Using the all-electron mixed-basis approach to the density-functional formalism for crystals, we calculate from first principles the electronic structure of zinc-blende ZnS, ZnSe, and ZnTe as well as that of their ordered pseudobinary alloys Zn. SSe, Zn&SeTe, and Zn&STe. For the latter we use as a model a CuAu Ilike structure (space group P4m2), and analyze the observed optical bowing in terms of three contributions: (i) a volume deformation of the band structure due to the replacement of the

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... ttice constants of the binary constituents by that of the alloy, (ii) a chemical-electronegativity contribution due to charge exchange in the alloy relative to its constituent binary subsystems, and (iii) a structural contribution due to the relaxation of the anion-cation bond lengths in the alloy. The total bowing eff'ect [the sum of (i) - (iii) above] agrees well with observations, yet the present analysis suggests a physical mechanism for optical bowing which differs profoundly from that offered by the popular virtual-crystal approach. The maximum contribution of disorder to the optical bowing is calculated for ZnS, Tel, using a cluster-averaging method, resulting in a reduction in the bowing of the fundamental gap. We further discuss the band structures, x-ray scattering factors, charge distribution, and deformation potentials of the binary zinc chalcogenides and their ordered alloys. I. INTRODUCTION: COMPOSITION DEPENDENCE OF SOME PROPERTIES OF PSEUDOBINARY SEMICONDUCTOR ALLOYS A. Phenomenology F(x) =xFqc + (1 x)Fiic-(la) by simple quadratic relationships of the type Soon after the development of modern techniques of crystal growth and purification of heteropolar binary semiconductors, it was realized that many pairs AC and BC of such semiconductors exhibit large ranges of mutual solid solubility, forming A, Bl,C alloys. ' It became apparent early on (see reviews of the early work in Refs. 1 -4), that if both AC and BC belong to the same iso-