Effect of carbon impurities on the density of states and the stability of hydrogenated amorphous silicon.

Abstract

The effect of trace carbon impurities on the density of states in amorphous silicon has been studied. The deep-defect densities and mobility-gap electronic structure were characterized with electron-spin resonance, drive-level capacitance profiling, and transient photocurrent measurements. Good quantitative agreement in the defect densities deduced from these three methods has been found. This implies a ratio between charged and neutral dangling bonds of at most 2 to 1. Light-induced changes in the mobility-gap electronic structure were also investigated in these films. A small but significant increase in the density of light-induced defects was observed for samples with carbon impurities at the 1 at. % level. The time to saturation of the light-induced degradation for the carbon containing samples was also significantly increased. We discuss the interpretation of these results in terms of two possible mechanisms: either from the presence of carbon-related precursor sites, or by widening of the band gap with carbon alloying.