arXiv subjects

Chris G. Van de Walle

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

At least 55 records · Page 3Linked to original sources

arXiv2020

Electronic structure and magneto-optical properties of silicon-nitrogen-vacancy complexes in diamond

The silicon-vacancy (SiV) and nitrogen-vacancy (NV) centers in diamond are commonly regarded as prototypical defects for solid-state quantum information processing. Here we show that when silicon and nitrogen are simultaneously introduced into the diamond lattice these defects can strongly interact and form larger complexes. Nitrogen atoms strongly bind to Si and SiV centers and complex formation can occur. Using a combination of hybrid density functional theory (DFT) and group theory, we analyze the electronic structure and provide various useful physical properties, such as hyperfine structure, quasi-local vibrational modes, and zero-phonon line, to enable experimental identification of these complexes. We demonstrate that the presence of substitutional silicon adjacent to nitrogen significantly shifts the donor level toward the conduction band, resulting in an activation energy for the SiN center that is comparable to phosphorus. We also find that the neutral SiNV center is of particular interest due to its photon emission at $\sim$1530 nm, which falls within the C band of telecom wavelengths, and its paramagnetic nature. In addition, the optical transition associated with the SiNV$^0$ color center exhibits very small electron--phonon coupling (Huang--Rhys factor~=~0.78) resulting in high quantum efficiency (Debye-Waller factor = 46\%) for single-photon emission. These features render this new center very attractive for potential application in scalable quantum telecommunication networks.

cond-mat.mtrl-sci↗

arXiv2019

Giant polarization charge density at lattice-matched GaN/ScN interfaces

Rocksalt ScN is a semiconductor with a small lattice mismatch to wurtzite GaN. Within the modern theory of polarization, ScN has a nonvanishing formal polarization along the [111] direction. As a result, we demonstrate that an interface between (0001) GaN and (111) ScN exihibts a large polarization discontinuity of $-$1.358 $\rm Cm^{-2}$. Interfaces between ScN and wurtzite III-nitrides will exhibit a high-density electron gas on the (000$\bar{1}$) GaN interface or a hole gas on the (0001) GaN interface, with carrier concentrations up to $8.5 \times 10^{14}$ cm$^{-2}$. The large polarization difference and small strain makes ScN a desirable choice for polarization-enhanced tunnel junctions within the III-nitride materials system. The large sheet carrier densities may also be useful for contacts or current spreading layers.

cond-mat.mtrl-sci↗

arXiv2019

Carbon dimer defect as a source of the 4.1 eV luminescence in hexagonal boron nitride

We propose that the carbon dimer defect in hexagonal boron nitride gives rise to the ubiquitous narrow luminescence band with a zero-phonon line of 4.08 eV (usually labeled the 4.1 eV band). Our first-principles calculations are based on hybrid density functionals that provide a reliable description of wide band-gap materials. The calculated zero-phonon line energy of 4.3 eV is close to the experimental value, and the deduced Huang-Rhys factor of ${S \approx 2.0}$, indicating modest electron-phonon coupling, falls within the experimental range. The optical transition occurs between two localized $π$-type defects states, with a very short radiative lifetime of 1.2 nanoseconds, in very good accord with experiments.

cond-mat.mtrl-sci↗

arXiv2019

Unusual Formation of Point Defect Complexes in the Ultra-wide Band Gap Semiconductor beta-Ga2O3

Understanding the unique properties of ultra-wide band gap semiconductors requires detailed information about the exact nature of point defects and their role in determining the properties. Here, we report the first direct microscopic observation of an unusual formation of point defect complexes within the atomic scale structure of beta-Ga2O3 using high resolution scanning transmission electron microscopy (STEM). Each complex involves one cation interstitial atom paired with two cation vacancies. These divacancy - interstitial complexes correlate directly with structures obtained by density functional theory, which predicts them to be compensating acceptors in beta-Ga2O3. This prediction is confirmed by a comparison between STEM data and deep level optical spectroscopy results, which reveals that these complexes correspond to a deep trap within the band gap, and that the development of the complexes is facilitated by Sn doping through the increase in vacancy concentration. These findings provide new insight on this emerging material's unique response to the incorporation of impurities that can critically influence their properties.

