Role of electronegativity in semiconductors: Isoelectronic S, Se, and O inZnTe

The photoluminescence and modulated reflectivity spectra of ZnSexTe1-x and ZnSxTe1-x with increasing x (0 less than or equal to x less than or equal t o 0.17) show a distinct blueshift for excitons bound to the oxygen center i n contrast to an unmistakable redshift for the free excitons. The binding e nergy of excitons bound to oxygen impurity present in these ternaries is pr oportional to the difference between the electronegativity of oxygen and th e concentration-weighted average of those of Se (or S) and Te, both scaling with x. Due to the strong localization of the excitons bound to isoelectro nic oxygen impurities, its no-phonon line exhibits a significantly faster a lloy broadening compared to that associated with excitons bound to shallow accepters. The Raman spectra show a contrasting mode behavior for their zon e-center optical phonons, "one mode" for ZnSexTe1-x and "two mode" for ZnSx Te1-x. A highly accurate frequency determination of the LO phonon in ZnSexT e1-x over a large composition range exhibits a large bowing as a function o f x.