Charge trap identification for proton-irradiated p+ channel CCDs

Abstract

Charge trapping in bulk silicon lattice structures is a source of charge transfer inefficiency (CTI) in CCDs. These traps can be introduced into the lattice by low-energy proton radiation in the space environment, decreasing the performance of the CCD detectors over time. Detailed knowledge of the inherent trap properties, including energy level and cross section, is important for understanding the impact of the defects on charge transfer as a function of operating parameters such as temperature and clocking speeds. This understanding is also important for mitigation of charge transfer inefficiency through annealing, software correction, or improved device fabrication techniques. In this paper, we measure the bulk trap properties created by 12.5 MeV proton irradiation on p+ channel, full-depletion CCDs developed at LBNL. Using the pocket pumping technique, we identify the majority trap populations responsible for CTI in both the parallel and serial transfer processes. We find the dominant parallel transfer trap properties are well described by the silicon lattice divacancy trap, in agreement with other studies. While the properties of the defects responsible for CTI in the serial transfer are more difficult to measure, we conclude that divacancy-oxygen defect centers would be efficient at our serial clocking rate and exhibit properties consistent with our serial pocket pumping data.