Hybrid Functional Calculations Research Articles

Self-consistent quasiparticle GW and hybrid functional calculations for Al/InAs/Al heterojunctions: Band offset and spin-orbit coupling effects

The electronic structure of surfaces and interfaces plays a key role in the properties of quantum devices. Here, we study the electronic structure of realistic Al/InAs/Al heterojunctions using a combination of density functional theory with hybrid functionals and state-of-the-art quasiparticle GW (QSGW) calculations. We find a good agreement between QSGW calculations and hybrid functional calculations, which themselves compare favorably well with angle-resolved photoemission spectroscopy experiments. Our paper confirms the need for well-controlled quality of the interfaces to obtain the needed properties of InAs/Al heterojunctions. A detailed analysis of the effects of spin-orbit coupling on the spin splitting of the electronic states shows a linear scaling in k space, related to the two-dimensional nature of some interface states. The good agreement by QSGW and hybrid functional calculations opens the door towards trustable use of an effective approximation to QSGW for studying very large heterojunctions. Published by the American Physical Society 2024

Prospect of Hexagonal CsMg(I1–xBrx)3 Alloys for Deep-Ultraviolet Light Emission

Abstract Materials for deep-ultraviolet (DUV) light emission are extremely rare, significantly limiting the development of efficient DUV light-emitting diodes. Here we report CsMg(I1−x Br x )3 alloys as potential DUV light emitters. Based on rigorous first-principles hybrid functional calculations, we find that CsMgI3 has an indirect bandgap, while CsMgBr3 has a direct bandgap. Further, we employ a band unfolding technique for alloy supercell calculations to investigate the critical Br concentration in CsMg(I1−x Br x )3 associated with the crossover from an indirect to a direct bandgap, which is found to be ∼ 0.36. Thus, CsMg(I1−x Br x )3 alloys with 0.36 ⩽ x ⩽ 1 cover a wide range of direct bandgap (4.38–5.37 eV; 284–231 nm), falling well into the DUV regime. Our study will guide the development of efficient DUV light emitters.

First-principles investigation of oxidized Si- and Ge-terminated diamond (100) surfaces.

Diamond is a semiconductor material with remarkable structural, thermal, and electronic properties that has garnered significant interest in the field of electronics. Although hydrogen (H) and oxygen (O) terminations are conventionally favored in transistor designs, alternative options, such as silicon (Si) and germanium (Ge), are being explored because of their resilience to harsh processing conditions during fabrication. Density-functional theory was used to examine the non-oxidized and oxidized group-IV (Si and Ge)-terminated diamond (100) surfaces. The (3 × 1) reconstructed surfaces feature an ether configuration and show relative stability compared with the bare surface. Hybrid-functional calculations of the electronic properties revealed reduced fundamental bandgaps (<1eV) and lower negative electron affinities (NEAs) than those of H-terminated diamond surfaces, which is attributed to the introduction of unoccupied Si (Ge) states and the depletion of negative charges. Furthermore, oxidation of these surfaces enhanced the stability of the diamond surfaces but resulted in two structural configurations: ether and ketone. Oxidized ether configurations displayed insulating properties with energy gaps of ∼4.3 ± 0.3eV, similar to H-terminated diamond (100) surfaces, whereas bridged ether configurations exhibited metallic properties. Oxidization of the metallic ketone configurations leads to the opening of relatively smaller gaps in the range of 1.1-1.7eV. Overall, oxidation induced a shift from NEAs to positive electron affinities, except for the reverse-ordered ketone surface with an NEA of -0.94eV, a value comparable to the H-terminated diamond (100) surfaces. In conclusion, oxidized group-IV-terminated diamond surfaces offer enhanced stability compared to H-terminated surfaces and display unique structural and electronic properties that are influenced by surface bonding.

