Showing papers on "Electronic band structure published in 1996"

Band structure, deformation potentials, and carrier mobility in strained Si, Ge, and SiGe alloys

Abstract: Using nonlocal empirical pseudopotentials, we compute the band structure and shear deformation potentials of strained Si, Ge, and SiGe alloys. Fitting the theoretical results to experimental data on the phonon‐limited carrier mobilities in bulk Si and Ge, the dilatation deformation potential Ξd is found to be 1.1 eV for the Si Δ minima, −4.4 eV for the Ge L minima, corresponding to a value for the valence band dilatation deformation potential a of approximately 2 eV for both Si and Ge. The optical deformation potential d0 is found to be 41.45 and 41.75 eV for Si and Ge, respectively. Carrier mobilities in strained Si and Ge are then evaluated. The results show a large enhancement of the hole mobility for both tensile and compressive strain along the [001] direction, but only a modest enhancement (approximately 60%) of the electron mobility for tensile biaxial strain in Si. Finally, from a fit to carrier mobilities in relaxed SiGe alloys, the effective alloy scattering potential is determined to be about 0...

Band-edge exciton in quantum dots of semiconductors with a degenerate valence band: Dark and bright exciton states

Abstract: We present a theoretical analysis of the band-edge exciton structure in nanometer-size crystallites of direct semiconductors with a cubic lattice structure or a hexagonal lattice structure which can be described within the framework of a quasicubic model. The lowest energy exciton, eightfold degenerate in spherically symmetric dots, is split into five levels by the crystal shape asymmetry, the intrinsic crystal field (in hexagonal lattice structures), and the electron-hole exchange interaction. Transition oscillator strengths and the size dependence of the splittings have been calculated. Two of the five states, including the ground state, are optically passive (dark excitons). The oscillator strengths of the other three levels (bright excitons) depend strongly on crystal size, shape, and energy band parameters. The relative ordering of the energy levels is also heavily influenced by these parameters. The distance between the first optically active state and the optically forbidden ground exciton state increases with decreasing size, leading to an increase of the Stokes shift in the luminescence. Our results are in good agreement with the size dependence of Stokes shifts obtained in fluorescence line narrowing and photoluminescence experiments in CdSe nanocrystals. Mixing of the dark and bright excitons in an external magnetic field allows the direct optical recombination of the dark exciton ground state. The observed shortening of the luminescence decay time in CdSe nanoncrystals in a magnetic field is also in excellent agreement with the theory, giving further support to the validity of our model. \textcopyright{} 1996 The American Physical Society.

Bloch oscillations of atoms in an optical potential.

Abstract: Ultracold cesium atoms are prepared in the ground energy band of the potential induced by an optical standing wave. We observe Bloch oscillations of the atoms driven by a constant inertial force. We measure the momentum distribution of Bloch states and effective masses different from the mass of the free atom. {copyright} {ital 1996 The American Physical Society.}

Photonic band structure of fcc colloidal crystals.

Abstract: Polystyrene colloidal crystals form three dimensional periodic dielectric structures which can be used for photonic band structure measurements in the visible regime. From transmission measurements the photonic band structure of an fcc crystal has been obtained along the directions between the $L$ point and the $W$ point. Kossel line patterns were used for locating the symmetry points of the lattice for exact positioning and orientation of the crystals. In addition, these patterns reveal the underlying photonic band structure of the crystals in a qualitative way.

Macroporous silicon with a complete two‐dimensional photonic band gap centered at 5 μm

Abstract: We have fabricated a two‐dimensional photonic band structure based on macroporous silicon with a gap common to both polarizations and centered at 5 μm. A triangular lattice of circular air rods with a lattice constant of 2.3 μm was etched 75 μm deep in an n‐type silicon substrate by electrochemical pore formation in hydrofluoric acid. The porous layer was then micromechanically structured in such a way that 200 μm thick free‐standing bars of porous material were left over on the silicon substrate. These bars were then used for measuring the transmission of the photonic lattice. The results showed an excellent agreement with the theoretically calculated structure.

Electronic structure and orbital ordering in perovskite-type 3d transition-metal oxides studied by Hartree-Fock band-structure calculations.

