Measurement of deep states in undoped amorphous silicon by current transient spectroscopy

Localized deep states in undoped a-Si:H have been investigated by current transient spectroscopy (CTS). After a trap-filling light pulse, current transients associated with thermal emission of trapped carriers are measured over a wide time range under the isothermal condition. Slow saturation of the CTS signal J×t, with increased filling-pulse duration is well explained by taking account of spatially and energy dependent carrier capture and emission processes, yielding an estimate of attempt-to-escape frequency. The attempt-to-escape frequency has been found to be energy dependent and to range from 1012 to 1013 s−1. Detailed analysis of the CTS signal has revealed that deep-states distribution displays a broad peak locating near the center of the gap and a less pronounced structure at 0.5–0.6 eV below the conduction band edge.