MOCVD growth of Si-doped α-(AlGa)2O3 on m-plane α-Al2O3 substrates

We reported the growth of (AlGa)2O3 layers on (10 10̅ ) α-Al2O3 substrates using cold-wall metalorganic chemical vapor deposition, and the electrical characterization of Si-doped (AlGa)2O3 layers. In the Ga2O3 growth, the α phase was dominant at low growth temperature, achieving the growth rate of 2.4 μm h−1 at 650 °C. Sheet resistance and electrical conductivity of the Ga2O3 layers with a Si concentration of 3 × 1020 cm−3 were 1 × 104 Ω/square and 8.3 S cm−1, respectively, at the measurement temperature of 500 °C. The Al composition in the (AlGa)2O3 layers was controlled from 0% to 74%. In our initial attempts, we obtained electrically conductive α-(AlGa)2O3 layers by Si doping (2 × 10−9 S cm−1 in the sample with an Al composition of 56%). Hybrid functional calculations suggest the conductivities are limited by compensation of Si through cation vacancy complexes, and not by the significant amounts of co-incorporated C and N that are predicted to be electrically passivated by hydrogen.

A first principles investigation of the electronic, mechanical and optical properties of XPbN2 (X=Mg, Zn)

The electronic, mechanical, optical and conductive properties of MgPbN2 and ZnPbN2 are investigated using hybrid-functional first-principles calculations. Our calculations show that these two compounds preferentially stabilize in the tetragonal chalcopyrite structure. The calculated phonon dispersions show that both compounds are thermodynamically stable. From the calculated elastic constants we infer that the compound are also mechanically stable. As for the electronic properties, MgPbN2 is computed to be a semiconductor with a direct band gap of 1.071 eV at the Γ point, while ZnPbN2 is semi-metallic with the valence band and conduction band crossing at the Γ point. Both MgPbN2 and ZnPbN2 show an enhanced optical absorption and electric conductivity compared to those of the related semiconductors MgSnN2 and ZnSnN2. ZnPbN2 gives a much stronger electric conductivity and a slightly larger light absorption than MgPbN2. In addition the influence of alloying is investigated for Mg1−xZnxPbN2 at different doping amount. When Mg was doped at 25% level, a band gap of 0.22 eV opens up and turns the system to semiconducting. When Sn was doped at Pb site, a similar phenomena was observed. On the other hand, the electric conductivity will increase with a small amount of Zn doping in MgPbN2. With large amount of doping, a conductivity higher than that of pristine ZnPbN2 is achieved. Our investigation widens the knowledge of II-IV-N2 family of ternary nitrides, and may hence help to boost their applications.

First-principles study of defects and doping limits in CaO

Calcium oxide (CaO) is a promising host for quantum defects because of its ultrawide bandgap and potential for long spin coherence times. Using hybrid functional calculations, we investigate the intrinsic point defects and how they limit Fermi-level positions and doping in CaO. We find calcium and oxygen vacancies to be the most common intrinsic defects, acting as compensating acceptors and donors, respectively. Oxygen interstitials are also prevailing under O-rich conditions and act as compensating donors. Due to compensation by these defects, O-poor conditions are required to dope CaO n-type, while O-rich conditions are required for p-type doping. We find that, at room temperature, intrinsic CaO can only achieve Fermi-level positions between 1.76 eV above the valence-band maximum (VBM) and 1.73 eV below the conduction-band minimum (CBM). If suitable shallow dopants are found, the allowed range of Fermi levels would increase to between VBM + 0.53 eV and CBM − 0.27 eV and is set by the compensating intrinsic defects. Additionally, we study hydrogen impurities, and show that hydrogen will not only limit p-type doping but can also act as shallow donor when substituting oxygen (HO defects).

Tucker Tensor Approach for Accelerating Fock Exchange Computations in a Real-Space Finite-Element Discretization of Generalized Kohn-Sham Density Functional Theory.

The evaluation of Fock exchange is often the computationally most expensive part of hybrid functional density functional theory calculations in a systematically improvable, complete basis. In this work, we employ a Tucker tensor based approach that substantially accelerates the evaluation of the action of Fock exchange by transforming three-dimensional convolutional integrals into a tensor product of one-dimensional convolution integrals. Our numerical implementation uses a parallelization strategy that balances the memory and communication bottlenecks, alongside overlapping compute and communication operations to enhance computational efficiency and parallel scalability. The accuracy and computational efficiency are demonstrated on various systems, including Pt clusters of various sizes and a TiO2 cluster with 3684 electrons.

