Evaluation of the quality of semi-insulating CdTe for radiation detectors by measurement of lux–ampere characteristics

Abstract

Accumulation of space charge on deep levels represents one of the major problems in fabrication of semi-insulating CdTe and CdZnTe X-ray and gamma-ray detectors, because it influences the applied electric field and can even result in a complete breakdown of the field in part of the sample (polarization and dead layer formation). The goal of the study was to evaluate possibilities of localization of areas of potential space charge accumulation in as grown crystals by steady state measurement of lux–ampere characteristics. All measurements were done at room temperature using He–Ne laser. Voltage was applied parallel to the direction of light propagation in the range 10–100 V. It was observed that all lux–ampere characteristics are sub-linear. Screening effects caused by space charge accumulated on deep levels explain these results. Crystals prepared by Vertical gradient freeze method in our laboratory are compared to a commercially available detector-grade sample prepared by Travelling heater method. Comparison of crystals grown from precursors of different starting purity shows an increase of the slope of lux–ampere characteristics with a decrease of impurity content. A correlation between the slope of lux–ampere characteristics and the mobility–lifetime product of electrons was observed, too.