@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{bryan_bryan_washiyama_reddy_gaddy_sarkar_breckenridge_guo_bobea_tweedie_et al._2018, title={Doping and compensation in Al-rich AlGaN grown on single crystal AlN and sapphire by MOCVD}, volume={112}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5011984}, DOI={10.1063/1.5011984}, abstractNote={In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the “knee behavior” in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range.}, number={6}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bryan, Isaac and Bryan, Zachary and Washiyama, Shun and Reddy, Pramod and Gaddy, Benjamin and Sarkar, Biplab and Breckenridge, M. Hayden and Guo, Qiang and Bobea, Milena and Tweedie, James and et al.}, year={2018}, month={Feb} }
 @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{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} }
 @article{bryan_bryan_gaddy_reddy_hussey_bobea_guo_hoffmann_kirste_tweedie_et al._2014, title={Fermi level control of compensating point defects during metalorganic chemical vapor deposition growth of Si-doped AlGaN}, volume={105}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4903058}, DOI={10.1063/1.4903058}, abstractNote={A Fermi-level control scheme for point defect management using above-bandgap UV illumination during growth is presented. We propose an extension to the analogy between the Fermi level and the electrochemical potential such that the electrochemical potential of a charged defect in a material with steady-state populations of free charge carriers may be expressed in terms of the quasi-Fermi levels. A series of highly Si-doped Al0.65Ga0.35N films grown by metalorganic chemical vapor deposition with and without UV illumination showed that samples grown under UV illumination had increased free carrier concentration, free carrier mobility, and reduced midgap photoluminescence all indicating a reduction in compensating point defects.}, number={22}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bryan, Zachary and Bryan, Isaac and Gaddy, Benjamin E. and Reddy, Pramod and Hussey, Lindsay and Bobea, Milena and Guo, Wei and Hoffmann, Marc and Kirste, Ronny and Tweedie, James and et al.}, year={2014}, month={Dec} }
 @article{gaddy_paisley_maria_irving_2014, title={Overcoming the polarization catastrophe in the rocksalt oxides MgO and CaO}, volume={90}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.90.125403}, abstractNote={Interfaces between dissimilar polar materials may provide a pathway to new device functionality, including high carrier mobility layers at the interface. The development of these materials has proven challenging, in part because of the high energy cost of forming polar surfaces. Our density functional theory calculations explore the mechanisms by which a real material satisfies the electrostatic criteria for stability imposed by a polar surface. The consequences of polarity are studied by comparing the formation energies, charge distribution, and electronic structure of a number of low-index surfaces of rocksalt MgO and CaO. These surfaces are explored both in their bare, undecorated form as well as with surface reconstructions and adsorbed foreign species. Our ground-state surface energies are extended to relevant environmental conditions by use of ab initio thermodynamics. We find that the high energy of bare polar surfaces is the result of the significant charge redistribution that arises to compensate the polarity and pushes electronic states into the forbidden band gap. Other mechanisms of polarity compensation (reconstruction or foreign species adsorption) are therefore seen more frequently. We explain the experimental observations of surface roughness during growth in the [111] direction. In typical epitaxial growth conditions, there is preferential formation of an octopolar reconstruction of the {111} surface, which exposes {001}-type nanofacets. The low energy of the {001} surface likely causes these facets to grow, leading to a rough surface morphology. Our results indicate that when water vapor is present during growth, a smooth, polar surface can be stabilized by the formation of a hydroxyl layer.}, number={12}, journal={PHYSICAL REVIEW B}, author={Gaddy, Benjamin E. and Paisley, Elizabeth A. and Maria, Jon-Paul and Irving, Douglas L.}, year={2014}, month={Sep} }