High-throughput hybrid-functional DFT calculations of bandgaps and formation energies and multifidelity learning with uncertainty quantification

High-throughput hybrid-functional DFT calculations of bandgaps and formation energies and multifidelity learning with uncertainty quantification

Nonempirical semilocal density functionals for correcting the self-interaction of polaronic states

Through the use of the piecewise-linearity condition of the total energy, we correct the self-interaction for the study of polarons by constructing nonempirical functionals at the semilocal level of theory. We consider two functionals, the γDFT and μDFT functionals, both of which are based on the addition of a weak local potential to the semilocal Hamiltonian to enforce the piecewise-linearity condition. We show that the resulting polaron properties are in good agreement with reference hybrid functional calculations. This supports the use of semilocal functionals for calculating polaron properties.

Surface termination modulation for superior S-Scheme Bi2WO6/BiOI heterojunction photocatalyst: a hybrid density functional study

The prevailing theoretical frameworks indicate that depending on the growth conditions, the Bi2WO6(001) surface can manifest in three distinct terminations—DL-O–Bi (DL: double layers), O–Bi, and O–W. In this study, we conduct a comprehensive examination of the interplay between these terminations on Bi2WO6(001) and the 1I-terminated BiOI(001) facet, especially focusing on their impact on the photocatalytic activity of Bi2WO6/BiOI heterostructure, applying hybrid functional calculations. The models formulated for this research are designated as Bi2WO6(O–Bi)/BiOI(1I), Bi2WO6(DL-O–Bi)/BiOI(1I), and Bi2WO6(O–W)/BiOI(1I). Our findings reveal that Bi2WO6(O–Bi)/BiOI(1I) shows a type II band alignment, which facilitates the spatial separation of photo-generated electrons and holes. Notably, the Bi2WO6(DL-O–Bi)/BiOI(1I) configuration has the lowest binding energy and results in an S-scheme (or Step-scheme) heterostructure. In contrast to the type II heterostructure, this particular configuration demonstrates enhanced photocatalytic efficiency due to improved photo-generated carrier separation, augmented oxidation capability, and better visible-light absorption. Conversely, Bi2WO6(O–W)/BiOI(1I) presents a type I projected band structure, which is less conducive for the separation of photo-generated electron–hole pairs. In summation, this investigation points out that one could significantly refine the photocatalytic efficacy of not only Bi2WO6/BiOI but also other heterostructure photocatalysts by modulating the coupling of different terminations via precise crystal synthesis or growth conditions.

An investigation on electronic and magnetic properties of Cr substituted MoS2 monolayer and multilayers—hybrid functional calculations

An investigation on electronic and magnetic properties of Cr substituted MoS2 monolayer and multilayers—hybrid functional calculations

Complex-Valued K-Means Clustering of Interpolative Separable Density Fitting Algorithm for Large-Scale Hybrid Functional Enabled Ab Initio Molecular Dynamics Simulations within Plane Waves.

K-means clustering, as a classic unsupervised machine learning algorithm, is the key step to select the interpolation sampling points in interpolative separable density fitting (ISDF) decomposition for hybrid functional electronic structure calculations. Real-valued K-means clustering for accelerating the ISDF decomposition has been demonstrated for large-scale hybrid functional enabled ab initio molecular dynamics (hybrid AIMD) simulations within plane-wave basis sets where the Kohn-Sham orbitals are real-valued. However, it is unclear whether such K-means clustering works for complex-valued Kohn-Sham orbitals. Here, we propose an improved weight function defined as the sum of the square modulus of complex-valued Kohn-Sham orbitals in K-means clustering for hybrid AIMD simulations. Numerical results demonstrate that the K-means algorithm with a new weight function yields smoother and more delocalized interpolation sampling points, resulting in smoother energy potential, smaller energy drift, and longer time steps for hybrid AIMD simulations compared to the previous weight function used in the real-valued K-means algorithm. In particular, we find that this improved algorithm can obtain more accurate oxygen-oxygen radial distribution functions in liquid water molecules and a more accurate power spectrum in crystal silicon dioxide compared to the previous K-means algorithm. Finally, we describe a massively parallel implementation of this ISDF decomposition to accelerate large-scale complex-valued hybrid AIMD simulations containing thousands of atoms (2,744 atoms), which can scale up to 5,504 CPU cores on modern supercomputers.

