The significance of charged defects in understanding the light-induced degradation of hydrogenated amorphous silicon-germanium alloys

We have characterized the defect state structure in a series of device quality glow discharge intrinsic, n-type doped, and p-type doped a-Si,Ge:H alloys with Ge content ranging from 20 at.% to 35 at.%. Our experimental methods include capacitance profiling, transient junction photocurrent and photocapacitance measurements. These methods have allowed us to identify one type of thermally induced defect transition plus two types of optical transitions from deep defects. Our results indicate that these transitions must involve at least two distinct defect subbands. Comparison of the magnitudes of these sub-bands for the intrinsic, n-type, and p-type alloys has allowed us to confirm that one of the optical transition belongs to D+ defect sub-band. All the optically and thermally induced bands of defect transitions are present with similar magnitudes for the most intrinsic a-Si,Ge:H alloys, which implies that charged defects play a significant role in glow discharge a-Si,Ge:H alloys. We then examined the changes in these defect densities, along with the changes in the hole mobility-lifetime products, that result from prolonged light exposure. By comparing the annealed state and light soaked state of each sample, we have been able to correlate the relative changes of the identified defect sub-band with the measured hole mobility-lifetime products. These data indicate that charged defects probably play a dominant role in determining the degradation of these a-Si,Ge:H alloys in device applications.