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

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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.