The quality of the coating is mainly manifested in the purity of chemical composition, integrity of crystal structure and physical and chemical properties.
1. Influence of partial pressure ratio of reaction mixture
The partial pressure ratio of the reaction mixture is one of the important reasons for determining the quality of the coating. The necessary condition for gas phase plating is to have a certain degree of gas phase supersaturation. The partial pressure of the gas phase species determines the nucleation and growth of the solid phase, which determines the deposition rate and the structural condition of the material. Usually, the partial pressure of the reactant is too high, because the surface reaction and nucleation are too fast and the structural integrity , and even lead to polycrystalline deposition; if the partial pressure is too small, the nucleation density is too small, and it is not easy to obtain a uniform epitaxial layer. When depositing compounds, the mutual ratio of the partial pressures of the reactants is particularly important, as it directly determines the composition of the deposit.
2. The influence of deposition temperature
Deposition temperature is one of the most important process conditions. For the same reaction system, at different temperatures, the formed coating can be single crystal, polycrystalline, amorphous, or even no coating. Generally, increasing the deposition temperature can lead to the conversion of surface control to mass transfer control, which is beneficial to improving the single crystal and surface morphology of the epitaxial layer. However, if the temperature is too high, the growth rate will be reduced, and the impurity contamination from the reactor material will be aggravated. Moreover, the deposition temperature will affect the supersaturation of the gas phase and the relative activity of the deposited solid phase, thereby affecting the chemical composition of the coating.
3. The influence of the substrate material
The substrate is a key factor affecting the quality of the coating, therefore, what material to choose as the substrate is an important issue . The quality of coatings obtained on substrates of different materials can vary greatly. When selecting heterogeneous epitaxial substrate materials, attention should be paid to the fact that at the deposition temperature, thermal decomposition does not occur, and it is not easily corroded by the reaction atmosphere. The lattice type, lattice constant and thermal expansion coefficient should be as close as possible to the epitaxial material, with good thermal conductivity, thermal shock resistance, and easy cutting, grinding, polishing and chemical cleaning.
The crystal plane orientation of the substrate will affect the nucleation and growth of new atoms on it, and it must be strictly controlled. Epitaxy on a crystal plane that deviates from a low-index crystal plane at an appropriate angle (2 ° ~ 5 °) can obtain better quality single crystal layer. Before epitaxy, the substrate needs to be subjected to strict cutting, grinding, and polishing mechanical damage to expose new crystal surfaces, which is conducive nucleation and growth.
4. The influence of total pressure and total gas flow in the system
In the reaction system, low-pressure deposition can improve the uniformity and adhesion of the coating. If the pressure is too low, the deposition rate will be low; if the pressure is too high, the coating will be rough and the adhesion will be poor. In the reaction system, the total flow of gas determines the control steps of the reaction process. Generally, increasing the total flow rate of gas will transform the reaction process from mass transfer control to surface control, which will significantly increase the growth rate.
5. The influence of the reaction device
Oxygen, water, etc. will have adverse effects on the reactive layer at high temperatures, so the reaction device is required to be well sealed , and the joints between the valve and the reaction pipeline should be minimized, and it is better to use a hard connection. The design of the reaction tube structure should be reasonable, it will affect the deposition rate and the uniformity of the coating.
6. The influence of raw material purity
The quality of the coating is closely related to the purity of raw materials (including carrier gas), so high-purity raw materials are required.