Broad luminescence from Zn acceptors in Zn doped β-Ga2O3

Zn-related defects in β-Ga2O3 were studied using photoluminescence (PL) spectroscopy combined with hybrid functional calculations and secondary ion mass spectrometry. We have in-diffused Zn by heat treatments of β-Ga2O3 in Zn vapor to promote the formation of the ZnGaZni complex as the dominating Zn configuration. Subsequently, we did heat treatment in oxygen ambient to study the dissociation of the donor complex ZnGaZni into the ZnGa acceptor. The PL spectra revealed a broad band centered at 2.5 eV. The signature has a minor contribution to the overall emission of as-grown and Zn-annealed samples but increases dramatically upon the subsequent heat treatments. The theoretical predictions from hybrid functional calculation show emission energies of 2.1 and 2.3 eV for ZnGa10/− and ZnGa20/−, respectively, and given that the previously observed deviation between the experimental and calculated values for the self-trapped holes in β-Ga2O3 is about 0.2 eV, we conclude that the 2.5 eV emission we observe herein is due to the Zn acceptor.

Absolute deformation potentials and absolute energy levels of III-N, ZnO, and II-IV-N2 semiconductors for optoelectronic applications

Absolute deformation potentials and absolute energy levels for III-N, ZnO, and II-IV-N2 semiconductors are systematically determined from hybrid-functional calculations. Separate bulk and slab calculations are combined and the vacuum level is taken as the common reference. The trends in the absolute deformation potentials are rationalized by the kinetic energy effect and the bonding (or antibonding) character of the band edge states. The calculated absolute energy levels can be used to obtain the natural band alignment between these semiconductors and are in accordance with the available results. The determined parameters are of practical importance to the optoelectronic devices designs.

Exploration of isoelectronic substitution in graphene dioxide for photocatalytic and photovoltaic applications - an ab-initio study.

Herein, we propose graphene dioxide (GDO) derivatives as promising materials for green hydrogen production by photocatalytic water splitting. The optoelectronic and photocatalytic properties of GDO, an insulator with a wide band gap, are tuned by designing new compositions through isovalent substitution of S/Se at the O site, Si and (B,N) at the C site. The newly predicted GDO derivatives were studied using hybrid functional calculations and our results show that several of these materials exhibit semiconducting behavior with a direct band gap value higher than 1.23 eV, hence appropriate for visible light-driven photocatalytic water splitting. The structural stability of these materials was analyzed by total energy and lattice dynamical calculations. The photo generated charge carriers possess lower effective mass and hence higher carrier mobility resulting in suppressed recombination rate and hence improving the water splitting efficiency. Apart from low excitonic binding energy, the electronic structure analysis shows that in several of these compounds the electrons and holes reside in two different atomic sites ensuring further reduction in recombination rate. The relatively higher absorption coefficient of GDO derivatives in the visible part of the solar spectrum indicates enhanced photoconversion efficiency suitable for solar cell applications also and it was further determined by photovoltaic performance parameter analysis. The band edge potential of GDO derivatives is well straddled by the water redox potential at different pHs, suggesting their potential for water splitting along with the possibility of CO2 reduction. Our findings indicate that the newly predicted compositions hold significant promise for photocatalytic as well as photovoltaic applications.

Modulation of the electronic and magnetic properties of an MnCrNO2 ferromagnetic semiconductor MXene.

