Theoretical study of Ga2Se3,Ga2Te3Ga2Se3,Ga2Te3 and Ga2(Se1-xTex)3Ga2(Se1-xTex)3: Band-gap engineering

The structure and band structure of Ga2Se3,Ga2Te3Ga2Se3,Ga2Te3 and Ga2(Se1-xTex)3Ga2(Se1-xTex)3 have been investigated systematically, providing a good and extensive foundation for further experimental and theoretical investigations. The upper bound of self-consistent (sc) GW band-gap variation due to vacancy ordering variation for Ga2Se3Ga2Se3 and Ga2Te3Ga2Te3 is 0.33 and 0.21 eV, respectively, whereas the upper bound of band-gap variation based on GGA (PBE) calculations is larger, 0.55 and 0.51 eV, respectively. Hybrid density functional theory calculations have little effect on the band-gap variation upper bound of Ga2Se3Ga2Se3, but reduce the band-gap variation upper bound of Ga2Te3Ga2Te3 by ∼∼0.1 eV (when the default mixing ratio (0.25) is used). Zigzag-line vacancy ordering and straight-line vacancy ordering produce a valence band maximum at the ΓΓ point and not at the ΓΓ point, respectively. The conduction band minimum is at ΓΓ point and not at ΓΓ point for Ga2Se3Ga2Se3 and Ga2Te3Ga2Te3, respectively. The calculated scGW band-gaps of mono-Ga2Se3Ga2Se3, ortho-Ga2Se3Ga2Se3, mono-Ga2Te3Ga2Te3 and ortho-Ga2Te3Ga2Te3 at 0 K are 2.53, 2.20, 1.54 and 1.33 eV, respectively. The calculated band-gap is noticeably dependent on the level of self-consistency of the GW methods. The band-gap is larger for a higher level of self-consistency of the GW method, from G0W0 to scGW. The calculated band-gap curve of Ga2(Se1-xTex)3Ga2(Se1-xTex)3 is noticeably convex downwards. The deviation between our experimental measured band-gap curve and the calculated band-gap curve based on completely disordered anion distribution and straight-line vacancy ordering is small and relatively large at the Te-rich end, for PBE + correction and HSE06, respectively. In addition, our previous method (Huang et al., 2013) based on the shift of plane layer atoms to generate disordered vacancies is extended in this paper to the shift of the zigzag plane layer of atoms, which is important for explaining the cubic symmetry of vacancy-disordered phases.