No abstract

By performing first-principles electronic structure calculations in frames of density functional theory we study the dependence of the valence band shape on the thickness of few-layer III-VI crystals (GaS, GaSe, and InSe). We estimate the critical thickness of transition from the bulklike parabolic to the ring-shaped valence band. Direct supercell calculations show that the ring-shaped extremum of the valence band appears in β-GaS and β-GaSe at a thickness below 6 tetralayers (∼4.6 nm) and 8 tetralayers (∼6.4 nm), respectively. Zone-folding calculations estimate the β-InSe critical thickness to be equal to 28 tetralayers (∼24.0 nm). The origin of the ring-shaped valence band maximum can be understood in terms of k·p theory, which provides a link between the curvature of the energy bands and the distance between them. We explain the dependence of the band shape on the thickness, as well as the transition between two types of extremes, by the k-dependent orbital composition of the topmost valence band. We show that in the vicinity of critical thickness the effective mass of holes in III-VI compounds depends strongly on the number of tetralayers.

International audienc

The electronic band structure of helical iodine chains was calculated within the empirical tight binding approach. The screw symmetry of the system was used to reduce the size of the problem to a single atom with four atomic orbitals. The overlap parameters were fitted to reproduce the DFT results for a linear iodine chain. The obtained results for helical chains have shown the energy band splitting due to the overlap between p orbitals introduced by the structure twisting. The splitting magnitude depends on the pitch of the helix.