Showing papers on "Electronic band structure published in 2010"

Transparent Conducting Oxides for Photovoltaics: Manipulation of Fermi Level, Work Function and Energy Band Alignment

Abstract: Doping limits, band gaps, work functions and energy band alignments of undoped and donor-doped transparent conducting oxides Zn0, In2O3, and SnO2 as accessed by X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) are summarized and compared. The presented collection provides an extensive data set of technologically relevant electronic properties of photovoltaic transparent electrode materials and illustrates how these relate to the underlying defect chemistry, the dependence of surface dipoles on crystallographic orientation and/or surface termination, and Fermi level pinning.

Zinc oxide : from fundamental properties towards novel applications

Abstract: Crystal Structure, Chemical Binding, and Lattice Properties.- Growth.- Band Structure.- Electrical Conductivity and Doping.- Intrinsic Linear Optical Properties Close to the Fundamental Absorption Edge.- Bound Exciton Complexes.- Influence of External Fields.- Deep Centres in ZnO.- Magnetic Properties.- Nonlinear Optics, High Density Effects and Stimulated Emission.- Dynamic Processes.- Past, Present and Future Applications.- Conclusion and Outlook.

Observation of Plasmarons in Quasi-Freestanding Doped Graphene

Abstract: A hallmark of graphene is its unusual conical band structure that leads to a zero-energy band gap at a single Dirac crossing point. By measuring the spectral function of charge carriers in quasi-freestanding graphene with angle-resolved photoemission spectroscopy, we showed that at finite doping, this well-known linear Dirac spectrum does not provide a full description of the charge-carrying excitations. We observed composite “plasmaron” particles, which are bound states of charge carriers with plasmons, the density oscillations of the graphene electron gas. The Dirac crossing point is resolved into three crossings: the first between pure charge bands, the second between pure plasmaron bands, and the third a ring-shaped crossing between charge and plasmaron bands.

Predicted band structures of III-V semiconductors in the wurtzite phase

Abstract: While non-nitride III-V semiconductors typically have a zinc-blende structure, they may also form wurtzite crystals under pressure or when grown as nanowhiskers. This makes electronic structure calculation difficult since the band structures of wurtzite III-V semiconductors are poorly characterized. We have calculated the electronic band structure for nine III-V semiconductors in the wurtzite phase using transferable empirical pseudopotentials including spin-orbit coupling. We find that all the materials have direct gaps. Our results differ significantly from earlier ab initio calculations, and where experimental results are available (InP, InAs, and GaAs) our calculated band gaps are in good agreement. We tabulate energies, effective masses, and linear and cubic Dresselhaus zero-field spin-splitting coefficients for the zone-center states. The large zero-field spin-splitting coefficients we find may facilitate the development of spin-based devices.

Band structure engineering of semiconductors for enhanced photoelectrochemical water splitting: The case of TiO 2

Abstract: Here, we propose general strategies for the rational design of semiconductors to simultaneously meet all of the requirements for a high-efficiency, solar-driven photoelectrochemical (PEC) water-splitting device. As a case study, we apply our strategies for engineering the popular semiconductor, anatase ${\text{TiO}}_{2}$. Previous attempts to modify known semiconductors such as ${\text{TiO}}_{2}$ have often focused on a particular individual criterion such as band gap, neglecting the possible detrimental consequence to other important criteria. Density-functional theory calculations reveal that with appropriate donor-acceptor coincorporation alloys with anatase ${\text{TiO}}_{2}$ hold great potential to satisfy all of the criteria for a viable PEC device. We predict that (Mo, 2N) and (W, 2N) are the best donor-acceptor combinations in the low-alloy concentration regime whereas (Nb, N) and (Ta, N) are the best choice of donor-acceptor pairs in the high-alloy concentration regime.

