Empirical Tight-binding Model Research Articles

Electronic structure of (001) GaN/AlN quantum wells

Abstract The electronic structure of (0 0 1) GaN/AlN quantum wells at the Γ ¯ point of the Brillouin zone is studied for 2 ⩽ n ⩽ 50, where n is the number of principal layers of GaN in the well region. The calculations are based on a surface Green-function matching analysis together with two different empirical tight-binding models. We study the influence of the strain and the spin–orbit coupling on the quantum well states.

Quantum Transport Through Multiterminal Phenalenyl Molecular Bridges

The quantum transport properties of multiterminal molecular bridge systems are theoretically studied with the Green's functions method based on an empirical tight-binding model. As an illustrated example, we adopt a phenalenyl molecule which has a nonbonding singly occupied molecular orbital (SOMO). For a comparative study, first the two-terminal molecular bridges, then the three- and four-terminal molecular bridges are calculated. For the two-terminal case, we find that the transmission spectra significantly depend on the terminal sites connected to the leads. For example, the transmission spectrum has a peak at E=0.0 (SOMO level) as long as both the source and drain are connected to the α sites, but otherwise a dip structure appears at this energy. As a general trend, even when the third and fourth terminals are connected, the transmission spectra do not change considerably from the corresponding spectra of the two-terminal cases. However, some attractive aspects, such as the disappearance of the dip at the SOMO level and a shift in the location of the large loop current, are newly found.

Valence band effective-mass expressions in thesp3d5s*empirical tight-binding model applied to a Si and Ge parametrization

Exact, analytic expressions for the valence band effective masses in the spin-orbit, ${\mathrm{sp}}^{3}{d}^{5}{s}^{*}$ empirical tight-binding model are derived. These expressions together with an automated fitting algorithm are used to produce improved parameter sets for Si and Ge at room temperature. Detailed examinations of the analytic effective-mass expressions reveal critical capabilities and limitations of this model in reproducing simultaneously certain gaps and effective masses. The [110] masses are shown to be completely determined by the [100] and [111] masses despite the introduction of $d$ orbitals into the basis.

Theory of spin waves in ultrathin ferromagnetic films: The case of Co on Cu(100)

We present theoretical studies of the nature of spin waves in ultrathin ferromagnetic Co films deposited on the Cu(100) surface. Motivation for the calculations are the recent experimental SPEELS data reported by Vollmer et al. Our calculations employ a realistic electronic structure for both the Co film and the semi-infinite Cu substrate, modeled through use of an empirical tight binding model with ferromagnetism in the Co film driven by intra-atomic Coulomb interactions within the Co $3d$ shell. Mean field theory describes the ground state, and we use the Random Phase Approximation to generate spin wave spectra. We obtain an excellent quantitative account of the value of the exchange stiffness reported in previous Brillouin lights scattering studies of epitaxially deposited Co films on Cu(100). As in the SPEELS experiment, we find a single very broad feature dominates the spin wave spectral density throughout much of the surface Brillouin zone, in contrast to the sequence of sharp standing spin wave resonances which result from generation of spin wave spectra via adiabatic approaches. Landau damping absent from such treatments influence the calculated spectra qualitatively, as in our earlier theoretical studies of related systems. It is the case, however, that the spin waves reported in the SPEELS experiments are very considerably softer than those generated by our theory.

Boundary conditions for the electronic structure of finite-extent embedded semiconductor nanostructures

The modeling of finite-extent semiconductor nanostructures that are embedded in a host material requires the numerical treatment of the boundary in a finite simulation domain. For the study of a self-assembled InAs dot embedded in GaAs, three kinds of boundary conditions are examined within the empirical tight-binding model: (i) the periodic boundary condition, (ii) raising the orbital energies of surface atoms, and (iii) raising the energies of dangling bonds at the surface. The periodic boundary condition requires a smooth boundary and consequently a larger GaAs buffer than the two nonperiodic boundary conditions. Between the nonperiodic conditions, the dangling-bond energy shift is more efficient than the orbital-energy shift, in terms of the elimination of nonphysical surface states in the middle of the gap. A dangling-bond energy shift bigger than 5 eV efficiently eliminates all of the mid-gap surface states and leads to interior states that are highly insensitive to the change of the energy shift.

Electronic spectra of quasi-regular heterostructures: simple versus realistic models

The electronic spectra of quasi-regular systems are investigated via simple one-dimensional, one-band models and compared with those obtained by means of realistic empirical tight-binding models, particularly for the Fibonacci sequence case.

Electronic spectra of quasi-regular heterostructures: simple versus realistic models

The electronic spectra of quasi-regular systems are investigated via simple one-dimensional, one-band models and compared with those obtained by means of realistic empirical tight-binding models, particularly for the Fibonacci sequence case.