MXenes are members of the rapidly expanding family of two-dimensional materials known for their electronic and magnetic properties and hold significant promise for advancements in electronics and spintronics technologies. In this study, we identified a stable MnCrNO2 MXene characterized by a band gap of 2.68 eV, a magnetic moment of 6μB, and a magnetic anisotropy energy of 78.6 μeV per transition metal atom. These properties were computed using density functional theory with an on-site Coulomb potential and HSE06 hybrid functional calculations. Manipulating the band gap and magnetic properties offers considerable advantages for tailoring MXenes for specific applications. Our investigation extended to exploring the property behaviors under biaxial strain, as well as the adsorption of Group-I and Group-II ions onto the newly discovered MXene. Our findings underscore a highly linear relationship between strain and band gap, supported by an impressive R2 score of 0.997 for the best-fit straight line. Moreover, we demonstrated the linear tunability of the material's magnetic anisotropy energy under biaxial strain, achieving an R2 score of 0.982. Adsorption of 2.2% Group-I and Group-II ions onto the MnCrNO2 MXene reveals the potential for a semiconductor-to-half-metal phase transition with K, Rb, Be, Mg, and Ca ions. These results provide pathways for leveraging MXenes for the development of next-generation electronic and spintronic devices.

Enhanced visible-light photocatalytic activity of BiOBr/BiOCl heterojunctions: A hybrid density functional investigation on the key roles of crystal facet and I-doping

In this work, (001) and (010) facets of BiOBr and BiOCl are selected to construct BiOBr(001)/BiOCl(001), BiOBr(010)/BiOCl(010), BiOBr(001)/BiOCl(010) and BiOBr(010)/BiOCl(001) heterostructure models for investigating the effect of crystal facet engineering on the photocatalytic activity of BiOBr/BiOCl through hybrid functional calculations. We found that the crystal facet is critical to the photocatalytic performance of BiOBr/BiOCl. The BiOBr(001)/BiOCl(001) and BiOBr(010)/BiOCl(010) with the same facet combinations in BiOBr/BiOCl present the unexpected type I band alignments and the poor photocatalytic activities. In contrast, the BiOBr(001)/BiOCl(010) and BiOBr(010)/BiOCl(001) with different facet combinations present the type II and direct Z-scheme characters, respectively, so that they can effectively separate the photo-excited electron-hole pairs in space. Especially, the direct Z-scheme BiOBr(010)/BiOCl(001) heterostructure can exhibit the stronger redox ability and better visible light absorption, further presenting excellent photocatalytic performance, in contrast to the type II BiOBr(001)/BiOCl(010). The investigations also show that Iodine (I) element doping can make type I BiOBr(001)/BiOCl(001) change to a direct Z-scheme band alignment and dramatically enhance its photocatalytic activity. This helps to interpret the related experimental results. Notably, I doping can significantly improve the visible light absorption of BiOBr(010)/BiOCl(001) heterostructure, enhance the solar-to-hydrogen (STH) efficiency, and still maintain its superior direct Z-scheme character. Therefore, these theoretical studies can provide valuable guidance in developing novel BiOBr/BiOCl heterostructures with high photocatalytic activities.

RPA, an Accurate and Fast Method for the Computation of Static Nonlinear Optical Properties.

The accurate computation of static nonlinear optical properties (SNLOPs) in large polymers requires accounting for electronic correlation effects with a reasonable computational cost. The Random Phase Approximation (RPA) used in the adiabatic connection fluctuation theorem is known to be a reliable and cost-effective method to render electronic correlation effects when combined with density-fitting techniques and integration over imaginary frequencies. We explore the ability of the RPA energy expression to predict SNLOPs by evaluating RPA electronic energies in the presence of finite electric fields to obtain (using the finite difference method) static polarizabilities and hyperpolarizabilities. We show that the RPA based on hybrid functional self-consistent field calculations yields accurate SNLOPs as the best-tuned double-hybrid functionals developed today, with the additional advantage that the RPA avoids any system-specific adjustment.

Dangling Bonds as Possible Contributors to Charge Noise in Silicon and Silicon-Germanium Quantum Dot Qubits.