cond-mat.mtrl-sci↗

arXiv2018

Defect identification based on first-principles calculations for deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) is used extensively to study defects in semiconductors. We demonstrate that great care should be exercised in interpreting activation energies extracted from DLTS as ionization energies. We show how first-principles calculations of thermodynamic transition levels, temperature effects of ionization energies, and nonradiative capture coefficients can be used to accurately determine actual activation energies that can be directly compared with DLTS. Our analysis is illustrated with hybrid functional calculations for two important defects in GaN that have similar thermodynamic transition levels, and shows that the activation energy extracted from DLTS includes a capture barrier that is temperature dependent, unique to each defect, and in some cases large in comparison to the ionization energy. By calculating quantities that can be directly compared with experiment, first-principles calculations thus offer powerful leverage in identifying the microscopic origin of defects detected in DLTS.

cond-mat.mtrl-sci↗

arXiv2018

Electron doping in $\text{Sr}_3\text{Ir}_2\text{O}_7$: collapse of band gap and magnetic order

The electron-doping-driven collapse of the charge gap and staggered magnetization of the spin-orbit-assisted Mott insulator Sr$_{3}$Ir$_{2}$O$_{7}$ is explored via first-principles computational methods. In the antiferromagnetic phase, the gap and magnetization are observed to decrease slowly with increasing doping, with an abrupt collapse of both the gap and the magnetization at an electron concentration corresponding to 4.8\% substitution of Sr with La, in excellent agreement with experiment. Additionally, we describe the structural effects of electron doping in Sr$_{3}$Ir$_{2}$O$_{7}$ via a competition between the steric effect from smaller La atoms substituted within the lattice and the dominant doping-driven deformation-potential effect. Curiously, our first-principles calculations fail to capture the low-temperature structural distortion reported in the low-gap phase of Sr$_{3}$Ir$_{2}$O$_{7}$, supporting the notion that this distortion arises as a secondary manifestation of an unconventional electronic order parameter in this material.

cond-mat.str-el↗

arXiv2018

Linear hyperfine tuning of donor spins in silicon using hydrostatic strain

We experimentally study the coupling of Group V donor spins in silicon to mechanical strain, and measure strain-induced frequency shifts which are linear in strain, in contrast to the quadratic dependence predicted by the valley repopulation model (VRM), and therefore orders of magnitude greater than that predicted by the VRM for small strains $|\varepsilon| < 10^{-5}$. Through both tight-binding and first principles calculations we find that these shifts arise from a linear tuning of the donor hyperfine interaction term by the hydrostatic component of strain and achieve semi-quantitative agreement with the experimental values. Our results provide a framework for making quantitative predictions of donor spins in silicon nanostructures, such as those being used to develop silicon-based quantum processors and memories. The strong spin-strain coupling we measure (up to 150~GHz per strain, for Bi-donors in Si), offers a method for donor spin tuning --- shifting Bi donor electron spins by over a linewidth with a hydrostatic strain of order $10^{-6}$ --- as well as opportunities for coupling to mechanical resonators.

quant-ph↗

arXiv2017

Posner molecules: From atomic structure to nuclear spins

We investigate "Posner molecules", calcium phosphate clusters with chemical formula Ca$_9$(PO$_4$)$_6$. Originally identified in hydroxyapatite, Posner molecules have also been observed as free-floating molecules $in$ $vitro$. The formation and aggregation of Posner molecules have important implications for bone growth, and may also play a role in other biological processes such as the modulation of calcium and phosphate ion concentrations within the mitochondrial matrix. In this work, we use a first-principles computational methodology to study the structure of Posner molecules, their vibrational spectra, their interactions with other cations, and the process of pairwise bonding. Additionally, we show that the Posner molecule provides an ideal environment for the six constituent $^{31}\text{P}$ nuclear spins to obtain very long spin coherence times. $In$ $vitro$, the spins could provide a platform for liquid-state nuclear magnetic resonance quantum computation. $In$ $vivo$, the spins may have medical imaging applications. The spins have also been suggested as "neural qubits" in a proposed mechanism for quantum processing in the brain.

