Effects of light induced degradation on the distribution of deep defects in hydrogenated amorphous silicon-germanium alloys

We have characterized the defect state structure in a series of device quality glow discharge produced a-Si,Ge:H alloys with Ge content ranging from 30 at.% to 50 at.% using capacitance profiling, modulated photocurrent, transient junction photocurrent and photocapacitance measurements. As previously reported, these methods have allowed us to identify two types thermally induced defect transitions plus two types of optical transitions from deep defects. In the current study we have examined the changes in these defects, along with the changes in the hole mobility-time products, that result from prolonged light exposure. By comparing these changes in the annealed state and light soaked state of the same sample, we attempt to correlate the changes in defects with the hole mobility-time product. In general, although all of the defect bands are found to increase after light soaking, the relative factor is found to be different for the various defect transitions within the same sample. We also try to identify a defect bands may be acting as a ''safe electron trap'', enhancing the lifetime of the minority carriers. We propose that the observed decrease of this defect band relative to the midgap defect band with light soaking could be a significant factor in determining the degradation of these a-Si,Ge:H alloys in the device performance.