Composition, growth mechanism, and stability of anodic fluoride films on Hg1−xCdxTe

Abstract

Fluoridic films produced by the anodization of Hg1−xCdxTe (x∼0.22) surfaces in nonaqueous solutions consist of three distinct regions. A thick uniform region, containing the fluorides of cadmium, mercury, and tellurium, as well as HgTe, is covered by a thin CdF2 layer. The third region—the film–substrate interface—poor in mercury, consists mainly of CdF2 and TeF4. The anodic film is grown by two mechanisms: the dominant one occurs by motion of the film–substrate interface into the semiconductor, consuming the original surface. There is, however, some growth at the film–electrolyte interface which forms the thin CdF2 layer on top of the structure. The CdF2 rich region acts as a diffusion barrier for the indiffusion of oxidizing species. Tellurium ions, on the other hand, diffuse to the outer surface to be oxidized there. This diffusion barrier is overcome when the fluorides are thermally stressed in the presence of traces of oxygen, and the film starts to oxidize. However, the film–substrate interface does not degrade unless the film is heavily oxidized. A model for the anodic fluoride structure is presented and discussed in light of the film stability with respect to room temperature post-growth oxidation and thermal treatments.