First-principles calculations of the band gap and optical properties of germanium sulfide

There are controversial results among the available experimental data of the germanium sulfide band gap, as well as between these results and the theoretical model-based band calculations published to date. To elucidate this situation, we performed an ab initio calculation of its electronic structure adopting the local density and generalized gradient approximations for the exchange-correlation potential. In addition, these calculations were carried out with and without the inclusion of the spin-orbit interaction. The main difference between our calculations and previous experimental and theoretical results is that we found several critical points in the valence and conduction bands that compete in defining the gap. This explains the diversity of the existent experimental results, which is also a consequence of the strong crystal anisotropy. Also, we suggest the important role of the s-Ge states contribution at the edge of the valence band. Based on our electronic structure, we discuss the experimental core spectra and optical properties of germanium sulfide. We found an excellent agreement between our results and the available experimental core spectra data. Furthermore, our calculated optical functions of GeS were satisfactorily compared against existing experimental data and they explain the origin of the optical transitions.