Abstract: We have studied transition-metal 3d-oxygen 2p lattice models, where full degeneracy of transition-metal 3d and oxygen 2p orbitals and on-site Coulomb and exchange interactions between 3d electrons are taken into account, by means of a spin- and orbital-unrestricted Hartree-Fock (HF) approximation. The electronic-structure parameters deduced from the cluster-model analyses of the photoemission spectra are used as input. We have applied this method to perovskite-type 3d transition-metal oxides, which exhibit various electrical and magnetic properties. It is shown that the HF results can explain the ground-state properties of insulating oxides. The relationship between spin- and orbital-ordered solutions and the Jahn-Teller-type and ${\mathrm{GdFeO}}_{3}$-type distortions in R${\mathrm{TiO}}_{3}$, R${\mathrm{VO}}_{3}$, R${\mathrm{MnO}}_{3}$, and R${\mathrm{NiO}}_{3}$ (R is a rare earth atom or Y) is extensively studied. Single-particle excitation spectra calculated using Koopmans' theorem give us an approximate but relevant picture on the electronic structure of the perovskite-type 3d transition-metal oxides. As a drawback, the HF calculations tend to overestimate the magnitude of the band gap compared with the experimental results and to predict some paramagnetic metals as antiferromagnetic insulators. \textcopyright{} 1996 The American Physical Society.

Electronic properties of three- and low-dimensional semiconducting materials with Pb halide and Sn halide units

Abstract: Band-structure calculations, semiempirical as well as ab initio, have been applied to study the electronic band gap of the new exotic natural low-dimensional MX systems (where M = Pb or Sn and X = I, Br or Cl). Moreover, variational calculations are employed to calculate the excitonic binding energies, whose amplification is due not only to the quantum confinement of the excitons but also to a dielectric enhancement effect. A single set of semiempirical parameters is sought to describe the materials; comparison of the calculations with experimental data shows this to be successful in the case of the PbI- and PbBr-containing compounds. .

Optical gain of strained wurtzite GaN quantum-well lasers

Abstract: A theory for the electronic band structure and the free-carrier optical gain of wurtzite-strained quantum-well lasers is presented. We take into account the strain-induced band-edge shifts and the realistic band structures of the GaN wurtzite crystals. The effective-mass Hamiltonian, the basis functions, the valence band structures, the interband momentum matrix elements, and the optical gain are presented with analytical expressions and numerical results for GaN-AlGaN strained quantum-well lasers. This theoretical model provides a foundation for investigating the electronic and optical properties of wurtzite-strained quantum-well lasers.

Analysis of atomic orbital basis sets from the projection of plane-wave results

Abstract: The projection of the eigenfunctions obtained in standard plane-wave first-principles calculations is used for analysing atomic orbital basis sets. The `spillage' defining the error in such a projection allows the evaluation of the quality of an atomic orbital basis set for a given system and its systematic variational optimization. The spillage is shown to correlate with the mean square error in the energy bands obtained from the projected Hamiltonian matrix. The method is applied to the characterization of finite-range pseudo-atomic orbitals (Sankey O F and Niklewski D J 1989 Phys. Rev. B 40 3979) in comparison to infinite-range pseudo-atomic and Slater-type orbitals. The bases are evaluated and optimized for several zinc-blende semiconductors and for aluminium; the finite-range orbitals display high quality in spite of the limited range. A simple scheme is proposed to systematically enlarge the basis without increasing its range.

Quantum-Mechanical Ab-initio Calculation of the Properties of Crystalline Materials

Abstract: 1 Crystal Lattices and Crystal Symmetry.- 2 The Language of Band Theory.- 3 Ab-Initio Approaches to the Quantum-Mechanical Treatment of Periodic Systems.- 4 Reciprocal Space Integration and Special-Point Techniques.- 5 Numerical Integration in Density Functional Methods with Linear Combination of Atomic Orbitals.- 6 Hartree-Fock Treatment of Spin-Polarized Crystals.- 7 The Quantum Theory of Periodic Systems on Modern Computers.- 8 The CRYSTAL Code.- 9 Description of an LAPW DF Program (WIEN95).- 10 A Pseudopotential Plane Waves Program (PWSCF) and some Case Studies.- 11 Total Energy and Related Properties.- 12 Lattice Dynamics and Thermodynamic Properties.- 13 Loss of Symmetry in Crystals: Surfaces and Local Defects.- 14 One-Electron Density Matrices and Related Observables.- 15 Macroscopic Dielectric Polarization: Hartree-Fock Theory.- 16 The Hubbard Models and Superconductivity.- A Schedule of the 1994 GICC School of Computational Chemistry.- B Subject Index.- C List of Acronyms.

