STEADY-STATE OPTICAL MODULATION SPECTROSCOPY OF P-TYPE A-SI-H

Steady-state optical modulation spectroscopy (OMS) was carried out to obtain information on the gap states of p-type hydrogenated amorphous silicon (a-Si:H). Doping was achieved by adding diborane (B2H6) to the silane (SiH4) gas in a conventional rf glow-discharge deposition. Measurements were performed at room temperature and at T similar or equal to 20 K. The OMS spectra show clear differences with the spectra of undoped a-Si:H. These differences can be associated with the dopant states, lying in the valence-band tail, created by the boron doping. The numerical calculations, based on a model for the density-of-states distribution, take into account several possible optical transitions. The energy positions of the dopant states, B-4(-) and B-4(0), and the doping-induced dangling-bond defects, D-B(+) and D-B(0), are determined. The dangling-bond states tend to shift further away from the valence-band edge with increasing doping concentration while the dopant states stay at the same energy position.