The effects of native and light induced defects on optoelectronic properties of hydrogenated amorphous silicon-germanium (a-SiGe:H) alloy thin films

Abstract

Effects of native and light induced defects states in hydrogenated amorphous silicon-germanium alloy thin films with different Ge concentrations have been investigated by using steady-state photoconductivity, dual beam photoconductivity (DBP), transmission spectroscopy and photothermal deflection spectroscopy (PDS) techniques. In the annealed state, sub-bandgap absorption spectra obtained from both PDS and DBP overlap very well at energies above 1.4 eV. However, differences in alpha (h nu) spectrum exist in the lower energy part of absorption spectrum. The alpha (h nu) value measured at 1.0 eV is the lowest for 10% Ge sample and increases gradually as Ge content of the sample increases. In the light soaked state, time dependence of photoconductivity decay obeys to t (-x) power law, where x changes from 0.30 to 0.60 for samples with low Ge content and 0.05-0.1 for samples with high Ge content. Correspondingly, the increase of the sub-bandgap absorption coefficient at lower energies obeys to t (y) power law, where y values are lower than the x value of the same sample. It can be inferred that sub-bandgap absorption and photoconductivity measurements are not controlled by the same set of defects created in the bandgap of alloys.