Magnon band structure of periodic composites

Abstract: We calculate the spin-wave spectra of two-dimensional composite materials consisting of periodic square arrays of parallel cylinders made of a ferromagnetic material embedded in a ferromagnetic background. Each material is described by its spontaneous magnetization MS and exchange constant A. An external static magnetic field is applied along the direction of the cylinders and both ferromagnetic materials are assumed to be magnetized parallel to this magnetic field. We consider the spin-waves propagation in the plane perpendicular to the cylinders. We reveal the existence of gaps in the magnon band structure of composite systems such as the periodic array of Fe cylinders in an EuO matrix. We investigate the existence of these gaps in relation to the physical parameters of the materials involved. We also study the influence of the lattice parameter (i.e., the square array periodicity) and the effect of the filling fraction of the cylinders on the magnon band structure.

Fundamental optical transitions in GaN

Abstract: A coherent picture for the band structure near the Γ point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time‐resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.

Electronic excitations in Bi2Sr2CaCu2O8: Fermi surface, Dispersion, and Absence of Bilayer Splitting

Abstract: From a detailed study, including polarization dependence, of the normal state angle-resolved photoemission spectra for Bi2Sr2CaCu2O8, We find only one CuO2 band related feature. All other spectral features can be ascribed either to unklapps from the superlattice or to ''shadow bands.'' Even though the dispersion of the peaks looks like band theory, the line shape is anomalously broad and no evidence is found for bilayer splitting. We argue that the ''dip feature'' in the spectrum below T-c arises not from bilayer splitting, but rather from many-body effects.

Empirical tight‐binding calculation of the branch‐point energy of the continuum of interface‐induced gap states

Abstract: The band lineup at metal–semiconductor contacts as well as at semiconductor heterostructures may be described by one and the same physical concept, the continuum of interface‐induced gap states. These intrinsic interface states derive from the virtual gap states (ViGS) of the complex semiconductor band structure and their character varies from predominantly donorlike closer to the valence band to mostly acceptorlike nearer to the conduction band. Calculations are presented of the respective branch points for elemental and binary as well as ternary compound semiconductors which make use of Baldereschi’s concept of mean‐value points in the Brillouin zone [Phys. Rev. B 7, 5212 (1973)], Penn’s idea of dielectric band gaps [Phys. Rev. 128, 2093 (1962)], and the empirical tight‐binding approximation (ETB). The results are as follows. First, at the mean‐value point the band gaps calculated in the GW approximation have the same widths as the dielectric band gaps. Second, the ETB approximation reproduces the GW va...

Electronic structure of divalent hexaborides

Abstract: Recent experiments suggest that divalent hexaborides like SrB6 and CaB6, traditionally considered small-gap semiconductors, can actually be semimetals. We calculate the structural and electronic properties of SrB6, CaB6 and of ferromagnetic EuB6, using the full-potential linearized augmented plane wave (FLAPW) method, within the local (spin) density approximation. The lattice constants and internal parameters are in very good agreement with the measured ones. Because of a small band overlap at the X point, all the materials are semimetals. The calculated Hall coefficient for SrB6 changes sign around zero doping, and has the freeelectron value for doping beyond ≈ 1.5%. The plasma frequency has a minimum at zero doping. We interpret the high-temperature transport properties of SrB6 and CaB6 in terms of a thermal gap deduced from the shape of the density of states around the Fermi energy. We also calculate the imaginary part of the dielectric function for SrB6, which can be compared to recent experiments.

