1/f noise in an amorphous selenium photo-detect

In direct-conversion, flat-panel, X-ray image detectors, stabilized amorphous selenium (a-Se) is used as the photoconductive layer because of its reasonably high X-ray absorption, low dark current, and good electronic transport properties. The signal-to-noise ratio of these detectors depends in part on the conductance noise in the selenium layer with the lower noise level providing better image quality. Modeling the performance of these detectors is hampered because noise studies of a-Se have not been reported in the literature perhaps because a-Se is a highly resistive material making such measurements difficult. We report here the first results for the low-frequency conductance noise in layers of a-Se under conditions similar to those found in the detectors. The noise power spectrum fits a 1/f power law with a in the range 0.77 to 1.5. Interpretation of the noise spectra is complicated due to the sample's highly nonlinear I-V relation. However, the noise spectrum depends critically on the type of metal evaporated on the a-Se surface for electrical contacts which indicates that the noise is controlled by the metal-semiconductor interface. Three types of metal electrodes have been measured-platinum, gold, and aluminum