C. Van De Walle - Academia.edu (original) (raw)

Papers by C. Van De Walle

We have calculated formation energies and position of the defect levels for all native defects an... more We have calculated formation energies and position of the defect levels for all native defects and for a variety of donor and acceptor impurities employing first-principles total-energy calculations. An analysis of the numerical results gives direct insight into defect concentrations and impurity solubility with respect to growth parameters (temperature, chemical potentials) and into the mechanisms limiting the doping levels in GaN. We show how compensation and passivation by native defects or impurities, solubility issues, and incorporation of dopants on other sites influence the acceptor doping levels.

Acta Physica Polonica A, 1999

We report a comprehensive investigation of native point defects and impurities in GaN, MN, and Al... more We report a comprehensive investigation of native point defects and impurities in GaN, MN, and AlGaN alloys, with the goal of understanding doping limitations in nitride semiconductors. Unintentional incorporation of impurities (mainly oxygen) explains the tendency of nitride semiconductors to exhibit n-type conductivity. Silicon is the n-type dopant of choice; it remains shallow in AlGaN up to high Al content, while oxygen undergoes a DX transition. Experimental evidence for DX centers will be discussed. In p-type material, Mg doping is hindered by an increase in ionization energy with increasing Al content in AlGaN, and by nitrogen vacancies acting as compensating centers. Complex formation between magnesium and oxygen and between magnesium and nitrogen vacancies will be discussed.

Barbara-Oxide heterostructures have been shown to exhibit unusual physics and hold the promise of... more Barbara-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, based on hybrid density functional theory, 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 present band alignments for 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.

Submitted for the MAR15 Meeting of The American Physical Society The impact of small polarons on ... more Submitted for the MAR15 Meeting of The American Physical Society The impact of small polarons on the properties of rare-earth titanates 1 CHRIS VAN DE WALLE, LARS BJAALIE, BURAK HIMMETOGLU, ANDERSON JANOTTI, Materials Department, Univ. of California-Santa Barbara-Optical conductivity measurements are frequently applied to determine the band gaps of complex oxides, such as the rare-earth titanate (RTiO 3) Mott insulators. The onset of the measured spectra, in the range of 0.2-0.7 eV, is commonly interpreted as the Mott-Hubbard gap. However, first-principles calculations that take strong electron-electron interactions into account [using either density functional theory (DFT) with a hybrid functional or DFT+U] produce band gaps close to 2 eV for GdTiO 3 and YTiO 3 [Himmetoglu et al., Phys. Rev. B 90, 161102 (2014)]. This raises the question of the origin of the absorption below 2 eV observed in optical experiments. We attribute this signal to excitation of small hole polarons. The rare-earth titanates commonly exhibit unintentional p-type conductivity, and we show that hole localization in the form of small polarons is energetically favorable. Configuration-coordinate diagrams enable us not only to assess the peak in optical absorption, but also to model the lineshape. Good agreement with experiment indicates that the infrared absorption is indeed likely to be polaron-related in GdTiO 3. The results probably apply to other rare-earth titanates as well.

Physical Review B, 1990

The existing discrepancy between theoretical models and experimental results for hydrogendonor co... more The existing discrepancy between theoretical models and experimental results for hydrogendonor complexes in crystalline silicon is resolved using first-principles pseudopotential-densityfunctional calculations for the hydrogen-phosphorus pair. In the configuration which is the global energy minimum, H is located on the extension of a P-Si bond on the Si side, with the Si-H pair relaxing away from P by 0.6 A, leaving the P atom threefold coordinated. The calculated stretching and~agging vibrational frequencies associated with this configuration are in accord with experiment. The structure and properties of hydrogen-impurity complexes in semiconductors have been studied intensively in the last few years using both experimental and theoretical methods. ' ' For the hydrogen-boron complex in silicon, which is the prototypal hydrogen-acceptor complex that has been studied most elaborately, a consistent picture has emerged (see, e.g. , Ref. 5 and references therein). In the equilibrium configuration of the complex, the H atom resides inside a Si-8 bond, forming

Physical Review B, 2001

The structural and electronic properties of beryllium substitutional acceptors and interstitial d... more The structural and electronic properties of beryllium substitutional acceptors and interstitial donors in GaN are investigated using first-principles calculations based on pseudopotentials and density-functional theory. In p-type GaN, Be interstitials, which act as donors, have formation energies comparable to that of substitutional Be on the Ga site, which is an acceptor. In thermodynamic equilibrium, incorporation of Be interstitials will therefore result in severe compensation. To investigate the kinetics of Be interstitial incorporation and outdiffusion we have explored the total-energy surface. The diffusivity of Be interstitials is highly anisotropic, with a migration barrier in planes perpendicular to the c axis of 1.2 eV, while the barrier for motion along the c axis is 2.9 eV. We have also studied complex formation between interstitial donors and substitutional acceptors, and between hydrogen and substitutional beryllium. The results for wurtzite GaN are compared with those for the zinc-blende phase. Consequences for p-type doping using Be acceptors are discussed.

