The magnetic electron-focusing effect (MEFE) has been investigated in mesoscopic multiterminal samples of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructures. The MEFE peaks are clearly observed even in a configuration where electrons seem to be reflected from an extra probe between an emitter and a collector probe. To clarify the reflection mechanism, the ‘‘reflection’’ peaks are studied in the samples with variously shaped extra probes and in the samples with two extra probes connected by a byway channel or an external circuit. In the reflection events, the electrons different from the electrons injected into the extra probe should be emitted from the extra probe, since the chemical potential of the extra probe becomes higher due to the accumulation of electrons in the extra probe.

Interface-state passivation during dc biased postmetallization annealing at ∼350 °C was studied in very thin oxide (∼31 Å) metal–tunnel-oxide–silicon structures. The gate metal was aluminum and the substrate was 〈111〉 oriented and of p type. Capacitance-voltage and tunnel-current–voltage measurements were used after the anneals to monitor the passivation of interface states. It was found that the passivation process of the initially present interface states is directly dependent on the surface potential but not on the average oxide electric field or the tunnel current. A negative gate voltage increases the passivation rate, whereas a positive gate voltage decreases it as compared to unbiased annealing. The interface states bear significant resemblances to ${\mathit{P}}_{\mathit{b}}$ centers, which are dangling bonds on trivalently bonded Si atoms at the interface. The present observations are found to agree well with the theoretical calculations by Edwards [Phys. Rev. B 44, 1832 (1991)] on the surface-potential dependence of the passivation of ${\mathit{P}}_{\mathit{b}}$ centers by molecular hydrogen. Furthermore, we report on the impact of biased annealing on the tunnel current and the flatband voltage and also on the behavior of electrically stressed devices during biased annealing. The beneficial effect of a negatively biased annealing makes it possible to find an optimum time and voltage, roughly 1000 s at a gate voltage of -1.2 V in our case, for simultaneously minimizing the dc tunnel current, flatband voltage shifts, and the density of fast interface states in these diodes. None of the passivation events was found to be promoted by annealing in a 10% ${\mathrm{H}}_2$/Ar ambient compared to annealing in an ${\mathrm{N}}_2$ ambient.

Thin films of high-quality ZnS were grown on (001) GaAs and (111) Si substrates by metal-organic chemical-vapor deposition. 2-K reflectivity was used to analyze various samples grown at different temperatures. The spectra show two structures at 3.801 eV corresponding to the free exciton and at 3.871 eV corresponding to the ${\mathit{E}}_0$+${\mathrm{\Delta }}_0$ transition. Theoretical reflectivity spectra were calculated using the spatial dispersion model with two oscillators. Thus, the transverse energies, the longitudinal transversal splitting, the oscillator strengths, and the damping parameters were determined for both the free exciton and the split-off exciton of ZnS. Photoluminescence measurements were also carried out using an excimer laser (308 nm). Both light-hole and heavy-hole excitons were observed, which allow for the determination of the strain that exists in the layer. This strain is due only to the difference between the thermal-expansion coefficients of the GaAs substrate and the ZnS epilayer, and was demonstrated to be a tensile strain. This paper also presents results on the band-gap energy variation as a function of the temperature and photoluminescence spectra when the excitation was varied from weak to very high densities (15 MW/${\mathrm{cm}}^2$).

The applicability of the infinite-barrier scheme to the study of the confinement mechanism for the optical phonons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructures is critically addressed. In the framework of a bond-charge-model calculation, we demonstrate that the line shape of ${\mathrm{LO}}_1$ GaAs-like displacement patterns differs from a sinelike function and depends on the Al content of the ternary barrier, preventing the extension of the infinite-barrier scheme to ternary systems. Finally, the analysis of Raman intensities is indicated as a suitable probe of the confinement efficiency because of its sensitivity to the displacement pattern line shape.

