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

We have studied a series of optimized glow discharge a-Si1-xGex:H films wit h Ge content 0.02 less than or equal to x less than or equal to 0.2. The dr ive level capacitance profiling method indicated defect densities <5 x 10(1 5) cm(-3) in these samples. Modulated photocurrent measurements detected tw o defect bands at 0.68 +/- 0.05 and 0.79 +/- 0.05 eV, compared to the singl e 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 ident ified these defect bands as Si and Ge neutral dangling bonds. The Ge dangli ng 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 th e density of Si dangling bonds was larger for all the alloys in this series . (C) 2000 Elsevier Science B.V. All rights reserved.