INSITU STUDIES OF THE EFFECT OF VARIOUS GASEOUS SPECIES AND ULTRAVIOLET ILLUMINATION ON THE DARK CURRENTS OF HGMNCDTE AND HGCDTE SHORT-WAVELENGTH INFRARED DIODES

This study deals with the effects of perturbing the surfaces of unpassivated HgMnCdTe and HgCdTe short-wavelength infrared photodiodes to further our understanding of these effects on dark currents. Perturbations include illuminating ultraviolet (UV) radiation in high vacuum and in various gaseous species. The change in dark current at 0.8 V reverse bias and its evolution with time was monitored in situ. By correlating the amount of excess dark current generated by UV radiation to the vacuum conditions, we conclude that it is a surface-related problem. A drastic improvement in device performance was found by illuminating its surface with UV radiation in high vacuum. This simple treatment prevents the generation of excess and persistent dark current and stabilizes the leakage current at low reverse bias. This can be understood in terms of photodesorption of surface contaminants which induce deep level surface states, giving rise to the persistent dark current generation. By studying the effects of various gaseous species, we identified water molecules as the key surface contaminants. We also discovered an anomalous degradation in HgCdTe diodes when they were exposed to hydrogen and UV radiation simultaneously. This anomalous degradation can be interpreted as UV-catalyzed chemical reduction of HgCdTe by hydrogen or the diffusion of hydrogen atoms into the bulk to change the junction characteristics.