Using atomic force microscopy (AFM), we have directly observed the progression of surface morphology of InAs deposited by molecular-beam epitaxy on GaAs(100). InAs self-assembled dots (coherent) or relaxed InAs islands (incoherent) are formed depending on the InAs coverage. The InAs coverage was varied continuously and AFM was used to monitor in detail the nucleation and resulting size and shape transition of the InAs self-assembled dots. Dots of uniform size were observed only at the initial stages of this Stranski-Krastanow growth-mode transition. The self-assembled dot density increased very abruptly with total deposited amount of InAs. Treating this InAs growth-mode transition as a first-order phase transition with InAs total coverage as the critical parameter, we extract a critical thickness for surface elastic relaxation of 1.50 ML.

We solve the one-dimensional Schrödinger wave equation in the effective-mass approximation for the transmission coefficient T(E) of electrons through asymmetric multibarrier semiconductor heterostructures in the presence of a constant applied electric field, using an exact Airy-function formalism and the transfer-matrix technique. In particular, we show that for appropriate choices of asymmetry in the barrier widths and heights of the semiconductor heterostructure, the transmission coefficient is enhanced to yield resonances that are stronger than those calculated in symmetric structures, thus giving further validity to Mendez’s concept of effective-barrier symmetry for obtaining optimal resonant tunneling in asymmetric double- and triple-barrier semiconductor heterostructures. These results should assist experimental efforts in designing resonant-tunneling systems that require optimum peaks both in the transmission spectrum and current density.

Optical vibrations of a thin ionic slab are described by a macroscopic theory involving the hybridization of LO, TO, and interface-polariton modes. The resultant modes are triple hybrids, which satisfy both elastic and electromagnetic boundary conditions at the two surfaces. Analytic expressions are derived for the relative ionic displacements and related electric fields, and for the dispersion relations, assuming elastic isotropy and neglecting retardation effects. Comparison of mode patterns with those obtained by Fuchs and Kliewer, who used electromagnetic boundary conditions only, show that Fuchs-Kliewer modes are a good approximation to the system of the triple-hybrid modes our theory describes.

We have observed a number of allowed and parity-forbidden Landau-level transitions in a modulation-doped GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As single heterojunction structure using photoluminescence-excitation spectroscopy at 2 K. The GaAs layer in this structure is 5000 Å thick. From the allowed Landau-level transitions we determine the reduced mass of the electron-hole pair to be 0.07${\mathit{m}}_{\mathit{e}}$. Using a heavy-hole mass of 0.45${\mathit{m}}_{\mathit{e}}$, we determine the average value of the electron effective mass to be 0.084${\mathit{m}}_{\mathit{e}}$. From the parity-forbidden transitions where the hole Landau level is the same but the electron Landau levels are different, we determine the average value of the electron mass to be 0.085${\mathit{m}}_{\mathit{e}}$, in excellent agreement with the value determined from the allowed transitions.

We derive a general expression for the low-temperature equilibrium orbital current distribution in a two-dimensional electron gas, subjected to a perpendicular magnetic field and in a confining potential that varies slowly on the scale of the magnetic length l. The analysis is valid within a self-consistent one-electron description, such as the Hartree or standard Kohn-Sham equations. Our expression, which correctly describes the current distribution on scales larger than l, has two components: One is an ‘‘edge current,’’ which is proportional to the local density gradient, and the other is a ‘‘bulk current,’’ which is proportional to the gradient of the confining potential. The direction of these currents generally displays a striking alternating pattern. In a compressible region at the edge of the nth Landau level, the edge current is simply j=-e${\mathrm{\omega }}_{\mathit{c}}$${\mathit{l}}^2$(n+1/2)∇ρ×${\mathbf{e}}_{\mathit{z}}$, where ${\mathrm{\omega }}_{\mathit{c}}$ is the cyclotron frequency and ρ is the electron sheet density. The bulk component, a Hall current, dominates in the incompressible regions. In the ideal case of perfect compressibility and incompressibility, only one type of current contributes to a given region, and the integrated orbital currents in these regions are universal, independent of the widths, positions, and geometry of the regions. The integrated orbital current in the nth edge channel is (n+1/2)e${\mathrm{\omega }}_{\mathit{c}}$/2π, whereas in an incompressible strip with integral filling factor ν it is νe${\mathrm{\omega }}_{\mathit{c}}$/2π with the opposite sign.

