arXiv subjects

Chris G. Van de Walle

Publications and source records attributed to Chris G. Van de Walle.

69 records · Page 4Linked to original sources

arXiv2014

LiH as a Li$^+$ and H$^-$ ion provider

We present a first-principles study of the formation and migration of native defects in lithium hydride (LiH), a material of interest in hydrogen storage and lithium-ion batteries. We find that the negatively charged lithium vacancy and positively charged hydrogen vacancy are the dominant defects in LiH with a formation energy of about 0.85 eV. With this low energy, LiH can be a good provider of Li$^+$ and/or H$^-$ ions. Based on our results, we discuss the role of LiH in its reactions with another reactant which are important in hydrogen storage and lithium-ion batteries, and explain the ionic conductivity and hydrogen surface loss as observed in experiments.

cond-mat.mtrl-sci↗

arXiv2014

Mechanisms for the decomposition and dehydrogenation of Li amide/imide

Reversible reaction involving Li amide (LiNH$_{2}$) and Li imide (Li$_{2}$NH) is a potential mechanism for hydrogen storage. Recent synchrotron x-ray diffraction experiments [W. I. David et al., J. Am. Chem. Soc. 129, 1594 (2007)] suggest that the transformation between LiNH$_{2}$ and Li$_{2}$NH is a bulk reaction that occurs through non-stoichiometric processes and involves the migration of Li$^{+}$ and H$^{+}$ ions. In order to understand the atomistic mechanisms behind these processes, we carry out comprehensive first-principles studies of native point defects and defect complexes in the two compounds. We find that both LiNH$_{2}$ and Li$_{2}$NH are prone to Frenkel disorder on the Li sublattice. Lithium interstitials and vacancies have low formation energies and are highly mobile, and therefore play an important role in mass transport and ionic conduction. Hydrogen interstitials and vacancies, on the other hand, are responsible for forming and breaking N$-$H bonds, which is essential in the Li amide/imide reaction. Based on the structure, energetics, and migration of hydrogen-, lithium-, and nitrogen-related defects, we propose that LiNH$_{2}$ decomposes into Li$_{2}$NH and NH$_{3}$ according to two competing mechanisms with different activation energies: one mechanism involves the formation of native defects in the interior of the material, the other at the surface. As a result, the prevailing mechanism and hence the effective activation energy for decomposition depend on the surface-to-volume ratio or the specific surface area, which changes with particle size during ball milling. These mechanisms also provide an explanation for the dehydrogenation of LiNH$_{2}$+LiH mixtures.

cond-mat.mtrl-sci↗

arXiv2014

The role of native defects in the transport of charge and mass and the decomposition of Li$_{4}$BN$_{3}$H$_{10}$

Li$_{4}$BN$_{3}$H$_{10}$ is of great interest for hydrogen storage and for lithium-ion battery solid electrolytes because of its high hydrogen content and high lithium-ion conductivity, respectively. The practical hydrogen storage application of this complex hydride is, however, limited due to irreversibility and cogeneration of ammonia (NH$_{3}$) during the decomposition. We report a first-principles density-functional theory study of native point defects and defect complexes in Li$_{4}$BN$_{3}$H$_{10}$, and propose an atomistic mechanism for the material's decomposition that involves mass transport mediated by native defects. In light of this specific mechanism, we argue that the release of NH$_{3}$ is associated with the formation and migration of negatively charged hydrogen vacancies inside the material, and it can be manipulated by the incorporation of suitable electrically active impurities. We also find that Li$_{4}$BN$_{3}$H$_{10}$ is prone to Frenkel disorder on the Li sublattice; lithium vacancies and interstitials are highly mobile and play an important role in mass transport and ionic conduction.

cond-mat.mtrl-sci↗

arXiv2014

First-principles study of the mobility of SrTiO$_3$

We investigate the electronic and vibrational spectra of SrTiO$_3$, as well as the coupling between them, using first-principles calculations. We compute electron-phonon scattering rates for the three lowest-energy conduction bands and use Boltzmann transport theory to calculate the room-temperature mobility of SrTiO$_3$. The results agree with experiment and highlight the strong impact of longitudinal optical phonon scattering. Our analysis provides important insights into the key factors that determine room temperature mobility, such as the number of conduction bands and the nature and frequencies of longitudinal phonons. Such insights provide routes to engineering materials with enhanced mobilities.

