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 inves tigated 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 dif ferent apparent ionized donor densities due to a spatial variation of the space-charge density. These observations are considered to be char acteristic of semiconductors with major deep gap states of large energ y width. The experimental results were analyzed using a simple model o f energy distribution of the gap state density of dividing the depleti on layer into two. In one of these deep depletion occurs. The variatio n of the apparent W-V relation observed upon lowering the exciting pho ton 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 consi stent with the electron-spin density observed in the same undoped a-Si :H.