Nonlinear photocarrier drift in hydrogenated amorphous silicon-germanium alloys

We have studied nonlinear effects of an electric field on the transport of both electrons and holes in a series of undoped hydrogenated amorphous silicon-germanium alloy specimens (a-${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ge}}_{\mathit{x}}$:H; 0<x<0.3). We measured the transient photocurrents in p-i-n diode structures as a function of electric field (${10}^4$–2.6×${10}^5$ V/cm) and temperature (90–300 K). Time-of-flight and charge-collection measurements were also conducted, from which we concluded that the quantum efficiency for photocarrier generation varied less than 10% for all specimens in this temperature and electric-field range. In all cases, transport was dispersive, but we found no evidence for a field dependence in the dispersion itself. For the hydrogenated amorphous silicon specimen (a-Si:H) a field of 2×${10}^5$ V/cm increased the electron drift mobility by a factor of 8 at 130 K. The characteristic electric field for the onset of nonlinear electron transport increased with Ge concentration and light soaking. In a-Si:H we found slight evidence for nonlinear transport of holes.