Epitaxial growth is actually mainly a chemical reaction process. The main sources of gas used for silicon epitaxial growth are hydrogen and chlorosilanes such as SiCl4, SiHCl3, and dichlorosilane (SiH2Cl2). In addition, silane is often used as a gas source in order to reduce the growth temperature. The selection of air source is mainly determined by the growth conditions and the epitaxial layer specifications, among which the growth temperature is the most important factor to be considered when choosing the type of air source. Relationship between growth rate and temperature of silicon epitaxial layer. The figure shows two obviously different growth regions. In the low temperature region (region A), the growth rate of silicon epitaxial layer is exponential with the temperature, indicating that they are controlled by the surface reaction. In the high temperature region (region B), there is almost no direct relationship between their growth rate and temperature, indicating that they are controlled by mass transport or diffusion. It is important to note that silicon thin films grown at low temperatures are polycrystalline. The formation temperature of silicon epitaxial layer is above the turning point of each curve. The turning point temperature varies with the molar ratio of reactants, the flow velocity and the type of reactor. It can be inferred from the figure that when SiH4 is used as the air source, the temperature of silicon epitaxial layer is about 900℃, while when SiCl4 is used as the air source, the temperature of silicon epitaxial layer is about 1 100℃.

      It is important to note that reduction and corrosion processes are competitive, depending on the molar ratio of the reactants and the growth temperature. At atmospheric pressure, with SiCl4 and H2 as reactants and at a total pressure of 1.01×l05Pa (1 atmospheric pressure), the boundary between corrosion and deposition is related to growth temperature and SiCl4 partial pressure. In addition, the relationship between growth rate and temperature was also given when SiCl4 and H2 were used as silicon epitaxial reactants, as shown in FIG. 2.2-31. It can be seen from the figure that corrosion processes occur at low and high temperatures. Therefore, in this case, the epitaxial temperature is usually chosen at 1, 100 ~ 1300℃. In order to obtain a thicker epitaxial layer, SiHCl3 is usually selected as the gas source, mainly because its deposition rate is faster than SiCl4.

      SiCl4 as an epitaxial gas source involves different chemical reactions. The thermal decomposition reaction with SiH4 gas source is irreversible. The process can be compared with any other chlorosilane. The main advantage of SiCl4 is that silicon epitaxial layers can be obtained at relatively low temperatures. However, due to the homogeneous reaction of silane, it is difficult to avoid the gas nucleation of silicon. Therefore, silicon particles will be formed during the growth process, resulting in rough surface morphology and even polycrystalline growth. This problem can be solved by controlling the growth temperature or by growing at low pressure. Silane is a gas that is easy to oxidize and explode, so it is not often used in conventional silicon epitaxy. Moreover, there is no HCl in the silane gas source growth process, so there is no corrosion of this process, resulting in a higher concentration of metal impurities in the epitaxial layer. Therefore, the use of silane as an epitaxial gas source requires a very careful pre-cleaning process.