Electronic structure of the ferroelectric layered perovskite SrBi2Ta2O9

Abstract: The band structure of the layered perovskite SrBi2Ta2O9 (SBT) was calculated by tight binding and the valence band density of states was measured by x‐ray photoemission spectroscopy. We find both the valence and conduction band edges to consist of states primarily derived from the Bi–O layer rather than the perovskite Sr–Ta–O blocks. The valence band maximum arises from O p and some Bi s states, while the conduction band minimum consists of Bi p states, with a wide band gap of 5.1 eV. It is argued that the Bi–O layers largely control the electronic response whereas the ferroelectric response originates mainly from the perovskite Sr–Ta–O block. Bi and Ta centered traps are calculated to be shallow, which may account in part for its excellent fatigue properties.

Terms linear in k in the band structure of wurtzite-type semiconductors

Abstract: Wurtzite has the space-group symmetry P${6}_{3}$mc. The absence of inversion symmetry allows linear-k terms in the electronic band structure when the spin-orbit interaction is included. Their existence has been confirmed in a number of experiments, but no microscopic calculations have been published. In the present paper, we discuss the origin of these linear-k terms using group theory and k\ensuremath{\cdot}p arguments. The various contributions to these terms are identified through band-structure models. We present an ab initio calculation, performed with the linear-muffin-tin-orbital method, of these spin splittings in CdS, CdSe, and ZnO. A renormalization of the valence-band spin-splitting coefficients obtained in the linear-muffin-tin-orbital calculations was found necessary to correct for errors in the relative energies of the uppermost valence bands as compared with the experimental values. We point out that a similar procedure should be used when evaluating masses and other band parameters from calculated local-density-approximation band structures. \textcopyright{} 1996 The American Physical Society.

Multiple Plasmon Satellites in Na and Al Spectral Functions from Ab Initio Cumulant Expansion.

Abstract: he valence photoemission spectra of alkali metals exhibit multiple plasmon satellite structure. The calculated spectral functions within the GW approximation show only one plasmon satellite at too large binding energy. In this Letter we use the cumulant expansion approach to obtain the spectral functions of Na and Al from ab initio calculations including the effects of band structure. The GW spectral functions are dramatically improved and the positions of the multiple plasmon satellites are in very good agreement with experiment while their intensities cannot be explained from intrinsic effects only.

Five-level k.p model for the conduction and valence bands of GaAs and InP

Abstract: The band structure of medium-gap semiconductors GaAs and InP is described theoretically using a five-level k\ensuremath{\cdot}p model and including far-level contributions as well as polaron effects. A corresponding theory is also developed to describe orbital and spin properties of charge carriers in both materials in the presence of an external magnetic field of arbitrary orientation. Field dependence of g factors, the spin doublet splittings of the cyclotron resonance, bands' anisotropies, and the energy dependences of the cyclotron masses are described and compared with existing experimental data all the way to the megagauss range of magnetic fields. A good theoretical fit to all the experiments is achieved, which is used to determine the band parameters for GaAs and InP. Values of the polar constants for both materials are discussed. The determined band parameters are employed to calculate the hole masses, which turn out to be in agreement with those found by other authors. It is concluded that the proposed description is adequate for both the conduction and valence bands of GaAs and InP, although there remains a certain ambiguity between the band-structure effects (related to the far-level contributions) and the polaron effects. \textcopyright{}1996 The American Physical Society.

Polarization fields and band offsets in GaInP/GaAs and ordered/disordered GaInP superlattices

Abstract: Using the first‐principles pseudopotential method we have calculated band offsets between ordered and disordered Ga0.5In0.5P and between ordered GaInP2 and GaAs. We find valence band offsets of 0.10 and 0.27 eV for the two interfaces with the valence band maximum on ordered GaInP2 and GaAs, respectively. Using experimental band gaps these offsets indicate that the ordered/disordered Ga0.5In0.5P interface has type I band alignment and that the ordered GaInP2/GaAs interface has type II alignment. Assuming transitivity of the band offsets, these results suggest a type I alignment between disordered Ga0.5In0.5P and GaAs and a transition from type I to type II as the GaInP side becomes more ordered. Our calculations also show that ordered GaInP2 has a strong macroscopic electric polarization. This polarization will generate electric fields in inhomogeneous samples, strongly affecting the electronic properties of the material.

