Browsing by Author "Van de Walle, Chris G."
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Item Correct Implementation of Polarization Constants in Wurtzite Materials and Impact on III-Nitrides(American Physical Society, 2016-06-20) Dreyer, Cyrus E.; Janotti, Anderson; Van de Walle, Chris G.; Vanderbilt, David; Cyrus E. Dreyer, Anderson Janotti, Chris G. Van de Walle, and David Vanderbilt; Janotti, AndersonAccurate values for polarization discontinuities between pyroelectric materials are critical for understanding and designing the electronic properties of heterostructures. For wurtzite materials, the zincblende structure has been used in the literature as a reference to determine the effective spontaneous polarization constants. We show that, because the zincblende structure has a nonzero formal polarization, this method results in a spurious contribution to the spontaneous polarization differences between materials. In addition, we address the correct choice of “improper” versus “proper” piezoelectric constants. For the technologically important III-nitride materials GaN, AlN, and InN, we determine polarization discontinuities using a consistent reference based on the layered hexagonal structure and the correct choice of piezoelectric constants, and discuss the results in light of available experimental data.Item Defects as qubits in 3C- and 4H-SiC(American Physical Society, 2015-07-20) Gordon, L.; Janotti, Anderson; Van de Walle, Chris G.; L. Gordon, A. Janotti, and C. G. Van de Walle; Janotti, AndersonWe employ hybrid density functional calculations to search for defects in different polytypes of SiC that may serve as qubits for quantum computing. We explore the divacancy in 4H- and 3C-SiC, consisting of a carbon vacancy adjacent to a silicon vacancy, and the nitrogen-vacancy (NV) center in 3C-SiC, in which the substitutional NC sits next to a Si vacancy (NC-VSi). The calculated excitation and emission energies of the divacancy in 4H-SiC are in excellent agreement with experimental data, and aid in identifying the four unique configurations of the divacancy with the four distinct zero-phonon lines observed experimentally. For 3C-SiC, we identify the paramagnetic defect that was recently shown to maintain a coherent quantum state up to room temperature as the spin-1 neutral divacancy. Finally, we show that the (NC-VSi)− center in 3C-SiC is highly promising for quantum information science, and we provide guidance for identifying this defect.Item Determination of the Mott-Hubbard gap in GdTiO3(American Physical Society, 2015-08-06) Bjaalie, L.; Verma, A.; Himmetoglu, B.; Janotti, Anderson; Raghavan, S.; Protasenko, V.; Steenbergen, E. H.; Jena, D.; Stemmer, S.; Van de Walle, Chris G.; L. Bjaalie, A. Verma, B. Himmetoglu, A. Janotti, S. Raghavan, V. Protasenko, E. H. Steenbergen, D. Jena, S. Stemmer, and C. G. Van de Walle; Janotti, AndersonThe band gaps of rare-earth titanates are commonly reported to be 0.2–0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report experimental and theoretical results on GdTiO3 (GTO) indicating that the gap is significantly larger. Photoluminescence (PL) measurements show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. First-principles calculations, based either on density-functional theory (DFT) with a hybrid functional or on DFT+U, consistently show that the gap is close to 2 eV. We also propose an interpretation of the previously reported optical absorption spectra. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials.Item First-principles study of surface charging in LaAlO3/SrTiO3 heterostructures(American Physical Society, 2015-08-19) Krishnaswamy, K.; Dreyer, C. E.; Janotti, Anderson; Van de Walle, Chris G.; K. Krishnaswamy, C. E. Dreyer, A. Janotti, and C. G. Van de Walle; Janotti, AndersonThe two-dimensional electron gas (2DEG) observed at the interface between LaAlO3 (LAO) and SrTiO3 (STO) is known to be very sensitive to the proximity of the LaAlO3 surface and the conditions to which the surface is exposed. We use first-principles calculations to study surface reconstructions on LAO films, taking into account that the LAO surface can be charged. The results for the charged surfaces and for the coupling between the surface and the 2DEG enable us to account not only for the behavior of the 2DEG as a function of thickness of the LAO layer, but simultaneously determine the stable terminations and reconstructions on the LAO surface under a variety of conditions. Our studies of charged surfaces are based on an extension of the methodology of A. Y. Lozovoi et al. [J. Chem. Phys. 115, 1661 (2001)]. From the calculated electronic structure of the unreconstructed (but relaxed) AlO2 and LaO surface terminations of LAO, we find surface states having excess holes (AlO2 termination) or excess electrons (LaO termination). This result is central to understanding the mechanism of 2DEG formation, and is consistent with a 2DEG of density 3.3 × 1014 cm−2 being intrinsic to the LaO-TiO2 interface in the LAO/STO system. We explore the effects of the Al-adatom, O-vacancy, and H-adatom surface reconstructions on the 2DEG density, and find that the stability of different reconstructions is tied to the thickness of the LAO layer as well as the surface exposure conditions. We find that including the effects of charging of the surface significantly stabilizes the AlO2 termination versus the LaO termination. Overall, our methodology has the advantage of decoupling first-principles calculations for the interface from those for the charged surface, and constitutes a