C. G. Van de Walle

University of California, Santa Barbara, Santa Barbara, California, United States

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Publications (184)333.89 Total impact

  • H Peelaers, C G Van de Walle
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    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.
    Journal of physics. Condensed matter : an Institute of Physics journal. 07/2014; 26(30):305502.
  • H. Peelaers, C. G. Van de Walle
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    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.
    The Journal of Physical Chemistry C. 05/2014; 118(22):12073–12076.
  • A. Janotti, L. Bjaalie, B. Himmetoglu, C. G. Van de Walle
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    ABSTRACT: Recent experiments have demonstrated the formation of a two-dimensional electron gas (2DEG) with a high density of 3 × 1014 cm–2 at the interface of a band insulator (SrTiO3) and a Mott insulator (GdTiO3), with potential application in electronic devices. This contrasted with the 2DEG at the SrTiO3/LaAlO3 interface, two band insulators, for which the reported electron densities are an order of magnitude lower. Understanding the differences between SrTiO3/LaAlO3 and SrTiO3/GdTiO3, and identifying other materials combinations that also give rise to such high 2DEG densities will add flexibility to materials design and contribute to advancing this field of research. Based on first-principles calculations, we propose that YTiO3 combined with SrTiO3 will also lead to a 2DEG with density of 3 × 1014 cm–2. YTiO3 is a Mott insulator, with lattice parameters and band structure similar to those of GdTiO3. We calculate the band alignment at the SrTiO3/YTiO3 interface, compare with the SrTiO3/GdTiO3 system, and discuss the origins of the differences between the rare-earth titanates and LaAlO3 regarding the 2DEG densities. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (RRL) - Rapid Research Letters 05/2014; · 2.39 Impact Factor
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    L Bjaalie, B Himmetoglu, L Weston, A Janotti, C G Van de Walle
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    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.
    New Journal of Physics 02/2014; 16(2):025005. · 4.06 Impact Factor
  • C. E. Dreyer, A. Janotti, C. G. Van de Walle
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    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.
    01/2014; 89(8).
  • J. B. Varley, A. Janotti, C. G. Van de Walle
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    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].
    01/2014; 89(7).
  • Applied Physics Letters 01/2014; 104(24):249902-249902-1. · 3.79 Impact Factor
  • J. L. Lyons, A. Janotti, C. G. Van de Walle
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    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.
    12/2013; 89(3).
  • J. L. Lyons, A. Janotti, C. G. Van de Walle
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    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.
    12/2013; 115(1).
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    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.
    Applied Physics Letters 10/2013; 104(20). · 3.79 Impact Factor
  • J. B. Varley, A. Schleife, A. Janotti, C. G. Van de Walle
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    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.
    Applied Physics Letters 08/2013; 103(8). · 3.79 Impact Factor
  • Physical review. B, Condensed matter 06/2013; 87(23). · 3.77 Impact Factor
  • C. E. Dreyer, A. Janotti, C. G. Van de Walle
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    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.
    Applied Physics Letters 04/2013; 102(14). · 3.79 Impact Factor
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    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.
    Journal of Applied Physics 04/2013; 113(14). · 2.21 Impact Factor
  • M. S. Miao, Q. M. Yan, C. G. Van de Walle
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    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.
    Applied Physics Letters 03/2013; 102(10). · 3.79 Impact Factor
  • Hartwin Peelaers, Chris G. van de Walle
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    ABSTRACT: Molybdenum disulfide (MoS2) is a layered semiconductor that shows great promise for devices such as field-effect transistors. It has an important advantage compared to graphene, namely that it has a band gap. However, a lot of crucial information about the band structure and electronic properties of this material is still lacking, hampering interpretation of experiments and preventing accurate device modeling. Here we use hybrid density functional theory to calculate key materials parameters such as band gaps and effective masses, as well as to investigate effects of strain. We show how strain allows engineering the nature (direct vs. indirect) and size of the band gap and the magnitude of effective masses. In addition, insight into the fundamental physics is provided by considering the transition between the bulk and the monolayer as a function of tensile uniaxial stress.
