The effect of trace C impurities upon the density of states in amorphous film samples was studied. The deep defect densities and mobility-gap electronic structure were characterized by means of electron spin resonance, drive-level capacitance profiling, and transient photocurrent measurements. Good quantitative agreement was found between the defect densities which were deduced by using these 3 methods. This implied that the ratio of charged to neutral dangling bonds was no greater than 2:1. Light-induced changes in the mobility-gap electronic structure were also investigated. A small but appreciable increase in the density of light-induced defects was observed for samples which had C contents that were of the order of 1at%. The time to saturation of light-induced degradation of C-containing samples was also significantly increased. The results were explained in terms of the presence of C-related precursor sites and of a widening of the band gap with C additions.

T.Unold, J.Hautala, J.D.Cohen: Physical Review B, 1994, 50[23], 16985-94