cond-mat.mtrl-sci↗

arXiv2014

Interband and polaronic excitations in YTiO3 from first principles

YTiO3, as a prototypical Mott insulator, has been the subject of numerous experimental investigations of its electronic structure. The onset of absorption in optical conductivity measurements has generally been interpreted to be due to interband transitions at the fundamental gap. Here we re-examine the electronic structure of YTiO3 using density functional theory with either a Hubbard correction (DFT+U) or a hybrid functional. Interband transitions turn out to be much higher in energy than the observed onset of optical absorption. However, in case of $p$-type doping, holes tend to become self-trapped in the form of small polarons, localized on individual Ti sites. Exciting electrons from the occupied lower Hubbard band to the small-polaron state then leads to broad infrared absorption, consistent with the onset in the experimental optical conductivity spectra.

cond-mat.mtrl-sci↗

arXiv2014

First-principles theory of nonradiative carrier capture via multiphonon emission

We develop a practical first-principles methodology to determine nonradiative carrier capture coefficients at defects in semiconductors. We consider transitions that occur via multiphonon emission. Parameters in the theory, including electron-phonon coupling matrix elements, are computed consistently using state-of-the-art electronic structure techniques based on hybrid density functional theory. These provide a significantly improved description of bulk band structures, as well as defect geometries and wavefunctions. In order to properly describe carrier capture processes at charged centers, we put forward an approach to treat the effect of long-range Coulomb interactions on scattering states in the framework of supercell calculations. We also discuss the choice of initial conditions for a perturbative treatment of carrier capture. As a benchmark, we apply our theory to several hole-capturing centers in GaN and ZnO, materials of high technological importance in which the role of defects is being actively investigated. Calculated hole capture coefficients are in good agreement with experimental data. We discuss the insights gained into the physics of defects in wide-band-gap semiconductors, such as the strength of electron-phonon coupling and the role of different phonon modes.

cond-mat.mtrl-sci↗

arXiv2014

First-principles theory of the luminescence lineshape for the triplet transition in diamond NV centre

In this work we present theoretical calculations and analysis of the vibronic structure of the spin-triplet optical transition in diamond nitrogen-vacancy centres. The electronic structure of the defect is described using accurate first-principles methods based on hybrid functionals. We devise a computational methodology to determine the coupling between electrons and phonons during an optical transition in the dilute limit. As a result, our approach yields a smooth spectral function of electron-phonon coupling and includes both quasi-localized and bulk phonons on equal footings. The luminescence lineshape is determined via the generating function approach. We obtain a highly accurate description of the luminescence band, including all key parameters such as the Huang-Rhys factor, the Debye-Waller factor, and the frequency of the dominant phonon mode. More importantly, our work provides insight into the vibrational structure of nitrogen vacancy centres, in particular the role of local modes and vibrational resonances. In particular, we find that the pronounced mode at 65 meV is a vibrational resonance, and we quantify localization properties of this mode. These excellent results for the benchmark diamond nitrogen-vacancy centre provide confidence that the procedure can be applied to other defects, including alternative systems that are being considered for applications in quantum information processing.

cond-mat.mes-hall↗

arXiv2013

First-Principles Calculations of Luminescence Spectrum Line Shapes for Defects in Semiconductors: The Example of GaN and ZnO

We present a theoretical study of broadening of defect luminescence bands due to vibronic coupling. Numerical proof is provided for the commonly used assumption that a multi-dimensional vibrational problem can be mapped onto an effective one-dimensional configuration coordinate diagram. Our approach is implemented based on density functional theory with a hybrid functional, resulting in luminescence lineshapes for important defects in GaN and ZnO that show unprecedented agreement with experiment. We find clear trends concerning effective parameters that characterize luminescence bands of donor- and acceptor-type defects, thus facilitating their identification.

cond-mat.mtrl-sci↗

arXiv2012

Phonon-assisted optical absorption in silicon from first principles

The phonon-assisted interband optical absorption spectrum of silicon is calculated at the quasiparticle level entirely from first principles. We make use of the Wannier interpolation formalism to determine the quasiparticle energies, as well as the optical transition and electron-phonon coupling matrix elements, on fine grids in the Brillouin zone. The calculated spectrum near the onset of indirect absorption is in very good agreement with experimental measurements for a range of temperatures. Moreover, our method can accurately determine the optical absorption spectrum of silicon in the visible range, an important process for optoelectronic and photovoltaic applications that cannot be addressed with simple models. The computational formalism is quite general and can be used to understand the phonon-assisted absorption processes in general.

