[Show abstract][Hide abstract] ABSTRACT: The 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.
Full-text · Article · Aug 2015 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Carbon is a common unintentional impurity in oxide semiconductors. We use hybrid density functional theory to calculate the electronic and structural properties of carbon impurities in ZnO, In2O3 , and Ga2O3 —materials that are used as transparent conductors. In each of these semiconducting oxides, we find that carbon is most likely to occupy the cation site under most electronic and chemical potential conditions. In ZnO, CZn acts as a shallow double donor and exhibits large local breathing-mode relaxations. In In2O3 and Ga2O3 , C acts as a shallow donor and moves off the cation site to become threefold oxygen coordinated. In all three oxides, Ccation exhibits modest formation energies, indicating that these species will be likely to incorporate. Our results indicate that C impurities are suitable donor dopants in these oxides and will contribute to background n -type conductivity if unintentionally present.
Full-text · Article · Nov 2014 · Physical Review Applied
[Show abstract][Hide abstract] ABSTRACT: The properties of materials at high temperatures are often determined by complex thermodynamic mechanisms. One of the most prominent examples is the stabilization of tetragonal and cubic zirconia, which we investigate using density functional theory. The results show that the minimum energy path for the tetragonal-to-cubic phase transformation differs significantly from the paths discussed in the literature so far. This provides insight into the properties of compositions codoped with yttria and titania, an approach that has recently been proposed for the design of thermal barrier coatings.
No preview · Article · Oct 2014 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Van der Waals interactions play an important role in layered materials such as MoS2 and MoO3. Within density functional theory, several methods have been developed to explicitly include van der Waals interactions. We compare the performance of several of these functionals in describing the structural and electronic properties of MoS2 and MoO3. We include functionals based on the local density or generalized gradient approximations, but also based on hybrid functionals. The coupling of the semiempirical Grimme D2 method with the hybrid functional HSE06 is shown to lead to a very good description of both structural and electronic properties.
No preview · Article · Jul 2014 · Journal of Physics Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: We use hybrid density functional theory, including van der Waals interactions, to study the elastic properties of bulk molybdenum disulfide (MoS2). We determine a complete, consistent set of accurate values for elastic constants, overcoming the inconsistencies in the reported values in the literature. We also elucidate the changes in materials properties that occur when hydrostatic pressure is applied. The compression ratio, the transition from semiconductor to semimetal, and the pressure dependence of the elastic constants are discussed. The good agreement with experiments for small hydrostatic pressures validates our methodology and our calculated values for the elastic constants.
No preview · Article · May 2014 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Oxide heterostructures have been shown to exhibit unusual physics and hold the promise of novel electronic applications. We present a set of criteria to select and design interfaces, particularly those that can sustain a high-density two-dimensional electron gas (2DEG). We describe how first-principles calculations can contribute to a qualitative and quantitative understanding, illustrated with the key issue of band alignment. Band offsets determine on which side of the interface the 2DEG will reside, as well as the degree of confinement. We use hybrid density functional calculations to determine the band alignments of a number of complex oxides, considering materials with different types of conduction-band character, polar or nonpolar character and band insulators as well as Mott insulators. We suggest promising materials combinations that could lead to a 2DEG with optimized properties, such as high 2DEG densities and high electron mobilities.
Full-text · Article · Feb 2014 · New Journal of Physics
[Show abstract][Hide abstract] ABSTRACT: We investigate the stability of O, Ti, and Sr vacancies in SrTiO3 and their interactions with hydrogen impurities. Based on density functional calculations with a hybrid functional, we analyze formation energies, binding energies, and H-related vibrational modes. We find that interstitial hydrogen (Hi+) and substitutional hydrogen on an oxygen site (HO) both act as shallow donors and are likely to contribute to unintentional n-type conductivity. Hydrogen can also bind to Ti vacancies in the form of (VTi-H)-3 and (VTi-2H)-2 complexes. Sr vacancies can form (VSr-H)- or accommodate an H2 molecule in the form of (VSr-H2)-2 complex. The latter provides an explanation for the "hidden" hydrogen recently observed in annealing experiments [M. C. Tarun and M. D. McCluskey, J. Appl. Phys. 109, 063706 (2011), 10.1063/1.3561867].
No preview · Article · Jan 2014 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Accurate values for absolute surface energies are required to understand bulk and thin-film growth. Using first-principles calculations based on hybrid density functional theory we determine energies for bare and hydrogenated surfaces of wurtzite GaN in polar and nonpolar orientations. We find that the energies of the nonpolar m and a planes are similar and constant over the range of Ga, N, and H chemical potentials studied. In contrast, the energies of the polar planes are strongly condition dependent. We find that the +c polar plane is systematically lower in energy than the -c plane.
No preview · Article · Jan 2014 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: We examine how hole localization limits the effectiveness of substitutional acceptors in oxide and nitride semiconductors and explain why p-type doping of these materials has proven so difficult. Using hybrid density functional calculations, we find that anion-site substitutional impurities in AlN, GaN, InN, and ZnO lead to atomic-like states that localize on the impurity atom itself. Substitution with cation-site impurities, on the other hand, triggers the formation of polarons that become trapped on nearest-neighbor anions, generally leading to large ionization energies for these acceptors. Unlike shallow effective-mass acceptors, these two types of deep acceptors couple strongly with the lattice, significantly affecting the optical properties and severely limiting prospects for achieving p-type conductivity in these wide-band-gap materials.
