Electron-drift-mobility measurements and exponential conduction-band tails in hydrogenated amorphous silicon-germanium alloys.

We have measured the temperature dependence of the electron drift mobility using the time-offlight technique for a series of undoped hydrogenated amorphous silicon-germanium alloys with band gaps spanning the range 1.47-1.72 eV. We also developed techniques for analyzing dispersion effects in such measurements, which permitted us to compare essentially all previous measurements with our own. We draw two main conclusions. First, there is substantial agreement between laboratories for the reduction in the electron drift mobility due to Ge alloying. Second, we are able to account for most of the features of the data using the standard multiple-trapping model by invoking only variations of an exponential conduction-band-tail width c2:; we find a fair linear correlation between this width and the optical band gap CT. The effects of alloying upon the microscopic mobility and the attempt-to-escape frequency were relatively minor.