Spin qubits based on Si and Si1-xGex quantum dot architectures exhibit among the best coherence times of competing quantum computing technologies, yet they still suffer from charge noise that limit their qubit gate fidelities. Identifying the origins of these charge fluctuations is therefore a critical step toward improving Si quantum-dot-based qubits. Here, we use hybrid functional calculations to investigate possible atomistic sources of charge noise, focusing on charge trapping at Si and Ge dangling bonds (DBs). We evaluate the role of global and local environment in the defect levels associated with DBs in Si, Ge, and Si1-xGex alloys, and consider their trapping and excitation energies within the framework of configuration coordinate diagrams. We additionally consider the influence of strain and oxidation in charge-trapping energetics by analyzing Si and GeSi DBs in SiO2 and strained Si layers in typical Si1-xGex quantum dot heterostructures. Our results identify that Ge dangling bonds are more problematic charge-trapping centers both in typical Si1-xGex alloys and associated oxidation layers, and they may be exacerbated by compositional inhomogeneities. These results suggest the importance of alloy homogeneity and possible passivation schemes for DBs in Si-based quantum dot qubits and are of general relevance to mitigating possible trap levels in other Si, Ge, and Si1-xGex-based metal-oxide-semiconductor stacks and related devices.

New Family of Interstitial Oxygen Ion Conductor Discovered By High-Throughput Computational Screening

Discovery and design of highly efficient oxygen-active materials which react with, absorb, and transport oxygen is essential for fuel cells and related applications. Currently, the predominant families of oxygen-active materials used in commercial devices are perovskites and fluorites, which diffuse oxygen via a vacancy mechanism and only have adequate oxygen diffusivity and surface exchange at high temperatures (e.g., ~800 ˚C), impeding their use at lower temperatures desirable for more cost-effective and long-lasting device operation. The development of alternative electrolyte/electrode materials with high oxygen ionic conductivity at low temperatures (e.g., room temperature to 400 °C) is of great interest. Interstitial oxygen ion conductors typically have significantly lower migration barriers than even state-of-the-art vacancy-mediated oxygen conductors, which can provide orders of magnitude kinetic enhancement at low temperatures. In addition, the interstitial oxygen defects typically become more thermodynamically favorable as the temperature is lowered, potentially providing an additional boost to the oxygen conductivity at low temperatures from a higher mediating defect concentration. The synergistic combination of low migration barriers and thermodynamically favorable interstitial oxygen content at low temperatures provides the tantalizing possibility that interstitial oxygen conductors may lead to transformative performance improvements in oxygen-active materials.In this work, we developed an approach to discover new interstitial-mediated oxygen conductors by performing high-throughput computational screening of the 34k oxide materials from the Materials Project database and discovered a new structural family of interstitial oxygen diffusers, perrierite-type oxide La4Mn5Si4O22+ 𝜹 (LMS). We used a hierarchy of screening criteria including the geometric free space, thermodynamic stability, synthesizability, redox-active elements, and presence of short diffusion pathways. This approach, based largely on simple, physically intuitive properties, quickly winnowed the field of 34k oxides down to just 345 oxides, an approximate 99% reduction in the search space considered, with essentially no significant computational cost. From this reduced set of promising materials, ab initio simulations were performed to calculate the interstitial oxygen formation energy and migration energy. Our screening has yielded several material families to date, among which the A4B5C4O22+ 𝜹, with La4Mn5Si4O22+ 𝜹 (LMS) as a representative material, was selected for investigation with higher-fidelity DFT hybrid functional calculations and experimental validations. The calculated formation energy of interstitial oxygen is -0.11 eV at a concentration of 2.3% under air condition, indicating that LMS is predicted to be oxygen hyperstoichiometric with an expected composition of La4Mn5Si4O22+0.5 in air. Two separate interstitial and interstitialcy (“kick-out”) diffusion mechanisms are revealed by ab initio molecular dynamics (AIMD) simulation. For the interstitial diffusion pathway, the moves through the channel in between the zigzag-arranged sorosilicate Si2O7 groups, with a barrier of 0.69 eV. For the interstitialcy diffusion pathway, the moves along the corner-sharing Si2O7-MnO2-Si2O7 framework by kicking out the lattice oxygen, with a barrier of 0.74 eV. These results align well with the experimental findings of stable interstitial oxygen content and high oxygen mobility in LMS (Please see a separate poster from Md Sariful Sheikhfor experimental results). This work provides a systematic approach to screening a large number of prospective materials and opens a new avenue of finding promising interstitial oxide ion conducting materials for potential applications in a range of fast oxygen ion conduction-based technologies.