DX behavior limits n-type carrier concentrations in ultrawide-bandgap nitrides such as aluminum nitride (AlN) and cubic boron nitride (c-BN). Instead of acting as effective-mass donors, DX centers capture two electrons, stabilizing a negative charge state that leads to self compensation. Silicon is the most effective n-type dopant in this class of materials; in AlN, its DX level [(i.e., the (+/-) transition level] is ∼270 meV from the conduction-band minimum. This implies that many silicon impurities incorporated into AlN will be negatively charged and compensate the intended n-type doping. By combining density functional theory calculations of temperature-dependent band gaps and Si dopant transition levels, we show here that significant compensation occurs in silicon-doped AlN, even in the absence of any other defects. This compensation strongly limits free electron concentrations which become independent of doping concentration, and donor activation is only significant for light doping scenarios. Higher free carrier concentrations can be achieved in AlGaN alloys or in c-BN, where the DX level sits closer to the conduction-band minimum.

Sub-bandgap optical absorption in AlN between 2 eV and 4 eV is widely observed, but its microscopic origin remains contested. Using photo-induced electron paramagnetic resonance (photo-EPR) and optical absorption spectroscopy on the same samples, we demonstrate a correlation between this absorption band and the neutral charge state of substitutional carbon on the nitrogen site (C_N). Hybrid functional calculations of the optical absorption spectra show that a transition involving C_N and the valence band occurs near 3.3 eV, which agrees well with a peak identified within the measured optical absorption between 2 eV and 4 eV. This conclusion requires the combined ability to manipulate the charge state of carbon using photo-EPR and to use first-principles calculations of the absorption line shape that account for the dispersion of the valence band and the energy dependence of the optical matrix elements.

The optical properties of defects in solids produce rich physics, from gemstone coloration to single-photon emission for quantum networks. Essential to describing optical transitions is electron-phonon coupling, which can be predicted from first principles but requires computationally expensive evaluation of all phonon modes in simulation cells containing hundreds of atoms. We demonstrate that this bottleneck can be overcome using machine learning interatomic potentials with negligible accuracy loss. A key finding is that atomic relaxation data from routine first-principles calculations suffice as a dataset for fine-tuning, though additional data can further improve models. The efficiency of this approach enables studies of defect vibrational properties with high-level theory. We fine-tune to hybrid functional calculations to obtain highly accurate spectra, comparing with explicit calculations and experiments for various defects. Notably, we resolve fine details of local vibrational mode coupling in the luminescence spectrum of the T center in Si, a prominent quantum defect.

We show with hybrid density functional theory calculations that chalcogen donors other than oxygen (i.e. SN, SeN, and TeN) give rise to deep donor states in aluminum nitride. These donors trap a localized electron in their neutral charge state, leading to deep (+/0) donor levels that are 0.45 eV or more from the conduction-band edge. As such, this behavior is distinct from theDXbehavior leads to deep (+/-) levels which affects other donors such as ONand SiAl. We highlight how these results hint at the formation of small electron polarons in AlN, which are found to be unstable in the bulk, but metastable when bound to donor dopants like SiAland the chalocogens, with activation energies on the order of 0.2-0.3 eV. These results indicate that S, Se, and Te are not shallow donor dopants in aluminum nitride and identify origins of the experimentally observed ∼200-300 meV activation energies for dopant activation in donor-doped samples.

We present photo-electron paramagnetic resonance (EPR) measurements and first-principles calculations that indicate germanium (Ge) is a DX-center in AlGaN. Our photo-EPR measurements on Ge-doped AlGaN samples show no EPR spectra in the dark, while persistent EPR spectra is observed upon photoexcitation with photon energies greater than 1.3 eV. Thermally annealing the samples decreased the EPR signal, with the critical temperature to quench the EPR signal being larger in the lower Al-content sample. Using detailed first-principles calculations of Ge in AlGaN, we show all of these observations can be explained by accounting for the DX configuration of Ge in AlGaN.

