Optical Properties and Electronic Structure of ZnSiAs2

We report electroreflectance spectra for the chalcopyrite crystal ZnSi${\mathrm{As}}_2$. Structure in the electroreflectance spectra is observed at 2.12, 2.22, and 2.47 eV due to direct energy gaps in ZnSi${\mathrm{As}}_2$ corresponding to the $E_0$ and $E_0+{\Delta }_0$ direct energy gaps in zinc blende. The ordering and splittings of these transitions as well as their strong polarization dependences are explained quantitatively by a simple model which regards the chalcopyrite lattice as a compressed version of its binary analog. We also observe structure at 2.74 and 2.90 eV corresponding to the $E_1$ and $E_1+{\Delta }_1$ structures in zinc blende. The splittings and polarization dependences agree with observations in stressed zinc-blende crystals. Much additional structure observed in ZnSi${\mathrm{As}}_2$ is attributed to "pseudodirect" band gaps which result from the doubling of the unit cell in the $Z$ direction in chalcopyrite relative to zinc blende. This change in the unit cell causes the Brillouin zone of zinc blende to be imbedded into the smaller Brillouin zone of chalcopyrite, and as a result new direct transitions can occur corresponding to indirect transitions in zinc blende. This is the first study of the electroreflectance spectra of a crystal which should have a pseudodirect transition as its lowest band gap. We do not observe this particular transition, and estimate that it is at least an order of magnitude weaker than the direct transitions derived from ${\Gamma }_{15}\to {\Gamma }_1$ in zinc blende. We have also observed the dichroism of ZnSi${\mathrm{As}}_2$ at a wavelength slightly below the lowest direct band gap. We find that the transmission is considerably higher for $\overrightarrow{\mathrm{E}}\perp Z$ than for $\overrightarrow{\mathrm{E}}\parallel Z$, as predicted by the quasicubic model.