Electronic and related properties of crystalline semiconducting iron disilicide

Abstract: Band structure calculations for β‐FeSi2 have been performed by the linear muffin‐tin orbital method within the local density approximation scheme including exchange and correlation effects. A detailed analysis of the conduction and valence band structure around high‐symmetry points has shown the existence of a quasidirect band gap structure in the material. It is experimentally confirmed that between the threshold energy of optical interband transition of 0.73 eV and the first direct gap transition with appreciable oscillator strength at about 0.87 eV there is a region in which direct transition of low oscillator strength and indirect transitions overlap. That explains the tricky behavior of β‐FeSi2 in experimental investigations demonstrating it to be either a direct or indirect gap semiconductor.

Bloch Oscillations of Atoms in an Optical Potential

Abstract: The early quantum theory of electrical conductivity in crystal lattices by Bloch and Zener [1,2] led to the striking prediction that a homogeneous static electric field induces an oscillatory rather than uniform motion of the electrons. These so-called Bloch oscillations have never been observed in natural crystals because the scattering time of the electrons by the lattice defects is much shorter than the Bloch period. In semiconductor superlattices the larger spatial period leads to a much shorter Bloch period (,600 fs) and Bloch oscillations have recently been observed through the emission of THz radiation by the electrons [3]. Here we present Bloch oscillations of atoms in the fundamental energy band of a periodic optical potential. We directly measure the atomic momentum distribution evolving in time under the influence of a constant inertial force for various potential depths. We experimentally observe oscillation periods in the millisecond range as well as positive and negative effective masses. Bloch oscillations are a pure quantum effect which can be explained in a simple one-dimensional model. The periodicity of the lattice (period d) leads to a band structure (Fig. 1) of the energy spectrum of the particle and the corresponding eigenenergies Ensqd and eigenstates jn, ql (Bloch states) are labeled by the discrete band index n and the continuous quasimomentum q; Ensqd and jn, ql are periodic functions of q with period 2pyd and q is conventionally restricted to the first Brillouin zone g2pyd, pydg [4]. Under the influence of a constant external force F, weak enough not to induce interband transitions, a given Bloch state jn, qs0dl evolves (up to a phase factor) into the state jn, qstdl according to qstd ­ qs0d 1 Ftyh . (1) This evolution is periodic with a period tB ­ hyjFjd corresponding to the time required for the quasimomentum to scan a full Brillouin zone. The mean velocity in jn, qstdl

Band Structure Calculations of Ferromagnetic Chromium Tellurides CrSiTe3 and CrGeTe3

Abstract: Band structure calculations using the spin-polarized ASW method were performed on the title compounds. Nonmetallic behavior is attributed to spin-polarization-induced opening of a gap at the Fermi level. Curie temperatures were calculated and found to be in reasonable agreement with the real evolution of silicon and germanium derivatives.

Band theory for electronic and magnetic properties of

Abstract: We report results of calculations that explain in the itinerant-electron picture magnetic and electronic properties of haematite, For this we use the local approximation to spin-density functional theory and the ASW method incorporating spin - orbit coupling and noncollinear moment arrangements The insulating character of the compound is obtained correctly and features in the density of states connected with Fe - O hybridization correlate well with experimental features seen in direct and inverse photoemission intensities The total energy correctly predicts the experimentally observed magnetic order of the ground state, and, using total energies of different magnetic configurations, we can give a rough estimate of the Neel temperature We also obtain a state showing weak ferromagnetism The rate of change is calculated for the decrease of the insulating gap when an external magnetic field is applied

Ground and excited state exciton spectra from GaN grown by molecular‐beam epitaxy

Abstract: The emission and reflection spectra of GaN have been investigated in the intrinsic region and the data have been interpreted in terms of the wurtzite crystal band structure. Three intrinsic exciton transitions have been observed, one associated with each of the valence bands. Exciton excited states associated with the two top valence bands were also observed. The exciton binding energies, the band‐gap energies, and the exciton Bohr radii are all reported along with the dielectric constant and the spin‐orbit and crystal‐field parameters for GaN.