Heavily electron-doped electronic structure and isotropic superconducting gap in AxFe2Se2 (A=K,Cs)

Abstract: The low energy band structure and Fermi surface of the newly discovered superconductor, AxFe2Se2 (A=K,Cs), have been studied by angle-resolved photoemission spectroscopy. Compared with iron pnictide superconductors, AxFe2Se2 (A=K,Cs) is the most heavily electron-doped with Tc~30 K. Only electron pockets are observed with an almost isotropic superconducting gap of ~10.3 meV, while there is no hole Fermi surface near the zone center, which indicates the inter-pocket hopping or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign changed s$_\pm$ pairing symmetry, a leading candidate proposed for iron-based superconductors, becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is a better description.

Band structure of ABC-stacked graphene trilayers

Abstract: The $ABC$-stacked $N$-layer-graphene family of two-dimensional electron systems is described at low energies by two remarkably flat bands with Bloch states that have strongly momentum-dependent phase differences between carbon $\ensuremath{\pi}$-orbital amplitudes on different layers and large associated momentum-space Berry phases. These properties are most easily understood using a simplified model with only nearest-neighbor interlayer hopping which leads to gapless semiconductor electronic structure and ${p}^{N}$ dispersion in both conduction and valence bands. We report on a study of the electronic band structures of trilayers which uses ab initio density-functional theory and $\mathbit{k}\ensuremath{\cdot}\mathbit{p}$ theory to fit the parameters of a $\ensuremath{\pi}$-band tight-binding model. We find that when remote interlayer hopping is retained, the triple Dirac point of the simplified model is split into three single Dirac points located along the three KM directions. External potential differences between top and bottom layers are strongly screened by charge transfer within the trilayer but still open an energy gap at overall neutrality.

Towards efficient band structure and effective mass calculations for III-V direct band-gap semiconductors

Abstract: The band structures and effective masses of III-V semiconductors (InP, InAs, InSb, GaAs, and GaSb) are calculated using the $GW$ method, the Heyd, Scuseria, and Ernzerhof hybrid functional, and modified Becke-Johnson combined with the local-density approximation (MBJLDA)---a local potential optimized for the description of the fundamental band gaps [F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 (2009)]. We find that MBJLDA yields an excellent description of the band gaps at high-symmetry points, on par with the hybrid functional and $GW$. However, the effective masses are generally overestimated by $20--30\text{ }\mathrm{%}$ using the MBJLDA local multiplicative potential. We believe this to be related to incorrect nearest-neighbor hopping elements, which are little affected by the choice of the local potential. Despite these shortcomings, the MBJLDA method might be a suitable approach for predicting or interpolating the full band dispersion, if only limited experimental data are available. Furthermore, the method is applicable to systems containing several thousand atoms where accurate quasiparticle methods are not applicable.

Ab initio studies on atomic and electronic structures of black phosphorus

Abstract: The atomic and electronic properties of black phosphorus (BP), which has been recently shown to have potential application as anode material for lithium ion batteries, are studied via ab initio calculations. The calculations reveal that the interlayer interaction in BP is Van der Waals Keesom force, which is critical to the formation of the layered structure. Interestingly, we also found that the small band gap of bulk BP (0.19 eV) when compared with that of single layer BP (0.75 eV) is partly because of the interlayer Van der Waals interaction in BP. The change in a materials band structure because of Van der Waals interaction is rarely reported in literature.

Comparison of Ab initio Low-Energy Models for LaFePO, LaFeAsO, BaFe2As2, LiFeAs, FeSe, and FeTe: Electron Correlation and Covalency

Abstract: Effective low-energy Hamiltonians for several different families of iron-based superconductors are compared after deriving them from the downfolding scheme based on first-principles calculations. Systematic dependences of the derived model parameters on the families are elucidated, many of which are understood from the systematic variation of the covalency between Fe-3 d and pnictogen-/chalcogen- p orbitals. First, LaFePO, LaFeAsO (1111), BaFe 2 As 2 (122), LiFeAs (111), FeSe, and FeTe (11) have overall similar band structures near the Fermi level, where the total widths of 10-fold Fe-3 d bands are mostly around 4.5 eV. However, the derived effective models of the 10-fold Fe-3 d bands ( d model) for FeSe and FeTe have substantially larger effective onsite Coulomb interactions U ∼4.2 and 3.4 eV, respectively, after the screening by electrons on other bands and after averaging over orbitals, as compared to ∼2.5 eV for LaFeAsO. The difference is similar in the effective models containing p orbitals of As, Se...

