@article{baker_bowes_harris_irving_2019, title={An informatics software stack for point defect-derived opto-electronic properties: the Asphalt Project}, volume={9}, ISSN={["2159-6867"]}, url={http://dx.doi.org/10.1557/mrc.2019.106}, DOI={10.1557/mrc.2019.106}, abstractNote={Computational acceleration of performance metric-based materials discovery via high-throughput screening and machine learning methods is becoming widespread. Nevertheless, development and optimization of the opto-electronic properties that depend on dilute concentrations of point defects in new materials have not significantly benefited from these advances. Here, the authors present an informatics and simulation suite to computationally accelerate these processes. This will enable faster and more fundamental materials research, and reduce the cost and time associated with the materials development cycle. Analogous to the new avenues enabled by current first-principles-based property databases, this type of framework will open entire new research frontiers as it proliferates.}, number={3}, journal={MRS COMMUNICATIONS}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Irving, Douglas L.}, year={2019}, month={Sep}, pages={839–845} }
 @article{baker_bowes_harris_irving_2019, title={Mechanisms governing metal vacancy formation in BaTiO3 and SrTiO3 (vol 124, 114101, 2018)}, volume={125}, ISSN={["1089-7550"]}, DOI={10.1063/1.5084251}, abstractNote={First Page}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Irving, Douglas L.}, year={2019}, month={Jan} }
 @article{harris_gaddy_collazo_sitar_irving_2019, title={Oxygen and silicon point defects in Al0.65Ga0.35N}, volume={3}, ISSN={["2475-9953"]}, DOI={10.1103/PhysRevMaterials.3.054604}, abstractNote={The formation energies of oxygen and silicon impurities have been examined explicitly in ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$ using hybrid exchange-correlation density-functional theory simulations. Both impurities were initialized in on-site substitutional and off-site DX configurations in a range of charge states. The ${\mathrm{O}}_{\mathrm{N}}^{+1}$ donor was found to always relax into an on-site configuration, and its formation energy is relatively independent of local chemistry (the configuration of Al and Ga atoms surrounding the defect). By contrast, the ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$ acceptor almost always relaxes into a DX configuration, with a formation energy that is strongly dependent on local chemistry. The differences in formation energy of distinct ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$ defect configurations are understood through the interplay of two qualitative trends in the types of nearest-neighbor bonds (O-Al or O-Ga), as well as the subtler influence of the lengths of the O-Al bonds. Knowledge of ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$ formation energies as well as the relative frequencies of sites with different local chemistry allows one to compute the relative site occupancies of ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$. Because the thermodynamic transition levels associated with different defect configurations are unique, the ${\mathrm{O}}_{\mathrm{N}}$ DX transition is associated with multiple defect levels. ${\mathrm{Si}}_{\mathrm{III}}$, where III represents the group III cation of Al or Ga, provides an interesting counterexample. ${\mathrm{Si}}_{\mathrm{III}}^{+1}$ is predicted to be the dominant charge state across the entire band gap of ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$, and little dependence of the formation energy on the composition of nearby cation sites was found. This is explained by the fact that the first-nearest neighbors are all of the same species (N), so the local environment is similar to a bulk III nitride, in which on-site ${\mathrm{Si}}_{\mathrm{III}}^{+1}$ is stable across the same Fermi level range (i.e., below the band gap of ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$). Thus, the trends in the energetics of ${\mathrm{O}}_{\mathrm{N}}$ and ${\mathrm{Si}}_{\mathrm{III}}$ in ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$ are both determined by the chemistry of the four nearest-neighbor sites surrounding the defect site.}, number={5}, journal={PHYSICAL REVIEW MATERIALS}, author={Harris, Joshua S. and Gaddy, Benjamin E. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2019}, month={May} }
 @article{bowes_wu_baker_harris_irving_2019, title={Space charge control of point defect spin states in AlN}, volume={115}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5099916}, DOI={10.1063/1.5099916}, abstractNote={One barrier to developing quantum information systems based on impurity point defects is that the desirable spin states of the defects are often unstable for Fermi levels obtained at increased impurity concentrations. The space charge induced band bending near the interface of Si/Mg aluminum nitride (AlN) homojunction is investigated computationally as a method to control the concentration, spin state, and position of such point defects. This is done by solving Poisson's equation with the charge density described by a grand canonical defect chemistry model informed by hybrid-functional density functional theory (DFT) calculations. Previous