Full-text · Article · Dec 2013 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Carbon is a common impurity in the group-III nitrides, often unintentionally incorporated during growth. Nevertheless, the properties of carbon impurities in the nitrides are still not fully understood. We investigate the impact of carbon impurities on the electrical and optical properties of GaN, AlN, and InN using density functional calculations based on a hybrid functional. We examine the stability of substitutional and interstitial configurations as a function of the Fermi-level position and chemical potentials. In all nitrides studied here, CN acts as a deep acceptor and gives rise to deep, broad photoluminescence bands. Carbon on the cation site acts as a shallow donor in InN and GaN, but behaves as a DX center in AlN. A split interstitial is the most stable configuration for the C impurity in InN, where it acts as a double donor and likely contributes to n-type conductivity.
Full-text · Article · Dec 2013 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Nanoscale semiconductor materials have been extensively investigated as the
channel materials of transistors for energy-efficient low-power logic switches
to enable scaling to smaller dimensions. On the opposite end of transistor
applications is power electronics for which transistors capable of switching
very high voltages are necessary. Miniaturization of energy-efficient power
switches can enable the integration with various electronic systems and lead to
substantial boosts in energy efficiency. Nanotechnology is yet to have an
impact in this arena. In this work, it is demonstrated that nanomembranes of
the wide-bandgap semiconductor gallium oxide can be used as channels of
transistors capable of switching high voltages, and at the same time can be
integrated on any platform. The findings mark a step towards using lessons
learnt in nanomaterials and nanotechnology to address a challenge that yet
remains untouched by the field.
Full-text · Article · Oct 2013 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Using first-principles calculations based on density functional theory, we investigate the impact of defects near interfaces of semiconductor channel materials and oxide dielectrics. We determine defect formation energies and transition levels, paying special attention to an accurate description of electronic structure by employing a hybrid functional. This methodology overcomes the band-gap problem inherent in traditional functionals and renders the approach more predictive. By calculating band alignments between the semiconductor material and the dielectric oxide, we are able to determine the position of defect levels in the oxide relative to the semiconductor band gap and assess how they will affect the device performance. We discuss results for vacancies in Ge, and for point defects in HfO2, ZrO2, and Al2O3. In addition to point defects, we have also investigated the properties of dangling bonds in Ge and Al2O3, which are likely to form in amorphous oxides as well as at semiconductor/oxide interfaces.
[Show abstract][Hide abstract] ABSTRACT: SnO is a promising oxide semiconductor that can be doped both p- and n-type, but the doping mechanisms remain poorly understood. Using hybrid functionals, we find that native defects cannot account for the unintentional p-type conductivity. Sn vacancies are shallow acceptors, but they have high formation energies and are unlikely to form. Unintentional impurities offer a more likely explanation for p-type doping; hydrogen is a likely candidate, and we find that it forms shallow-acceptor complexes with Sn vacancies. We also demonstrate that the ambipolar behavior of SnO can be attributed to the high position of the valence-band on an absolute energy scale.
No preview · Article · Aug 2013 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: There is considerable excitement recently in the field of transparent conducting-oxide-semiconductors due to the successful realization of large-area single crystals of the wide-bandgap semiconductor β-Ga2O3 by bulk growth methods . The availability of bulk β-Ga2O3 crystals led to the rapid demonstration of high-voltage metal-semiconductor field-effect transistors (MESFETs) by controlled Sn-doped epilayers grown by molecular beam epitaxy (MBE) . β-Ga2O3 has an energy bandgap of ~4.9 eV, significantly larger than both GaN and SiC. Coupled with the availability of low-cost bulk crystals, this material is highly attractive for high-voltage switching applications. Here we show preliminary results that show that similar to layered crystals  and rather surprisingly, one can peel-off nanoscale layers of β-Ga2O3 from a nominally undoped bulk single-crystal. Conducting channels can then be created electrostatically in these nanomembranes with a back-gate, and the resulting transistors are able to sustain very high voltages and still switch by several orders of magnitude.
[Show abstract][Hide abstract] ABSTRACT: Stress is known to strongly alter the effective mass in semiconductors, changing the mobility of carriers. Transport measurements on AlGaN/GaN heterostructures indicated a large increase in mobility under tensile strain [M. Azize and T. Palacios, J. Appl. Phys. 108, 023707 (2010)]. Using first-principles methods, we calculate the variation of electron effective mass in GaN and AlN under hydrostatic and biaxial stress. Unexpected trends are found, which are explained within k·p theory through a variation of the interband momentum matrix elements. The magnitude of the effective-mass reduction is too small to explain the experimentally reported increase in mobility.
[Show abstract][Hide abstract] ABSTRACT: The experimental determination of valence band offsets (VBOs) at interfaces in complex-oxide heterostructures using conventional soft x-ray photoelectron spectroscopy (SXPS, hν ≤ 1500 eV) and reference core-level binding energies can present challenges because of surface charging when photoelectrons are emitted and insufficient probing depth to clearly resolve the interfaces. In this paper, we compare VBOs measured with SXPS and its multi-keV hard x-ray analogue (HXPS, hν > 2000 eV). We demonstrate that the use of HXPS allows one to minimize charging effects and to probe more deeply buried interfaces in heterostructures such as SrTiO3/LaNiO3 and SrTiO3/GdTiO3. The VBO values obtained by HXPS for these interfaces are furthermore found to be close to those determined by first-principles calculations.
[Show abstract][Hide abstract] ABSTRACT: Using first-principles methods and 8-band k·p simulations, we study the electronic structure of an ultrathin quantum-well system consisting of a single layer of InN inserted in GaN matrix. Experimental photoluminescence and electroluminescence emission peaks for such structures have been reported in the wavelength region between 380 to 450 nm. In contrast, our calculations show an energy difference between the electron and hole states around 2.17 eV (573 nm). Possible origins of the experimental light emission are examined. We suggest that the experimental emission may be due to recombination of electrons (holes) in GaN with holes (electrons) in the quantum well.
No preview · Article · Mar 2013 · Applied Physics Letters