cond-mat.mtrl-sci↗

arXiv2011

Electrostatic carrier doping of GdTiO3/SrTiO3 interfaces

Heterostructures and superlattices consisting of a prototype Mott insulator, GdTiO3, and the band insulator SrTiO3 are grown by molecular beam epitaxy and show intrinsic electronic reconstruction, approximately 1/2 electron per surface unit cell at each GdTiO3/SrTiO3 interface. The sheet carrier densities in all structures containing more than one unit cell of SrTiO3 are independent of layer thicknesses and growth sequences, indicating that the mobile carriers are in a high concentration, two-dimensional electron gas bound to the interface. These carrier densities closely meet the electrostatic requirements for compensating the fixed charge at these polar interfaces. Based on the experimental results, insights into interfacial band alignments, charge distribution and the influence of different electrostatic boundary conditions are obtained.

cond-mat.mtrl-sci↗

arXiv2009

Formation and migration of native defects in NaAlH$_4$

We present a first-principles study of native defects in NaAlH$_4$. Our analysis indicates that the structure and energetics of these defects can be interpreted in terms of elementary building blocks, which include $V_{\rm{AlH_4}}^+$, $V_{\rm{Na}}^-$, $V_{\rm{H}}^+$, H$_i^-$, and (H$_2$)$_i$. We also calculate migration barriers for several key defects, in order to compare enthalpies of diffusion to experimentally measured activation energies of desorption. From this, we estimate activation energies for diffusion of defects and defect pairs. We suggest that $V_{\rm{AlH_4}}^+$ and H$_i^-$, or $V_{\rm{Na}}^-$ and $V_{\rm{H}}^+$, may be responsible for diffusion necessary for desorption. We discuss the possible role of $V_{\rm{H}}^+$-H$_i^-$ complex formation. The values we find are in the range of activation energies reported for catalyzed desorption.

cond-mat.mtrl-sci↗

arXiv2009

Auger recombination rates in nitrides from first principles

We report Auger recombination rates for wurtzite InGaN calculated from first principles density-functional and many-body-perturbation theory. Two different mechanisms are examined -- inter- and intra-band recombination -- that affect different parts of the emission spectrum. In the blue to green spectral region and at room temperature the Auger coefficient can be as large as 2x10^-30cm^6s^-1; in the infrared even larger. Since Auger recombination scales with the cubic power of the free-carrier concentration it becomes an important non-radiative loss mechanism at high current densities. Our results indicate that Auger recombination may be responsible for the loss of quantum efficiency that affects InGaN-based light emitters.

cond-mat.mtrl-sci↗

arXiv2008

Defect Formation Energies without the Band-Gap Problem: Combining DFT and GW for the Silicon Self-Interstitial

We present an improved method to calculate defect formation energies that overcomes the band-gap problem of Kohn-Sham density-functional theory (DFT) and reduces the self-interaction error of the local-density approximation (LDA) to DFT. We demonstrate for the silicon self-interstitial that combining LDA with quasiparticle energy calculations in the G0W0 approach increases the defect formation energy of the neutral charge state by ~1.1 eV, which is in good agreement with diffusion Monte Carlo calculations (E. R. Batista et al. Phys. Rev. B 74, 121102(R) (2006), W.-K. Leung et al. Phys. Rev. Lett. 83, 2351 (1999)). Moreover, the G0W0-corrected charge transition levels agree well with recent measurements.

cond-mat.mtrl-sci↗

arXiv2008

Multiplets Matter: The Electronic Structure of Rare-Earth Semiconductors and Semimetals

We demonstrate that a theoretical framework fully incorporating intra-atomic correlations and multiplet structure of the localized 4f states is required in order to capture the essential physics of rare-earth semiconductors and semimetals. We focus in particular on the rare-earth semimetal erbium arsenide (ErAs), for which effective one-electron approaches fail to provide a consistent picture of both high and low-energy electronic states. We treat the many-body states of the Er 4f shell within an atomic approximation in the framework of dynamical mean-field theory. Our results for the magnetic-field dependence of the 4f local moment, the influence of multiplets on the photoemission spectrum, and the exchange splitting of the Fermi surface pockets as measured from Shubnikov-de Haas oscillations, are found to be in good agreement with experimental results.

cond-mat.str-el↗

arXiv1998

Clean and As-covered zinc-blende GaN (001) surfaces: Novel surface structures and surfactant behavior

We have investigated clean and As-covered zinc-blende GaN (001) surfaces, employing first-principles total-energy calculations. For clean GaN surfaces our results reveal a novel surface structure very different from the well-established dimer structures commonly observed on polar III-V (001) surfaces: The energetically most stable surface is achieved by a Peierls distortion of the truncated (1x1) surface rather than through addition or removal of atoms. This surface exhibits a (1x4) reconstruction consisting of linear Ga tetramers. Furthermore, we find that a submonolayer of arsenic significantly lowers the surface energy indicating that As may be a good surfactant. Analyzing surface energies and band structures we identify the mechanisms which govern these unusual structures and discuss how they might affect growth properties.

cond-mat↗