experimental works have found unintentional carbon and oxygen impurities pervade AlN homojunctions. First principles calculations have predicted the neutral complex between an aluminum vacancy and oxygen impurity on a neighboring nitrogen site (vAl-1ON)0 has a spin triplet configuration, which is stable in a region when the Fermi level is below midgap. From defect equilibrium simulations considering 602 possible defects, vAl-1ON was found to be unstable on the Mg-doped side of the homojunction and isolated oxygen impurities are preferred. On the Si-doped side, vAl-1ON forms but as (vAl-1ON)–2, not (vAl-1ON)0. This makes vAl-1ON a prototypical test case for the proposed strategy. Simulations of the Si/Mg:AlN homojunction showed (vAl-1ON)0 is stabilized within 6 nm of the interface in the Si-doped portion. This result indicates space charge induced band bending enables control over the concentration, spin state, and position of point defects, which is critical to realizing point defect based quantum information systems.}, number={5}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bowes, Preston C. and Wu, Yifeng and Baker, Jonathon N. and Harris, Joshua S. and Irving, Douglas L.}, year={2019}, month={Jul} }
 @article{bowes_baker_harris_behrhorst_irving_2018, title={Influence of impurities on the high temperature conductivity of SrTiO3}, volume={112}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.5000363}, DOI={10.1063/1.5000363}, abstractNote={In studies of high temperature electrical conductivity (HiTEC) of dielectrics, the impurity in the highest concentration is assumed to form a single defect that controls HiTEC. However, carrier concentrations are typically at or below the level of background impurities, and all impurities may complex with native defects. Canonical defect models ignore complex formation and lump defects from multiple impurities into a single effective defect to reduce the number of associated reactions. To evaluate the importance of background impurities and defect complexes on HiTEC, a grand canonical defect model was developed with input from density functional theory calculations using hybrid exchange correlation functionals. The influence of common background impurities and first nearest neighbor complexes with oxygen vacancies (vO) was studied for three doping cases: nominally undoped, donor doped, and acceptor doped SrTiO3. In each case, conductivity depended on the ensemble of impurity defects simulated with the extent of the dependence governed by the character of the dominant impurity and its tendency to complex with vO. Agreement between simulated and measured conductivity profiles as a function of temperature and oxygen partial pressure improved significantly when background impurities were included in the nominally undoped case. Effects of the impurities simulated were reduced in the Nb and Al doped cases as both elements did not form complexes and were present in concentrations well exceeding all other active impurities. The influence of individual impurities on HiTEC in SrTiO3 was isolated and discussed and motivates further experiments on singly doped SrTiO3.}, number={2}, journal={APPLIED PHYSICS LETTERS}, author={Bowes, Preston C. and Baker, Jonathon N. and Harris, Joshua S. and Behrhorst, Brian D. and Irving, Douglas L.}, year={2018}, month={Jan} }
 @article{baker_bowes_harris_irving_2018, title={Mechanisms governing metal vacancy formation in BaTiO3 and SrTiO3}, volume={124}, ISSN={["1089-7550"]}, url={http://dx.doi.org/10.1063/1.5044746}, DOI={10.1063/1.5044746}, abstractNote={Barium titanate (BTO) and strontium titanate (STO) are often treated as close analogues, and models of defect behavior are freely transferred from one material to the other with only minor modifications. On the other hand, it is often reported that B-site vacancies (vB) are the dominant metal vacancy in BTO, while A-site vacancies (vA) dominate in STO. This difference precludes the use of analogous defect models for BTO and STO, begging the question: how similar are the defect chemistries of the two materials? Here, we address this question with density functional theory calculations using a state-of-the-art hybrid exchange correlation functional, which more accurately describes the electronic structure and charge localization than traditional functionals. We find that vA is the dominant metal vacancy in STO but that different combinations of vA, vB, and vB-vO complexes are present in BTO depending on processing and doping. Mechanistically, this occurs for two reasons: thermodynamic differences in the accessible processing conditions of the two materials and energy differences in the bonds broken when forming the vacancies. These differences can also lead to widely differing responses when impurity dopants are intentionally added. Therefore, the response of metal vacancy behavior in BTO and STO to the inclusion of niobium and iron, two typical dopants in these systems, is examined and compared.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Irving, Douglas L.}, year={2018}, month={Sep} }
