Heyd Scuseria Ernzerhof Hybrid Research Articles

Unravelling the Role of the Central Metal Ion in the Electronic Structure of Tris(8-hydroxyquinoline) Metal Chelates: Photoemission Spectroscopy and Hybrid Functional Calculations

The electronic structures of tris(8-hydroxyquinolinato)-erbium(III) (ErQ(3)) and tris(8-hydroxyquinolinato)-aluminum(III) (AlQ(3)) have been studied by means of core level and valence band photoemission spectroscopy with the theoretical support of hybrid Heyd-Scuseria-Ernzerhof density functional theory, to investigate the role played by the central metal atom. A lower binding energy (0.2 eV and 0.3 eV, respectively) of the O 1s and N 1s core levels has been observed for ErQ(3) with respect to AlQ(3). Differences in the valence band spectra, mainly related to the first two peaks next to the highest occupied molecular orbital (HOMO), have been ascribed to an energetic shift (to 0.4 eV lower energies for ErQ(3)) of the σ molecular orbital between the oxygen atoms and the central metal atom. A lower (by 0.5 eV) ionization energy has been measured for the ErQ(3). The interpretation of these results is based on a reduced interaction between the central metal atom and the ligands in ErQ(3), with increased electronic charge around the ligands, due to the higher ionic radius and the lower electronegativity of Er with respect to Al.

A systematic study of polarons due to oxygen vacancy formation at the rutile TiO2(110) surface by GGA + U and HSE06 methods

The polaronic nature of excess electrons accompanying an oxygen vacancy in a TiO2(110) surface has been studied by several theoretical approaches. According to previous studies, DFT + U and hybrid functional methods predict different sites of localization of the polarons. In this paper, we conducted a thorough comparison of the results obtained by GGA + U (generalized gradient approximation + Hubbard U) and HSE06 (Heyd–Scuseria–Ernzerhof hybrid functional) approximations. Considering initial symmetry breaking in the geometry optimization process, we show that regardless of the approximations used, electrons localize at two particular subsurface Ti sites in a state with mixed dx2−y2/dz2 character in the global coordinate frame with a spatial extent of the order of 7 Å. The lowest state of the polarons is a singlet, but the triplet is only about 0.1 meV higher in energy. Our results agree with previous experiments and calculations, wherever available. We stress that the hybrid functional has been first applied on this surface with a realistic coverage of oxygen vacancies corresponding to the experimental situation (∼12.5%).

A simplified approach to the band gap correction of defect formation energies: Al, Ga, and In-doped ZnO

The calculation of defect levels in semiconductors within a density functional theory approach suffers greatly from the band gap problem. We propose a band gap correction scheme that is based on the separation of energy differences in electron addition and relaxation energies. We show that it can predict defect levels with a reasonable accuracy, particularly in the case of defects with conduction band character, and yet is simple and computationally economical. We apply this method to ZnO doped with group III elements (Al, Ga, In). As expected from experiment, the results indicate that Zn substitutional doping is preferred over interstitial doping in Al, Ga, and In-doped ZnO, under both zinc-rich and oxygen-rich conditions. Further, all three dopants act as shallow donors, with the +1 charge state having the most advantageous formation energy. Also, doping effects on the electronic structure of ZnO are sufficiently mild so as to affect little the fundamental band gap and lowest conduction bands dispersion, which secures their n-type transparent conducting behavior. A comparison with the extrapolation method based on LDA+U calculations and with the Heyd–Scuseria–Ernzerhof hybrid functional (HSE) shows the reliability of the proposed scheme in predicting the thermodynamic transition levels in shallow donor systems.

