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

The photoluminescence and modulated reflectivity spectra of ZnSeₓTe_1-x and ZnSₓTe_1-x with increasing $x (0<~x<~0.17)$ show a distinct blueshift for excitons bound to the oxygen center in contrast to an unmistakable redshift for the free excitons. The binding energy of excitons bound to oxygen impurity present in these ternaries is proportional to the difference between the electronegativity of oxygen and the 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 isoelectronic oxygen impurities, its no-phonon line exhibits a significantly faster alloy broadening compared to that associated with excitons bound to shallow acceptors. The Raman spectra show a contrasting mode behavior for their zone-center optical phonons, ``one mode'' for ZnSeₓTe_1-x and ``two mode'' for ZnSₓTe_1-x. A highly accurate frequency determination of the LO phonon in ZnSeₓTe_1-x over a large composition range exhibits a large bowing as a function of x.