Temperature Dependence of Optical Reflection Spectra of Cuinse₂ Single Crystals with the Chalcopyrite Structure

Resonances of free excitons at energies A ~ 1.0409 eV, B ~ 1.0445 eV, and C ~ 1.2690 eV were detected in reflection spectra of single crystals of the direct-gap compound CuInSe₂ at a temperature of 8.6 K. The appearance of the three reflection resonances A, B, and C could be explained by removal of degeneracy from valence-band energy levels due to the influence of crystal field and spin–orbit interaction on the tetragonal lattice of CuInSe₂ with Δ_CF ~ 5.4 meV and Δ_SO ~ 224 meV. Measurements of the temperature dependence of the reflection spectra in the range 8.6–90 K showed the energies of free-exciton resonances A and B increased because of deformation of the unit cell (tetragonal stretching) of the CuInSe₂ lattice with the chalcopyrite structure. The binding energies of free excitons A and B were determined as 10.7 and 152 meV based on temperature quenching of exciton resonances A ~ 1.0409 eV and B ~ 1.0445 eV