Deposition mechanism of hydrogenated amorphous silicon

The surface and subsurface processes occurring during the growth of hydrogenated amorphous silicon $\mathrm{(a-}\mathrm{Si:H})$ are analyzed to understand how dangling bond defects and weak bonds form. It is found that the abstraction and addition of adsorbed ${\mathrm{SiH}}_3$ radicals gives a surface defect density which decreases continuously with decreasing temperature with no minimum near 250 °C. Hence it cannot be the process that defines defect densities in the bulk. Hydrogen elimination to create the bulk Si–Si network occurs because the chemical potential of hydrogen causes the expulsion of hydrogen from the bulk. Hydrogen elimination is the rate-limiting step at lower temperatures, as its diffusion is slow. The difficulty of eliminating hydrogen leads to the formation of weak bonds. Weak bonds arise at higher deposition temperatures from thermal disorder. The dangling bond defects arise from weak bonds by the defect pool process, and this process must continue at lower temperatures than normal in the growth zone. Plasma processes which dehydrogenate the surface layers, such as ion bombardment, can lower weak bond densities.