Structural and electronic properties of layered semiconductor chalcogenide crystals: TIGaSe2, TIGaS2 and TIInS2

Abstract

We report the density functional theory structure optimization and electronic structure calculations of TIGaSe2, TIGaS2, and TlInS2 compounds. We find that there is a weak bonding between TI and the chalcogen while Ga and In make covalent bonds with the chalcogen. The s orbitals of the chalcogen hybridize with the d orbitals of TI, bringing additional directionality to the TI-chalcogen interaction. There is no direct TI-TI bond. Although these three compounds are extremely similar, the band structures exhibit quantitative and qualitative differences. The optimized TIGaSe2 and TIInS2 crystals have direct band gaps while TIGaS2 has an indirect gap. The lowest unoccupied states have a bonding character between the p orbitals of the adjacent TI atoms. These states form conduction channels along the TI chains. The valence band maximum originates from TI-s orbitals and its position in the Brillouin zone is robust. Almost degenerate TI-p related bands are affected differently by the inter layer interaction. Therefore, minute changes in the structure may shuffle their energy levels, resulting in qualitatively and quantitatively different band gaps in these extremely similar compounds. External pressure and defects may initiate the same type of electronic structure changes resulting in contradicting results in experiments. A small down-shifting of the unoccupied parabolic bands lead to several valleys in the conduction band of TIGaSe2.