Energy gap tuning in graphene on hexagonal boron nitride bilayer system

Abstract: We use a tight-binding approach and density functional theory calculations to study the band structure of graphene/hexagonal boron nitride bilayer system in the most stable configuration We show that an electric field applied in the direction perpendicular to the layers significantly modifies the electronic structure of the whole system, including shifts, anticrossing and other deformations of bands, which can allow to control a value of the energy gap It is shown that band structure of biased system may be tailored for specific requirements of nanoelectronics applications The carriers' mobilities are expected to be higher than in the bilayer graphene devices

The evolution of electronic structure in few-layer graphene revealed by optical spectroscopy

Abstract: The massless Dirac spectrum of electrons in single-layer graphene has been thoroughly studied both theoretically and experimentally. Although a subject of considerable theoretical interest, experimental investigations of the richer electronic structure of few-layer graphene (FLG) have been limited. Here we examine FLG graphene crystals with Bernal stacking of layer thicknesses N = 1,2,3,...8 prepared using the mechanical exfoliation technique. For each layer thickness N, infrared conductivity measurements over the spectral range of 0.2-1.0 eV have been performed and reveal a distinctive band structure, with different conductivity peaks present below 0.5 eV and a relatively flat spectrum at higher photon energies. The principal transitions exhibit a systematic energy-scaling behavior with N. These observations are explained within a unified zone-folding scheme that generates the electronic states for all FLG materials from that of the bulk 3D graphite crystal through imposition of appropriate boundary conditions. Using the Kubo formula, we find that the complete infrared conductivity spectra for the different FLG crystals can be reproduced reasonably well within the framework a tight-binding model.

Strain effect on the optical conductivity of graphene

Abstract: Within the tight-binding approximation, we study the dependence of the electronic band structure and of the optical conductivity of a graphene single layer on the modulus and direction of applied uniaxial strain. While the Dirac-cone approximation, albeit with a deformed cone, is robust for sufficiently small strain, band dispersion linearity breaks down along a given direction, corresponding to the development of anisotropic massive low-energy excitations. We recover a linear behavior of the low-energy density of states, as long as the cone approximation holds, while a band gap opens for sufficiently intense strain, for almost all, generic strain directions. This may be interpreted in terms of an electronic topological transition, corresponding to a change in topology of the Fermi line, and to the merging of two inequivalent Dirac points as a function of strain. We propose that these features may be observed in the frequency dependence of the longitudinal-optical conductivity in the visible range, as a function of strain modulus and direction, as well as of field orientation.

Band structure and optical gain of tensile-strained germanium based on a 30 band k⋅p formalism

Abstract: We have investigated the band structure of tensile-strained germanium using a 30 band k⋅p formalism. This multiband formalism allows to simultaneously describe the valence and conduction bands, including the L, Δ, and Γ valleys. We calculate the energy band variation as a function of strain and obtain that the crossover from indirect to direct band gap occurs for a tensile in-plane strain of 1.9%. The effective masses of density of states are deduced from the calculated conduction and valence band density of states. Significant deviations are observed as compared to the effective masses of density of states values of unstrained bulk germanium. We finally calculate the optical gain that can be achieved with tensile-strained bulk germanium. An optical gain larger than 3000 cm−1 is predicted for a carrier density of 1×1018 cm−3 and a 3% in-plane biaxial strain. This optical gain is larger than the one of GaAs calculated with the same formalism and is much larger than the experimental free-carrier absorption ...