    03/2013;
  • Patrick McBride, Qimin Yan, Chris van de Walle
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    ABSTRACT: We investigate the effects of different InGaN quantum well (QW) profiles in c-plane InGaN/GaN 3-QW blue light-emitting diodes (LEDs) by employing a semi-empirical drift-diffusion model. Our results show that changing the typically assumed square indium profile to one with a smoother interfacial transition leads to a significant modification of the band diagram, carrier overlap, and current-voltage characteristics. In previous works, an ad hoc reduction of the polarization field has often been used to generate simulated results that match experiment while the realistic indium profile is not taken into account. However, our results indicate that the indium profile plays an important role in determining the current vs. voltage characteristics of InGaN/GaN heterostructure LEDs.
    03/2013;
  • Chris van de Walle, Lars Bjaalie, Luke Gordon, Anderson Janotti
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    ABSTRACT: The formation of a two-dimensional electron gas (2DEG) at the interface between two insulators, SrTiO3(STO) and LaAlO3 (LAO), has sparked huge interest in oxide electronics. In spite of almost a decade of research, the mechanisms that determine the density of this 2DEG have not yet been unravelled. The polar discontinuity at the STO/LAO interface can in principle sustain an electron density of 3.3x10^14cm-2(0.5 electrons per unit cell). However, experimentally observed densities are more than an order of magnitude lower. Using a combination of first-principles and Schr"odinger-Poisson simulations we investigate the origin of the electrons in the 2DEG at the STO/LAO interface. We analyze the asymmetric nature of the heterostructures, i.e., the inability to form a second LAO/STO interface that is a mirror image of the first, and the effects of passivation of the LAO surface. Our results apply to oxide interfaces in general, and explain why the SrTiO3/GdTiO3interface has been found to exhibit the full density of 0.5 electrons per unit cell.
    03/2013;
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    ABSTRACT: The use of TiO2 in photocatalysis, photosensitized solar cells, and memristors strongly depends on the behavior of conduction-band electrons, prompting a more profound understanding of conduction mechanisms. The reported results for the behavior of excess electrons in TiO2 are contradictory. High carrier mobilities, characteristic of delocalized electrons, have been observed in Hall measurements, whereas optical spectra indicate the presence of localized, small polarons. Using first-principles calculations based on a hybrid functional we study the formation of small polarons, comparing it to delocalized electrons in the conduction band of TiO2. From the calculated configuration coordinate diagram and migration energy barriers, we discuss the coexistence of small polarons with delocalized electrons, and address how the observed behavior depends on the type of experiment being conducted. The interaction of small polarons with intrinsic defects such as the oxygen vacancy and donor impurities will also be discussed.
    03/2013;
  • J. R. Weber, A. Janotti, C. G. Van de Walle
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    ABSTRACT: The quest for metal-oxide-semiconductor field-effect transistors (MOSFETs) with higher carrier mobility has triggered great interest in germanium-based MOSFETs. Still, the performance of germanium-based devices lags significantly behind that of their silicon counterparts, possibly due to the presence of defects such as dangling bonds (DBs) and vacancies. Using screened hybrid functional calculations we investigate the role of DBs and vacancies in germanium. We find that the DB defect in germanium has no levels in the band gap; it acts as a negatively charged acceptor with the (0/−1) transition level below the valence-band maximum (VBM). This explains the absence of electron-spin-resonance observations of DBs in germanium. The vacancy in germanium has a much lower formation energy than the vacancy in silicon and is stable in a number of charge states, depending on the position of the Fermi level. We find the (0/−1) and (−1/−2) transition levels at 0.16 and 0.38 eV above the VBM; the spacing of these levels is explained based on the strength of intraorbital repulsion. We compare these results with calculations for silicon, as well as with available experimental data.
    Physical review. B, Condensed matter 01/2013; 87(3). · 3.77 Impact Factor

Publication Stats

2k Citations
333.89 Total Impact Points

Institutions

  • 2005–2014
    • University of California, Santa Barbara
      • Department of Physics
      Santa Barbara, California, United States
  • 2002–2004
    • National Renewable Energy Laboratory
      • National Center for Photovoltaics
      Golden, Colorado, United States
  • 1994–2004
    • Palo Alto Research Center
      Palo Alto, California, United States
  • 2003
    • University of Leicester
      • Department of Physics and Astronomy
      Leicester, ENG, United Kingdom
  • 2000
    • Max Planck Society
      München, Bavaria, Germany