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We use photo-electron paramagnetic resonance (EPR) measurements and first-principles calculations to identify and explain the properties of carbon in AlN. We present clear evidence for carbon substitution on the nitrogen site (C$_{\rm N}$). We also clarify the origin of a widely observed EPR spectra in AlN that, although often attributed to a deep donor defect, we demonstrate is surprisingly due to C$_{\rm N}$. Finally, we show the presence of C$_{\rm N}$ is consistent with the absorption spectra at 4.7 eV observed in AlN., Comment: 5 pages, 3 figures

We find the recently developed strongly constrained and appropriately normed (SCAN) functional, now widely used in calculations of many materials, is not able to reliably describe the properties of deep defects and small polarons in a set of wide-bandgap semiconductors and insulators (ZnO, ZnSe, GaN, Ga$_2$O$_3$, and NaF). By comparing first-principles calculations using the SCAN functional against established experimental information and first-principles calculations using a hybrid functional, we find that the SCAN functional systematically underestimates the magnitude of the structural distortions at deep defects and tends to delocalize the charge density of these defect states., Comment: 10 pages, 8 figures

While nanoscale semiconductor crystallites provide versatile fluorescent materials for light-emitting devices, such nanocrystals suffer from the "dark exciton"$\unicode{x2014}$an optically inactive electronic state into which the nanocrystal relaxes before emitting. Recently, a theoretical mechanism was discovered that can potentially defeat the dark exciton. The Rashba effect can invert the order of the lowest-lying levels, creating a bright excitonic ground state. To identify materials that exhibit this behavior, here we perform an extensive high-throughput computational search of two large open-source materials databases. Based on a detailed understanding of the Rashba mechanism, we define proxy criteria and screen over 500,000 solids, generating 173 potential "bright-exciton" materials. We then refine this list with higher-level first-principles calculations to obtain 28 candidates. To confirm the potential of these compounds, we select five and develop detailed effective-mass models to determine the nature of their lowest-energy excitonic state. We find that four of the five solids (BiTeCl, BiTeI, Ga$_2$Te$_3$, and KIO$_3$) can yield bright ground-state excitons. Our approach thus reveals promising materials for future experimental investigation of bright-exciton nanocrystals., Comment: 19 pages, 4 figures

Ga$_2$O$_3$ is a wide-bandgap material of interest for a wide variety of devices, many of these requiring heterostructures, for instance to achieve carrier confinement. A common method to create such heterostructures is to alloy with In$_2$O$_3$ or Al$_2$O$_3$. However, the lattice constants of these materials are significantly different from those of Ga$_2$O$_3$, leading to large amounts of strain in the resulting heterostructure. If the thickness of the heterostructure is increased, this can lead to cracking. By considering alloys of In$_2$O$_3$ and Al$_2$O$_3$, the lattice constants can be tailored to those of Ga$_2$O$_3$, while still keeping a sizable conduction-band offset. We use density functional theory with hybrid functionals to investigate the structural and electronic properties of In$_2$O$_3$ and Al$_2$O$_3$ alloys in the bixbyite, corundum, and monoclinic structures. We find that the lattice constants increase with In incorporation. Bandgaps decrease nonlinearly with increasing In concentration. We find the (In$_{\rm 0.25}$Al$_{\rm 0.75}$)$_{\rm 2}$O$_{\rm 3}$ monoclinic structure to be of particular interest, as it closely matches the Ga$_2$O$_3$ lattice constants while providing an indirect/direct bandgap of 5.94/5.70 eV and a conduction-band offset of 1 eV compared to Ga$_2$O$_3$., Comment: 6 pages, 5 figures

Chalcogen vacancies in the semiconducting monolayer transition-metal dichalcogenides (TMDs) have frequently been invoked to explain a wide range of phenomena, including both unintentional p-type and n-type conductivity, as well as sub-band gap defect levels measured via tunneling or optical spectroscopy. These conflicting interpretations of the deep versus shallow nature of the chalcogen vacancies are due in part to shortcomings in prior first-principles calculations of defects in the semiconducting two-dimensional (2D) TMDs that have been used to explain experimental observations. Here we report results of hybrid density functional calculations for the chalcogen vacancy in a series of monolayer TMDs, correctly referencing the thermodynamic charge transition levels to the fundamental band gap (as opposed to the optical band gap). We find that the chalcogen vacancies are deep acceptors and cannot lead to n-type or p-type conductivity. Both the (0/$-1$) and ($-$1/$-$2) transition levels occur in the gap, leading to paramagnetic charge states S=1/2 and S=1, respectively, in a collinear-spin representation. We discuss trends in terms of the band alignments between the TMDs, which can serve as a guide to future experimental studies of vacancy behavior.

In this comment on "Origin of correlated isolated flat bands in copper-substituted lead phosphate apatite" (arXiv:2307.16892 [cond-mat.supr-con]), we discuss the flat half-occupied two-band manifold that appears in $\mathrm{Pb}_9\mathrm{Cu}(\mathrm{PO}_4)_6(\mathrm{OH})_2$ when using a semilocal DFT functional. We argue that the flat band is an artifact of the functional's overestimation of the energy of the oxygen p states in the valence band. When using the HSE hybrid functional, the energy of the oxygen p states is reduced, and the copper-derived manifold splits into one fully occupied and one empty band. While these results do not rule out the possibility of superconductivity in doped LK-99, they do predict that stoichiometric $\mathrm{Pb}_9\mathrm{Cu}(\mathrm{PO}_4)_6(\mathrm{OH})_2$ is an insulator, not a superconductor. Furthermore, we have shown that future first-principles studies of these materials should employ hybrid functionals or other advanced methods to ensure that the oxygen-derived valence-band energies are correctly described., Comment: 2 pages, 1 figure

The lone-pair s states of germanium, tin, and lead underlie many of the unconventional properties of the inorganic metal halide perovskites. Dynamic stereochemical expression of the lone pairs is well established for perovskites based on all three metals, but previously only the germanium perovskites were thought to express the lone pair crystallographically. In this work, we use advanced first-principles calculations with a hybrid functional and spin-orbit coupling to predict stable monoclinic polar phases of $\mathrm{CsSnI}_3$ and $\mathrm{CsSnBr}_3$, which exhibit a ferroelectric distortion driven by stereochemical expression of the tin lone pair. We also predict similar metastable ferroelectric phases of $\mathrm{CsPbI}_3$ and $\mathrm{CsPbBr}_3$. In addition to ferroelectricity, these phases exhibit the Rashba effect. Spin splitting in both the conduction and valence bands suggests that nanostructures based on these phases could host bright ground-state excitons. Finally, we discuss paths toward experimental realization of these phases via electric fields and tensile strain., Comment: 21 pages, 4 figures