Ionizing Radiation Effects in HgCdTe MIS Capacitors

Ionization-induced charge buildup in HgCdTe MIS capacitors has been examined. Most of the capacitors studied contained ZnS as the insulator and had different native oxide layers between the HgCdTe and the ZnS. The net charge buildup measured in these devices following 77°K irradiation is attributed to the charge trapped in the interfacial region (i.e., between the HgCdTe and the ZnS). Either electron or hole trapping may occur in that region, depending on the sign of the electric field applied during irradiation. This finding implies that at least a fraction of both the electrons and holes produced by ionization are mobile in ZnS at 77°K. The amount of charge trapped in the interfacial region depended on the nature of that region, particularly on the thickness of the native oxide, and on the magnitude of the field applied during irradiation. Some studies of capacitors with a CVD SiO2 insulator were also performed. The results indicated that both holes and electrons are trapped in the oxide during 77°K irradiation, with the resulting net trapped charge being relatively small. A room-temperature anneal evidently removes at least a portion of the trapped electrons, resulting in increased net positive trapped charge.