Electronic and magnetic properties of the graphene ferromagnet interface

Abstract: This paper presents our work on the investigation of the surface structure and the electronic and magnetic properties of the graphene layer on the lattice-matched surface of a ferromagnetic material, Ni(111). Scanning tunneling microscopy imaging shows that perfectly ordered epitaxial graphene layers can be prepared by elevated temperature decomposition of hydrocarbons, with domains larger than the terraces of the underlying Ni(111). In some exceptional cases, graphene films do not show rotational alignment with the metal surface, leading to moire structures with small periodicities. We discuss the crystallographic structure of graphene with respect to the Ni(111) surface relying both on experimental results and on recent theoretical studies. X-ray absorption spectroscopy investigations of empty valence-band states demonstrate the existence of interface states, which originate from the strong hybridization between the graphene π and Ni 3d valence-band states with the partial charge transfer of the spin-polarized electrons to the graphene π* unoccupied states. The latter leads to the appearance of an induced magnetic moment of carbon atoms in the graphene layer, which is unambiguously confirmed by both x-ray magnetic circular dichroism and spin-resolved photoemission. Further angle-resolved photoemission investigations indicate a strong interaction between graphene and Ni(111), showing considerable modification of the valence-band states of Ni and graphene due to strong hybridization. A detailed analysis of the Fermi surface of the graphene/Ni(111) system shows very good agreement between experimental and calculated two-dimensional maps of the electronic states around the Fermi level, supporting the idea of spin-filtering. We analyze our spectroscopic results relying on the currently available band structure calculations for the graphene/Ni(111) system and discuss the perspectives of the realization of graphene/ferromagnet-based devices.

Topological insulators and Mott physics from the Hubbard interaction

Abstract: We investigate the Hubbard model on the honeycomb lattice with intrinsic spin-orbit interactions as a paradigm for two-dimensional topological band insulators in the presence of interactions Applying a combination of Hartree-Fock theory, slave-rotor techniques, and topological arguments, we show that the topological band insulating phase persists up to quite strong interactions Then we apply the slave-rotor mean-field theory and find a Mott transition at which the charge degrees of freedom become localized on the lattice sites The spin degrees of freedom, however, are still described by the original Kane-Mele band structure Gauge-field effects in this region play an important role When the honeycomb layer is isolated then the spin sector becomes already unstable toward an easy-plane Neel order In contrast, if the honeycomb lattice is surrounded by extra ``screening'' layers with gapless spinons, then the system will support a fractionalized topological insulator phase with gapless spinons at the edges For large interactions, we derive an effective spin Hamiltonian

Effective Band Structure of Random Alloys

Abstract: Random substitutional A(x)B(1-x) alloys lack formal translational symmetry and thus cannot be described by the language of band-structure dispersion E(k(→)) Yet, many alloy experiments are interpreted phenomenologically precisely by constructs derived from wave vector k(→), eg, effective masses or van Hove singularities Here we use large supercells with randomly distributed A and B atoms, whereby many different local environments are allowed to coexist, and transform the eigenstates into an effective band structure (EBS) in the primitive cell using a spectral decomposition The resulting EBS reveals the extent to which band characteristics are preserved or lost at different compositions, band indices, and k(→) points, showing in (In,Ga)N the rapid disintegration of the valence band Bloch character and in Ga(N,P) the appearance of a pinned impurity band

Electronic band structure of zirconia and hafnia polymorphs from the GW perspective

Abstract: The electronic structure of crystalline ZrO2 and HfO2 in the cubic, tetragonal, and monoclinic phase has been investigated using many-body perturbation theory in the GW approach based on density-functional theory calculations in the local-density approximation LDA. ZrO2 and HfO2 are found to have very similar quasiparticle band structures. Small differences between them are already well described at the LDA level indicating that the filled f shell in HfO2 has no significant effect on the GW corrections. A comparison with direct and inverse photoemission data shows that the GW density of states agrees very well with experiment. A systematic investigation into the structural and morphological dependence of the electronic structure reveals that the internal displacement of the oxygen atoms in the tetragonal phase has a significant effect on the band gap.

