We report the temperature dependences of the dielectric function ε = ε₁ + iε₂ and critical point (CP) energies of the uniaxial crystal GaSe in the spectral energy region from 0.74 to 6.42 eV and at temperatures from 27 to 300 K using spectroscopic ellipsometry. The fundamental bandgap and strong exciton effect near 2.1 eV are detected only in the c-direction, which is perpendicular to the cleavage plane of the crystal. The temperature dependences of the CP energies were determined by fitting the data to the phenomenological expression that incorporates the Bose–Einstein statistical factor and the temperature coefficient to describe the electron–phonon interaction. To determine the origin of this anisotropy, we perform first-principles calculations using the mBJ method for bandgap correction. The results clearly demonstrate that the anisotropic dielectric characteristics can be directly attributed to the inherent anisotropy of p orbitals. More specifically, this prominent excitonic feature and fundamental bandgap are derived from the band-to-band transition between s and p_z orbitals at the Γ-point.

The effect of C dopant on the electronic structure and thermoelectric properties of SnSe has been studied using density functional theory (DFT) and by solving the Boltzmann transport equation (BTE). We found that the substitution of C into the Sn site creates a strongly localized defect state with a sharp density for the state peak. This strong modification of band structure reduces the band gap from 0.61 eV to 0.42 eV. It was found that the maximal Seebeck coefficient can be enhanced up to 17.1% and the maximal temperature reduces from 525 K to 280 K. These behaviors can be understood using the Mott equation and Goldsmid – Sharp relation.