Compensating deep level defect states that influence the electrical properties of beta-phase gallium oxide (beta-Ga2O3) can be introduced intentionally through doping, or unintentionally through unwanted impurities and intrinsic defects. Understanding the complex behavior of these compensating centers is essential for optimizing beta-Ga2O3 for both high voltage and RF electronics. Applications in harsh environments, such as in space, add further complications due to defect creation resulting from high energy irradiation. Due to its inherent n-type conductivity, compensating deep acceptor states intentionally introduced by doping with Nitrogen, Iron or Magnesium to create semi-insulating regions of device structures are of great interest. However, inadvertent impurities such as Carbon are also predicted to create deep acceptors and can be problematic. The presence of intrinsic defects that form during growth or introduced by harsh environments makes the understanding of how defects impact material and device properties very complex. Here we use deep level transient (thermal) spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) to identify, compare and characterize individual deep acceptor states created by both extrinsic and intrinsic sources in beta-Ga2O3 and compare with theoretical predictions. Compensation efficiencies between acceptor choices are compared, and some of the more unusual behaviors of very deep states present below midgap are described, where both conduction and valence band transitions are observed and explained. Radiation studies used to differentiate extrinsic from intrinsic sources will be discussed in the context of unraveling the comprehensive impacts of defects in beta-Ga2O3.

Oxygen is a common impurity in AlN samples. Using hybrid density functional calculations, we investigate the role of substitutional oxygen (ON) in the optical absorption. We construct configuration coordination diagrams for ON and related complexes. Our results indicate that an optical transition involving ON− (a DX center) gives rise to an absorption band peaked at 2.22 eV, suggesting it is a source of the absorption band with an onset at ∼ 2 eV observed in oxygen-containing samples. We also propose that neutral ON–DX complexes can form, which would give rise to absorption peaking at 3.06 eV. In addition, we find that oxygen, in spite of its DX character, may behave as an “optically shallow donor” and be involved in optical transitions from deep defect states to the conduction band. This observation provides an alternative physical mechanism for the optical absorption bands observed in AlN samples in the visible and ultraviolet (UV) region.

Nearly all ultrawide-bandgap oxides are affected by hole localization that limits $p$-type conductivity and thus potential applications for these materials. Highly localized holes, also known as hole polarons, trap in the vicinity of acceptor dopants, giving rise to large ionization energies and severely constraining free hole concentrations. Though this hole-trapping behavior affects wurtzite zinc oxide, rocksalt zinc oxide was recently found to be resistant to the formation of hole polarons. Moreover, $p$-type doping using lithium acceptors was predicted to be achievable. While rocksalt zinc oxide is metastable and has a band gap near $\sim$3 eV, here it is found that zinc magnesium oxide (Zn$_{\rm x}$Mg$_{\rm 1-x}$O) alloys remain $p$-type dopable within the stable rocksalt crystal structure, in addition to exhibiting band gaps in excess of 4 eV. As in rocksalt zinc oxide, alkali acceptors are shallow in zinc magnesium oxide and do not appear to be affected by donor compensation. These results indicate that alkali-doped Zn$_{\rm x}$Mg$_{\rm 1-x}$O alloys are a promising system for achieving a $p$-type dopable ultrawide-bandgap oxide.

The inorganic tin halide perovskites (such as CsSnBr_{3} and CsSnI_{3}) exhibit high hole concentrations, which have been attributed to the presence of intrinsic defects such as cation vacancies. Despite their promise as photovoltaic materials, this high intrinsic conductivity limits deployment. Using hybrid density functional theory, we examine the impact of these intrinsic defects on the electronic properties of CsSnBr_{3} and CsSnI_{3}. We find that tin and cesium vacancies have low formation energies, especially under Sn-poor conditions, in agreement with prior work, and that the presence of these native acceptor defects can lead to high hole concentrations. However, effective donor doping of these systems can be obtained via Sc or Y incorporation. These impurities substitute on the Sn site, where they act as single donors. By examining the formation energy of these dopants versus the native defects, we show that Sc or Y doping should lead to a strong compensation, reducing hole concentrations under Sn-rich conditions.