 @article{paisley_gaddy_lebeau_shelton_biegalski_christen_losego_mita_collazo_sitar_et al._2014, title={Smooth cubic commensurate oxides on gallium nitride}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4861172}, abstractNote={Smooth, commensurate alloys of ⟨111⟩-oriented Mg0.52Ca0.48O (MCO) thin films are demonstrated on Ga-polar, c+ [0001]-oriented GaN by surfactant-assisted molecular beam epitaxy and pulsed laser deposition. These are unique examples of coherent cubic oxide|nitride interfaces with structural and morphological perfection. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100× reduction in leakage current density for the surfactant-assisted samples. HAADF-STEM images of the MCO|GaN interface show commensurate alignment of atomic planes with minimal defects due to lattice mismatch. STEM and DFT calculations show that GaN c/2 steps create incoherent boundaries in MCO over layers which manifest as two in-plane rotations and determine consequently the density of structural defects in otherwise coherent MCO. This new understanding of interfacial steps between HCP and FCC crystals identifies the steps needed to create globally defect-free heterostructures.}, number={6}, journal={JOURNAL OF APPLIED PHYSICS}, author={Paisley, Elizabeth A. and Gaddy, Benjamin E. and LeBeau, James M. and Shelton, Christopher T. and Biegalski, Michael D. and Christen, Hans M. and Losego, Mark D. and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and et al.}, year={2014}, month={Feb} }
 @article{gaddy_bryan_bryan_xie_dalmau_moody_kumagai_nagashima_kubota_kinoshita_et al._2014, title={The role of the carbon-silicon complex in eliminating deep ultraviolet absorption in AlN}, volume={104}, ISSN={["1077-3118"]}, DOI={10.1063/1.4878657}, abstractNote={Co-doping AlN crystals with Si is found to suppress the unwanted 4.7 eV (265 nm) deep ultraviolet absorption associated with isolated carbon acceptors common in materials grown by physical vapor transport. Density functional theory calculations with hybrid functionals demonstrate that silicon forms a stable nearest-neighbor defect complex with carbon. This complex is predicted to absorb at 5.5 eV and emit at or above 4.3 eV. Absorption and photoluminescence measurements of co-doped samples confirm the presence of the predicted CN-SiAl complex absorption and emission peaks and significant reduction of the 4.7 eV absorption. Other sources of deep ultraviolet absorption in AlN are also discussed.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Gaddy, Benjamin E. and Bryan, Zachary and Bryan, Isaac and Xie, Jinqiao and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and Kubota, Yuki and Kinoshita, Toru and et al.}, year={2014}, month={May} }
 @article{gaddy_kingon_irving_2013, title={Effects of alloying and local order in AuNi contacts for Ohmic radio frequency micro electro mechanical systems switches via multi-scale simulation}, volume={113}, ISSN={["0021-8979"]}, DOI={10.1063/1.4804954}, abstractNote={Ohmic RF-MEMS switches hold much promise for low power wireless communication, but long-term degradation currently plagues their reliable use. Failure in these devices occurs at the contact and is complicated by the fact that the same asperities that bear the mechanical load are also important to the flow of electrical current needed for signal processing. Materials selection holds the key to overcoming the barriers that prevent widespread use. Current efforts in materials selection have been based on the material's (or alloy's) ability to resist oxidation as well as its room-temperature properties, such as hardness and electrical conductivity. No ideal solution has yet been found via this route. This may be due, in part, to the fact that the in-use changes to the local environment of the asperity are not included in the selection criteria. For example, Joule heating would be expected to raise the local temperature of the asperity and impose a non-equilibrium thermal gradient in the same region expected to respond to mechanical actuation. We propose that these conditions should be considered in the selection process, as they would be expected to alter mechanical, electrical, and chemical mechanisms in the vicinity of the surface. To this end, we simulate the actuation of an Ohmic radio frequency micro electro mechanical systems switch by using a multi-scale method to model a current-carrying asperity in contact with a polycrystalline substrate. Our method couples continuum solutions of electrical and thermal transport equations to an underlying molecular dynamics simulation. We present simulations of gold-nickel asperities and substrates in order to evaluate the influence of alloying and local order on the early stages of contact actuation. The room temperature response of these materials is compared to the response of the material when a voltage is applied. Au-Ni interactions are accounted for through modification of the existing Zhou embedded atom method potential. The modified potential more accurately captures trends in high-temperature properties, including the enthalpy of mixing and melting temperatures. We simulate the loading of a contacting asperity to several substrates with varying Ni alloying concentrations and compare solid solution strengthening to a phase-separated system. Our simulations show that Ni concentration and configuration have an important effect on contact area, constriction resistance, thermal profiles, and material transfer. These differences suggest that a substrate with 15 at. % Ni featuring phase segregation has fewer early markers that experimentally have indicated long-term failure.}, number={20}, journal={JOURNAL OF APPLIED PHYSICS}, author={Gaddy, Benjamin E. and Kingon, Angus I. and Irving, Douglas L.}, year={2013}, month={May} }