The fabrication of pseudomorphic Ge layers in the interface region of ZnSe-GaAs heterostructures was recently shown to be an effective method of reducing the overall valence-band discontinuity. We investigate here the microscopic mechanism behind this effect, drawing on measurements and theoretical calculations of the band offsets for individual isolated heterojunctions between the different semiconductor constituents. We find that the formation of different interface configurations in individual neutral heterojunctions can account for the observed deviations from the predictions of the transitivity rule and hence for the effect of Ge interlayers.

A microscopic theory of defect-induced emission of atoms from nonmetallic surfaces is presented. The results agree very well with the experimental results for the emission yield with the number of laser shots, laser fluence, and incident photon energy. Our results show that the emission of atoms at low photon energy and laser fluence originates only from the surface defects like adatoms and steps. Such emissions cannot be observed from the defect-free surfaces of semiconductors considered here.

We have incorporated electric fields into the eight-band k⋅p theory, which we have applied to heterostructures, in conjunction with the envelope-function approximation. We use the method of Baraff and Gershoni to implement the electric-field effects in a computer program that calculates the optical properties of direct-band-gap heterostructures in one, two, and three dimensions. Using this method, we calculate the interband and intersubband electroabsorption of multiple quantum wells as well as the interband electroabsorption in superlattices. We illustrate the evolution of the Stark localization of the electron wave function under the application of an external electric field in superlattices. Comparison with experimental data, available in the literature, exhibits very good agreement between theory and experiment, with respect to the spectral shape, the absolute magnitude, and the electric-field dependence of the absorption.

${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/InP quantum wires with widths down to about 10 nm have been investigated by high-excitation luminescence spectroscopy. The one-dimensional electron-hole plasma spectra of quantum wires with widths between 50 and 30 nm exhibit shoulders on the high-energy edge of the emission. With decreasing quantum-wire widths the shoulders shift consistently to higher energy. The low- and high-energy features are identified by model calculations with the first, second, and third lateral-subband transitions in the structures. A simple model based on a square-well confinement by the semiconductor vacuum interface and the measured widths of the structures is in quantitative agreement with the experimental data.

The surface-phonon dispersion of cleaved GaAs(110) surfaces was studied by use of high-resolution electron-energy-loss spectroscopy along the symmetry directions Γ¯ X¯ and Γ¯ X¯ ’, i.e., parallel and perpendicular to the surface Ga-As chains. The energy of the incident electrons was varied between 4 and 80 eV. Besides two acoustic-surface-phonon branches with energies lower than 9 meV, four almost flat phonon bands were recorded at 10.5, 16.5, 21.5, and 35.8 meV. The experimental results agree very well with recent ab initio calculations of GaAs(110) surface-phonon dispersion.

The spin-flip Raman scattering of electrons bound to donors in CdTe/${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te multiple-quantum-well structures at 1.6 K in magnetic fields up to 6 T has been studied for a range of samples forming two series in which, first, the CdTe quantum-well width and second, the ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te barrier composition x were varied systematically. For structures with x<0.1, two spin-flip Raman bands are observed, which can be assigned to electrons located in the quantum wells and bound to donors located either in the quantum wells themselves or in the barriers of the structure. Measurements of the excitation spectra of the two Raman bands and of the quantum-well photoluminescence support this assignment. A variational calculation allows us to simulate the form of the observed spectra and also gives a quantitative description of the dependence of the Raman peak positions on the well width and barrier composition. The calculation allows us to derive a value of 0.060±0.005 eV for the conduction-band offset when x=0.07.

We have implemented a dynamical stochastic scheme to determine from first principles the structure of complex low-symmetry atomic systems such as surfaces and clusters. The method is based on Langevin molecular dynamics and quantum-mechanical interactions derived from ab initio pseudopotential calculations. No fictitious electron dynamics is employed, and insulating as well as metallic or charged systems can be handled in a straightforward manner. We apply this method together with the simulated annealing strategy to small neutral and charged silicon clusters, and show that the ground-state structures can be efficiently obtained with this approach. We also exploit this scheme to perform first-principles isothermal molecular-dynamics simulations, and examine the adsorption of a Si atom on a cluster.