Nearly all ultrawide-bandgap oxides are affected by hole localization that limits p-type conductivity and thus potential applications for these materials. Highly localized holes, also known as hole polarons, trap in the vicinity of acceptor dopants, giving rise to large ionization energies and severely constraining free hole concentrations. Though this hole-trapping behavior affects wurtzite zinc oxide, rocksalt zinc oxide was recently found to be resistant to the formation of hole polarons. Moreover, p-type doping using lithium acceptors was predicted to be achievable. While rocksalt zinc oxide is metastable and has a band gap near ∼3 eV, here it is found that zinc magnesium oxide (Zn_ xMg_ 1-xO) alloys remain p-type dopable within the stable rocksalt crystal structure, in addition to exhibiting band gaps in excess of 4 eV. As in rocksalt zinc oxide, alkali acceptors are shallow in zinc magnesium oxide and do not appear to be affected by donor compensation. These results indicate that alkali-doped Zn_ xMg_ 1-xO alloys are a promising system for achieving a p-type dopable ultrawide-bandgap oxide.

A description of electron-phonon coupling at a defect or impurity is essential to characterizing and harnessing its functionality for a particular application. Electron-phonon coupling limits the amount of useful light produced by a single-photon emitter and can destroy the efficiency of optoelectronic devices by enabling defects to act as recombination centers. Information on atomic relaxations in the excited state of the center is needed to assess electron-phonon coupling but may be inaccessible due to failed convergence or computational expense. Here we develop an approximation technique to quantify electron-phonon coupling using only the forces of the excited state evaluated in the equilibrium geometry of the ground state. The approximations are benchmarked on well-studied defect systems, namely C_ N in GaN, the nitrogen-vacancy center in diamond, and the carbon dimer in h-BN. We demonstrate that the zero-phonon line energy can be approximated with just a single mode, while the Huang-Rhys factor converges by including displacements up to the second nearest neighbors. This work also provides important insight into the success of the widely utilized one-dimensional accepting-mode approximation, specifically demonstrating that the accepting-mode Huang-Rhys factor is a strict upper bound on the full multidimensional Huang-Rhys factor.

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.

Ga2O3 is a wide-band-gap 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 In2O3 or Al2O3. However, the lattice constants of these materials are significantly different from those of Ga2O3, 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 In2O3 and Al2O3, the lattice constants can be tailored to those of Ga2O3, while still keeping a sizable conduction-band offset. We use density functional theory with hybrid functionals to investigate the structural and electronic properties of In2O3 and Al2O3 alloys in the bixbyite, corundum, and monoclinic structures. We find that the lattice constants increase with In incorporation. Band gaps decrease nonlinearly with increasing In concentration. We find the (In0.25Al0.75)2O3 monoclinic structure to be of particular interest, as it closely matches the Ga2O3 lattice constants while providing an indirect/direct band gap of 5.94/5.70 eV and a conduction-band offset of 1 eV compared to Ga2O3.

While semiconductor nanocrystals provide versatile fluorescent materials for light-emitting devices, their brightness suffers from the "dark exciton"─an optically inactive electronic state into which nanocrystals relax before emitting. Recently, a theoretical mechanism, the Rashba effect, was discovered that can overcome this limitation by inverting the lowest-lying levels and creating a bright excitonic ground state. However, no methodology is available to systematically identify materials that exhibit this inversion, hindering the development of superbright nanocrystals and their devices. Here, based on a detailed understanding of the Rashba mechanism, we demonstrate a procedure that reveals previously unknown "bright-exciton" nanocrystals. We first define physical criteria to reduce over 500,000 known solids to 173 targets. Higher-level first-principles calculations then refine this list to 28 candidates. From these, we select five with high oscillator strength and develop effective-mass models to determine the nature of their lowest excitonic state. We confirm that four of the five solids yield bright ground-state excitons in nanocrystals. Thus, our results provide a badly needed roadmap for experimental investigation of bright-exciton nanomaterials.

A major shortcoming of ultrawide-bandgap (UWBG) semiconductors is unipolar doping, in which eithern-type orp-type conductivity is typically possible, but not both within the same material. For UWBG oxides, the issue is usually thep-type conductivity, which is inhibited by a strong tendency to form self-trapped holes (small polarons) in the material. Recently, rutile germanium oxide (r-GeO2), with a band gap near 4.7 eV, was identified as a material that might break this paradigm. However, the predicted acceptor ionization energies are still relatively high (∼0.4 eV), limitingp-type conductivity. To assess whether r-GeO2is an outlier due to its crystal structure, the properties of a set of rutile oxides are calculated and compared. Hybrid density functional calculations indicate that rutile TiO2and SnO2strongly trap holes at acceptor impurities, consistent with previous work. Self-trapped holes are found to be unstable in r-SiO2, a metastable polymorph that has a band gap near 8.5 eV. Group-III acceptor ionization energies are also found to be lowest among the rutile oxides and approach those of GaN. Acceptor impurities have sufficiently low formation energies to not be compensated by donors such as oxygen vacancies, at least under O-rich limit conditions. Based on the results, it appears that r-SiO2has the potential to exhibit the most efficientp-type conductivity when compared to other UWBG oxides.

