Showing papers on "Electronic band structure published in 2001"

Band parameters for III–V compound semiconductors and their alloys

Abstract: We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Superconductivity of metallic boron in MgB2.

Abstract: Boron in MgB2 forms stacks of honeycomb layers with magnesium as a space filler. Band structure calculations indicate that Mg is substantially ionized, and the bands at the Fermi level derive mainly from B orbitals. Strong bonding with an ionic component and considerable metallic density of states yield a sizable electron-phonon coupling. Together with high phonon frequencies, which we estimate via zone-center frozen phonon calculations to be between 300 and 700 cm(-1), this produces a high critical temperature, consistent with recent experiments. Thus MgB2 can be viewed as an analog of the long sought, but still hypothetical, superconducting metallic hydrogen.

Bulk electronic structure of SrTiO3: Experiment and theory

Abstract: Valence electron-energy loss spectroscopy (VEELS) in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO3 Assignments of the interband transitions in the electronic structure of bulk SrTiO3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states The experimentally determined indirect band gap energy is 325 eV, while the direct band gap energy is 375 eV The conduction bands in SrTiO3 correspond to the bands composed of mainly Ti 3d t2g and eg states, followed at higher energies by the bands of Sr 4d t2g and eg states, and free electron like states dominating at energies above 15 eV The upper valence band (UVB) contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 325 eV up to 15 eV The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV Interband transitions from the O 2s band to the conduction bands appear at 26 eV A very narrow band at −33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons

Experimental and Theoretical Evidence for the Existence of Absolute Acoustic Band Gaps in Two-Dimensional Solid Phononic Crystals

Abstract: Experimental measurements of acoustic transmission through a solid-solid two-dimensional binary-composite medium constituted of a triangular array of parallel circular steel cylinders in an epoxy matrix are reported. Attention is restricted to propagation of elastic waves perpendicular to the cylinders. Measured transmitted spectra demonstrate the existence of absolute stop bands, i.e., band gaps independent of the direction of propagation in the plane perpendicular to the cylinders. Theoretical calculations of the band structure and transmission spectra using the plane wave expansion and the finite difference time domain methods support unambiguously the absolute nature of the observed band gaps.

A Three-Dimensional Synthetic Metallic Crystal Composed of Single-Component Molecules

Abstract: Molecular metals normally require charge transfer between two different chemical species. We prepared crystals of [Ni(tmdt) 2 ] (tmdt, trimethylenetetrathiafulvalenedithiolate) and carried out crystal structure analyses and resistivity measurements. The analyses and measurements revealed that these single-component molecular crystals are metallic from room temperature down to 0.6 kelvin. Ab initio molecular orbital calculations suggested that π molecular orbitals form conduction bands. The compact molecular arrangement, intermolecular overlap integrals of the highest occupied and lowest unoccupied molecular orbitals, and tight-binding electronic band structure calculation revealed that [Ni(tmdt) 2 ] is a three-dimensional synthetic metal composed of planar molecules.

Monte Carlo simulation of electron transport in the III-nitride wurtzite phase materials system: binaries and ternaries

Abstract: We present a comprehensive study of the transport dynamics of electrons in the ternary compounds, Al/sub x/Ga/sub 1-x/N and In/sub x/Ga/sub 1-x/N. Calculations are made using a nonparabolic effective mass energy band model. Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models. The effects of alloy scattering on the electron transport physics are examined. The steady state velocity field curves and low field mobilities are calculated for representative compositions of these alloys at different temperatures and ionized impurity concentrations. A field dependent mobility model is provided for both ternary compounds AlGaN and InGaN. The parameters for the low and high field mobility models for these ternary compounds are extracted and presented. The mobility models can be employed in simulations of devices that incorporate the ternary III-nitrides.

Photoemission study of energy-band alignments and gap-state density distributions for high-k gate dielectrics

Abstract: The determination of the energy band gaps of thin-gate insulators has been demonstrated from the onsets of the energy-loss spectra of O 1s (or N 1s) photoelectrons. The valence-band lineups of thin high-dielectric-constant (high-k) dielectrics such as Ta2O5, Al2O3, and ZrO2 formed on metals and Si(100) have also been determined by measuring the energy difference between the valence-band density-of-states curves. The energy band diagrams for metal/high-k dielectrics/Si(100) systems have been derived explicitly from considering the measured band gaps, valence-band lineups, electron affinities, and metal work functions in the systems. It is also demonstrated that total photoelectron yield spectroscopy can be used to quantify the energy distributions of both the defect states in high-k gate dielectrics and at the dielectric/Si(100) interfaces over the entire Si band gap without gate formation.

