MODIFICATION OF THE OPTICAL-PROPERTIES OF HYDROGENATED AMORPHOUS-SILICON BY ALLOYING WITH A1, GA AND S

Alloys of hydrogenated amorphous silicon (a-Si:H) with Al, Ga and S respectively, have been prepared. The alloy films were grown by the plasma enhanced chemical vapor deposition (PECVD) method. The chemical composition of the alloys has been characterized by electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS). The EPMA results show that the alloying element is incorporated in the films more efficiently at low input gas flux than at the higher fluxes. A topological model has been proposed to explain the observed behaviour. The Al and Ga alloys were found to be low band gap materials whereas alloying with S had the effect of widening the energy gap. The energy gap of the alloys could be varied in the 0.90 to 1.92 eV range. The trends in the band gap composition dependence were quantitatively analyzed on the basis of White's random network model. The photoresponse of the alloys was measured under white light illumination. The highest light to dark conductivity ratio of 4 x 10(2) was obtained for the S alloys. The Al and Ga alloys showed at best a one order change in magnitude between the light and dark conductivity. The Si:S alloys are promising window-materials for multibandgap solar cells.