general approach that can be applied to the commonly occurring problem of charge exchange between the surface and the interface of a thin film with a substrate, or between the surface and defects/impurities in the bulk of a material. DOI: 10.1103/PhysRevB.Item (InxGa1−x)2O3 alloys for transparent electronics(American Physical Society, 2015-08-31) Peelaers, Hartwin; Steiauf, Daniel; Varley, Joel B.; Janotti, Anderson; Van de Walle, Chris G.; Hartwin Peelaers, Daniel Steiauf, Joel B. Varley, Anderson Janotti, and Chris G. Van de Walle; Janotti, Anderson(InxGa1−x )2O3 alloys show promise as transparent conducting oxides. Using hybrid density functional calculations, band gaps, formation enthalpies, and structural parameters are determined for monoclinic and bixbyite crystal structures. In the monoclinic phase the band gap exhibits a linear dependence on alloy concentration, whereas in the bixbyite phase a large band-gap bowing occurs. The calculated formation enthalpies showthat the monoclinic structure is favorable for In compositions up to 50% and bixbyite for larger compositions. This is caused by In strongly preferring sixfold oxygen coordination. The formation enthalpy of the 50:50 monoclinic alloy is much lower than the formation enthalpy of the 50:50 bixbyite alloy and also lower than most monoclinic alloys with lower In concentration; these trends are explained in terms of local strain. Consequences for experiment and applications are discussed.Item Small polarons and point defects in barium cerate(American Physical Society, 2015-12-28) Swift, Michael; Janotti, Anderson; Van de Walle, Chris G.; Michael Swift, Anderson Janotti, and Chris G. Van de Walle; Janotti, AndersonBarium cerate (BaCeO3) is a well-known ionic conductor of both hydrogen and oxygen. In applications, it is frequently doped (for instance with Y) to increase stability and promote diffusion. However, the effects of doping and native defects are not fully understood. Computational studies have been stymied by the nature of the conduction band, which is made up of cerium 4f states. These states present a challenge to ab initio techniques based on density functional theory within the standard approximations for exchange and correlation. Using a hybrid functional, we investigate the effects of hydrogen impurities and native defects on the electrical and optical properties of BaCeO3. We discuss the tendency of excess electrons or holes to localize in the form of small polarons. We also explore the interactions of polarons with hydrogen impurities and oxygen vacancies, and their impact on luminescence properties.Item Structural and electronic properties of SrZrO3 and Sr(Ti,Zr)O3 alloys(American Physical Society, 2015-08-11) Weston, L.; Janotti, Anderson; Cui, X. Y.; Himmetoglu, B.; Stampfl, C.; Van de Walle, Chris G.; L. Weston, A. Janotti, X. Y. Cui, B. Himmetoglu, C. Stampfl, and C. G. Van de Walle; Janotti, AndersonUsing hybrid density functional calculations, we study the electronic and structural properties of SrZrO3 and ordered Sr(Ti,Zr)O3 alloys. Calculations were performed for the ground-state orthorhombic (Pnma) and high-temperature cubic (Pm3m) phases of SrZrO3. The variation of the lattice parameters and band gaps with Ti addition was studied using ordered SrTix Zr1−x O3 structures with x = 0, 0.25, 0.5, 0.75, and 1. As Ti is added to SrZrO3, the lattice parameter is reduced and closely follows Vegard’s law. On the other hand, the band gap shows a large bowing and is highly sensitive to the Ti distribution. For x = 0.5, we find that arranging the Ti and Zr atoms into a 1 × 1 SrZrO3/SrTiO3 superlattice along the [001] direction leads to interesting properties, including a highly dispersive single band at the conduction-band minimum (CBM), which is absent in both parent compounds, and a band gap close to that of pure SrTiO3. These features are explained by the splitting of the lowest three conduction-band states due to the reduced symmetry of the superlattice, lowering the band originating from the in-plane Ti 3dxy orbitals. The lifting of the t2g orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. Our results demonstrate how short-period SrZrO3/SrTiO3 superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk SrTiO3.Item Tuning bad metal and non-Fermi liquid behavior in a Mott material: Rare-earth nickelate thin films(American Association for the Advancement of Science (AAAS), 2015-11-06) Mikheev, Evgeny; Hauser, Adam J.; Himmetoglu, Burak; Moreno, Nelson E.; Janotti, Anderson; Van de Walle, Chris G.; Stemmer, Susanne; Evgeny Mikheev, Adam J. Hauser, Burak Himmetoglu, Nelson E. Moreno, Anderson Janotti, Chris G. Van de Walle, Susanne Stemmer; Janotti, AndersonResistances that exceed the Mott-Ioffe-Regel limit (known as bad metal behavior) and non-Fermi liquid behavior are ubiquitous features of the normal state of many strongly correlated materials. We establish the conditions that lead to bad metal and non-Fermi liquid phases in NdNiO3, which exhibits a prototype bandwidth-controlled metalinsulator transition. We show that resistance saturation is determined by the magnitude of Ni eg orbital splitting, which can be tuned by strain in epitaxial films, causing the appearance of bad metal behavior under certain conditions. The results shed light on the nature of a crossover to a non-Fermi liquid metal phase and provide a predictive criterion for Anderson localization. They elucidate a seemingly complex phase behavior as a function of film strain and confinement and provide guidelines for orbital engineering and novel devices.