Densities of states below midgap determined from the space-charge-limited currents of holes in intrinsic hydrogenated amorphous silicon

Space-charge-limited currents of holes have been obtained in intrinsic hydrogenated amorphous silicon layers using p-i-p structures and have been used to derive the densities of states distribution between 1.0 and 0.7 eV from the hole mobility edge. The densities obtained near midgap are consistent with the measured densities of dangling bond defects located over an energy range of about 0.3 eV. At 0.7 eV from the valence-band mobility edge, the densities are consistent with the densities expected from the Urbach edge tail states. Such space-charge-limited currents offer a powerful tool for the direct measurement of the densities of states below the Fermi level in a-Si:H and its alloys.