@article{wu_mirrielees_irving_2022, title={Defect Chemistry of Halogen Dopants in ZnSe}, volume={13}, ISSN={["1948-7185"]}, DOI={10.1021/acs.jpclett.2c01976}, abstractNote={Halogen dopants in ZnSe have been a research focus for quantum applications utilizing excitonic emissions, wherein point defects play a critical role. To provide a full first-principles perspective on the defect chemistries of halogen-doped ZnSe, Cl- and F-doped ZnSe were explored via hybrid functional density functional theory calculations involving all possible isolated defects and defect-defect complexes. Cl and F both exhibit more complicated defect chemistries than just forming a shallow substitutional donor on the Se site. For Cl, the complex of Cl substituting for Se with a neighboring Zn vacancy was also found to be prevalent. For F, its interstitial in the Zn tetrahedron was found to be stable in addition to the complex of such interstitial with an adjacent F atom substituting for Se. The explicitly simulated emission photoluminescence lineshapes of the self-activated centers exhibited both a peak value and a broad line width consistent with the experiment.}, number={35}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Wu, Yifeng and Mirrielees, Kelsey J. and Irving, Douglas L.}, year={2022}, month={Sep}, pages={8380–8385} }
 @article{wu_mirrielees_irving_2022, title={On native point defects in ZnSe}, volume={120}, ISSN={["1077-3118"]}, DOI={10.1063/5.0092736}, abstractNote={Aiming at a fundamental understanding of the defect chemistry of pure ZnSe for optical and quantum applications, systematic density functional theory calculations with hybrid exchange-correlation functionals were performed to build an accurate database of native defects in ZnSe, including isolated defects and first nearest-neighbor defect–defect complexes. From the defect formation energies, zinc vacancy is found to be the most prevalent defect as the Fermi level approaches the conduction band edge, while zinc interstitial in the selenium tetrahedron and selenium vacancy become the most prevalent defects as the Fermi level approaches the valence band maximum. The divacancy complex, consisting of first nearest-neighboring zinc and selenium vacancies, is also found to have a favorable binding energy across the entire bandgap. Its formation energy is, however, always higher than either the isolated zinc or selenium vacancy, meaning it will never be the predominant defect in equilibrium. Finally, a point defect with extended spin coherence in Fluorine-implanted ZnSe was recently discovered, and it was found to exhibit a broad emission peak centered at 2.28 eV. The identity of this defect was determined to be either zinc vacancy or its associated complex according to the electron paramagnetic resonance measurements. Explicit simulations of the optical signatures of all zinc vacancy-related native defects were conducted here, showing that both zinc vacancy and divacancy are the most likely native defect contributors to that peak.}, number={23}, journal={APPLIED PHYSICS LETTERS}, author={Wu, Yifeng and Mirrielees, Kelsey J. and Irving, Douglas L.}, year={2022}, month={Jun} }
 @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{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{wu_irving_2021, title={Prediction of chemical ordering in refractory high-entropy superalloys}, volume={119}, ISSN={["1077-3118"]}, DOI={10.1063/5.0059453}, abstractNote={Refractory high-entropy superalloys (RHESs) exhibit impressive nanostructure-property relationships and have promise in next-generation high-temperature structural applications, which has motivated extensive research into these materials. The design space, however, is compositionally vast and complex due to the presence of multiple phases that differ in the composition and chemical order. To address these obstacles in a computationally efficient manner, an advanced approach combining the mean-field density functional theory with parameters determined using machine learning tools has been developed. This approach was implemented here to investigate AlMo0.5NbTa0.5TiZr, which exhibits a nanostructure consisting of cuboidal BCC precipitates coherently embedded within the B2 matrix. It was found that Al and Zr were responsible for the formation of the B2 matrix. In addition, the matrix and the precipitate were found to have very different elastic characteristics. The matrix has a small elastic moduli and large anisotropy while the precipitate is elastically stiff and nearly isotropic. Beyond the current findings, the parameters for the mean field approach are given in the supplementary material and these can be used in future efforts to predict chemical orders, phase partitioning, and elastic properties of RHESs as a function of chemical composition.}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Wu, Yifeng and Irving, Douglas L.}, year={2021}, 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{wu_irving_2019, title={Finite temperature elastic properties of equiatomic CoCrFeNi from first principles}, volume={162}, ISSN={["1872-8456"]}, DOI={10.1016/j.scriptamat.2018.11.010}, abstractNote={The finite temperature elastic properties of the equiatomic CoCrFeNi medium-entropy alloy has been studied by density functional theory. Besides atomic vibrations and electronic free energy, the predictive model developed here includes contributions from spin fluctuations (SFs) in determining the elastic properties of CoCrFeNi. Including SFs changes the magnitude of the temperature derivatives of the poly-crystal elastic moduli, resulting in a close agreement between simulation and experimentally measured trends. How the single-crystal elastic moduli depend on SFs and how these dependencies influence changes in the poly-crystal elastic moduli are analyzed systematically. Finally, the elemental sources to the simulated trends are identified.}, journal={SCRIPTA MATERIALIA}, author={Wu, Yifeng and Irving, Douglas L.}, year={2019}, month={Mar}, pages={176–180} }
 @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} }