 @article{harris_baker_gaddy_bryan_bryan_mirrielees_reddy_collazo_sitar_irving_2018, title={On compensation in Si-doped AlN}, volume={112}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5022794}, DOI={10.1063/1.5022794}, abstractNote={Controllable n-type doping over wide ranges of carrier concentrations in AlN, or Al-rich AlGaN, is critical to realizing next-generation applications in high-power electronics and deep UV light sources. Silicon is not a hydrogenic donor in AlN as it is in GaN; despite this, the carrier concentration should be controllable, albeit less efficiently, by increasing the donor concentration during growth. At low doping levels, an increase in the Si content leads to a commensurate increase in free electrons. Problematically, this trend does not persist to higher doping levels. In fact, a further increase in the Si concentration leads to a decrease in free electron concentration; this is commonly referred to as the compensation knee. While the nature of this decrease has been attributed to a variety of compensating defects, the mechanism and identity of the predominant defects associated with the knee have not been conclusively determined. Density functional theory calculations using hybrid exchange-correlation functionals have identified VAl+nSiAl complexes as central to mechanistically understanding compensation in the high Si limit in AlN, while secondary impurities and vacancies tend to dominate compensation in the low Si limit. The formation energies and optical signatures of these defects in AlN are calculated and utilized in a grand canonical charge balance solver to identify carrier concentrations as a function of Si content. The results were found to qualitatively reproduce the experimentally observed compensation knee. Furthermore, these calculations predict a shift in the optical emissions present in the high and low doping limits, which is confirmed with detailed photoluminescence measurements.}, number={15}, journal={APPLIED PHYSICS LETTERS}, author={Harris, Joshua S. and Baker, Jonathon N. and Gaddy, Benjamin E. and Bryan, Isaac and Bryan, Zachary and Mirrielees, Kelsey J. and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2018}, month={Apr} }
 @article{alden_harris_bryan_baker_reddy_mita_callsen_hoffmann_irving_collazo_et al._2018, title={Point-Defect Nature of the Ultraviolet Absorption Band in AIN}, volume={9}, ISSN={["2331-7019"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85047735459&partnerID=MN8TOARS}, DOI={10.1103/physrevapplied.9.054036}, abstractNote={We present an approach, where point defects and defect complexes are identified using power dependent photoluminescence excitation spectroscopy, impurity data from SIMS and DFT-based calculations accounting for the total charge balance in the crystal. Employing the capabilities of such experimental-computational approach, in this work, the UVC absorption band at 4.7 eV, as well as the 2.7 eV and 3.9 eV luminescence bands in AlN single crystals grown via physical vapor transport (PVT) are studied in detail. Photoluminescence excitation spectroscopy measurements demonstrate the relationship between the defect luminescent bands centered at 3.9 eV and 2.7 eV to the commonly observed absorption band centered at 4.7 eV. Accordingly, the thermodynamic transition energy for the absorption band at 4.7 eV and the luminescence band at 3.9 eV is estimated at 4.2 eV, in agreement with the thermodynamic transition energy for the C N– point defect. Finally, the 2.7 eV PL band is the result of a donor-acceptor pair transition between the V N and C N point defects since nitrogen vacancies, is predicted to be present in the crystal in concentrations similar to carbon employing charge balance constrained DFT calculations. Power dependent photoluminescence measurements reveal the presence of the deep donor state with a thermodynamic transition energy of 5.0 eV, which we hypothesize to be nitrogen vacancies in agreement with predictions based on theory. The charge state, concentration and type of impurities in the crystal is calculated considering a fixed amount of impurities and using a density functional theory (DFT) based defect solver, which considers their respective formation energies and the total charge balance in the crystal. The presented results show that nitrogen vacancies are the most likely candidate for the deep donor state involved in the donor acceptor pair transition with peak emission at 2.7 eV for the conditions relevant to PVT growth.}, number={5}, journal={PHYSICAL REVIEW APPLIED}, author={Alden, D. and Harris, J. S. and Bryan, Z. and Baker, J. N. and Reddy, P. and Mita, S. and Callsen, G. and Hoffmann, A. and Irving, D. L. and Collazo, R. and et al.}, year={2018}, month={May} }
