Various defect states ina-Si:H studied by modulated photoconductivity spectroscopy

Abstract

Various kinds of defect state distributions dominating electron trapping and recombination in a-Si:H films from different laboratories are studied by analyzing modulated photocurrent measurements obtained by varying the frequency or the bias light level. The characteristic features of the experimental modulated photocurrent spectra are compared with those predicted theoretically and a clear physical meaning is obtained. Two kinds of defect states are extracted directly from the spectra without prior assumptions about the form of the density of states. The energetic distributions of these defect states at the quasi-Fermi level and above it are calculated and their magnitude was found to differ by about an order of magnitude, while their capture coefficients differ by as much as 2--3 orders of magnitude. We assign the two kinds of defects to silicon dangling bonds with three backbonded silicon atoms and silicon dangling bonds where one backbond is substituted by hydrogen.