Optical dispersion of ternary II–VI semiconductor alloys

The wavelength dependence of the refractive indices $n$ of a series of II–VI ternary alloys—ZnCdSe, ZnBeSe, ZnMgSe, ZnMnSe, ZnCdTe, ZnMnTe, ZnMgSe, and ZnSeTe—were measured at frequencies below their respective energy gaps using the combined techniques of optical reflectivity and the prism coupler method. To facilitate the analysis of the results—including those obtained in the wavelength region near the fundamental energy gap—we have modified the semiempirical single-effective-oscillator (SEO) model of $n$ by introducing an additional term that explicitly accounts for interband transitions at the fundamental gap. Using the SEO model modified in this manner to fit the wavelength dependence of $n,$ a set of semiempirical parameters was established for the above ternary II–VI-based alloys, where the fitting parameters for each alloy family are themselves expressed as functions of the alloy composition. The availability of these parameters makes it possible to calculate the index of refraction of any given II–VI ternary alloy for any composition and at any wavelength. Furthermore, these parameters provide valuable physical insights, such as the relationship between the covalency (or ionicity) of the material and its refractive index. In addition to its fundamental usefulness, this approach can—by appropriate extrapolation—also be used for obtaining the dispersion properties for “hypothetical” zinc blende compounds that do not form under equilibrium crystal growth conditions (such as MnTe, MnSe, or BeSe).