 @article{gaddy_bryan_bryan_kirste_xie_dalmau_moody_kumagai_nagashima_kubota_et al._2013, title={Vacancy compensation and related donor-acceptor pair recombination in bulk AlN}, volume={103}, ISSN={["1077-3118"]}, DOI={10.1063/1.4824731}, abstractNote={A prominent 2.8 eV emission peak is identified in bulk AlN substrates grown by physical vapor transport. This peak is shown to be related to the carbon concentration in the samples. Density functional theory calculations predict that this emission is caused by a donor-acceptor pair (DAP) recombination between substitutional carbon on the nitrogen site and a nitrogen vacancy. Photoluminescence and photoluminescence-excitation spectroscopy are used to confirm the model and indicate the DAP character of the emission. The interaction between defects provides a pathway to creating ultraviolet-transparent AlN substrates for optoelectronics applications.}, number={16}, journal={APPLIED PHYSICS LETTERS}, author={Gaddy, Benjamin E. and Bryan, Zachary and Bryan, Isaac and Kirste, Ronny and Xie, Jinqiao and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and Kubota, Yuki and et al.}, year={2013}, month={Oct} }
 @article{collazo_xie_gaddy_bryan_kirste_hoffmann_dalmau_moody_kumagai_nagashima_et al._2012, title={On the origin of the 265 nm absorption band in AlN bulk crystals}, volume={100}, ISSN={["1077-3118"]}, DOI={10.1063/1.4717623}, abstractNote={Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep ultraviolet light emitting and laser diodes. An absorption band centered around 4.7 eV (∼265 nm) with an absorption coefficient above 1000 cm−1 is observed in these substrates. Based on density functional theory calculations, substitutional carbon on the nitrogen site introduces absorption at this energy. A series of single crystalline wafers were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN.}, number={19}, journal={APPLIED PHYSICS LETTERS}, author={Collazo, Ramon and Xie, Jinqiao and Gaddy, Benjamin E. and Bryan, Zachary and Kirste, Ronny and Hoffmann, Marc and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and et al.}, year={2012}, month={May} }
 @article{paisley_losego_gaddy_tweedie_collazo_sitar_irving_maria_2011, title={Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions}, volume={2}, ISSN={["2041-1723"]}, DOI={10.1038/ncomms1470}, abstractNote={Property coupling at interfaces between active materials is a rich source of functionality, if defect densities are low, interfaces are smooth and the microstructure is featureless. Conventional synthesis techniques generally fail to achieve this when materials have highly dissimilar structure, symmetry and bond type—precisely when the potential for property engineering is most pronounced. Here we present a general synthesis methodology, involving systematic control of the chemical boundary conditions in situ, by which the crystal habit, and thus growth mode, can be actively engineered. In so doing, we establish the capability for layer-by-layer deposition in systems that otherwise default to island formation and grainy morphology. This technique is demonstrated via atomically smooth {111} calcium oxide films on (0001) gallium nitride. The operative surfactant-based mechanism is verified by temperature-dependent predictions from ab initio thermodynamic calculations. Calcium oxide films with smooth morphology exhibit a three order of magnitude enhancement of insulation resistance. Property coupling by heteroepitaxy is severely limited in material combinations with highly dissimilar bonding. This report presents a chemical boundary condition methodology to actively engineer two-dimensional film growth in such systems that otherwise collapse into island formation and rough morphologies.}, journal={NATURE COMMUNICATIONS}, author={Paisley, Elizabeth A. and Losego, Mark. D. and Gaddy, Benjamin E. and Tweedie, James S. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L. and Maria, Jon-Paul}, year={2011}, month={Sep} }
 @inproceedings{alden_bryan_gaddy_bryan_callsen_koukitu_kumagai_hoffmann_irving_sitar_et al., title={On the origin of the 4.7 eV absorption and 2.8 eV emission bands in bulk AlN substrates}, volume={72}, number={5}, booktitle={Wide bandgap semiconductor materials and devices 17}, author={Alden, D. and Bryan, Z. and Gaddy, B. E. and Bryan, I. and Callsen, G. and Koukitu, A. and Kumagai, Y. and Hoffmann, A. and Irving, D. L. and Sitar, Z. and et al.}, pages={31–40} }