@article{baker_irving_2022, title={Inverse Materials Design of Doping Strategies with AI, Thermodynamics, and Density Functional Theory}, ISSN={["1543-1851"]}, DOI={10.1007/s11837-021-05087-x}, journal={JOM}, author={Baker, Jonathon N. and Irving, Douglas L.}, year={2022}, month={Jan} }
 @article{mirrielees_baker_bowes_irving_2021, title={Computational approaches to point defect simulations for semiconductor solid solution alloys}, volume={154}, ISBN={1089-7690}, url={https://doi.org/10.1063/5.0041127}, DOI={10.1063/5.0041127}, abstractNote={Despite their technological importance, studying the properties of alloys with first principles methods remains challenging. In cases of AlxGa1-xN and BaxSrx-1TiO3 (BST), whose most important properties are governed by point defects, explicit simulation can be a computationally demanding task due to the random occupation of Al and Ga on cation sites in AlGaN and Ba and Sr on A-sites in BST. In this work, interpolation between end member compounds is used as a first approximation to defect properties and concentrations in intermediate alloy compositions in lieu of explicit simulation. In AlGaN, the efficacy of Si and Ge as dopants for n-type Al-rich AlGaN is explored by considering self-compensating defects such as multi-donor vacancy complexes and Si and Ge DX configurations. In BST, variation of the high temperature defect chemistry of Mg and Fe is examined. The approach presented here is expected to be generally appropriate for first approximation of defect properties in semiconductors and dielectrics where the alloy is a random solid solution of the end members.}, number={9}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Mirrielees, Kelsey J. and Baker, Jonathon N. and Bowes, Preston C. and Irving, Douglas L.}, year={2021} }
 @article{bowes_ryu_baker_dickey_irving_2021, title={Fermi level pinning in Co-doped BaTiO3: Part II. Defect chemistry models}, volume={7}, ISSN={["1551-2916"]}, url={https://doi.org/10.1111/jace.17938}, DOI={10.1111/jace.17938}, abstractNote={Abstract}, number={11}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, publisher={Wiley}, author={Bowes, Preston C. and Ryu, Gyung Hyun and Baker, Jonathon N. and Dickey, Elizabeth C. and Irving, Douglas L.}, year={2021}, month={Jul} }
 @article{bowes_wu_baker_irving_2021, title={Modeling the spatial control over point defect spin states via processing variables}, volume={129}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0039972}, DOI={10.1063/5.0039972}, abstractNote={Contemporary models that are used to search for solid-state point defects for quantum-information applications tend to focus on the defect’s intrinsic properties rather than the range of conditions in which they will form. In this work, a first-principles based multi-scale device model is used to explore how the conditions (i.e., growth temperature, doping concentration, unintentional impurity concentration) influence the formation of a neutral aluminum vacancy complexed with an oxygen impurity at a neighboring nitrogen site vAl-1ON in an Si/Mg:AlN homojunction. Varying the donor (Si) concentration is predicted to lead to the greatest change in both the maximum height and shape of the (vAl-1ON)0 profile. The shape is found to depend on the acceptor (Mg) concentration as well, and a critical ratio between the acceptor and unintentional impurities below which the (vAl-1ON)0 center would not form was identified. A detailed analysis of the electrostatic potential, electric field, and defect chemistry obtained with the model was used to reveal the underlying causes of these changes. These results show the potential of varying processing parameters to manipulate the local electronic structure as a means to control the properties of point defects for quantum-information applications.}, number={22}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bowes, Preston C. and Wu, Yifeng and Baker, Jonathon N. and Irving, Douglas L.}, year={2021}, month={Jun} }
 @article{mirrielees_dycus_baker_reddy_collazo_sitar_lebeau_irving_2021, title={Native oxide reconstructions on AlN and GaN (0001) surfaces}, volume={129}, ISSN={["1089-7550"]}, DOI={10.1063/5.0048820}, abstractNote={Properties of AlN/GaN surfaces are important for realizing the tunability of devices, as the presence of surface states contributes to Fermi level pinning. This pinning can influence the performance of high electron mobility transistors and is also important for passivation of the surface when developing high-power electronic devices. It is widely understood that both AlN and GaN surfaces oxidize. Since there are many possible reconstructions for each surface, it is a challenge to identify the relevant surface reconstructions in advance of a detailed simulation. Because of this, different approaches are often employed to down select initial structures to reduce the computational load. These approaches usually rely on either electron counting rules or oxide