Giant Anharmonicity and Nonlinear Electron-Phonon Coupling in MgB 2 : A Combined First-Principles Calculation and Neutron Scattering Study

Abstract: First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB{sub 2} are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E{sub 2g} in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B {sigma} bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T{sub c} and boron isotope effect in MgB{sub 2} .

Elasticity of hexagonal BeO

Abstract: We study the elastic properties, electronic structure, and equation of state of BeO using a first-principles pseudopotential method within the gradient-corrected approximation of the density functional theory. Comparison of the calculated and experimental properties of BeO shows good agreement for all the properties studied here: ground-state structure, linear and bulk compressibilities, and elastic moduli. Calculations are also performed with the local density approximation and the differences in elastic properties are interpreted in terms of a uniform compression. Analysis of the pressure effect on the lattice parameters and on the atomic coordinates shows that the structure changes are close to isotropic from zero to 100 GPa.

Electronic band structure of indium tin oxide and criteria for transparent conducting behavior

Abstract: Indium-based transparent conductors, notably indium tin oxide (ITO), have a wide range of applications due to a unique combination of visible light transparency and modest conductivity. A fundamental understanding of such an unusual combination of properties is strongly motivated by the great demand for materials with improved transparent conducting properties. Here we formulate conditions for transparent conducting behavior on the basis of the local density full-potential linear muffin-tin orbital electronic band structure calculations for Sn-doped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ and available experimental data. We conclude that the position, dispersion, and character of the lowest conduction band are the key characteristics of the band structure responsible for its electro-optical properties. Further, we find that this lowest band is split with Sn doping due to the strong hybridization with dopant s-type states and this splitting contributes to both the decrease of the plasma frequency and the mobility of the carriers.

Anomalously high thermoelectric figure of merit in Bi1−xSbx nanowires by carrier pocket alignment

Abstract: Electronic transport calculations were carried out for Bi1−xSbx nanowires (0⩽x⩽0.30) of diameters 10 nm⩽dW⩽100 nm at 77 K. A band structure phase diagram was generated, showing the dependence of the relative band edge positions on diameter and composition. Calculations of the thermoelectric figure-of-merit (ZT) predict that the performance of Bi1−xSbx nanowires is superior to that of Bi nanowires and to that of the bulk alloy. An exceptionally high value of ZT for p-type nanowires at 77 K was found for dW∼40 nm and x∼0.13, which is explained by the coalescence in energy of up to ten valence subband edges to maximize the density-of-states at the Fermi energy.

The electronic structure of tantalum (oxy)nitrides TaON and Ta3N5

Abstract: Detailed results of ab initio band structure calculations for tantalum (oxy)nitrides (TaON and Ta3N5) are reported. The calculations are performed within the framework of density functional theory (DFT). We compare results obtained with the molecular dynamics pseudopotential (PP) approach of the Vienna Ab initio Simulation Program (VASP) and the Full Potential Linearized Augmented Plane Waves method (FP-LAPW) using the WIEN97 program. In agreement with neutron diffraction measurements, we show an ordering of the anions in TaON. The calculations also show that the valence band is composed mainly of the anion 2p orbitals hybridized with Ta 5d states. For TaON the top of the valence band is dominated by N 2p states. The bottom of the conduction band is mainly composed of Ta 5d states. Both TaON and Ta3N5 are semiconductors with calculated indirect band gaps of respectively 1.8 and 1.1 eV (VASP calculations) and 2.0 and 1.2 eV (WIEN97 calculations). Optical diffuse-reflectance spectra show an energy gap of 2.08 eV for Ta3N5.

Full potential linearized augmented plane wave calculations of structural and electronic properties of BN, BP, BAs and BSb

Abstract: A theoretical study of structural and electronic properties of boron compounds BN, BP, BAs and BSb is presented, using the full potential linearized augmented plane wave method. In this approach, the generalized gradient approximation was used for the exchange-correlation potential. Ground state properties such as lattice parameter, bulk modulus and its pressure derivative are calculated as well as structural transition pressure. The band structure is obtained for both zincblende and rocksalt structures. We also give the valence charge density at equilibrium lattice constant and at transition pressure. We show from the latter quantity the inverse role between cation and anion for BP, BAs and BSb. Results are discussed and compared with experimental and other theoretical data with reasonable agreement.

