Ionizing Radiation Effects in HgCdTe MIS Capacitors Containing a Photochemically-Deposited SiO2 Layer

Ionization-induced charge buildup in photochemically-deposited SiO2 on HgCdTe substrates has been examined. Irradiation at 95 K produces a net positive trapped charge which is relatively independent of the oxide field applied during exposure and the preparation of the HgCdTe surface prior to oxide deposition. Annealing experiments indicate that both electrons and holes are trapped in photo SiO2 at 95 K. Annealing of the trapped charge occurs over a broad temperature region between 150 K and 300 K, with behavior in the vicinity of 275 K indicating a complex process in which trapped electrons and holes anneal, though not simultaneously. Ionizing radiation also produces interface states at a rate which is also relatively independent of the oxide field and HgCdTe surface preparation. The ionization-induced interface states have a peaked distribution (peaking near midgap) superimposed on a normal U-shaped distribution. The states with a peaked distribution anneal in a single stage between 200 K and 225 K. The flatband voltage shift in photo-SiO2-n-HgCdTe devices irradiated at 95 K is found to be less than that in hardened, thermally-grown SiO2-on-silicon devices irradiated at 300 K. Further, the shifts produced in the photo-SiO2 case are less than those produced in devices containing ZnS under many conditions, indicating that photo SiO2 has good potential for use in the fabrication of hardened HgCdTe devices.