@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{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{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{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{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{dycus_mirrielees_grimley_kirste_mita_sitar_collazo_irving_lebeau_2018, title={Structure of Ultrathin Native Oxides on III-Nitride Surfaces}, volume={10}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.8b00845}, abstractNote={When pristine material surfaces are exposed to air, highly reactive broken bonds can promote the formation of surface oxides with structures and properties differing greatly from bulk. Determination of the oxide structure is often elusive through the use of indirect diffraction methods or techniques that probe only the outermost layer. As a result, surface oxides forming on widely used materials, such as group III-nitrides, have not been unambiguously resolved, even though critical properties can depend sensitively on their presence. In this study, aberration corrected scanning transmission electron microscopy reveals directly, and with depth dependence, the structure of ultrathin native oxides that form on AlN and GaN surfaces. Through atomic resolution imaging and spectroscopy, we show that the oxide layers are comprised of tetrahedra-octahedra cation-oxygen units, in an arrangement similar to bulk θ-Al2O3 and β-Ga2O3. By applying density functional theory, we show that the observed structures are more stable than previously proposed surface oxide models. We place the impact of these observations in the context of key III-nitride growth, device issues, and the recent discovery of two-dimensional nitrides.}, number={13}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Dycus, J. Houston and Mirrielees, Kelsey J. and Grimley, Everett D. and Kirste, Ronny and Mita, Seiji and Sitar, Zlatko and Collazo, Ramon and Irving, Douglas L. and LeBeau, James M.}, year={2018}, month={Apr}, pages={10607–10611} }