Zinc Oxide as a Model Transparent Conducting Oxide: A Theoretical and Experimental Study of the Impact of Hydroxylation, Vacancies, Interstitials, and Extrinsic Doping on the Electronic Properties of the Polar ZnO (0002) Surface

The technology-relevant zinc-terminated zinc oxide (0002) polar surface has been studied at the density-functional theory level using both Perdew–Burke–Ernzerhof (PBE) and hybrid Heyd–Scuseria–Ernzerhof (HSE06) functionals. We have considered a number of surface conditions to better understand the impact of surface hydroxylation and intrinsic and extrinsic surface defects, including zinc vacancies, oxygen vacancies, zinc interstitials, and aluminum dopants on the surface electronic properties. Our calculations point to large variations in surface work function and energy band gap as a function of the surface model; these variations can be attributed to changes in surface charge carrier density and to additional surface states induced by the defects. The calculated shifts in O(1s) core-level binding energy of the surface oxygens in different bonding configurations are in good agreement with experimental X-ray photoelectron spectroscopy data and point to the presence of two distinct OH-species on the ZnO su...

Electronic Structure of Some Wurtzite Semiconductors: Hybrid Functionals vs. Ab Initio Many Body Calculations

Using the rst-principles projector augmented wave method, the structural and electronic properties of wurtzite crystals, AlN, GaN, InN and ZnO have been calculated. Di erent exchange-correlation approximations: LDA, LDA+U , GGA, GGA+U and hybrid Heyd Scuseria Ernzerhof method were used. We also present the values of band gap calculated within di erent GW approximations (G0W0, GW0, GW and U+G0W0, the last one for materials with shallow d states). In case of structural parameters the best agreement with experiment was obtained for hybrid Heyd Scuseria Ernzerhof functional and in case of band gap the best agreement was for GW and U+G0W0 approximation.

Magnetism in Sc-doped ZnO with zinc vacancies: A hybrid density functional and GGA + U approaches

We investigate the zinc vacancy effects on the electronic structures and magnetic properties of Sc-doped ZnO, by performing first-principles calculations within both GGA+U and Heyd–Scuseria–Ernzerhof hybrid functional methods. We find that Sc impurities stabilize considerably Zn vacancies. The electronic and magnetic analysis shows a half metallic ferromagnetic character with a total magnetic moment of 2.01μB. The magnetism mainly stems from the O 2p states around the Zn vacancies. Calculations with the hybrid density functional agree with the GGA+U results but give an accurate description of the electronic structure for pure ZnO and Sc-doped ZnO with Zn vacancies.

Barrier height determination of silicide-silicon contact by hybrid density functional simulation

Accurate prediction of the Schottky barrier heights (SBHs) from first principles is important for contact and device technologies. Ab initio simulations based on widely used local density approximation and generalized gradient approximation significantly underestimate the SBHs. By modeling CoSi2-Si and NiSi2-Si contacts using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations, we demonstrate that HSE hybrid functional simulations can predict the SBHs in agreement with experiments. Furthermore, the effects of interfacial dopants and metal induced gap states are examined by ab initio hybrid functional simulations.

HSE hybrid functional within the FLAPW method and its application to GdN

We present an implementation of the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional within the full-potential linearized augmented-plane-wave (FLAPW) method. Pivotal to the HSE functional is the screened electron-electron interaction, which we separate into the bare Coulomb interaction and the remainder. Both terms give rise to exchange potentials, which sum up to the screened nonlocal exchange potential of HSE. We evaluate the former with the help of an auxiliary basis, defined in such a way that the bare Coulomb matrix becomes sparse. The latter, which is a slowly varying function in real space, is computed efficiently in reciprocal space. This approach is general and can be applied to a whole class of screened hybrid functionals. We obtain excellent agreement of band gaps and lattice constants for prototypical semiconductors and insulators with electronic-structure calculations using plane-wave or Gaussian basis sets. We apply the HSE hybrid functional to examine the ground-state properties of rocksalt GdN, which have been controversially discussed in literature. Our results indicate that there is a half-metal to insulator transition occurring between the theoretically optimized lattice constant at $0\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and the experimental lattice constant at room temperature. Overall, we attain good agreement with experimental data for band transitions, magnetic moments, and the Curie temperature.

