Influence of the CdZnTe Substrate Thickness on the Response of HgCdTe Detectors Under Irradiation: Modeling of the Substrate Luminescence

The most extensively used infrared (IR) detectors in astrophysics are based on mercury cadmium telluride (MCT) technology: the MCT light-sensitive layer is grown on a cadmium zinc telluride (CZT) substrate. When launched on a satellite, these detectors are subjected to ionizing radiation from cosmic rays or solar flares (mainly protons) which degrade the detector performance. Indeed, an elevation of the detector background was noted under irradiation, which is believed to be associated with the luminescence of the CZT substrate. Complete removal of the substrate eliminates the problem, but it is a challenging step in the fabrication process. A deeper understanding of the response of IR detectors under irradiation, when the substrate is fully removed, will enable the optimization of substrate design for high-performance space-based scientific imaging. Here, the first results of proton irradiation modeling in MCT detectors, including energy deposition in the CZT substrate, are presented. The estimation of image pollution relies on GEANT4 (GEometry ANd Tracking 4) Monte Carlo simulations as well as analytical and numerical calculations of carrier transport inside the detector structure. In particular, recombination processes in the CZT substrate are taken into account to model the luminescence effect induced by proton irradiation. According to this model, considering published material properties, the diffusion of the carriers generated inside the CZT substrate toward the MCT layer is the main source of pollution. As the substrate thickness increases, more pixels are impacted by a proton impact on the IR. Consequently, depending on the targeted application, either partial or complete removal may be chosen.