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 ($a-\mathrm{S}\mathrm{i}:\mathrm{H}$) 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 ∼0.9 eV for both undoped and phosphorus-doped $a-\mathrm{S}\mathrm{i}:\mathrm{H}$. For excitations less than ∼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 ∼1.5 eV. A model is proposed to explain the results, resolving a discrepancy between secondary- and primary-photoconductivity measurements of the optical absorption.