Electronic and Band Structure Tuning of Ternary Semiconductor Photocatalysts by Self Doping: The Case of BiOI

Abstract: Foreign nonmetal or metal element doping has been widely used to tailor the electronic and band structures of wide band gap binary oxide semiconductor photocatalysts, extending their absorption edges into the visible light range for better utilization of solar light. Besides doping with foreign elements, self-doping can also tune the electronic and band structures of semiconductor photocatalysts but only limited to binary metal oxides, such as oxygen-deficient TiOx (x < 2). In this study, we demonstrate that self-doping is able to tune the electronic and band structures of ternary semiconductor photocatalysts and thus significantly enhance their photocatalytic activities by utilizing BiOI as the example. Density functional theory calculations revealed that iodine self-doping could effectively tune the electronic structures of BiOI. Motivated by the calculations, iodine self-doped bismuth oxyiodide photocatalysts were synthesized with a soft chemical method to illustrate this band structure tailoring appro...

Soluble semiconductors AAsSe2 (A = Li, Na) with a direct-band-gap and strong second harmonic generation: a combined experimental and theoretical study.

Abstract: AAsSe(2) (A = Li, Na) have been identified as a new class of polar direct-band gap semiconductors These I-V-VI(2) ternary alkali-metal chalcoarsenates have infinite single chains of (1/infinity)[AsQ(2)(-)] derived from corner-sharing pyramidal AsQ(3) units with stereochemically active lone pairs of electrons on arsenic The conformations and packing of the chains depend on the structure-directing alkali metals This results in at least four different structural types for the Li(1-x)Na(x)AsSe(2) stoichiometry (alpha-LiAsSe(2), beta-LiAsSe(2), gamma-NaAsSe(2), and delta-NaAsSe(2)) Single-crystal X-ray diffraction studies showed an average cubic NaCl-type structure for alpha-LiAsSe(2), which was further demonstrated to be locally distorted by pair distribution function (PDF) analysis The beta and gamma forms have polar structures built of different (1/infinity)[AsSe(2)(-)] chain conformations, whereas the delta form has nonpolar packing A wide range of direct band gaps are observed, depending on composition: namely, 111 eV for alpha-LiAsSe(2), 160 eV for LiAsS(2), 175 eV for gamma-NaAsSe(2), 223 eV for NaAsS(2) The AAsQ(2) materials are soluble in common solvents such as methanol, which makes them promising candidates for solution processing Band structure calculations performed with the highly precise screened-exchange sX-LDA FLAPW method confirm the direct-gap nature and agree well with experiment The polar gamma-NaAsSe(2) shows very large nonlinear optical (NLO) second harmonic generation (SHG) response in the wavelength range of 600-950 nm The theoretical studies confirm the experimental results and show that gamma-NaAsSe(2) has the highest static SHG coefficient known to date, 3379 pm/V, among materials with band gaps larger than 10 eV

Effective band gap narrowing of anatase TiO2 by strain along a soft crystal direction

Abstract: Due to its large band gap (3.2 eV), TiO2 cannot absorb sun light effectively. To reduce its band gap, various approaches have been attempted; most of them are using doping to modify its band structure. Using first-principles band structure calculations, we show that unlike the rutile phases, the band gap of TiO2 in the anatase phase can be effectively reduced by applying stress along a soft direction. We propose that this approach of tuning the band gap by applying stress along soft direction of a layered semiconductor is general and should be applicable to other anisotropic materials.

Evolution of the Fermi Surface of BaFe2(As1-xPx)2 on Entering the Superconducting Dome

Abstract: Using the de Haas-van Alphen effect we have measured the evolution of the Fermi surface of BaFe2(As1-xPx){2} as a function of isoelectric substitution (As/P) for 0.41

Electronic and defect structure of CuSCN

Abstract: Copper thiocyanate (CuSCN) is a candidate as a transparent solid p-type conductor for optoelectronic and photovoltaic applications, such as solar cells. We calculate the band structure, bonding characteristics, and basic native defect configurations of hexagonal β-CuSCN. β-CuSCN is predicted to be an indirect-gap semiconductor with an unusual orbital character: although the highest valence bands have the expected character of Cu 3d levels hybridized with S 3p states, the conduction band minimum (at the K point of the hexagonal Brillouin zone) has mostly cyanide antibonding character. This quasi-molecular character results in some unusual properties, including that the electron effective masses are comparable to or even larger than the hole effective masses. Calculated results match well with the valence band spectrum of thin film CuSCN, although optical absorption measurements do not conclusively confirm the predicted indirect nature of the lowest transitions. The dominant p-type character of this materia...