Ultra-wide bandgap semiconductors, with bandgaps greater than 3.5 eV, have immense potential in power-switching electronic applications and ultraviolet light emitters. But the development of these materials faces a number of challenges, many of which relate to controlling electrical conductivity. In this work, we review the major obstacles for a set of these materials (focusing on AlGaN, AlN, BN, Ga2O3, Al2O3, and diamond) including limitations in n- and p-type doping and the effects of impurities and native point defects. We present an in-depth discussion on ultra-wide-bandgap nitride and oxide semiconductors, which face several similar challenges, as well as diamond, which presents a more unique scenario. The biggest obstacle for these semiconductors is attaining bipolar electrical conductivity, which means achieving both n-type and p-type conductivity within the same material. Toward this end, we also discuss potential future research directions that may lead to the development of bipolar ultra-wide bandgap semiconductor devices.

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-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-N. Finally, we show the presence of C-N is consistent with the absorption spectra at 4.7 eV observed in AlN.

Recently, LiGa5O8 was claimed to be a p-type dopable ultrawide-bandgap oxide, based on measurements of undoped material. Here, the electronic properties of potential acceptor dopant impurities in LiGa5O8 are calculated using hybrid density functional theory to evaluate their potential for causing p-type conductivity. As with the related compound LiGaO2, the heavy oxygen-derived valence bands lead to stable self-trapped holes in LiGa5O8. Acceptor defects and dopants also bind trapped holes (or small polarons), which lead to large acceptor ionization energies. The calculations here indicate that neither native acceptor defects (such as cation vacancies or antisites) nor impurity dopants can give rise to p-type conductivity in LiGa5O8. Optical transitions associated with these defects are also calculated, in order to allow for possible experimental verification of their behavior.

Doping can tailor the electronic, optical, and magnetic properties of colloidal nanocrystals for optimal performance in devices. We present a novel synthesis of copper (Cu)-doped PbS colloidal nanocrystals based on Cu-to-Pb cation exchange. These nanocrystals exhibit both a blueshift of the energy of the lowest exciton peak, relative to undoped PbS nanocrystals, and photoluminescence. This blueshift of the lowest exciton energy is unique, as only redshifts are observed in Cu-doped II-VI and III-V nanocrystals. Furthermore, retention of the strong infrared photoluminescence is noteworthy because the photoluminescence in other doped PbS nanocrystals is quenched. Three possible reasons for these results are discussed. Cu-doping is confirmed using inductively coupled plasma optical emission spectroscopy (ICP-OES), in addition to high-angle annular dark-field imaging scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS). Coupled with photoluminescence quantum yields (PLQYs) comparable to undoped PbS, the shorter photoluminescence lifetimes measured from time-correlated single photon counting (TCSPC) of the Cu-doped PbS nanocrystals show an increase in the radiative and nonradiative rates by 3-4x as compared to pure oleate-capped PbS nanocrystals. These improved radiative rates could lead to brighter nanocrystal infrared-emitting devices such as single-photon emitters.

First-principles calculations of defects and electron–phonon interactions play a critical role in the design and optimization of materials for electronic and optoelectronic devices. The late Audrius Alkauskas made seminal contributions to developing rigorous first-principles methodologies for the computation of defects and electron–phonon interactions, especially in the context of understanding the fundamental mechanisms of carrier recombination in semiconductors. Alkauskas was also a pioneer in the field of quantum defects, helping to build a first-principles understanding of the prototype nitrogen-vacancy center in diamond, as well as identifying novel defects. Here, we describe the important contributions made by Alkauskas and his collaborators and outline fruitful research directions that Alkauskas would have been keen to pursue. Audrius Alkauskas’ scientific achievements and insights highlighted in this article will inspire and guide future developments and advances in the field.