Isotropy of drift mobilities in hydrogenated amorphous silicon

We report transient photocurrent and time-of-flight measurements in undoped hydrogenated amorphous silicon (a-Si:H) for photocarrier motion both parallel and perpendicular to the thin-film growth axis. These measurements were analyzed to obtain the electron drift mobility and deep-trapping mobility-lifetime product at room temperature. We found good agreement of the electron-drift-mobility measurements for both field directions in two specimens prepared in several light-soaking states. Fifteen pairs of mobility-lifetime product estimates for the two field directions were also measured in a larger number of specimens. The data exclude an electron-transport anisotropy in a-Si:H greater than a factor of 2 in our specimens. We also studied the effects of absorption depth upon estimates of deep-trapping mobility-lifetime products for the standard sandwich electrode structure; results using 520-nm illumination yield estimates that are typically half the value estimated with uniformly absorbed illumination. We also present data on the correlation of the electron and hole deep-trapping mobility-lifetime products for these specimens.