Energy dependence of the carrier mobility-lifetime product in hydrogenated amorphous silicon

The results of optical-absorption measurements determined by photothermal deflection spectroscopy, primary photoconductivity, and secondary photoconductivity on undoped and phosphorus-doped hydrogenated amorphous silicon films (a-Si:H) are reported. A normalization procedure for obtaining photocurrent spectra at constant generation rate is demonstrated. From these measurements, the efficiency-mobility-lifetime product (eta..mu..tau) for electrons is found to be constant from 2.0 to approx.0.9 eV for both undoped and phosphorus-doped a-Si:H. For excitations less than approx.0.9 eV, there is evidence that the product eta..mu..tau for electrons drops rapidly; similarly, the eta..mu..tau for holes exhibits a rapid decrease in undoped material for photon energies less than approx.1.5 eV. A model is proposed to explain the results, resolving a discrepancy between secondary- and primary-photoconductivity measurements of the optical absorption.