Using the transfer-matrix method, we calculate the transmission coefficient versus the frequency of the incident electromagnetic waves propagating in photonic-band-gap structures constructed from dispersive and highly absorbing materials. We study how the photonic band gaps are affected by the presence of polariton gaps and/or absorption. Also, the possible difficulties of their experimental investigation are discussed.

Although the number of dynamical low-energy-electron-diffraction (LEED) calculations that need to be performed in the linear LEED technique scales linearly with the number N of structural parameters, the number of combinations of the calculated intensities that need to be compared with experiment scales exponentially with N, and could be a serious limitation when N is large. In this paper, we suggest an algorithm for overcoming this residual exponential scaling, thus allowing linear LEED to achieve its true potential for linear scaling. We demonstrate our algorithm with model calculations on the Si(111)(√3 × √3 )R30°-Al surface.

The lamellar semiconductors GaSe and InSe have been studied with k-resolved inverse-photoemission spectroscopy along two major symmetry directions (Γ¯ K¯ and Γ¯ M¯) of the surface Brillouin zone. Three bands with well-resolved features are observed from which the dispersion of the conduction bands can be determined with good precision. The minimum of the conduction band is found at M¯ in GaSe and at Γ¯ in InSe. These results are compared with theoretical studies using pseudopotential and tight-binding calculations.

Control of plane-wave scattering is examined using designed potential structures in solid-state devices with dimensions of the electron coherence length. Reflection coefficients at specified incident electron energies are controlled by exploiting the quantum interference effects associated with the wavelike nature of the electrons through optimally designed manipulation of the solid-generated scattering potential. This work is motivated by the increasing ability to fabricate semiconductor structures with controlled layer thickness and lateral features, and here the goal is to demonstrate the degree of coherent electron control achievable through the employment of optimal design tools. We examine the case where the potential form is restricted to a fixed number of rectangular barriers. Here, the optimization of the design is performed with respect to the barrier width and spacings in order to achieve the desired reflection coefficients at one or more incident energies. We also examine the case where the potential is not restricted to any particular form, and here optimal control theory is employed to optimize the scattering potential form in order to achieve the desired reflection coefficients over a range of incident electron energies. The possibility of extending this work to controlling electron wave-packet structures is also discussed.

The low-temperature electron mobility in δ-doped GaAs is calculated by using the Boltzmann equation and the relaxation-time approximation. It is assumed that the electrons are scattered from ionized impurities. Screening of charged impurities by electrons occupying several subbands is described with the help of (i) the random-phase approximation, (ii) the Thomas-Fermi method, and (iii) the bulk dielectric constant only. Among those methods mentioned above, the random-phase approximation has proved quite successful in studying the screening while the other two methods are inadequate. The mobility exhibits a drop when the excited subbands become occupied. It is shown, however, that as a consequence of the parity of the subband wave functions, the drop in the mobility when the Fermi level coincides with the bottom of the first excited subband is negligible.

The deposition of Si on oxidized GaAs(110) surfaces produces the complete reduction of the GaAs oxides and the formation of ${\mathrm{SiO}}_2$. These chemical reactions have been monitored in situ from synchrotron photoemission measurements. To discern the stoichiometry of the formed Si-oxide is a difficult task, because the Si(2p) core level shifts ∼1.2 eV to lower kinetic energies during the chemical process. A detailed study of the system based on the comparison of the core-level binding energies and the work-function changes permits us to separate the interfacial voltage effects from the bonding charge transference and final-state relaxation contributions to the core-level shift. Thus, it can be concluded that the observed energy shift corresponds to a band-lineup variation at the ${\mathrm{SiO}}_2$/GaAs interface, and that ${\mathrm{SiO}}_2$ is formed from the beginning of the process. The band-offset variation is consistent with the decrease of an interfacial voltage related to an As excess produced in the first stage of the reduction of the GaAs oxides, and a tentative model is proposed. After the reduction of the substrate oxides, Si grows on the ${\mathrm{SiO}}_2$ layer with a modification of 0.7 eV in the Si-${\mathrm{SiO}}_2$ chemical shift. Cluster size effects and band lineup at this interface are discussed as possible explanations.