2008 IEEE 20th International Conference on Indium Phosphide & Related Materials (IPRM), 2008

Low off-state current (10 nA/μm) for low static dissipation Low off-state current (10 nA/μm) for ... more Low off-state current (10 nA/μm) for low static dissipation Low off-state current (10 nA/μm) for low static dissipation → minimum subthreshold slope→ minimum L g / T ox low gate tunneling, low band-band tunneling Low delay C FET ΔV/I d in gates where transistor capacitances dominate. Parasitic capacitances are 0.5-1.0 fF/μm → while low C gs is good, high I d is much better g d Low delay C wire ΔV/I d in gates where wiring capacitances dominate. wiring capacitances dominate. large FET footprint → long wires between gates → need high I d / W g ; target ~6 mA/μm short transit time alone (low C gs, int ΔV gs /ΔI d) is not sufficient Process Process Development p Process Flow with MBE Source/Drain Regrowth Process Flow with MBE Source/Drain Regrowth Gate Dielectrics ALD Al O f IBM (D S d) & St f d (P M I t) ALD Al 2 O 3 from IBM (D. Sedana) & Stanford (P. McIntyre) ALD ZrO 2 from Intel (S. Koveshnikov et al, DRC 2008) Al 2 O 3 is more robust in processing. → initial process development → initial process development Process modules being developed for ZrO Process modules being developed for ZrO 2. Gate Definition: Challenges Must scale to 22 nm Dielectric cap on gate for source/drain regrowth ε r p g g Metal & Dielectric etch must stop in 5 nm channel Semiconductor etch must not etch through 5 nm InP subchannel well ε r Process must leave surfaces ready for S/D regrowth barrier InP well Gate Stack: Multiple Layers & Selective Etches Key: stop etch before reaching dielectric then gentle low-power etch to stop on dielectric Key: stop etch before reaching dielectric, then gentle low-power etch to stop on dielectric

Physical Review B, 2003

The effects of H on the interaction between Ga vacancies V Ga and N in GaAs 1Ϫx N x dilute alloys... more The effects of H on the interaction between Ga vacancies V Ga and N in GaAs 1Ϫx N x dilute alloys are studied through first-principles total-energy calculations. We find that N binds to Ga vacancies and that in the presence of H this binding is enhanced. The formation energy of V Ga bonded to N and H ͑resulting in a N-H-V Ga complex͒ can be more than 2 eV lower than that of the isolated Ga vacancy V Ga in GaAs. Our finding that the concentration of V Ga increases with N and even more in the presence of H allows us to interpret several recent experiments.

The Journal of Physical Chemistry C, 2013

Titania (TiO 2) is frequently used in photovoltaic and photocatalytic applications, despite the f... more Titania (TiO 2) is frequently used in photovoltaic and photocatalytic applications, despite the fact that its main optical absorption occurs only at ∼4 eV. Absorption across the band gap of 3 eV is dipole-forbidden in rutile TiO 2. By means of first-principles theoretical spectroscopy calculations, we demonstrate that alloying with TiS 2 introduces an absorption band into the fundamental gap of TiO 2. In addition, band-edge transitions contribute to optical absorption because the S incorporation breaks the symmetry of the TiO 2 lattice. Both effects lead to pronounced absorption of visible light for S concentrations as low as 1.5%.

Semiconductor Science and Technology, 2010

We present consistent sets of band parameters (including band gaps, crystal-field splittings, eff... more We present consistent sets of band parameters (including band gaps, crystal-field splittings, effective masses, Luttinger, and E P parameters) for AlN, GaN, InN, and ZnO in the wurtzite phase. For band-energy differences we observe a pronounced nonlinear dependence on strain. Consistent and complete sets of deformation potentials are then derived for realistic strain conditions in the linear regime around the experimental equilibrium volume. To overcome the limitations of density-functional theory (DFT) in the local-density or generalizedgradient approximations (LDA and GGA) we employ the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional as well as exact exchange (OEPx) based quasi-particle energy calculations in the G 0 W 0 approach.