Tunable band gap in hydrogenated bilayer graphene.

Abstract: We have studied the electronic structural characteristics of hydrogenated bilayer graphene under a perpendicular electric bias using first-principles density functional calculations. The bias voltage applied between the two hydrogenated graphene layers allows continuous tuning of the band gap and leads to transition from semiconducting to metallic state. Desorption of hydrogen from one layer in the chair conformation yields a ferromagnetic semiconductor with a tunable band gap. The implications of tailoring the band structure of biased system for future graphene-based device applications are discussed.

Subgap states, doping and defect formation energies in amorphous oxide semiconductor a-InGaZnO4 studied by density functional theory

Abstract: Amorphous In-Ga-Zn-O (a-IGZO) is expected for channel layers in thin-film transistors (TFTs). It is known that a-IGZO is sensitive to an O/H-containing atmosphere; therefore, it is important to clarify the roles of oxygen and hydrogen in a-IGZO. This paper provides atomic and electronic structures, formation energies of defects and bond energies in a-IGZO calculated by first-principles density functional theory (DFT). It was confirmed that oxygen deficiencies having small formation energies (2-3.6 eV) form either deep fully-occupied localized states near the valence band maximum or donor states, which depend on their local structures. All the hydrogen doping form -OH bond and work as a donor. The stable -OH bonds have small formation energy of ~0.45 eV and consist of three metal cations coordinated to the O ion. The bond energy of Ga-O is calculated to be ~2.0 eV, which is the largest among the chemical bonds in a-IGZO (1.7 eV for In-O and 1.5 eV for ZnO). This result supports the idea that the incorporation of Ga stabilizes a-IGZO TFTs.

Quasiparticle self-consistent GW calculations for PbS, PbSe, and PbTe: Band structure and pressure coefficients

Abstract: The electronic band structures of PbS, PbSe, and PbTe in the rocksalt structure are calculated with the quasiparticle self-consistent GW (QSGW) approach with spin-orbit coupling included. The semiconducting gaps and their deformation potentials as well as the effective masses are obtained. The GW approximation provides a correct description of the electronic structure around the gap, in contrast to the local-density approximation, which leads to inverted gaps in the lead chalcogenides. The QSGW calculations are in good quantitative agreement with experimental values of the gaps and masses. At moderate hole doping a complex filamental Fermi-surface structure develops with ensuing large density of states. The pressure-induced gap closure leads to linear (Dirac-type) band dispersions around the L point.

The electronic structure of β-Ga2O3

Abstract: β-Ga2O3 has the widest energy gap of the transparent conducting oxides. The interest in its electronic properties has recently increased because of its applications in various optoelectronic devices, semiconductor lasers, and ultrasensitive gas detecting systems. In contrast, information on the electronic structure of β-Ga2O3 is very scarce. Here, we present the experimental valence-band structure of β-Ga2O3 single crystals determined by high-resolution angle-resolved photoelectron spectroscopy utilizing synchrotron radiation. We find good matching of the experimental band structure with the advanced density functional theory calculations employing hybrid functionals and projector augmented wave potentials.

Phase diagram and gap anisotropy in iron-pnictide superconductors

Abstract: Using the fluctuation-exchange approximation, we study an effective five-band Hubbard model for iron-pnictide superconductors obtained from the first-principles band structure. We preclude deformations of the Fermi surface due to electronic correlations by introducing a static potential, which mimics the effect of charge relaxation. Evaluating the Eliashberg equation for various dopings and interaction parameters, we find that superconductivity can sustain higher hole than electron doping. Analyzing the symmetry of the superconducting order parameter we observe clear differences between the hole- and electron-doped systems. We discuss the importance of the pnictogen height for superconductivity. Finally, we dissect the pairing interaction into various contributions, which allows us to clarify the relationship between the superconducting transition temperature and the proximity to the antiferromagnetic phase.