Electroreflectance Spectra of CdSiAs2and CdGeAs2

Abstract

We report electroreflectance spectra for the chalcopyrite crystals CdSi${\mathrm{As}}_2$ and CdGe${\mathrm{As}}_2$. These compounds are characterized by large built-in compressions and internal displacements of the As anions due to the difference in the cation covalent radii, Cd being 27% larger than Si and 21% larger than Ge. We find that CdSi${\mathrm{As}}_2$ has a direct band gap at 1.55 eV. The simple quasicubic model for the crystal field splitting of the fundamental band gap in chalcopyrite crystals breaks down in CdSi${\mathrm{As}}_2$ because of a contribution (∼ 50% of the compressional splitting) of opposite sign due to the difference in the pseudopotentials of the cations Cd and Si. However, the quasicubic model quantitatively explains the observed polarization dependences in terms of the measured valence band splittings. Transitions corresponding to the $\Lambda$ transitions in zinc-blende crystals are not observed in CdSi${\mathrm{As}}_2$ and CdGe${\mathrm{As}}_2$. Instead, a new doublet is observed for $\overrightarrow{\mathrm{E}}\perp Z$ in both crystals, and we assign this new structure to transitions at the point $N$ in the chalcopyrite Brillouin zone.