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 α=104 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.