Electronic and structural properties of the layered ternary carbide Ti3AlC2

Abstract: The electronic and structural properties of the layered ternary compound Ti3AlC2 have been determined using the ab initio pseudopotential method based on density functional theory. We have obtained the equilibrium lattice parameters, the equilibrium atomic positions in the unit cell, and interatomic distances. The calculated bulk modulus is 190 GPa and is comparable to that of TiC. The band structure, density of states (DOS) and effective charges are presented and compared with those of TiC. The band structure indicates that Ti3AlC2 is an electronic conductor. The electronic structure discloses that the bonding in Ti3AlC2 is anisotropic and metallic–covalent–ionic in nature. Compare to the structure of TiC, the presence of Al changes the Ti–C–Ti–C covalent bond chain into a Ti–C–Ti–C–Ti–Al bond chain through its reaction with Ti, forming the layered structure. Effective charge calculations suggest the ionic formula of Ti3AlC2 to be (Ti1.18+)(Ti0.59+)2(Al0.52−)(C0.92−)2.

Relationship between photonic band structure and emission characteristics of a polymer distributed feedback laser

Abstract: We present an experimental study of the emission characteristics and photonic band structure of a distributed feedback polymer laser, based on the material poly[${2\ensuremath{-}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y}\ensuremath{-}5\ensuremath{-}(2}^{\ensuremath{'}}$-ethylhexyloxy)-1,4-phenylene vinylene]. We use measurements of the photonic band dispersion to explain how the substrate microstructure modifies both spontaneous and stimulated emission. The lasing structure exhibits a one-dimensional photonic band gap around 610 nm, with lasing occurring at one of the two associated band edges. The band edge (frequency) selection mechanism is found to be a difference in the level of output coupling of the modes associated with the two band edges. This is a feature of the second-order distributed feedback mechanism we have employed and is clearly evident in the measured photonic band structure.

Electronic band structure of single-crystal and single-layer WS 2 : Influence of interlayer van der Waals interactions

Abstract: The valence band structure of the layered transition metal dichalcogenide ${\mathrm{WS}}_{2}$ has been determined experimentally by angle resolved photoelectron spectroscopy and theoretically by augmented spherical wave band structure calculations as based on density functional theory. Good agreement between experimental and calculated band structure is observed for single crystal ${\mathrm{WS}}_{2}.$ An experimental band structure of a single layer was determined from an electronically decoupled film prepared on a single crystalline graphite substrate by metal-organic van der Waals epitaxy. The polarization dependent photoemission selection rules of the single layer film are appropriate for a free standing film. The experimental single layer band structure shows some differences compared to band structure calculations using bulk atomic positions within the layer. We conclude that relaxation of the single layer occurs as a consequence of the missing interlayer interactions leading to close agreement between electronic structure of the single layer and single crystal. As a consequence of the missing interlayer interactions the valence band maximum for the single layer is located at the K point of the Brillouin zone.

Stacking fault band structure in 4H–SiC and its impact on electronic devices

Abstract: First principles calculations of the stacking fault (SF) in 4H–SiC indicate the occurrence of an interface band in the gap with maximum depth of 0.2–0.3 eV below the conduction band minimum at the M point. The energy of formation of SFs in 3C–, 4H–, and 6H–SiC on the other hand is found to be of order a few meV/pair. Thus, there is a thermodynamic driving force promoting growth of SF area in an n-type sample. Radiationless recombination of electrons trapped at the SF with holes is proposed to provide sufficient energy to overcome the partial dislocation motion barriers towards formation of additional SF area in a device under forward bias.

Antiferromagnetic band structure of La 2 CuO 4 : Becke- 3-Lee-Yang-Parr calculations

Abstract: Using the Becke-3-LYP functional, we have performed band structure calculations on the high temperature superconductor parent compound, La2CuO4. Under the restricted spin formalism (rho(alpha) equal to rho(beta)), the R-B3LYP band structure agrees well with the standard LDA band structure. It is metallic with a single Cu x2-y2/O p(sigma) band crossing the Fermi level. Under the unrestricted spin formalism (rho(alpha) not equal to rho(beta)), the UB3LYP band structure has a spin polarized antiferromagnetic solution with a band gap of 2.0 eV, agreeing well with experiment. This state is 1.0 eV (per formula unit) lower than that calculated from the R-B3LYP. The apparent high energy of the spin restricted state is attributed to an overestimate of on-site Coulomb repulsion which is corrected in the unrestricted spin calculations. The stabilization of the total energy with spin polarization arises primarily from the stabilization of the x2-y2 band, such that the character of the eigenstates at the top of the valence band in the antiferromagnetic state becomes a strong mixture of Cu x2-y2/O p(sigma) and Cu z2/O' p(z). Since the Hohenberg-Kohn theorem requires the spin restricted and spin unrestricted calculations give exactly the same ground state energy and total density for the exact functionals, this large disparity in energy reflects the inadequacy of current functionals for describing the cuprates. This calls into question the use of band structures based on current restricted spin density functionals (including LDA) as a basis for single band theories of superconductivity in these materials.

