Luminescence Properties of CdTe and CdZnTe Materials When Used as Substrate for IR Detectors

In astrophysics, in the infrared domain, the most widely used detectors are based on HgCdTe technology, where the light-sensitive HgCdTe layer is grown on a CdZnTe substrate. When located on space-based instruments, these detectors are submitted to ionizing particle irradiation. It has been shown in the literature that, when the CdZnTe substrate is not fully removed, an increase in the detector background is observed. This increase was suspected to be linked to CdZnTe substrate luminescence: carriers excited by the passage of the ionizing particle lose their energy by emitting photons, which are in turn detected by the HgCdTe detection layer. We validate this assumption with a model and an irradiation campaign performed on real detectors, and demonstrate that the pollution mainly comes from low-energy photons emitted within the substrate. The application of the model relies on CdZnTe material properties. In particular, luminescence characteristics are of prime importance. No data were available in the literature at 100 K (detector operating temperature) with 4% zinc concentration. Thus, we performed optical measurements on CdZnTe substrates identical to those used in IR detector fabrication. Measurement results are presented within this paper. Three samples were submitted to different sets of measurements: one CdTe sample used as a reference and two CdZnTe samples. We present photoluminescence measurements from 4 to 50 K and cathodoluminescence spectra acquired at 80 K, 100 K, and 300 K. We show that excitonic recombination dominates up to 100 K in the CdTe and CdZnTe material. We have also performed ellipsometry measurements at 80 K, 100 K, and 300 K. Each measurement has been carefully analyzed and compared to published data. These measurements helped us to understand the luminescence properties of the CdZnTe material, and then were directly used in the application of the model to infer the response of infrared detectors under irradiation.