Band Structure of ZnGeP2and ZnSiP2— Ternary Compounds with Pseudodirect Energy Gaps

We have observed electroreflectance structure in ZnGe${\mathrm{P}}_2$ at the pseudodirect energy gap, the direct energy gap derived from the indirect gap in GaP. The direct energy gap is also observed at higher energy. We also observe the direct energy gap of ZnSi${\mathrm{P}}_2$ some 1.0 eV above the absorption edge previously reported. The polarization selection rules observed at the direct gap in ZnSi${\mathrm{P}}_2$ are opposite to those found in all other II-IV-${\mathrm{V}}_2$ compounds investigated, and raise a fundamental question as to the physical origin of the crystal field splitting of the valence bands in these compounds.