Photoconductivity Mapping of Semi-Insulating CdZnTe

Semi-insulating CdZnTe crystals were studied by photoconductivity mapping using both the contactless method and measurement with evaporated Au contacts. The contact quality was tested by measurement of voltampere characteristics by a three-point method. We evaluated the distribution of space charge analyzing the slope of lux-ampere characteristics. Mobility- lifetime (μτ) product maps were extracted fitting the Hecht relation to voltampere characteristics measured with a weak light at wavelength 750 nm. Correlation analysis of contactless resistivity and photoconductivity maps shows, that both these parameters are anticorrelated. This fact can be explained by a shift of Fermi level changing the average occupation of a midgap level. A decrease of occupation with an increasing resistivity results in an increase of electron trapping and decreased photoconductivity. This qualitative explanation is supported by simulating the dependence of the photocurrent density on the Fermi level position near the midgap using parameters of midgap levels assumed in state-of-the art radiation detectors. The map of electron mobility-lifetime product correlates with the maps of photocurrent intensity measured with and without the Au contact. The comparison of maps displaying slopes of lux-ampere characteristics with the map of μτ shows, that areas with a higher accumulation of space charge have a smaller μτ .