High-field electron-drift measurements and the mobility edge in hydrogenated amorphous silicon

We report electron photocarrier time-of-flight measurements at high electric fields for two thick hydrogenated amorphous silicon (a-Si:H) p-i-n diodes. At 77 K an exponential increase in the electron mobility of more than 100 is observed as the field is increasd to E=400 kV/cm. The dispersion parameter was field independent. We discuss previous reports of field-dependent dispersion in terms of interface effects. We propose a model for high-field effects based on an electric-field-dependent mobility edge which accounts satisfactorily for the measured electric field and temperature dependence of the electron-drift mobility. Effective-temperature models do not account for our measurements since they predict field-dependent dispersion. The microscopic electron mobility μ0∼3 cm2/V s is remarkably independent of electric field, temperature, and germanium alloying.