Quasiparticle electronic structure of copper in the GW approximation.

Abstract: We show that the results of photoemission and inverse photoemission experiments on bulk copper can be quantitatively described within band-structure theory, with no evidence of effects beyond the single-quasiparticle approximation. The well-known discrepancies between the experimental band structure and the Kohn-Sham eigenvalues of density functional theory are almost completely corrected by self-energy effects. Exchange-correlation contributions to the self-energy arising from 3s and 3p core levels are shown to be crucial.

Electronic structure of ScN determined using optical spectroscopy, photoemission, and ab initio calculations

Abstract: Experimental and ab initio computational methods are employed to conclusively show that ScN is a semiconductor rather than a semimetal; i.e., there is a gap between the N $2p$ and the Sc $3d$ bands. Previous experimental investigators reported, in agreement with band structure calculations showing a band overlap of 0.2 eV, that ScN is a semimetal while others concluded that it is a semiconductor with a band gap larger than 2 eV. We have grown high quality, single crystalline ScN layers on MgO(001) and on TiN(001) buffer layers on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition. ScN optical properties were determined by transmission, reflection, and spectroscopic ellipsometry while in-situ x-ray and ultraviolet valence band photoelectron spectroscopy were used to determine the density of states (DOS) below the Fermi level. The measured DOS exhibits peaks at 3.8 and 5.2 eV stemming from the N $2p$ bands and at 15.3 eV due to the N $2s$ bands. The imaginary part of the measured dielectric function ${\ensuremath{\varepsilon}}_{2}$ consists of two primary features due to direct X- and \ensuremath{\Gamma}-point transitions at photon energies of 2.7 and 3.8 eV, respectively. For comparison, the ScN band structure was calculated using an ab initio Kohn--Sham approach which treats the exchange interactions exactly within density-functional theory. Calculated DOS and the complex dielectric function are in good agreement with our ScN valence-band photoelectron spectra and measured optical properties, respectively. We conclude, combining experimental and computational results, that ScN is a semiconductor with an indirect $\ensuremath{\Gamma}--X$ bandgap of $1.3\ifmmode\pm\else\textpm\fi{}0.3\mathrm{eV}$ and a direct X-point gap of $2.4\ifmmode\pm\else\textpm\fi{}0.3\mathrm{eV}.$

Diffraction of light from thin-film polymethylmethacrylate opaline photonic crystals.

Abstract: Photonic crystals in the form of large area thin films consisting of closely packed polymethylmethacrylate beads were sedimented on glass substrates. The high ordering of the opaline films made it possible to observe a number of fine features in the optical diffraction, including Fabry-Perot oscillations of the reflectivity and branching of the angular dispersion of the Bragg resonances with increase of the angle of incidence of the light beam. Results of calculations of the photonic band structure and simulations of the reflectance spectra agree well with experimental observations.

Band Anticrossing in III-N-V Alloys

Abstract: Recent high hydrostatic pressure experiments have shown that incorporation of small amounts of nitrogen into conventional III–V compounds to form III–N–V alloys leads to splitting of the conduction band into two subbands. The downward shift of the lower subband edge is responsible for the observed, large reduction of the fundamental band gaps in III–N–V alloys. The observed effects were explained by an anticrossing interaction between the conduction band states close to the center of the Brillouin zone and localized nitrogen states. The interaction leads to a change in the nature of the fundamental from the indirect gap in GaP to a direct gap in GaNP. The predictions of the band anticrossing model of enlarged electron effective mass and enhanced donor activation efficiency were confirmed by experiments in GaInNAs alloys.

Structural, electronic, and effective-mass properties of silicon and zinc-blende group-III nitride semiconductor compounds

Abstract: The electronic band structures of silicon and the zinc-blende-type III-N semiconductor compounds BN, AlN, GaN, and InN are calculated by using the self-consistent full potential linear augmented plane wave method within the local-density functional approximation. Lattice constant, bulk modulus, and cohesive energy are obtained from full relativistic total-energy calculations for Si and for the nitrides. Band structures and total density of states (DOS) are presented. The role played by relativistic effects on the bulk band structures and DOS is discussed. In order to provide important band structure-derived properties, such as effective masses and Luttinger parameters, the ab initio band structure results are linked with effective-mass theory. Electron, heavy-, light-, and split-off-hole effective masses, as well as spin-orbit splitting energies are extracted from the band-structure calculations. By using the Luttinger-Kohn $6\ifmmode\times\else\texttimes\fi{}6$ effective-mass Hamiltonian we derive the corresponding Luttinger parameters for the materials. A comparison with other available theoretical results and experimental data is made.

