Gap state distribution in amorphous hydrogenated silicon carbide films deduced from photothermal deflection spectroscopy

摘要

The density of gap states distribution in silicon (Si) rich hydrogenated amorphous silicon carbide (a-Si_(1-x)C_(x):H) films with varying carbon (C) fraction (x) is investigated by the photothermal deflection spectroscopy (PDS). The films are grown using the Electron Cyclotron Resonance Chemical Vapor Deposition (ECR-CVD) technique. By using different methane-to-silane gas flow ratios, a-Si_(1-x)C_(x):H with x ranging from 0 to 0.36 are obtained. A deconvolution procedure is performed based on a proposed DOS model for these Si rich a-Si_(1-x)C_(x):H. Good fits between the simulated and experimental spectra are achieved, thus rendering support to the model proposed. Deduction of the DOS enables us to obtain various parameters, including the optical gap and the valence band tail width. The fitted mobility gap E_(g) is found to be well correlated to the Tauc gap E_(tauc) and E_(04) gap deduced from the optical absorption spectra. A correlation is also seen between the fitted valence band tail width E_(vu), the Urbach energy E_(u) and the defect density. All these parameters are seen to increase with C alloying. A shift in the defect energy level in the midgap with increasing C incorporation is observed, together with a broadening of the defect distribution and a stronger correlation between the defect bands, which can be accounted for in terms of the influence of C dangling bonds on the deep defect density distribution.