Combinatorial Investigations of ZnTe_xSe_1-x Alloys for Applications as CO₂ Reduction Photocathodes

Semiconductor photoabsorbers in the ZnTe_xSe_1-xspace show promise for use in photoelectrochemical CO₂ reduction. Bandgaps, band edge positions, and good aqueous stability meet stringent selection criteria for suitable photocathodes, and isostructural zincblende alloys present an opportunity for tunability. In addition, chemically and structurally similar II-VI semiconductors, such as CdTe, already have technological penetration as optoelectronic materials. Combinatorial ZnTe_xSe_1-xalloy thin films were fabricated by RF sputtering with an anionic compositional gradient across sample libraries. A nitrogen dopant was introduced in several concentrations by varying the gaseous N₂ flow into the sputtering chamber, up to the point where material crystallinity was lost. It was found that the nitrogen dopant induces a stacking polytype transformation from the usual II-VI zincblende structure into wurtzite. The position of the phase transformation on the compositional gradient, and the crystallinity of both wurtzite and zincblende phases were dependent on dopant concentration and growth temperature. Powder diffraction data confirmed the presence of the wurtzite structure in anionic compositions ranging from Se/Te ≈ 0.35 to nearly pure ZnSe. An increase in the p-type conductivity of doped Te rich ZnTe_xSe_1-xwas observed in both polytypes, and effects of nitrogen flow and growth temperature on optoelectronic properties will be discussed.