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Quasiparticle self-consistent $GW$ calculations are presented for the band structures of $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$ in the orthorhombic $Pna{2}_{1}$ tetrahedrally coordinated crystal structures, which are closely related to the wurtzite structure of ZnO. Symmetry labeling of the bands near the gap is carried out, and effective-mass tensors are extracted for the conduction-band minimum and crystal-field split valence-band maxima (VBM) at $\mathrm{\ensuremath{\Gamma}}$. The gap is found to be direct at $\mathrm{\ensuremath{\Gamma}}$ and is 5.81 eV in $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and 5.46 eV in $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$. Electron-phonon coupling zero-point normalization is estimated to lower these gaps by about $0.2\ifmmode\pm\else\textpm\fi{}0.1$ eV. Optical response functions are calculated within the independent-particle long-wavelength limit, and they show the expected anisotropy of the absorption onsets due to the crystal-field splitting of the VBM. The results show that both materials are promising candidates as ultrawide-gap semiconductors with wurtzite-based tetrahedrally bonded crystal structures. Direct transitions from the lowest conduction band to higher bands, relevant to $n$-type doped material and transparent conduction applications, are found to start only above 3.9 eV and are allowed for only one polarization, and several higher band transitions are forbidden by symmetry. Alternative crystal structures, such as $R\overline{3}m$ and a rocksalt-type phase with a tetragonally distorted $P4/mmm$ spacegroup, both with octahedral coordination of the cations, are also investigated. They are found to have higher energy but about 20% smaller volume per formula unit. The transition pressures to these phases are determined, and for $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ they are found to be in good agreement with experimental studies. The $R\overline{3}m$ phase also has a comparably high but slightly indirect band gap, while the rocksalt-type phase is found to have a considerably smaller gap of about 3.1 eV in $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and 1.0 eV in $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$.

Orhorhombic $\alpha$-MoO$_3$ is a layered oxide with various applications and with excellent potential to be exfoliated as a 2D ultrathin film or monolayer. In this paper, we present a first-principles computational study of its vibrational properties. Our focus is on the zone center modes which can be measured by a combination of infared and Raman spectroscopy. The polarization dependent spectra are simulated. Calculations are also performed for a monolayer form in which "double layers" of Mo$_2$O$_6$ which are weakly van der Waals bonded in the $\alpha$-structure are isolated. Shift in phonon frequencies are analyzed., Comment: 13 pages, 13 figures, 14 tables

Alloys between group III nitrides and the corresponding heterovalent II–IV-N 2 compounds have recently been proposed as a further way of tuning the properties of nitride semiconductors. At 50% composition, a new ordered compound with composition ZnGeGa 2N 4 and space group P m n 2 1 was found, which has the lowest energy among wurtzite based structures. This structure obeys the local octet rule that every N is surrounded by two Ga, one Zn, and one Ge, ensuring local charge neutrality. Here, we investigate the vibrational properties of this new compound and provide predictions for its related infrared and Raman spectra, which may become useful for the characterization of this material. A group theoretical analysis, phonon frequencies and related Born effective charges, dielectric constants, infrared oscillator strengths, and Raman tensors are presented. Polarized infrared and Raman spectra for different scattering geometries are presented, as well as the phonon band structure and density of states.

𝛽-Ga2O3 has recently received attention as an ultra-wide-band-gap material, promising as transparent conductor and for high-power device applications. In this talk, we review some recent computational work on the electronic band structure and it defects. The electronic band structure was calculated using the quasiparticle self-consistent GW method including a correction of the screened Coulomb interaction W for electron-hole interaction and lattice-polarization effect. The selection rules of the optical transition help understand the anisotropy of the optical absorption.1 Among the point defects, which have already been discussed in various papers,2 we here focus on the two types of Ga–vacancies, which can occur on both the tetrahedral and octahedral Ga site. Electron paramagnetic resonance (EPR) related to the Ga-vacancy has been reported.3,4 Here we present first-principles calculations of the g-factor and hyperfine structure, which allow us to identify the EPR spectrum observed after high energy particle irradiation, with the tetrahedral Ga vacancy in the 2- charge state. Modification of the EPR spectrum under low temperature photoexcitation are explained by the following model. The EPR spectrum of the tetrahedral Ga vacancy shows superhyperfine interaction with two Ga atoms which are nearest neighbors to the oxygen on which the defect localizes in the q=2- charge state. The g-tensor has its largest deviation from the free-electron value for the crystallographic b-direction in which the defect Ga-O-Ga complex is oriented. According to Ref. 2, the tetrahedral Ga vacancy has its 2-/3- transition level closer to the conduction band minimum than the octahedral one. Upon illumination, this defect may become inactivated while the octahedral one is activated. The latter has a different orientation of its main largest g-tensor component but similar superhyperfine splitting with two Ga atoms. These computational findings explain the corresponding experimental observations. 1 Amol Ratnaparkhe and Walter R. L. Lambrecht, Appl. Phys. Lett. 110, 132103 (2017) 2 Peter Deak, Quoc Duy Ho, Florian Seemann, Balint Aradi, Michael Lorke, and Thomas Frauenheim, Phys. Rev. B 95, 075208 (2017) and references therein. 3 B. E. Kananen , L. E. Halliburton , K. T. Stevens , G. K. Foundos , and N. C. Giles, Appl. Phys. Lett. 110, 202104 (2017) 4 H.Jurgen von Bardeleben, unpublished

Quasiparticle self-consistent GW calculations are presented for the band structure of β-Ga2O3, including a lattice polarization correction of the screened Coulomb interaction W. It is found that this correction is of the order of 0.5 eV. When an estimated zero-point motion correction is also included, the direct gap is found to be 4.8 ± 0.1 eV in good agreement with experiment. The indirect gap is found to be 0.1 eV smaller. The origin of the anisotropy of the absorption edge is interpreted in terms of selection rules and the symmetry labeling of the bands at Γ.