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Hybrid functional calculations are presented for defects in LiGaO$_2$ with the fraction of non-local exchange adjusted to reproduce the recently reported exciton gap of 6.0 eV. We study how the defect transition levels of the main native defects change with respect to the band edges compared to earlier calculations which assumed a smaller band gap near 5.1 eV. In addition, we consider defect complexes formed by combining the main native donor Ga$_\mathrm{Li}$ with the main acceptors, $V_\mathrm{Li}$ and Li$_\mathrm{Ga}$ antisites as function of their relative position. These results are used to tentatively identify the photoluminescence bands previous assigned to donor-acceptor-pair recombination.

The theoretical discovery of new and stable 2D penta materials has stimulated the technological advancement due to the anticipated exotic properties of such structure, including the recent $\alpha$ phase and $\beta$ phase of penta-NiPS based on first-principles calculations. Inspired by the similarity between the theoretically proposed penta-NiPS and the experimentally synthesized ($\alpha$ phase) of penta-PdPSe, we preposed herein the $\beta$ phase penta-PdPSe as a new member of the penta-2D materials. Comprehensive analysis indicated that the $\beta$ phase penta-PdPSe is thermodynamically, dynamically, mechanically, and thermally stable, similar to the NiPS analog. It was found that $\beta$ penta-PdPSe is a wide band gap semiconductor with an indirect band gap of 1.58 eV, significantly lower than 2.15 eV for the $\alpha$ phase. Moreover, the two polymorphs of penta-PdPSe are a soft material with 2D Young's modulus of $E_a$ = 151 Nm$^-1$ and $E_b$ = 123 Nm$^-1$ for the $\beta$ phase, to be compared with $E_a$ = 155 Nm$^-1$ and $E_b$ = 113 Nm$^-1$ for the $\alpha$ phase. The calculated adsorption coefficient shows that $\beta$ phase penta-PdPSe is acceptable for electronic and optical nanodevice., Comment: arXiv admin note: substantial text overlap with arXiv:2210.08267

Various defects in ZnO, focused on substitutional N$_O$ and N$_2$ in various sites, O-site, interstitial and Zn-site are studied using first-principles calculations with the goal of understanding the electron paramagnetic resonance (EPR) center reported for N$_2$ in ZnO and substitutional N on the O-site. The $g$ tensors are calculated using the gauge including projector augmented wave (GIPAW) method and compared with experiments. The $g$-tensor of the free N$_2^+$ and N$_2^-$ radicals and their various contributions within the GIPAW theory are analyzed first to provide a baseline reference for the accuracy of the method and for understanding the N$_2$ behavior in ZnO. Previous controversies on the site location of N$_2$ in ZnO for this EPR center and on the shallow or deep nature and donor or acceptor nature of this center are resolved. We find that the N$_2$ on the Zn site is mostly zinc-vacancy like in its spin density and $g$-tensor, while for the O-site, a model with the N$_2$ axis lying in-the basal plane and the singly occupied $\pi_g$-orbital along the {\bf c} axis provides good agreement with experiment. For the interstitial location, if the N$_2$ is not strongly interacting with the surroundings, no levels in the gap are found and hence also no possible EPR center. The calculated $g$-tensors for N$_O$ and $V_ Zn$ are also found to be in good agreement with experiment. The effects of different functionals affecting the localization of the spin density are shown to affect the $g$-tensor values.

The discovery of new and stable two-dimensional (2D) materials with exotic properties is essential for technological advancement. Inspired by the recently reported penta-PdPSe, we proposed penta-NiPS as a new member of the penta-2D materials based on first-principles calculations. The penta-NiPS monolayer is stable in two polymorphs including the alpha phase with identical structure as penta-PdPSe, and the newly proposed beta phase with rotated sublayers. Comprehensive analysis indicated that both phases are thermodynamically, dynamically, mechanically, and thermally stable. The penta-NiPS is a soft material with 2D Young's modulus of Ea=208 Nm^-1 and Eb=187 Nm^-1 for the alpha phase and Ea=184 Nm^-1 and Eb=140 Nm^-1 for the beta phase. Interestingly, the alpha-penta-NiPS showed nearly zero Poisson's ratios along the in-plane direction, where its dimensions would be maintained when being extended. For electronic applications, we demonstrated that penta-NiPS is the wide band gap semiconductor with an indirect band gap of 2.35 eV for alpha phase, and 2.20 eV for beta phase.

Doping of the ultrawide band gap semicodnctor LiGaO$_2$ ($E_g=5.6$ eV) with N$_2$, NO and O$_2$ molecules placed in either Ga or Li-vacancies is studied using first-principles calculations. These molecular dopants are considered as potential acceptors for $p$-type doping. Their optimal placement and orientation relative to the lattice is studied as well as their transition levels and energy of formation. Unfortunately, they are found to have deep acceptor level transition states. We discuss the relation of the transition levels to the one-electron levels, their spin state and defect wave functions. They are found to be closely related to those of the corresponding vacancies., Comment: 8 pages, 8 figures, 1 supplementary material

A computational study of the Electron Paramagnetic Resonance (EPR) $g$-tensors and hyperfine tensors of Li and Ga vacancies in LiGaO$_2$ is presented. Density Functinal Theory (DFT) calculations are carried out of the Ga and Li vacancies using the DFT+U approach in the charge states which carry an unpaired spin. In both vacancies the hole is located on one oxygen $p$-orbital adjacent to the vacancy. Apical and different basal plane O are considered. The magnetic resonance parameters of the defects are determined using the Gauge Including Projector Augmented Wave (GIPAW) method. The EPR spectra of $V_\mathrm{Ga}^{2-}$ is characterized by a quasi-isotropic superhyperfine (SHF) interaction with one Ga nucleus and for the apical O spin gives a $g$-tensor with maximum oriented along the bond direction from that O to its other Ga neighbor. For $V_\mathrm{Li}^0$ there is a quasi-isotropic SHF interaction with two Ga nuclei and the $g$-tensor maximum is along ${\bf c}$ for the basal plane O spin. Both of these are in agreement with experiment but we predict also the $g$-tensors for the other possible localization of the spins as well as the small hyperfine splittings (as yet not observed) on Li. The energies of formation and transition levels of the corresponding defects provide insight into the conditions required to activate these EPR spectra.

The native point defects are studied in LiGaO2 using hybrid functional calculations. We find that the relative energy of formation of the cation vacancies and the cation antisite defects depends strongly on the chemical potential conditions. The lowest energy defect is found to be the Ga_Li^2+ donor. It is compensated mostly by V_Li^-1and in part by Li_Ga^-2 in the more Li-rich conditions. The equilibrium carrier concentrations are found to be negligible because the Fermi level is pinned deep in the gap and this is consistent with insulating behavior in pure LiGaO2. The V_Ga has high energy under all reasonable conditions. Both the Ga_Li and the V_O are found to be negative U centers with deep 2+/0 transition levels., Comment: 6 pages, 5 figures