Photoluminescence in hydrogenated amorphous silicon with sulfur

Photoluminescence (PL) has been measured at 80 K in a series of samples of sulfur doped a-Si:H using above- and below-bandgap excitation energies (2.4 and 1.38 eV). In addition, the absorption coefficients at room temperature have been obtained using photothermal deflection spectroscopy (PDS). At light sulfur doping levels (S/Si <10(-2)), the Urbach slopes of the absorption coefficients on a semilog plot and the optical energy gaps, as measured by the points at which alpha=10(4) cm(-1), are independent of sulfur concentration. The slopes decrease and optical gaps increase with increasing doping level for doping levels above 10(-2). At light sulfur doping levels the PL spectra excited with both above- and below-gap light are independent of sulfur concentration. For larger sulfur concentrations the shapes of the PL spectra vary. In particular, for S/Si >10(-2) the peak of the PL spectrum shifts to below 0.8 eV using below-gap excitation at 1.38 eV, and the defect PL band dominates. Comparing the PL spectra of sulfur- and phosphorus-doped samples, the PL spectra change for sulfur doping above 10(-2) and for phosphorus doping above 1 ppm. This trend is consistent with inefficient sulfur doping.