Steady-state optical modulation spectroscopy ofp-typea-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 (${\mathrm{B}}_2$ ${\mathrm{H}}_6$) to the silane (${\mathrm{SiH}}_4$) gas in a conventional rf glow-discharge deposition. Measurements were performed at room temperature and at T≃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, ${\mathit{B}}_4^{\mathrm{-}}$ and ${\mathit{B}}_4^0$, and the doping-induced dangling-bond defects, ${\mathit{D}}_{\mathit{B}}^{+}$ and ${\mathit{D}}_{\mathit{B}}^0$, 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.