Longitudinal-optic (LO) phonon and Fermi-sea shake-up excitations are studied in the magneto-optical spectra of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As-InP quantum wells. The final-state excitations are observed as low-energy satellites in photoluminescence spectra in magnetic fields up to 20 T. Clear resonant interactions between the LO-phonon and shake-up satellites are observed. In the low-density sample (${\mathit{n}}_{\mathit{s}}$=1.15×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$) studied, interaction between the shake-up satellite, corresponding to promotion of an electron from the ${\mathit{N}}_{\mathit{e}}$=0 to the ${\mathit{N}}_{\mathit{e}}$=1 Landau level, and the LO-phonon satellite of the ${\mathit{N}}_{\mathit{e}}$=0 Landau-level transition, is observed. The other two samples investigated have high carrier density of 9.2×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$; one sample has strong disorder and strong hole localization and the other weak disorder and weak hole localization. The sample with strong hole localization exhibits a Fermi-energy-edge singularity in its photoluminescence (PL) spectrum at zero magnetic field, and has strong phonon and shake-up satellites. The other high-density sample has a very weak PL signal at the Fermi energy and phonon and shake-up satellites at high field at least an order of magnitude weaker than in the sample with strong hole localization. This behavior arises since the strength of the Fermi edge singularity and the coupling to the LO phonon and shake-up excitations both depend on the degree of hole localization in the system.

In addition to resonant interaction between the LO-phonon and shake-up satellites, interaction of the phonon satellites of partially filled Landau levels with the ${\mathit{N}}_{\mathit{e}}$=0 electron to ${\mathit{N}}_{\mathit{h}}$=0 hole recombination line is reported. A marked variation of the intensities of these satellites with magnetic field is observed. It is suggested that the variation of Landau-level filling with field plays an important role in determining the intensities of these satellites of partially filled Landau levels. A theoretical treatment that explains the variation of the intensities of the shake-up satellites with field, at low magnetic field in the low-density sample, is also presented.

A model describing a system in which Landau electronic modes are coupled with phonons is proposed and discussed. A simplified version of the model is further analyzed, with special attention to the dynamical symmetries that characterize it. In particular, the corresponding equations of motion are thoroughly examined: they provide a variety of behaviors, ranging from completely integrable (in both the classical and quantum case) to chaotic (in the semiclassical approximation). The chaotic regime is believed to be suitable to eventually represent the stochastic behavior of the longitudinal voltage versus time, recently observed in several quantum Hall effect experiments.

We have investigated the optical properties of excitons in GaAs quantum wires grown on nonplanar patterned substrates (V-shaped quantum wires) by photoluminescence and magnetophotoluminescence. The experimentally observed transitions are in agreement with the theoretical evaluation of the quantized energy levels based on TEM measurements. The clear evidence of the ${\mathit{n}}_{\mathit{y}}$=1 and 2 excitonic recombination in the photoluminescence spectra is exploited to directly measure the anisotropy of the optical matrix elements.

A series of silicon nitride thin films prepared by different techniques are investigated with respect to their optical properties with spectroscopic ellipsometry in the energy region 1.5–9.5 eV, using conventional and synchrotron-radiation light sources. The dielectric-function spectra of the films are analyzed with the microscopic Si-centered tetrahedron model and thus the applied growth techniques are compared, with respect to the resulting tetrahedron types and volume fractions, and the superiority of the chemical-vapor deposition techique is distinctly denoted. Moreover, a model is developed for the deduction of the films’ stoichiometry from the above analysis and the results are compared with corresponding ones from Rutherford backscattering spectroscopy and elastic recoil detection. The effect of stoichiometry on the film quality and optical parameters is discussed. In addition, the effect of hydrogen on the fundamental and mean optical gaps of silicon nitride is investigated and it is concluded that in both cases hydrogen causes a shift to the red, which is interpreted as due to the existence of a significant number of Si-H bonds. Temperature-dependent optical studies on SiN bulk materials and thin films are performed in order to examine the temperature shifts of the fundamental and mean optical gaps, which are found to be redshifted and blueshifted, respectively. The effect of stoichiometry on the observed temperature coefficients is discussed.