A theoretical and experimental study of the valence-band electronic structure and optical constants of quaternary copper mercury tin sulfide, Cu2HgSnS4, a potential material for optoelectronics and solar cells

Abstract

Experimental and theoretical studies on the electronic-band structure and optical constants are made for the first time for quaternary copper mercury tin sulfide, Cu2HgSnS4. The present X-ray photoelectron spectroscopy studies reveal low hygroscopicity of Cu2HgSnS4 alloy surface and its sensitivity to 3 keV Ar+-ion irradiation that decreases substantially the content of mercury atoms in topmost analyzing layers. Performed within density functional theory (DFT) the present band-structure calculations show that the best coincidence with the experiment is derived when the computational procedure is made employing for exchange correlation potential modified Becke-Johnson functional and also including Hubbard parameter U and spin-orbit coupling. The present DFT calculations indicate that S 3p states are the main contributors to the broad region of the valence band of Cu2HgSnS4 (in the central and upper portions of the band), while in its lower part contributions of Hg 6s and Sn 5s states prevail and Hg 5d states dominate at its bottom. A conduction band bottom is formed due to contributions of unoccupied Sn 5s states with also substantial input of unoccupied S 3p and Hg 6s states too. The DFT calculations allow for concluding that the Cu2HgSnS4 compound is a direct bandgap semiconductor with valence band maximum and conduction band minimum positioned at Г point. The calculations indicate that Cu2HgSnS4 is a very prospective material for application in optoelectronic devices and absorber layers of thin films solar cells.