 @article{tian_wang_harris_irving_zhao_vitos_2017, title={Alloying effect on the elastic properties of refractory high-entropy alloys}, volume={114}, ISSN={["1873-4197"]}, DOI={10.1016/j.matdes.2016.11.079}, abstractNote={Ab initio total energy calculations are used to determine the elastic properties of TiZrVNb, TiZrNbMo and TiZrVNbMo high-entropy alloys in the body centered cubic (bcc) crystallographic phase. Calculations are performed using the Vienna Ab initio Simulation Package and the Exact Muffin-Tin Orbitals methods, and the compositional disorder is treated within the frameworks of the special quasi-random structures technique and the coherent potential approximation, respectively. Special emphasis is given to the effect of local lattice distortion and trends against composition. Significant distortion can be observed in the relaxed cells, which result in an overlap of the first and second nearest neighbor (NN) shells represented in the histograms. When going from the four-component alloys TiZrVNb and TiZrNbMo to the five-component TiZrVNbMo, the changes in the elastic parameters follow the expected trends, except that of C44 which decreases upon adding equiatomic Mo to TiZrVNb despite of the large shear elastic constant of elemental Mo. Although the rule of mixtures turns out to be a useful tool to estimate the elastic properties of the present HEAs, to capture the more delicate alloying effects one needs to resort to ab initio results.}, journal={MATERIALS & DESIGN}, author={Tian, Li-Yun and Wang, Guisheng and Harris, Joshua S. and Irving, Douglas L. and Zhao, Jijun and Vitos, Levente}, year={2017}, month={Jan}, pages={243–252} }
 @article{baker_bowes_long_moballegh_harris_dickey_irving_2017, title={Defect mechanisms of coloration in Fe-doped SrTiO3 from first principles}, volume={110}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4978861}, DOI={10.1063/1.4978861}, abstractNote={To understand the underlying defect mechanisms governing the coloration of Fe-doped SrTiO3 (Fe:STO), density functional theory calculations were used to determine defect formation energies and to interpret optical absorption spectra. A grand canonical defect equilibrium model was developed using the calculated formation energies, which enabled connection to annealing experiments. It was found that FeTi0 is stable in oxidizing conditions and leads to the optical absorption signatures in oxidized Fe:STO, consistent with experiment. Fe:STO was found to transition from brown to transparent as PO2 was reduced during annealing. The defect equilibrium model reproduces a consistent PO2 of this coloration transition. Most critical to reproducing the PO2 of the coloration transition was inclusion of a FeTi-VO first nearest neighbor complex, which was found to be strongly interacting. The coloration transition PO2 was found to be insensitive to the presence of minority background impurities, slightly sensitive to Fe content, and more sensitive to annealing temperature.}, number={12}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Baker, Jonathon N. and Bowes, Preston C. and Long, Daniel M. and Moballegh, Ali and Harris, Joshua S. and Dickey, Elizabeth C. and Irving, Douglas L.}, year={2017}, month={Mar}, pages={122903} }
 @article{dycus_harris_sang_fancher_findlay_oni_chan_koch_jones_allen_et al._2015, title={Accurate Nanoscale Crystallography in Real-Space Using Scanning Transmission Electron Microscopy}, volume={21}, ISSN={["1435-8115"]}, DOI={10.1017/s1431927615013732}, abstractNote={Abstract}, number={4}, journal={MICROSCOPY AND MICROANALYSIS}, author={Dycus, J. Houston and Harris, Joshua S. and Sang, Xiahan and Fancher, Chris M. and Findlay, Scott D. and Oni, Adedapo A. and Chan, Tsung-ta E. and Koch, Carl C. and Jones, Jacob L. and Allen, Leslie J. and et al.}, year={2015}, month={Aug}, pages={946–952} }
 @article{sachet_shelton_harris_gaddy_irving_curtarolo_donovan_hopkins_sharma_sharma_et al._2015, title={Dysprosium-doped cadmium oxide as a gateway material for mid-infrared plasmonics}, volume={14}, ISSN={1476-1122 1476-4660}, url={http://dx.doi.org/10.1038/NMAT4203}, DOI={10.1038/nmat4203}, abstractNote={The interest in plasmonic technologies surrounds many emergent optoelectronic applications, such as plasmon lasers, transistors, sensors and information storage. Although plasmonic materials for ultraviolet-visible and near-infrared wavelengths have been found, the mid-infrared range remains a challenge to address: few known systems can achieve subwavelength optical confinement with low loss in this range. With a combination of experiments and ab initio modelling, here we demonstrate an extreme peak of electron mobility in Dy-doped CdO that is achieved through accurate 'defect equilibrium engineering'. In so doing, we create a tunable plasmon host that satisfies the criteria for mid-infrared spectrum plasmonics, and overcomes the losses seen in conventional plasmonic materials. In particular, extrinsic doping pins the CdO Fermi level above the conduction band minimum and it increases the formation energy of native oxygen vacancies, thus reducing their populations by several orders of magnitude. The substitutional lattice strain induced by Dy doping is sufficiently small, allowing mobility values around 500 cm(2) V(-1) s(-1) for carrier densities above 10(20) cm(-3). Our work shows that CdO:Dy is a model system for intrinsic and extrinsic manipulation of defects affecting electrical, optical and thermal properties, that oxide conductors are ideal candidates for plasmonic devices and that the defect engineering approach for property optimization is generally applicable to other conducting metal oxides.}, number={4}, journal={Nature Materials}, publisher={Springer Science and Business Media LLC}, author={Sachet, Edward and Shelton, Christopher T. and Harris, Joshua S. and Gaddy, Benjamin E. and Irving, Douglas L. and Curtarolo, Stefano and Donovan, Brian F. and Hopkins, Patrick E. and Sharma, Peter A. and Sharma, Ana Lima and et al.}, year={2015}, month={Feb}, pages={414–420} }