Light-induced change in photoconductivity of a-SiGe-H alloys

The Staebler-Wronski effect (SWE) of a-SiGe:H for solar cells has been examined for highly photoconductive alloys prepared by the microwave-excited plasma CVD (chemical vapor deposition) method. It was found that the light-induced change in photoconductivity depends upon the optical gap of films. i.e. the Ge content in alloys, rather than upon the initial value of the photoconductivity. The light-induced change in photoconductivity decreased with the decrease in optical gap and disappeared for the alloys with an optical gap around 1.4 eV, irrespective of the initial value of the photoconductivity. This dependence of the SWE upon the optical gap was not continuous to a-Si:H, and the light-induced change in photoconductivity of the alloys with small amounts of Ge was larger than that of a-Si:H. The photosensitivity slightly decreased under light soaking, which suggests a structural change during illumination. The changes in infrared spectra and electron-spin-resonance spin densities of a-SiGe:H alloys caused by light soaking were examined. The Si-H and Ge-H stretching bands (centered at 2000 and 1880 cm^-1) decreased by the same amount without any shift in wavenumber.