physics.chem-ph↗

arXiv2017

Conditions for $T^2$ resistivity from electron-electron scattering

Many complex oxides (including titanates, nickelates and cuprates) show a regime in which resistivity follows a power law in temperature ($ρ\propto T^2$). By analogy to a similar phenomenon observed in some metals at low temperature, this has often been attributed to electron-electron (Baber) scattering. We show that Baber scattering results in a $T^2$ power law only under several crucial assumptions which may not hold for complex oxides. We illustrate this with sodium metal ($ρ_\text{el-el}\propto T^2$) and strontium titanate ($ρ_\text{el-el}\not\propto T^2$). We conclude that an observation of $ρ\propto T^2$ is not sufficient evidence for electron-electron scattering.

cond-mat.str-el↗

arXiv2016

First-principles analysis of electron transport in BaSnO$_3$

BaSnO$_3$ (BSO) is a promising transparent conducting oxide (TCO) with reported room-temperature (RT) Hall mobility exceeding 320 cm$^{2}$V$^{-1}$s$^{-1}$. Among perovskite oxides, it has the highest RT mobility, about 30 times higher than that of the prototypical SrTiO$_3$. Using first-principles calculations based on hybrid density functional theory, we elucidate the physical mechanisms that govern the mobility by studying the details of LO-phonon and ionized impurity scattering. A careful numerical analysis to obtain converged results within the relaxation-time approximation of Boltzmann transport theory is presented. The ${\bf k}$ dependence of the relaxation time is fully taken into account. We find that the high RT mobility in BSO originates not only from a small effective mass, but also from a significant reduction in the phonon scattering rate compared to other perovskite oxides; the origins of this reduction are identified. Ionized impurity scattering influences the total mobility even at RT for dopant densities larger than $5\times10^{18}$ cm$^{-3}$, and becomes comparable to LO-phonon scattering for $1\times10^{20}$ cm$^{-3}$ doping, reducing the drift mobility from its intrinsic LO-phonon-limited value of $\sim$594 cm$^{2}$V$^{-1}$s$^{-1}$ to less than 310 cm$^{2}$V$^{-1}$s$^{-1}$. We suggest pathways to avoid impurity scattering via modulation doping or polar discontinuity doping. We also explicitly calculate the Hall factor and Hall mobility, allowing a direct comparison to experimental reports for bulk and thin films and providing insights into the nature of the dominant mechanisms that limit mobility in state-of-the art samples.

cond-mat.mtrl-sci↗

arXiv2016

Correct implementation of polarization constants in wurtzite materials and impact on III-nitrides

Accurate 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.

cond-mat.mtrl-sci↗

arXiv2016

Role of excited states in Shockley-Read-Hall recombination in wide-band-gap semiconductors

Defect-assisted recombination is an important limitation on efficiency of optoelectronic devices. However, since nonradiative capture rates decrease exponentially with energy of the transition, the mechanisms by which such recombination can take place in wide-band-gap materials are unclear. Using electronic structure calculations we uncover the crucial role of electronic excited states in nonradiative recombination processes. The impact is elucidated with examples for the group-III nitrides, for which accumulating experimental evidence indicates that defect-assisted recombination limits efficiency. Our work provides new insights into the physics of nonradiative recombination, and the mechanisms are suggested to be ubiquitous in wide-band-gap semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

Structural investigation of the bilayer iridate Sr3Ir2O7

A complete structural solution of the bilayer iridate compound Sr3Ir2O7 presently remains outstanding. Previously reported structures for this compound vary and all fail to explain weak structural violations observed in neutron scattering measurements as well as the presence of a net ferromagnetic moment in the basal plane. In this paper, we present single crystal neutron diffraction and rotational anisotropy second harmonic generation measurements unveiling a lower, monoclinic symmetry inherent to Sr3Ir2O7 . Combined with density functional theory, our measurements identify the correct structural space group as No. 15 (C2/c) and provide clarity regarding the local symmetry of Ir 4+ cations within this spin-orbit Mott material.

