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 mobi lity using the time-of-flight technique for a series of undoped hydrog enated amorphous silicon-germanium alloys with band gaps spanning the range 1.47-1.72 eV. We also developed techniques for analyzing dispers ion effects in such measurements, which permitted us to compare essent ially all previous measurements with our own. We draw two main conclus ions. First, there is substantial agreement between laboratories for t he 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 th e standard multiple-trapping model by invoking only variations of an e xponential conduction-band-tail width. epsilon(C)0; we find a fair lin ear correlation between this width and the optical band gap epsilon(T) . The effects of alloying upon the microscopic mobility and the attemp t-to-escape frequency were relatively minor.