Band structures and thermoelectric properties of the clathrates Ba8Ga16Ge30,Sr8Ga16Ge30,Ba8Ga16Si30, and Ba8In16Sn30

Abstract: Density functional calculations in the generalized gradient approximation are used to study the transport properties of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30. The band structures of these clathrates indicate that they are all semiconductors. Seebeck coefficients, conductivities and Hall coefficients are calculated, to assess the effects of carrier concentration on the quantity S2σ/τ (where S is the Seebeck coefficient, σ is the conductivity, and τ the electron relaxation time) which is proportional to the thermoelectric power factor. In each compound we find that both p- and n-doping will significantly enhance the thermoelectric capabilities of these compounds. For p-doping, the power factors of all four clathrates are of comparable magnitude and have similar temperature dependence, while for n-doping we see significant variations from compound to compound. We estimate that room-temperature ZT values of 0.5 may be possible for optimally n-doped Sr8Ga16Ge30 or Ba8In16Sn30; a...

Anisotropy of quasiparticle lifetimes and the role of disorder in graphite from ultrafast time-resolved photoemission spectroscopy

Abstract: Femtosecond time-resolved photoemission of photoexcited electrons in highly oriented pyrolytic graphite (HOPG) provides strong evidence for anisotropies of quasiparticle (QP) lifetimes. Indicative of such anisotropies is a pronounced anomaly in the energy dependence of QP lifetimes between 1.1 and 1.5 eV---the vicinity of a saddle point in the graphite band structure. This is supported by recent ab initio calculations and a comparison with experiments on defect-enriched HOPG which reveal that disorder, e.g., defects or phonons, increases electron energy relaxation rates.

Band structure and optical parameters of the SnO 2 ( 110 ) surface

Abstract: With a first-principles-density-functional method, combined with two different pseudopotentials, ideal oxidized and reduced surfaces of tin oxide are studied. The band structures of bulk and the surface systems are calculated and compared. The nature of the surface ${\mathrm{Sn}}^{2+}$ ions, their outward relaxation, associated ``dangling bonds'' and band gap states are considered. Also ultraviolet optical constants are determined by using the electric dipole approximation with a scissor correction, and noted to agree with experiments. The presence of the surface, and more significantly, its removed bridging oxygen atoms, becomes apparent in a formation of a new absorption feature. This is predicted to cause about 0.7 eV decrease of the absorption edge.

Hole trapping at Al impurities in silica: a challenge for density functional theories.

Abstract: The atomic geometry and electronic structure around a neutral substitutional Al impurity in silica is investigated using either the unrestricted Hartree-Fock (UHF) approximation, or Beckes three-parameter hybrid functional (B3LYP). It is found that the B3LYP functional fails to describe the structural distortions around the Al impurity, while the UHF results are consistent with experimental information. We argue that the failure of the B3LYP functional is caused by the incomplete self-interaction cancellation usually present in density functional theories.

Optical properties of copper and silver in the energy range 2.5-9.0 eV

Abstract: The optical properties of copper and silver bulk crystals with atomically clean surfaces have been determined experimentally and interpreted in terms of ab initio band structure calculations. The dielectric functions are evaluated from spectroscopic ellipsometry data taken in ultrahigh vacuum (UHV) in the spectral range of 2.5--9.0 eV (at room temperature). The data are corrected for surface roughness using results from ex situ atomic force microscopy (AFM). Significant differences of detail in the amplitudes and line shape are attributed to the better surface quality of our samples. Density functional calculations of the dielectric functions of copper and silver are carried out, based on models of the valence bands deduced by fitting to experimental Fermi surface and quasiparticle mass data. Small energy shifts, which take into account many-body effects in the final states of the optical transitions in an extended scissors approximation, are needed to bring the calculated dielectric functions into good agreement with the experimental data. The interband transitions associated with individual features in the dielectric function are identified by comparing the energy derivatives of the measured and calculated dielectric functions.

Ultrafast optical nonlinear properties of metal nanoparticles

Abstract: The physical origin and the dynamics of the ultrafast optical nonlinear response of noble metal nanoparticles are analyzed around the surface plasmon resonance frequency using extension of the bulk metal electron kinetics and band structure models. The computed spectral and temporal responses are found to be in very good agreement with the measured ones in silver when taking into account the impact of electron excitation on both the interband absorption and electron optical scattering rate. A good reproduction of the strong excitation regime experimental results is also obtained in the case of gold, with a dominant contribution of the interband effect.