Modeling the static and dynamic properties of liquid tellurium with an empirical tight-binding Hamiltonian

The structure and dynamics of liquid and supercooled tellurium are investigated using molecular dynamics and an empirical tight-binding Hamiltonian model designed to reproduce ab initio cohesive energies and band structures of various solid phases. Despite the well-known short comings of the reference density-functional theory ab initio calculations, the fitted tight-binding model yields a semiquantitatively correct picture of the dynamics and a relatively good description of the microscopic structure, in fair accordance with the experimental data.

Molecular dynamics study of the local order and defects in quenched states

Empirical tight binding model potentials have been used in molecular dynamics simulations of local order and defects in liquid and glass under high and normal pressures. Results are reported for some solidlike cluster and for structural properties of liquid and glass. Not only are Frank-Kasper polyhedra and Bernal hole polyhedra detected, but also a variety of defective icosahedra. A dramatic split of the second peak of $g(r)$ is associated with the glass transition. The split of the second peak is caused by the fluidity of the liquid phase in glass. The nearest distance is shortened, and the nearest-neighbor coordination number increases under high pressure. High pressure favors glass formation. The microstructure of glass is very similar to medium range order icosahedra structure.

Empirical tight-binding model for the electronic structure of dilute GaNAs alloys

We present an empirical tight-binding (TB) model for the electronic structure of ${\mathrm{GaN}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ with nitrogen content $x<5%,$ over the entire Brillouin zone. The model upgrades existing TB schemes, introducing an additional s orbital to account for the N-induced change of conduction-band states. The resulting band structure is analyzed using an ${\mathrm{sp}}^{3}{d}^{5}{s}^{*}$ TB parametrization and compared to experimental and other theoretical data. The model reproduces the results of the band anticrossing model at the \ensuremath{\Gamma} point of the Brillouin zone, and the dependence of the X and L band-edge states on the N concentration is in good agreement with experimental data. The model is readily applicable to electronic structure calculations of other III-N-V compounds and heterostructures.

Calculations on Electronic States in QDs with Saturated Shapes

Electronic States of Si and Ge QDs of 5 to 3127 atoms with saturated shapes in a size range of 0.57 to 4.92 nm for Si and 0.60 to 5.13 nm for Ge are calculated by using an empirical tight-binding model combined with the irreducible representations of the group theory. The results are compared with those of Si and Ge quantum dots with spherical shape. The effects of the shapes on electronic states in QDs are discussed.

Diagonal parameter shifts due to nearest-neighbor displacements in empirical tight-binding theory

Nanoscale heterostructures are generally characterized by local strain variations. Because the atoms in such systems can be irregularly positioned, theroretical models and parameterizations that are restricted to hydrostatic and uniaxial strain are generally not applicable. To address this shortcoming, a method that enables the incorporation of general distortions into the empirical tight binding model is presented. The method shifts the diagonal Hamiltonian matrix elements due to displacements of neighboring atoms from their ideal bulk positions. The new, efficient, and flexible method is developed for zincblende semiconductors and employed to calculate gaps for GaAs and InAs under hydrostatic and uniaxial strain. Where experimental and theoretical data are available our new method compares favorably with other methods, yet it is not restricted to the cases of uniaxial or hydrostatic strain. Because our method handles arbitrary nearest-neighbor displacements it permits the incorporation of diagonal parameter shifts in general, three-dimensional nanoscale electronic structure simulations, such as the nanoelectronic modeling tool (NEMO 3D).

Electron-hole correlations and optical excitonic gaps in quantum-dot quantum wells: Tight-binding approach

Electron-hole correlation in quantum-dot quantum wells (QDQW's) is investigated by incorporating Coulomb and exchange interactions into an empirical tight-binding model. Sufficient electron and hole single-particle states close to the band edge are included in the configuration to achieve convergence of the first spin-singlet and triplet excitonic energies within a few meV. Coulomb shifts of about 100 meV and exchange splittings of about 1 meV are found for CdS/HgS/CdS QDQW's (4.7 nm CdS core diameter, 0.3 nm HgS well width and 0.3 nm to 1.5 nm CdS clad thickness) which have been characterized experimentally by Weller and co-workers [ D. Schooss, A. Mews, A. Eychmuller, H. Weller, Phys. Rev. B, 49, 17072 (1994)]. The optical excitonic gaps calculated for those QDQW's are in good agreement with the experiment.

Finite-size versus periodic effects in Ni/Co multilayers

The periodic effects on the electronic properties of Ni/Co magnetic multilayers grown in the (111) direction are investigated by studying superlattices and finite bilayers formed with the same unit block. Band structures and Fermi surfaces are calculated using a Green-function-matching method within a self-consistent empirical tight-binding model. Our results show strong sp-d hybridization of the electronic levels, the existence of nested multisheet Fermi surfaces, and the occurrence of d-derived spatially extended states in both superlattices and finite bilayers. Nevertheless, the superlattice periodicity increases the band degeneracy in the in-plane direction and changes the energy position of the superlattice electron levels with respect to those of the bilayer. The discrepancies result in different Fermi wave vectors for both systems, which may yield different transport and dynamical coupling characteristics for superlattices and bilayers.

