Electroreflectance, Absorption Coefficient, and Energy-Band Structure of CdGeP2near the Direct Energy Gap

Abstract

We report electroreflectance and absorption measurements near the direct energy gap of CdGe${\mathrm{P}}_2$ at 1.72 eV. The ordering, splittings, and polarization properties of the three closely spaced energy gaps derived from ${\Gamma }_{15}\to {\Gamma }_1$ in zinc-blende crystals are quantitatively explained by a quasicubic model taking into account the built-in uniaxial compression alone. Although the absorption coefficient is ∼45 times larger for $\overrightarrow{\mathrm{E}}\parallel Z$, at low temperatures we observe in the absorption spectrum for $\overrightarrow{\mathrm{E}}\perp Z$, a free exciton with a binding energy of 7.7 meV near the lowest energy gap. The observation of this exciton is independent evidence that the dichroism of the absorption edge in II-IV-${\mathrm{V}}_2$ crystals results entirely from the lowest energy gap having a highly anisotropic oscillator strength. The dichroism of the absorption edge is opposite to that recently assumed by Goryunova et al. in the process of optically orienting single crystals. Hence their conclusion that CdGe${\mathrm{P}}_2$ is negatively birefringent is in error, and in fact, CdGe${\mathrm{P}}_2$ is positively birefringent (extraordinary index larger than ordinary index). After allowing for this sign error, we show that the dispersion of the birefringence of CdGe${\mathrm{P}}_2$ can be readily explained by a theoretical model, taking into account the built-in uniaxial compression alone.