stoichiometry, as both of these models tend to lead to structures that are energetically favorable. Here we explore models from these approaches but also explore a reconstruction of the (0001) surface directly observed using scanning transmission electron microscopy with predictive density functional theory simulations. Two compositions of the observed surface reconstruction—one which obeys oxide stoichiometry and one which is cation deficient and obeys electron counting—are compared to reconstructions from the previous work. Furthermore, surface states are directly calculated using hybrid exchange-correlation functionals that correct for the underestimation of the bandgaps in AlN and GaN and improve the predicted positions of surface states within the gap. It is found that cation deficiency in the observed reconstruction yields surface states consistent with the experiment. Based on all of these results, we provide insight into the observed properties of oxidized AlGaN surfaces.}, number={19}, journal={JOURNAL OF APPLIED PHYSICS}, author={Mirrielees, Kelsey J. and Dycus, J. Houston and Baker, Jonathon N. and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko and LeBeau, James M. and Irving, Douglas L.}, year={2021}, month={May} }
 @article{wu_bowes_baker_irving_2021, title={Photochromism of UV-annealed Fe-doped SrTiO3}, volume={119}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0068523}, DOI={10.1063/5.0068523}, abstractNote={High-temperature annealing coupled with above bandgap UV illumination is an emerging approach to manipulate defect chemistries and resultant properties of electroceramics. To explore defect-processing-property relationships in these materials, an advanced multiphysics and multiscale model has been developed, which involves (a) high-fidelity first principles simulations of defect energies, (b) grand canonical thermodynamics of defect equilibria, (c) UV-perturbed defect formation energies from Shockley–Read–Hall generation and recombination, and (d) finite-element analyses of electrostatic potential and defect redistribution. Using this model, bottom-up insights into defect mechanisms associated with the UV-induced brown photochromism of Fe-doped SrTiO3 at high temperatures are provided. It is found that UV illumination leads to dissociation of the FeTi-vO complex and reduction in the oxygen vacancy concentration through exchange with the gas reservoir. Changes to these defect populations cause reionization of the FeTi defect from −1 to 0 charge state to maintain charge neutrality. This collectively gives rise to an increased concentration of FeTi0, which is the source of brown chromism. In addition, this model reproduces the experimentally observed electrical resistance degradation of samples annealed in this manner due to the increasing hole concentration in the material with time. The present model itself offers a route to guide and facilitate future efforts in this field.}, number={26}, journal={APPLIED PHYSICS LETTERS}, author={Wu, Yifeng and Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2021}, month={Dec} }
 @article{washiyama_mirrielees_bagheri_baker_kim_guo_kirste_guan_breckenridge_klump_et al._2021, title={Self-compensation in heavily Ge doped AlGaN: A comparison to Si doping}, volume={118}, ISSN={["1077-3118"]}, DOI={10.1063/5.0035957}, abstractNote={Self-compensation in Ge- and Si-doped Al0.3Ga0.7N has been investigated in terms of the formation of III vacancy and donor-vacancy complexes. Both Ge- and Si-doped AlGaN layers showed a compensation knee behavior with impurity compensation (low doping regime), compensation plateau (medium doping regime), and self-compensation (high doping regime). A maximum free carrier concentration of 4–5 × 1019 cm−3 was obtained by Ge doping, whereas Si doping resulted in only half of that value, ∼2 × 1019 cm−3. A DFT calculation with the grand canonical thermodynamics model was developed to support the hypothesis that the difference in self-compensation arises from the difference in the formation energies of the VIII-n•donor complexes relative to their onsite configurations. The model suggested that the VIII-2•donor and VIII-3•donor complexes were responsible for self-compensation for both Ge- and Si-doped AlGaN. However, a lower free carrier concentration in Si-doped samples was due to a high VIII-3•Si concentration, resulting from a lower energy of formation of VIII-3•Si.}, number={4}, journal={APPLIED PHYSICS LETTERS}, author={Washiyama, Shun and Mirrielees, Kelsey J. and Bagheri, Pegah and Baker, Jonathon N. and Kim, Ji-Hyun and Guo, Qiang and Kirste, Ronny and Guan, Yan and Breckenridge, M. Hayden and Klump, Andrew J. and et al.}, year={2021}, month={Jan} }