Physical Review Letters, 2008

Physical Review Letters, 1991

Wide-band-gap semiconductors typically can be doped either n-type or p-type, but not both. Compen... more Wide-band-gap semiconductors typically can be doped either n-type or p-type, but not both. Compensation by native defects has often been invoked as the source of this difficulty. Using first-principles total-energy calculations we show that, for ZnSe and diamond, native-defect concentrations are too low to cause compensation. For nonstoichiometric ZnSe, native defects compensate both n-type and p-type material; thus deviations from stoichiometry cannot explain why ZnSe can be doped only one way. In the absence of a generic mechanism, specific dopants should be examined case by case.

Physical Review B, 2007

We identify hydrogen-related point defects as a dominant defect species involved in ͑de͒hydrogena... more We identify hydrogen-related point defects as a dominant defect species involved in ͑de͒hydrogenation of sodium alanate, a viable hydrogen storage material. These defects are positively or negatively charged, and hence their formation energies are Fermi-level dependent-an important feature that has not been recognized in past studies. This dependence enables us to explain why small amounts of transition-metal additives drastically alter the kinetics of dehydrogenation. The rate-limiting step for hydrogen release is the creation of charged hydrogen-related defects, while transition-metal additives act as electrically active impurities that lower the formation energy of these defects.

New Journal of Physics, 2011

Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the purs... more Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In 2 O 3 and Ga 2 O 3 , two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ∼10 19 cm −3 (In 2 O 3) and ∼10 17 cm −3 (Ga 2 O 3). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence-and conduction-band regions by exploiting photoemission cross

Materials Science and Engineering: B, 1999

We demonstrate room temperature, pulsed, current-injected operation of InGaAlN heterostructure la... more We demonstrate room temperature, pulsed, current-injected operation of InGaAlN heterostructure laser diodes with mirrors fabricated by chemically assisted ion beam etching. The multiple quantum well devices were grown by organo-metallic vapor phase epitaxy on c-face sapphire substrates. The emission wavelengths of the gain-guided laser diodes were 400 nm. The lowest threshold current density obtained was 6 kA cm − 2 with maximum output powers of 50 mW per facet. Optically-pumped distributed-feedback laser operation was also achieved.

Applied Physics Letters, 2000

The performance and degradation characteristics of continuous-wave (cw) InGaN multiple-quantum-we... more The performance and degradation characteristics of continuous-wave (cw) InGaN multiple-quantum-well laser diodes are reported. A cw threshold current as low as 62 mA was obtained for ridge-waveguide laser diodes on epitaxially laterally overgrown GaN on sapphire substrates grown by metalorganic chemical vapor deposition. Transmission electron microscopy reveals a defect density <5×107 cm−2 in the active region. The emission wavelength was near 400 nm with output powers greater than 20 mW per facet. Under cw conditions, laser oscillation was observed up to 70 °C. The room-temperature cw operation lifetimes, measured at a constant output power of 2 mW, exceeded 15 h. From the temperature dependence of the laser diode lifetimes, an activation energy of 0.50 eV±0.05 eV was determined.

Applied Physics Letters, 2012

We have derived consistent sets of band parameters (bandgaps, crystal-field splittings, effective... more We have derived consistent sets of band parameters (bandgaps, crystal-field splittings, effective masses, Luttinger, and EP parameters) and strain deformation potentials for MgO, ZnO, and CdO in the wurtzite phase. To overcome the limitations of density-functional theory in the local-density and generalized gradient approximations, we employ a hybrid functional as well as exact-exchange-based quasiparticle energy calculations in the G0W0 approach. We demonstrate that the band and strain parameters derived in this fashion are in very good agreement with the available experimental data and provide predictions for all parameters that have not been determined experimentally so far.

Angewandte Chemie International Edition, 2011

ZnO is an extremely attractive material for a number of optoelectronic and electronic application... more ZnO is an extremely attractive material for a number of optoelectronic and electronic applications. Among its advantages is its radiation hardness, which is even greater than that of GaN. Based on our comprehensive investigations of intrinsic point defects [1,2], we have developed a model for defect creation and annihilation during and after irradiation. The calculations, based on pseudopotential-densityfunctional theory combined with LDA+U [2] produce formation energies, stability of charge states as a function of Fermi level, and migration barriers for each of the point defects. Migration barriers allow us to determine annealing temperatures at which we predict various defects to be mobile. In ZnO, the key factors responsible for radiation hardness are (1) the low migration barriers of point defects and (2) the charge-state matching of dominant defect pairs. Quantitative arguments for both ZnO and GaN will be presented, and the results compared with experimental observations. The insights provided by our modeling can be fruitfully applied to understand irradiation effects in semiconductors and insulators in general. [1] S.