We report on the lattice deformation of quantum-wire heterostructures with cubic symmetry deposited on arbitrarily oriented substrate surfaces. The elasic strain and the stress-tensor components are calculated by using Hooke’s law and applying the appropriate boundary conditions at the wire-substrate interface. We assume that the lattice coherence at the wire-substrate interface is achieved only along the wire direction, due to the limted lateral extension of the wires. We show that either a tetragonal or an orthorhombic or a lower-symmetry lattice deformation of the quantum-wire structure may occur depending on the coherence direction and substrate orientation. In some particular cases, even for high-symmetry substrate orientations ([001] and [110]), it is possible to obtain one shear strain element different from zero, leading to a monoclinic lattice deformation. Moreover, for the [111] or low-symmetry substrate orientations a triclinic lattice deformation can also be obtained. In the case of an orthorhombic lattice deformation our theoretical results are found to be in good agreement with the few experimental data available.

Solid-phase epitaxy of iron silicides on Si(111)7×7 substrates is studied by comparing photoemission spectroscopy of the extended states and core levels, and Fe ${\mathit{L}}_{2,3}$ x-ray-absorption spectroscopy, for the interface phases and the bulk stoichiometric silicides. Epitaxial growth favors the low-temperature formation of metallic tetragonal α-${\mathrm{FeSi}}_2$ which starts decomposing irreversibly into semiconducting orthorhombic β-${\mathrm{FeSi}}_2$ at above 600 °C. This interface chemistry is opposite to the bulk phase diagram where the α phase is the high-temperature stable disilicide which reversibly transforms into the β phase when the temperature is lowered to 950 °C. No evidence of metastable interface phases other than the bulk phases is found. The coexistence of bulk epitaxial phases over an extended temperature range indicates that local properties of the interface strongly influence the silicide phase transitions.

We present two original methods which yield the small-signal response around the dc bias in bulk semiconductors, using direct numerical resolutions of the perturbed Boltzmann equation. The first method operates in the frequency domain. An ac sinusoidal electric-field perturbation superimposed to the dc field produces an ac perturbation of the distribution function which is computed at each frequency. The second method operates in the time domain. A step electric-field perturbation is superimposed at time t=0 to the dc field. The resulting perturbations of the distribution function and of the average velocity are then computed as functions of time. These methods are applied to the case of holes in silicon at T=300 K under hot-carrier conditions and are used to compute the perturbed distribution function and the differential mobility spectrum.

Periodic Hartree-Fock total-energy calculations on two-dimensional slabs have been used to study the symmetry-conserving relaxation of the nonpolar (101¯0) surface of ZnO. We find that it is energetically favorable for the Zn-O surface dimers to tilt slightly (by 2.3 °) and move downwards towards the slab, and for the dimer bond to shorten significantly. Our results agree fairly well with those of a recent density-functional calculation, but disagree with empirical tight-binding theory which predicts surface bonds to shorten only slightly while the surface dimers undergo a large tilt (18 °). The available experimental data lies between the ab initio and tight-binding results with large error bars. We have tested the effects of several refinements of our Hartree-Fock calculation, including improvements of the orbital basis set and precision tolerances, the use of thicker slabs in approximating the semi-infinite crystal, and post-self-consistent-field density-functional correlation corrections to the total energy. None of these refinements significantly changed our results. We discuss possible reasons for the disagreement between our results and those of tight-binding theory.

The chemical composition of thin native oxide layers grown on GaAs and InAs by ultraviolet (UV)/ozone and thermal oxidation is investigated using x-ray-photoelectron spectroscopy. Core-level binding energies, core-level intensities, and valence-band spectra are compared with data for bulk crystalline binary or ternary As, In, and Ga oxides. The chemical compositions, which vary strongly from GaAs to InAs and from thermal to UV oxidation, appear to be controlled by both thermodynamic and kinetic factors. Only for GaAs thermal oxidation are the products predicted at thermodynamic equilibrium obtained. In all cases the native oxides can be described as single phase nonstoichiometric compounds and not as a macroscopic mixture of stoichiometric binary oxides.