 @article{baker_bowes_harris_collazo_sitar_irving_2020, title={Complexes and compensation in degenerately donor doped GaN}, volume={117}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0013988}, DOI={10.1063/5.0013988}, abstractNote={Gallium nitride is an increasingly technologically relevant material system. While donor doping GaN to low and intermediate dopant concentrations using silicon and germanium has become routine, compensation mechanisms activate under very high donor doping, limiting the maximum electron concentration achievable with either dopant in the degenerate doping regime. This effect, and how it differs between the two dopants, is investigated by hybrid functional density functional theory calculations and grand canonical thermodynamics models and is found to be due to the onset of multi-member Ga vacancy-donor substitutional complexes under degenerate doping conditions. The differing energetics of Ge- and Si-related complexes leads to different responses, ultimately making Ge the more effective donor in degenerate conditions.}, number={10}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2020}, month={Sep} }
 @article{wu_bowes_baker_irving_2020, title={Influence of space charge on the conductivity of nanocrystalline SrTiO3}, volume={128}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0008020}, DOI={10.1063/5.0008020}, abstractNote={A grand canonical multiscale space-charge model has been developed to study and predict the electrical properties of polycrystalline perovskites with complex defect chemistries. This model combines accurate data from hybrid exchange-correlation functional density functional theory calculations (defect formation energies, resultant grand canonical calculations of defect concentrations, and ionization states) with finite-element simulation of the electric field and its coupling to defect redistribution and reionization throughout the grain. This model was used to simulate the evolution of the oxygen partial pressure-dependent conductivity of polycrystalline acceptor-doped strontium titanate as the grain size decreases, and the results were compared to previous experiments. These results demonstrate that as the grain size is reduced from the microscale to nanoscale, the experimentally observed disappearance of ionic conductivity and forward shift of the oxygen partial pressure of the n–p crossover are successfully reproduced and explained by the model. Mechanistically, the changes to conductivity stem from the charge transfer from the grain boundary core into the grain interior, forming a space-charge layer near the grain boundary core that perturbs the local defect chemistry. The impact of the grain size on the electrical conductivity and the underlying defect chemistry across the grain are discussed. In addition to the findings herein, the model itself enables exploration of the electrical response of polycrystalline semiconductor systems with complex defect chemistries, which is critical to the design of future electronic components.}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Wu, Yifeng and Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2020}, month={Jul} }
 @article{bowes_baker_irving_2020, title={Site preference of Y and Mn in nonstoichiometric BaTiO3 from first principles}, volume={4}, ISSN={["2475-9953"]}, url={https://doi.org/10.1103/PhysRevMaterials.4.084601}, DOI={10.1103/PhysRevMaterials.4.084601}, abstractNote={$\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ is often doped with Y and/or Mn to realize a range of desired properties. Yet, existing canonical models of their defect chemistry cannot explain various observed phenomena outside of the high temperature electrical conductivity measurements to which they were fit. Existing models assume Y substitutes exclusively for Ba or Ti despite experiments showing Y is amphoteric, substituting predominantly for Ba or Ti depending on the cation nonstoichiometry. Existing models assume Mn forms isolated ${\mathrm{Mn}}_{\text{Ti}}$ exclusively, but experiments have shown complexes of ${\mathrm{Mn}}_{\mathrm{Ti}}$ with native oxygen vacancies (${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$) form in significant concentrations. Additionally, recent computational works in ${\mathrm{SrTiO}}_{3}$ suggest A-site substitutional defects may form in greater concentrations in a configuration with reduced symmetry relative to the on-site geometry. To address these inconsistencies, we developed a hybrid functional density functional theory informed grand canonical defect model with the ability to simulate specific nonstoichiometries without ad hoc assumptions about the bulk chemical potentials. Using this model, the site preference of Y and Mn in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ as a function of the Ba/Ti ratio was evaluated in the context of a more complete set of defects including: native cation vacancies, on-site and reduced symmetry A-site defects, isolated B-site defects, and ${\mathrm{X}}_{\text{B}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$ defects. The results reproduce experimental observations of yttrium's amphotericity and significant concentrations of ${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$. Both and Y and Mn are found to substitute predominantly for Ba at Ba/Ti = 0.99 and Ti at Ba/Ti = 1.01, but neither are found to substitute exclusively for Ba or Ti within the range of experimentally accessible Ba/Ti ratios at 1400 ${}^{\ensuremath{\circ}}\mathrm{C}$.}, number={8}, journal={PHYSICAL REVIEW MATERIALS}, author={Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2020}, month={Aug} }
