SURFACE-CHEMISTRY OF SULFIDIZED MERCURY CADMIUM TELLURIDE AS PROBED BY VOLTAMMETRY AND PHOTOELECTROCHEMISTRY

The sulfidation of mercury cadmium telluride (MCT) single crystal surfaces was studied by cyclic/linear sweep voltammetry and photoelectrochemical techniques. The study was done mainly m aqueous polysulfide solutions, although a brief comparison with nonaqueous ethylene glycol medium is also provided, particularly with respect to oxide contamination. The evolution of the surface composition with the electrode potential was mapped by using (a) reference voltammograms for sulfidized Hg and Te surfaces along with the voltammetric behavior of polysulfide solutions at Pt, (b) linear sweep photovoltammetry, combining wavelength-selective light excitation with voltammetric scanning of the MCT surface in polysulfide, and (c) photocurrent spectroscopy (i(ph) vs lambda) at selected potentials. With these data as a unit, it is shown that CdS forms first on sulfidation of MCT followed by HgS in two stages. Subsequently, the polysulfide solution undergoes oxidation to elemental sulfur at the MCT surface along with partial oxidation of Hg to HgO (the latter only in aqueous media). At higher potentials, HgS dissolves as HgS22-, and Te sulfidizes to TeS2 followed by its dissolution as TeS32-. The last stage (again only in aqueous media) comprises the generation of higher oxidation states of Te4+ (as TeO(x)) and S0. At positive potentials, the net result of the sulfidation is the formation of Cd-rich MCT (contrasting with the starting composition, Hg0.8Cd0.2Te). Supportive data from X-ray photoelectron spectroscopy and differential scanning calorimetry are also presented.