We have studied the effects of disorder on energy transport by confined optical phonons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As semiconductor superlattices. A model which includes both interface and alloy disorder has been studied using diagrammatic perturbation techniques. We find that true phonon localization is not expected for realistic systems but rather that a transition from ballistic to diffusive transport occurs for increasing barrier widths. The implications of these results for recent experimental data are discussed.

The optical properties of different atomic arrangements of ${\mathrm{Al}}_{0.5}$${\mathrm{Ga}}_{0.5}$As, deposited by molecular-beam epitaxy and having nominally identical average composition, have been compared. Markedly different signatures are seen in steady-state, time-resolved, and microwave-modulated photoluminescence, photoreflectance, and photoluminescence-excitation spectroscopies for a random pseudobinary alloy and (AlAs${)}_{\mathit{n}}$(GaAs${)}_{4\mathrm{-}\mathit{n}}$ superlattices where n=2 (ordered) or n is randomly chosen (disordered) from the sets [1,2,3] or [0,1,2,3,4]. When n=[0, 1, 2, 3, or 4] the optical properties are dominated by quantum confinement effects due to the presence of layers with thicknesses up to 14 monolayers. When n=[1, 2, or 3] the optical properties are consistently described by the presence of disorder-induced localized states and a hopping-assisted recombination mechanism.

The feasibility of the cluster method is studied using two-dimensional ZnS surface models. We used the translational symmetry of electron integrals in the Hartree-Fock valence method as it enables studies of large systems, containing several hundred atoms. The results show that the influence of boundaries and empty vacancies are limited only to a small area, and thus relatively small models can be used to simulate infinite systems. The properties are converging monotonically as the size of the model increases. However, in some cases small, size-dependent variations exist. Augmenting the minimal valence basis set with Zn 4p polarization functions decreases the influence of boundaries, and the charge distribution of surface becomes more uniform.

Local-density-functional calculations of the pairing of H atoms on the (100)-2×1 surface of Si are presented. We find that significant interactions between neighboring dimers along a row are present, favoring configurations where H-unpaired dimers are in the same row and have the H-atoms all on the same side. We discuss how these effects can contribute to the effective H-pairing energies determined experimentally.

Three-wave-mixing spectroscopy was used to probe the interfacial electronic structure of ZnSe/GaAs(001) heterojunctions. The interface spectra exhibit two sharp features at 2.92 and 2.72 eV. The former resonance was assigned to the ${\mathit{E}}_1$ transition of buried GaAs. This assignment was consistent with thickness-dependent measurements of the resonance intensity. The strong interfacial feature at 2.72 eV resulted from a virtual crossover transition connecting a resonance state of the interfacial quantum well to the ZnSe valence band. The interfacial quantum well was produced by band bending near the junction. Photomodulation second-harmonic measurements corroborate the assignment of the 2.72 eV resonance. A quantitative microscopic calculation of the relevant matrix elements suggests that the strong interfacial resonance at 2.72 eV is probably the result of a two-level nonlinear process involving only the quantum well resonance state and the ZnSe valence band. Finally we demonstrate that the interfacial deformation potential could not be responsible for the generation of the interfacial resonance.

