Rate-determining reactions and surface species in CVD of silicon: II. The SiH2Cl2-H2-N2-HCL system

The growth rate of CVD silicon from SiH2Cl2-nitrogen-hydrogen mixtures is found to be a linear function of the partial pressure of hydrogen at temperatures between 800 and 1000°C. At temperatures below 900°C, the SiH2Cl2-H2 system shows a saturation in growth rate for input concentrations above 0.5 vol%. This is at variance with the SiH2Cl2-N2 system, which shows a linear dependence between the growth rate and the input concentration of SiH2Cl2 at 800°C. Addition of HCl to the SiH2Cl2-H2 system reduces the growth rate at temperatures below 1000°C due to the formation of SiHCl3 in the gas phase. The formation of SiHCl3 is directly related to the dissociation of SiH2Cl2 in the gas phase into SiCl2 and H2. It is concluded that in a Si-H-N-Cl system the surface concentration of SiCl2 is always much larger than the Si adatom concentration. The reduction of SiCl2 at step sites is proposed to be the rate-limiting reaction, leading to a surface concentration of SiCl2 far above the equilibrium concentration and favouring desorption of SiCl2. The high SiCl2 concentration accounts for the saturation in growth rate as a function of input concentration of SiH2Cl2 in hydrogen; the linear increase in growth rate when nitrogen is used as a carrier gas can also be explained along these lines.