Inter-band transitions in Zn₃P₂

Optical methods such as measurements of transmissivity, reflectivity and photoconductivity were used to investigate inter-band transitions in Zn3P2 at room temperature. The optical measurements were performed within the 1.4-5 eV energy range, for light polarised both parallel and perpendicular to the c axis of the crystal; with reflectivity, the measurements extended up to 11 eV, but for unpolarised light only. The birefringence of Zn3P2 crystal was also measured and was discussed in terms of band-to-band transitions. Transitions observed within the 4.5-11 eV energy range were interpreted on the basis of the pseudo-cubic energy-band structure model. However, this model could not be adopted to explain low-energy transitions. The band-structure model at the Gamma point was suggested based on the results of measurements for polarised light and applying selection rules obtained from group-theory analysis. The direct energy gap of Zn3P2 at the Gamma point was estimated in this model as 1.6 eV at 300 K, whereas crystal-field and spin-orbit splittings of the valence band were equal to 0.03 eV and 0.11 eV, respectively.