Interband magneto-optical studies have been performed on a series of strained single and coupled ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$Sg/GaSb quantum wells. We have investigated the dependence of interwell coupling on the well and barrier thicknesses and on the In content of the wells. Superlattice miniband formation is discussed where we observe a large anisotropy in carrier masses along and perpendicular to the growth direction. A reduced in-plane mass of the highest valence band (‖${\mathit{m}}_{\mathit{J}}$‖=3/2, conventional heavy-hole band) is observed, dependent on the degree of the heavy-hole–light-hole (‖${\mathit{m}}_{\mathit{J}}$‖=1/2) decoupling. An extra Landau level transition appears between the conventional l=0 and l=1 Landau level transitions, increasing in intensity as the magnetic field perpendicular to the layer increases. This is interpreted as a result of mixing between the heavy-hole and light-hole states induced by the magnetic field. Interband spin splitting is observed in narrow ${\mathrm{Ga}}_{0.88}$${\mathrm{In}}_{0.12}$Sb/GaSb single quantum wells but not wide wells. The difference arises from the valence-band spin splittings, which are very dependent on the heavy-hole–light-hole coupling, and therefore on the quantum-well width. These last two observations are explained well by Landau level calculations with the Luttinger k⋅p Hamiltonian.

The purpose of this work is to develop and demonstrate the practical use of very efficient algorithms which can be readily used to study systems with significant inhomogeneities. We have developed two algorithms which can be used to study inhomogeneous discrete systems. The first one is an extension of known algorithms for homogeneous media and rests on the notion of transfer matrices, which are then used to evaluate the desired elements of the Green-function matrices to be employed in surface Green-function matching calculations. The second one is totally different and yields the Green-function matrices directly. Both work quite efficiently. When tested in practice for a graded-composition quantum well they give the same results for the local density of state at the interfaces. We apply the algorithms to the study of quantum wells consisting of AlAs in the barriers and ${\mathit{N}}_{\mathit{w}}$ layers ${\mathrm{Al}}_{\mathit{x}\left(\mathit{n}\right)}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}\left(\mathit{n}\right)}$As in the well region, and x varying linearly from x=0.3 to x=0. The ${\mathit{sp}}^3$${\mathit{s}}^{\mathrm{*}}$ empirical tight-binding model and the virtual-crystal approximation are used. We studied three wells of different thicknesses (${\mathit{N}}_{\mathit{w}}$=21,35,51). The ground-state and some excited-state energies of the conduction and valence bands are studied in detail: Spatial dependence and orbital composition of the corresponding spectral strengths show all the expected features.

Recent integer quantum Hall effect experiments on silicon samples have shown deviations from the quantized Hall resistance despite a vanishing longitudinal resistance. Here we argue that single short-range elastic scatterers at the edges can lead to observable deviations in the Hall conductivity without backscattering, at the large currents typical experimentally. We do so within an approach to steady-state mesoscopic transport which is not restricted to the linear-response regime (i.e., to small currents).

We present a theoretical study of the electronic structure of short-period ${\mathrm{Sn}}_{\mathit{m}}$${\mathrm{Ge}}_{\mathit{n}}$ strained-layer superlattices (SLS’s) grown along the [001] and [111] directions both on Ge and on zinc-blende-like SnGe. The total energies of SnGe and of ${\mathrm{Sn}}_2$/${\mathrm{Ge}}_2$ SLS’s as well as the band structures of various Sn/Ge SLS’s are calculated by means of the relativistic linear-muffin-tin-orbital method within the atomic-spheres approximation. The error in the excitation energies inherent in the local-density approximation is compensated by means of external potentials chosen such that the gaps of bulk Ge and α-Sn agree well with experiments. The total energy of Sn/Ge SLS’s increases with increasing concentration of homopolar bonds (Ge-Ge,Sn-Sn). The structural energy differences per formula unit are of the order of 0.1 eV. We have not found any direct-gap [001] SLS (with the exception of zinc-blende-type SnGe) but some of the [111] SLS’s have a direct gap. The ${\mathrm{Sn}}_1$/${\mathrm{Ge}}_3$ [111] SLS grown on Ge, for example, has a direct gap 0.22 eV. The Γ-Γ optical transitions in this SLS could have a considerable strength. The band structures of [001] SLS’s with both m and n even reflect the orthorhombic anisotropy of the SLS unit cell. Most of the results are successfully interpreted in terms of a tight-binding model.