Optical properties of zinc-blende CdSe andZnxCd1−xSe films grown on GaAs

We present room-temperature ellipsometric measurements of the dielectric function of ${\mathrm{Zn}}_{\mathit{x}}$ ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$Se single-crystal films grown on (001)GaAs in the 1.5–6.0-eV energy region x ranging from 0 to 1. We identify the ${\mathit{E}}_0$, ${\mathit{E}}_0$+${\mathrm{\Delta }}_0$, ${\mathit{E}}_1$, ${\mathit{E}}_1$+${\mathrm{\Delta }}_1$, and ${\mathit{E}}_2$ threshold energies using the CdSe band structure calculated with a nonlocal empirical pseudopotential method. We find that the contact exciton effect has to be included in the calculated dielectric function of CdSe in order to obtain good agreement with our measurements. A compositon-dependent critical-point analysis of the ${\mathit{E}}_1$ and ${\mathit{E}}_1$+${\mathrm{\Delta }}_1$ structures has been performed. We also find that the spin-orbit-splitting band gap ${\mathrm{\Delta }}_1$(x), the linewidths, and the excitonic angles are about maximum at x=0.5, which we attribute to the statistical fluctuation of the alloy composition. Finally, the ${\mathit{E}}_1$ and ${\mathit{E}}_1$+${\mathrm{\Delta }}_1$ critical-point amplitudes cannot be understood by the one-electron approximation, confirming the existence of strong excitonic effects.