First -principles DFT computation and X-ray spectroscopy study of the electronic band structure and optical constants of Cu 2 HgGeS 4

We present results of a complex study of the electronic structure and optical constants from experimental and theoretical points of view of quaternary copper mercury germanium sulfide, Cu2HgGeS4. Partly, based on measurements employing X-ray photoelectron (XP) spectroscopy, we evaluate binding energy of core-level electrons for atoms constituting the sulfide under study using a high-quality Cu2HgGeS4 crystal. Furthermore, the energy distribution of the electronic valence states in Cu2HgGeS4 is evaluated on measurements of the valence band XP spectrum of the crystal under study. To find the best correspondence of the shape of the valence band XP spectrum to theoretical total density of states (DOS) curve, different approaches for exchange-correlation (XC) potential are explored in the present work. We find that the best correlation of the shapes of the theoretical total DOS curve and the experimental valence band XP spectrum is obtained when the first-principles calculation within a framework of density functional theory (DFT) are made using modified Becke-Johnson (mBJ) functional and treating also the correction parameter U (Hubbard parameter) and effect of spin-orbit (SO) coupling (mBJ + U + SO technique). Based on the present mBJ + U + SO calculation, we explore in detail projected DOS curves, bands dispersion, and the main optical constants. The projected DOS data are found to be in excellent agreement with X-ray emission bands measured for the main contributors to the valence-band region of the compound under study. The present experimental and theoretical results allow for statement that Cu2HgGeS4 is a promising photovoltaic absorber material. © 2020 Elsevier Masson SAS