cond-mat.str-el↗

arXiv2015

Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films

We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

cond-mat.mtrl-sci↗

arXiv2015

First-principles calculations of indirect Auger recombination in nitride semiconductors

Auger recombination is an important non-radiative carrier recombination mechanism in many classes of optoelectronic devices. The microscopic Auger processes can be either direct or indirect, mediated by an additional scattering mechanism such as the electron-phonon interaction. Phonon-assisted Auger recombination is particularly strong in nitride materials and affects the efficiency of nitride optoelectronic devices at high powers. Here we present a first-principles computational formalism for the study of direct and indirect Auger recombination in direct-band-gap semiconductors and apply it to the case of nitride materials. We show that direct Auger recombination is weak in the nitrides and cannot account for experimental measurements. On the other hand, carrier scattering by phonons and alloy disorder enables indirect Auger processes that can explain the observed loss in devices. We analyze the dominant phonon contributions to the Auger recom- bination rate and the influence of temperature and strain on the values of the Auger coefficients. Auger processes assisted by charged-defect scattering are much weaker than the phonon-assisted ones for realistic defect densities and not important for the device performance. The computational formalism is general and can be applied to the calculation of the Auger coefficient in other classes of optoelectronic materials.

cond-mat.mtrl-sci↗

arXiv2015

Tuning bad metal and non-Fermi liquid behavior in a Mott material: rare earth nickelate thin films

Resistances 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. Here we establish the conditions that lead to bad metal and non-Fermi liquid phases in NdNiO3, which exhibits a prototype, bandwidth-controlled metal-insulator transition. We show that resistance saturation is determined by the magnitude of the 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 non-Fermi liquid metal phase and provide a predictive criterion for strong 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.

cond-mat.str-el↗

arXiv2014

Decomposition mechanism and the effects of metal additives on the kinetics of lithium alanate

First-principles density functional theory studies have been carried out for native defects and transition-metal (Ti and Ni) impurities in lithium alanate (LiAlH$_{4}$), a potential material for hydrogen storage. On the basis of our detailed analysis of the structure, energetics, and migration of lithium-, aluminum-, and hydrogen-related defects, we propose a specific atomistic mechanism for the decomposition and dehydrogenation of LiAlH$_{4}$ that involves mass transport mediated by native point defects. We also discuss how Ti and Ni impurities alter the Fermi-level position with respect to that in the undoped material, thus changing the concentration of charged defects that are responsible for mass transport. This mechanism provides an explanation for the experimentally observed lowering of the temperature for the onset of decomposition and of the activation energy for hydrogen desorption from LiAlH$_{4}$.

cond-mat.mtrl-sci↗

arXiv2014

Mechanism for the decomposition of lithium borohydride

We report first-principles density functional theory studies of native defects in lithium borohydride (LiBH$_{4}$), a potential material for hydrogen storage. Based on our detailed analysis of the structure, energetics, and migration of lithium-, boron-, and hydrogen-related defects, we propose a specific mechanism for the decomposition and dehydrogenation of LiBH$_{4}$ that involves mass transport mediated by native defects. In this mechanism, LiBH$_{4}$ releases borane (BH$_{3}$) at the surface or interface, leaving the negatively charged hydrogen interstitial (H$_{i}^{-}$) in the material, which then acts as the nucleation site for LiH formation. The diffusion of H$_{i}^{-}$ in the bulk LiBH$_{4}$ is the rate-limiting step in the decomposition kinetics. Lithium vacancies and interstitials have low formation energies and are highly mobile. These defects are responsible for maintaining local charge neutrality as other charged defects migrating along the material, and assisting in the formation of LiH. In light of this mechanism, we discuss the effects of metal additives on hydrogen desorption kinetics.

cond-mat.mtrl-sci↗