Defect bands in a-Si–Ge:H alloys with low Ge content

We have studied a series of optimized glow discharge a-Si1−xGex:H films with Ge content 0.02⩽x⩽0.2. The drive level capacitance profiling method indicated defect densities <5×1015 cm−3 in these samples. Modulated photocurrent measurements detected two defect bands at 0.68±0.05 and 0.79±0.05 eV, compared to the single band normally observed in pure a-Si:H. The magnitudes of these bands vary with Ge content and with the state of light-induced degradation. Based on electron spin resonance (ESR) measurements on matched samples we have identified these defect bands as Si and Ge neutral dangling bonds. The Ge dangling bond concentration was found to be larger in the annealed state for all samples, even at 2 at.% Ge. However, in the light-induced degraded state the density of Si dangling bonds was larger for all the alloys in this series.