Measurement of the density of gap states in hydrogenated amorphous silicon by space charge spectroscopy

We report a comprehensive study of the dynamic response of junction space-charge layers in undoped and PH₃-doped a-Si:H films grown by the rf-glow-discharge technique. By using the numerical analysis methods discussed in the adjoining theory paper, we are able consistently to interpret a variety of transient response and ac admittance measurements in terms of a bulk density of gap states $g(E)$ which is characteristic of each sample. While the general shape of $g(E)$ seems to be a characteristic property of a-Si:H, the overall concentration of gap states depends on growth conditions and doping. The density of states at approximately midgap is observed to vary between values as low as about 2×{}10¹⁵ cm^-3 eV^-1 in undoped films and as high as 1×{}10¹⁸ cm^-3 eV^-1 in some PH₃-doped films. The general shape of our $g(E)$ is dominated by a deep minimum (10¹⁶ cm^-3 eV^-1) between 0.3 and 0.6 eV from the conduction band and a broad shoulder of states extending from the valence band up to midgap. The significant difference between this type of bulk $g(E)$ and previous models for the density of states in a-Si:H may be explained by the effects of states at or near the surface of the films which strongly influence the previous types of measurements. We discuss recent transport and optical measurements and show that they provide strong support for our density of states as opposed to previous models for $g(E)$.