Effect of deposition conditions on the nucleation and growth of glow‐discharge a‐Si:H

In situ ellipsometry experiments have been used to study the initial nucleation and growth of glow‐discharge hydrogenated amorphous silicon (a‐Si:H) deposited on c‐Si substrates under different conditions. The substrate temperature (T_s), dc bias, and silane concentration in both hydrogen and inert gas diluents were varied. In the first ∼30–60 Å of film growth, deviations of the in situ ellipsometry data from models assuming thickness independent dielectric functions are observed for a‐Si:H prepared under conditions that lead to a low density of electronic defects. The form of the deviations in the in situ data appear to be consistent with the development and convergence of densely packed clusters with a lateral extent of ∼20–30 Å, not with a well‐dispersed distribution. The deviations disappear for T_s less than ∼200 °C and for flow ratios less than 1:20 (SiH₄):(diluent). The low T_s effect is attributed to the lack of development of the initial growth microstructure caused by (1) the low mobility and desorption rate of surface radicals and/or (2) the low average coordination number due to ineffective hydrogen elimination. The low SiH₄:diluent ratio effect is attributed to the lack of effective convergence of the initial growth microstructure since film formation may be limited, not by hydrogen elimination and surface reaction, but by radical flux arriving at the surface. The effect of deposition parameters on the bulk bonding density, surface roughness, and the interface structure is also presented. At very low SiH₄:diluent ratios, it has been instructive to consider the damage mechanisms induced by the pure diluent plasmas.