First-principles study of Cu2ZnSnS4 and the related band offsets for photovoltaic applications

First-principles calculations of the band offsets betweenCu2ZnSnS4 (CZTS)and XS (X = Cd, Zn) are performed. While the interface dipole contribution for the band offsets iscalculated using the Perdew–Burke–Ernzerhof functional, the Heyd–Scuseria–Ernzerhofhybrid functional is employed to introduce the quasiparticle corrections to the band offsets.The calculated conduction band offset between CZTS and CdS is 0.2 eV, validating CdS forthe buffer layer of the CZTS solar cell. The small conduction band offset stems from theband gap narrowing of CdS under the interface strain caused by the lattice misfit withCZTS. A large valence band offset over 0.9 eV between CZTS and ZnS indicates thatprecipitated ZnS is regarded as an inactive insulator phase in CZTS absorbers.

Electronic structure of cation-codoped TiO2 for visible-light photocatalyst applications from hybrid density functional theory calculations

The electronic structures of Mg/Ca- and/or Mo/W- (mono- and co-) doped anatase TiO2 have been investigated via generalized Kohn–Sham theory with the Heyd–Scuseria–Ernzerhof hybrid functional for exchange-correlation {J. Heyd et al., [J. Chem. Phys. 118, 8207 (2003)], J. Heyd et al., [J. Chem. Phys. 124, 219906 (2006)], and J. Paier et al., [J. Chem. Phys. 125, 249901 (2006)]}, in the context of density functional theory. Gap narrowing is small for monodoping, which also creates impuritiy bands in the “forbidden gap,” either as acceptor or donor states, limiting possible utility as visible-light photocatalysts. However, codoping of Mg/Ca and Mo/W not only induces appreciable gap narrowing, but also serves to passivate the impurity bands, which can harvest visible-light to a greater extent. Considering ionic radii, Mg and Mo should constitute the best cation-pair.

Band parameters and strain effects in ZnO and group-III nitrides

We present consistent sets of band parameters (including band gaps, crystal-field splittings, effective masses, Luttinger and EP 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 in the local-density and generalized-gradient approximations we employ the Heyd–Scuseria–Ernzerhof hybrid functional as well as exact exchange (OEPx)-based quasi-particle energy calculations in the G0W0 approach.

Role of strain in polarization switching in semipolar InGaN/GaN quantum wells

The effect of strain on the valence-band structure of (112¯2) semipolar InGaN grown on GaN substrates is studied. A k⋅p analysis reveals that anisotropic strain in the c-plane and shear strain are crucial for deciding the ordering of the two topmost valence bands. The shear-strain deformation potential D6 is calculated for GaN and InN using density functional theory with the Heyd–Scuseria–Ernzerhof hybrid functional [J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 124, 219906 (2006)]. Using our deformation potentials and assuming a pseudomorphically strained structure, no polarization switching is observed. We investigate the role of partial strain relaxation in the observed polarization switching.

Accurate ab initio predictions of III–V direct-indirect band gap crossovers

We report the compositional dependence of the electronic band structure for a range of III–V alloys. Standard density functional theory is insufficient to mimic the electronic gap energies at different symmetry points of the Brillouin zone. The Heyd–Scuseria–Ernzerhof hybrid functional with screened exchange accurately reproduces the experimental band gaps and, more importantly, the alloy concentration of the direct-indirect gap crossovers for the III–V alloys studied here: AlGaAs, InAlAs, AlInP, InGaP, and GaAsP.

Calculation of semiconductor band gaps with the M06-L density functional

The performance of the M06-L density functional has been tested for band gaps in seven semiconductors plus diamond and MgO. Comparison with the local spin density approximation (LSDA), Becke-Lee-Yang-Parr (BLYP), Perdew-Burke-Eernzerhof (PBE), Tao-Perdew-Staroverov-Scuseria (TPSS), and Heyd-Scuseria-Ernzerhof (HSE) functionals shows that M06-L has improved performance for calculating band gaps as compared to other local functionals, but it is less accurate than the screened hybrid HSE functional for band gaps.