OPTICAL ABSORPTION IN ZnSiP2

The polarised absorption coefficient of ZnSiP? near the fundamental absorption edge has been studied. Three singularities were found for E l c which are attributed to pseudodirect transitions. The absorption for El/ c is mainly due to indirect transitions. A theoretical curve was fitted to the experimental results with the aid of the relative theoretical intensities for pseudodirect transitions calculated from the quasicubic model. This yields approximate values for the exciton binding energy of 22 meV, and for the matrix element for pseudodirect transitions about a thousand times smaller than that for direct transitions in 111-V compounds. The splitting5 of the valence bands give values for the spin-orbit splitting of 0.056 eV and of the crystal field splitting of 0.130 eV. The lowest band gap in ZnSiPz is pseudodirect at 2.082 eV at room temperature after making allowance for the exciton binding energy.