Modelling Electronic Spectra of Crystals in Electric Fields with Various Orientations

The present work considers the electronic spectra of a simple two-dimensional (2D) cubic crystal and a three-dimensional (3D) GaAs crystal when a constant electric field is applied. The 2D case is treated within the empirical tight-binding (TB) model, taking one s-orbital per atom and accounting for the interactions with neighbours up to fourth order. For the 3D case a semiempirical sp 3 s * TB model is used, taking into account the spin and first neighbours. The local densities of states (LDOSs) have been calculated for different field intensities, showing that for a sufficiently strong field the 2D LDOS turns into the 1D LDOS corresponding to chains perpendicular to the field orientation. This is a novel observation achieved through the use of neighbours farther than the first. The 3D LDOS of a GaAs crystal projected by a strong field on the [001] direction is found to coincide with the 2D LDOS of an isolated atomic layer.

Absorption coefficient of wurtzite GaN calculated from an empirical tight binding model

The linear absorption coefficient for polarizations parallel and perpendicular to the c-axis of wurtzite GaN is calculated by integrating the momentum matrix element over the full Brillouin zone using a fundamental numerical technique. The energy band structure and wave functions required for this calculation are obtained using an sp 3d 5–sp 3 empirical tight-binding method (ETBM) that includes the spin–orbit and up to second-nearest-neighbor interactions. The model reproduces the selection rules expected from symmetry considerations.

Theory of electronic and optical properties of bulk AlSb and InAs and InAs/AlSb superlattices

An empirical tight-binding model with an orthogonal ${\mathrm{sp}}^{3}$ set of orbitals, interactions up to second neighbor, and spin-orbit coupling is used to describe the electronic and optical properties of bulk InAs and AlSb as well as InAs/AlSb superlattices. The tight-binding parameters required for the model are obtained by fitting existing band structures for bulk materials. It is found that with the evaluated tight-binding parameters the model describes very accurately the band structure and the optical properties of the bulk materials. Then the model is used in order to investigate the electronic and optical properties of InAs/AlSb superlattices. It is shown that interface states located in the InSb-like interface quantum wells produce a narrow upper valence band. The effective masses and the variation of the energy gap with interface structure and lattice periodicity are also examined. Additionally the optical functions for the superlattice are calculated, and from the second derivative of ${\ensuremath{\varepsilon}}_{2}$ the critical points are obtained. Finally the optical anisotropy in the superlattice plane is investigated.

Role of tight-binding parameters and scaling laws on effective charges in semiconductors

We compute the transverse effective charges for several compound semiconductors by using the empirical tight-binding model and the Berry-phase approach. We compare different parametrizations showing that a suitable tuning of the scaling laws for the radial part of the hopping parameters provides a fairly good prediction of the effective charges even with the minimal ${\mathrm{sp}}^{3}$ basis set. In contrast new and refined parametrizations that reproduce very well the dispersion of the conduction band at equilibrium by including d and/or ${s}^{*}$ polarization orbitals may underestimate the effective charges. We suggest that the root of such a discrepancy may rest with the difficulty of determining the scaling laws for polarization orbital.

Calculation of Electronic States in Semiconductor Heterostructures with an Empirical spds* Tight-Binding Model

We illustrate how the tight-binding formalism can be used to accurately compute the electronic states in semiconductor quantum wells and superlattices. To this end we consider a recently developed empirical tight-binding model which carefully reproduces ab initio pseudopotential calculations and experimental results of bulk semiconductors. The present approach is particularly suited both for short-period superlattices and large and complex unit cells where the transferability of the hopping parameters is required. First applications for ultrathin GaAs/AlAs superlattices and InGaAs/AlAs heterostructures are presented and discussed.

Electronic and optical properties ofSi1−yCyalloys

We investigate the electronic and optical properties of free-standing and epitaxially strained ${\mathrm{Si}}_{1\ensuremath{-}y}{\mathrm{C}}_{y}$ alloys. We first determine the microscopic atomic structure of the alloys using the semi-grand-canonical Monte Carlo method and empirical interatomic potentials. For the calculation of the electronic and optical properties of the alloys we employ a supercell geometry and an empirical tight-binding model, which was fitted to reproduce the relevant band structures obtained from first-principles calculations. Our approach allows for a thorough investigation of C alloying and strain effects on the band gap, the band offsets, the effective masses of the upper valence and lowest conduction bands, and the optical properties. Our results are in very good agreement with experiment.