Electroreflectance Study of the Energy-Band Structure of CdSnP2

Abstract

We report electroreflectance spectra for the chalcopyrite crystal CdSn${\mathrm{P}}_2$, the ternary analog of InP. Structure in the electroreflectance spectra is observed at 1.17, 1.25, and 1.33 eV due to direct energy gaps in CdSn${\mathrm{P}}_2$ corresponding to the $E_0$ and $E_0+{\Delta }_0$ direct energy gaps in InP. From the polarization dependences of this structure, we have established an unusual ordering of the valence bands, which we explain quantitatively by a simple model for the chalcopyrite lattice. The valence bands of CdSn${\mathrm{P}}_2$ are regarded as equivalent to those which would occur in a strained version of its binary analog InP, could one strain InP sufficiently to achieve the lattice constants of CdSn${\mathrm{P}}_2$. We also observe structure at 2.56 and 2.69 eV corresponding to the $E_1$ and $E_1+{\Delta }_1$ peaks in zinc blende. The polarization dependences of these peaks agree with observations in stressed zinc-blende crystals. Much additional structure observed in CdSn${\mathrm{P}}_2$ 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. Hence at every point in the chalcopyrite Brillouin zone, new direct transitions appear. We refer to these transitions as "pseudodirect," since their strength will depend upon the degree of difference of the pseudopotentials of the two cations.