a technique of growing crystals having various cross-sectional shapes directly from the melt. The Czochralski method is to heat and melt the raw materials in a crucible, and then pulls the seed crystals on the surface of the melt. Under controlled conditions, the rearrangement of atoms or molecules is continuously performed at the interface between the seed crystal and the melt, and a single crystal gradually grows as the temperature decreases.
2.1.3 Zone melting method
The prominent feature of the zone melting method is that it does not need to contain molten silicon, instead, it depends on the surface tension and electromagnetic force of silicon to support the locally melted silicon liquid in high-frequency electromagnetic field. Therefore, the zone melting method is also referred to as float-zone method. The biggest advantage of the zone melting method is that its energy consumption is reduced by more than 60% compared with the traditional methods. The biggest drawback of this method is that it is difficult to meet the requirements of high-purity electronic grade polysilicon. During the zone melting process, the solid and liquid phases of the material are transported because of the density difference. Therefore, soluble impurities or phases present in the raw materials can be controlled or redistributed by zone melting method. Using one or several melting zones in the same direction repeatedly through the raw material, sintering to remove harmful impurities and to effectively eliminate the segregation effect.
2.2 Solution growth method
A homogeneous mixture of two or more substances is called a solution and the solution consists of a solvent and a solute. The solutions used for the synthesis of crystals include normal temperature solutions (such as aqueous solutions, organic solutions, gel solutions, etc.) and high temperature solutions (ie molten salts).
2.2.1 Normal temperature solution method
In a solution using water, heavy water, or liquid organic substances as a solvent, it is possible to grow a uniform and large-sized single crystal. Under certain temperature conditions, the concentration of the solution is greater than the equilibrium concentration (ie, saturation concentration) at that temperature. This phenomenon is called “supersaturation” and its degree of excess is called “supersaturation”. It is the driving force for crystal growth in solution.
2.2.2 High temperature solution method
High-temperature solution methods including temperature differential method, falling temperature method or raising temperature method and isothermal method. At present, temperature differential method is mainly used for crystallization.
2.3 Phase growth method
Phase growth can be divided into single-component system growth and multi-component system growth. Vapor growth of a single component requires the vapor phase to have a sufficiently high vapor pressure to grow on the principle that the crystal vaporizes and sublimates in the high-temperature region and condenses and grows in the low-temperature region. However, this method is not widely used, and most of the crystals grown are acicular and flaky single crystals. Multicomponent vapor growth is generally used for epitaxial thin film. It mainly used in the production of electronic components, magnetic memory devices, photonic and optical devices.
There are many ways to prepare single crystals, but the principle is similar to the above methods. With the development of science, single crystals have become more and more widely used in daily life and industry, and thus the preparation methods of single crystal will become more industrialized.