 @article{bowes_baker_irving_2020, title={Survey of acceptor dopants in SrTiO3: Factors limiting room temperature hole concentration}, volume={103}, ISSN={["1551-2916"]}, url={https://doi.org/10.1111/jace.16784}, DOI={10.1111/jace.16784}, abstractNote={Abstract}, number={2}, journal={JOURNAL OF THE AMERICAN CERAMIC SOCIETY}, publisher={Wiley}, author={Bowes, Preston C. and Baker, Jonathon N. and Irving, Douglas L.}, year={2020}, month={Feb}, pages={1156–1173} }
 @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{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{long_cai_baker_bowes_bayer_wang_wang_chen_randall_irving_et al._2018, title={Conductivity of iron‐doped strontium titanate in the quenched and degraded states}, volume={102}, ISSN={0002-7820 1551-2916}, url={http://dx.doi.org/10.1111/jace.16212}, DOI={10.1111/jace.16212}, abstractNote={Abstract}, number={6}, journal={Journal of the American Ceramic Society}, publisher={Wiley}, author={Long, Daniel M. and Cai, Biya and Baker, Jonathon N. and Bowes, Preston C. and Bayer, Thorsten J.M. and Wang, Jian‐Jun and Wang, Rui and Chen, Long‐Qing and Randall, Clive A. and Irving, Douglas L. and et al.}, year={2018}, month={Dec}, pages={3567–3577} }
 @article{baker_bowes_irving_2018, title={Hydrogen solubility in donor-doped SrTiO3 from first principles}, volume={113}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.5047793}, DOI={10.1063/1.5047793}, abstractNote={Hydrogen contamination of strontium titanate (STO) during processing and usage is a known problem. However, it is relatively little-studied due to the difficulty in quantifying the amount of hydrogen that dissolves in the lattice. Here, we use hybrid exchange-correlation density functional theory calculations as input to a grand canonical thermodynamics framework to estimate hydrogen solubility and site preferences in donor-doped STO. Our results provide clear theoretical evidence that hydrogen contamination in donor-doped STO occurs at a low enough level to essentially ignore. But, this simple conclusion belies hydrogen's rich behavior; unlike many dopants, it is able to easily change its incorporation site in response to changes in processing conditions. Overall, the findings are consistent with prevailing wisdom and suggest that the presented first principles approach could be used for systematic exploration of hydrogen's impact as a function of doping and processing in this and other wide bandgap materials.}, number={13}, journal={APPLIED PHYSICS LETTERS}, author={Baker, Jonathon N. and Bowes, Preston C. and Irving, Douglas L.}, year={2018}, month={Sep} }
 @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{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{bayer_wang_carter_moballegh_baker_irving_dickey_chen_randall_2016, title={The relation of electrical conductivity profiles and modulus data using the example of STO:Fe single crystals: A path to improve the model of resistance degradation}, volume={117}, ISSN={1359-6454}, url={http://dx.doi.org/10.1016/j.actamat.2016.07.024}, DOI={10.1016/j.actamat.2016.07.024}, abstractNote={Resistance degradation in perovskites is characterized by an increase in current over time with applied electric field. This behavior can be simulated and spatially resolved conductivity profiles can be measured, but some inconsistencies remain. A new approach to address these problems is presented that utilizes time-resolved impedance spectroscopy with an applied DC voltage to provide new insight into the resistance degradation phenomenon. In particular, this method allows the in-situ acquisition of spatio-temporal variations in conductivity. In SrTiO3 a single bulk-dominated maximum of the imaginary part of the modulus M″ transitions to two maxima during degradation, reflecting the hole conductivity in the anode region and the electron conductivity in the cathode region. To clarify the influence of conductivity profiles on impedance data, the reversed route is presented by using simulated conductivity profiles to calculate impedance data. It will be emphasized that this methodology is not limited to the perovskite system considered here, but can be adapted to any kind of system characterized by a spatially varying conductivity.}, journal={Acta Materialia}, publisher={Elsevier BV}, author={Bayer, Thorsten J.M. and Wang, Jian-Jun and Carter, Jared J. and Moballegh, Ali and Baker, Jonathon and Irving, Douglas L. and Dickey, Elizabeth C. and Chen, Long-Qing and Randall, Clive A.}, year={2016}, month={Sep}, pages={252–261} }