Investigations of the origins of metastable, light-induced changes in hydrogenated amorphous silicon

This report presents results of an investigation into the metastable changes in the density of deep mobility gap states in hydrogenated amorphous silicon, using a variety of junction capacitance techniques. This work extends previous studies (using drive-level capacitance profiling) to some new samples. With these measurements, researchers examined how the distribution of occupied gap states changes with light soaking and partial annealing through a series of intermediate states between states A and B. The results on two phosphorus-free samples indicate no qualitative change from previous results, which revealed a large metastable increase in the concentration of D (sup -) centers with light soaking, so phosphorus contamination is not crucial to this increase. Because such an increase is inconsistent with the Si-Si bond-breaking model, this result favors a model in which local configurational changes shift the gap-state energies of existing defects. Transient photocapacitance was applied to undoped films to investigate metastable changes in more detail. Results seemed to confirm the magnitude of the metastable defect creation found by drive-level analysis on the same samples.