Surface treatment of HgCdTe in $H_2S$ gas for improving the HgCdTe/ZnS interface properties

Abstract

In order to remove the surface contaminants formed on the HgCdTe surface during the exposure of the wafer to atmosphere and to improve the interface properties between p-type HgCdTe and ZnS, some surface treatments have been introduced in this thesis in low temperature surface cleaning process. UV exposure in vacuum chamber is effective to remove $CO_2$ and water molecules absorbed on the surface and decreases the fixed interface charge and the interface trap density. The formation of native sulfide is observed by AES and XPS analyses, when the HgCdTe wafer is exposed to $H_2S$ gas ambient. The XPS analysis identifies that the native sulfide consists of Te-S bonding state. The native sulfide improves the quality of the ZnS film being evaporated on the HgCdTe. UV exposure in $H_2S$ gas ambient inside vacuum chamber decreases effectively the insulator trap charge density, in addition to the surface cleaning effect by UV exposure. HCl gas must be avoided because Cl element acts as deep donor states and increases the interface charge, even if it removes the native oxide which forms high interface charge. Electrochemical reduction in pH5 acetate buffer solution at -0.9V chemical potential produces a mirror-like surface and inhibits the formation of the native oxide film. These surface treatments can be used to clean the HgCdTe surface, but they are effective to reduce only a part of interface charges. To effectively utilize the merits of these surface treatments, they are combined with the following sequence. The sequence starts with the etching of HgCdTe wafer in methanol solution with 1% bromine, and continues with electrochemical reduction for 1 h in pH5 acetate buffer solution at -0. 9V chemical potential, UV exposure for 10 min in $H_2S$ gas ambient inside the vacuum chamber for evaporating ZnS film, exposure for 10 min to $H_2S$ gas ambient heated by filament. After these processes, ZnS film is evaporated inside the same vacuum chamber without exposing the...