Depletion layer width in undoped a-Si:H Schottky barrier revealed by reverse bias photocurrent

Reverse bias photocurrent of undoped a-Si:H Schottky barrier was investigated under excitation with uniformly absorbed light. Dependence of the photocurrent on the applied voltage V gives unambiguous dependence of the depletion layer width W on voltage V, which cannot be obtained by conventional capacitance-voltage measurements for undoped a-Si:H. There are two voltage ranges giving parabolic dependence of W with different apparent ionized donor densities due to a spatial variation of the space-charge density. These observations are considered to be characteristic of semiconductors with major deep gap states of large energy width. The experimental results were analyzed using a simple model of energy distribution of the gap state density by dividing the depletion layer into two. In one of these deep depletion occurs. The variation of the apparent W-V relation observed upon lowering the exciting photon energy was analyzed to be related to a decrease in the absorption coefficient with the applied voltage due to an emptying of the initial states of the gap state absorption. Results of the analysis are consistent with the electron-spin density observed in the same undoped a-Si:H.