We have investigated the carrier spin segregation process in a ZnSe/${\mathrm{Zn}}_{0.96}$${\mathrm{Fe}}_{0.04}$Se quantum-well structure by studying the energy and relative intensity of the ground-state exciton components as a function of applied magnetic field. The ${\mathit{e}}_1$${\mathit{h}}_1$ heavy-hole exciton initially tends towards type-II behavior as the field is applied but the trend reverses itself at higher fields as the conduction-band splitting of the magnetic barriers becomes significant. The interplay between the electron-hole Coulomb attraction and the tunable type-II confining potentials is described in the context of a variational model.

We present an analysis of the vibrational properties and Raman response of (113)-oriented GaAs/AlAs superlattices with periodically corrugated interfaces. The vibrational properties have been calculated using the bond-charge model and the Raman response is evaluated in the framework of the bond polarizability model. We have measured the Raman spectra of samples grown by molecular-beam epitaxy, where an additional in-plane periodicity was expected, and of samples grown by atomic-layer molecular-beam epitaxy, which did not present this new in-plane periodicity. The comparison between the experimental results and the theoretical simulations suggests that faceting of the surface observed in the samples grown by molecular-beam epitaxy is not maintained in the buried interface.

We present a parametrization of the ground-state energy ${\mathit{E}}_{\mathit{g}}$(ν) of the two-dimensional electron gas as a function of the filling factor ν, in the fractional quantum Hall regime for the lowest Landau level. The cusp structure is included via an approximate model of the quasiparticle energies ${\mathrm{\mu }}_{\pm }$(ν) at the cusps. The parametrization depends only on a single parameter fitted to the known ${\mathit{E}}_{\mathit{g}}$ at ν=1/3. The energy functional is useful in density-functional calculations for inhomogeneous systems and for approximate predictions of ${\mathit{E}}_{\mathit{g}}$, ${\mathrm{\mu }}_{\pm }$(ν) at values of ν for which more fundamental calculations are not available.

Nanocrystallite Ge (nc-Ge) embedded in a glassy ${\mathrm{SiO}}_2$ matrix is fabricated and examined by x-ray photoelectron spectrometry, Raman spectrometry, and high-resolution transmission-electron microscopy. The precipitation and growth of nc-Ge are found to be related to a thermodynamical reduction of ${\mathrm{GeO}}_2$, the diffusion of Si atoms from the Si substrate into the glassy matrix, and an aggregation of small-sized nc-Ge. The size inhomogeneity can be precisely controlled by a double annealing, and the average size can be changed in the range of 2–6 nm. Broadband photoluminescence (PL) spectra are observed in the visible wavelength range at room temperature, and they exhibit pronounced blueshifts of the peak energies and broadening of the PL spectra, which can be correlated to the change in the size. The PL excitation spectra show a Stokes energy smaller than 0.1 eV and dependence on the measurement energy. The visible PL also shows a strong correlation to the presence and actual condition (i.e., size) of nc-Ge. Possible origins of the visible PL such as a quantum-confinement model in quantum dots, the presence of luminescent centers (Ge:E’) in silica glass, or a structural transition of nc-Ge are discussed. The present experimental data are concluded to be more consistent with a quantum-confinement model than with the other models.

Molecular-dynamics simulations have been performed to investigate the defect formation associated with the Staebler-Wronski (SW) effect in undoped a-Si:H and the role that H plays in this process. Semiempirical Si-Si and Si-H total-energy functionals were used to obtain the forces needed for these simulations. Two a-Si:H random networks proposed by Guttman and Fong [Phys. Rev. B 26, 6756 (1982)], a monohydride system and a dihydride system, both of which contain 54 Si and 6 H atoms, were used as initial configurations. The bond-breaking model of the SW effect was assumed, and a localized excitation was used to model the nonradiative energy transfer from photoexcited electron-hole pairs to the system. Our results indicate that the monohydride system is considerably more stable against localized excitations than the dihydride system. We also find that, at least within the bond-breaking model, H is probably not involved in the defect formation associated with the SW effect in undoped a-Si:H.

We have developed a theory of one-phonon Raman scattering in semiconductor microcrystallites and quantum dots. In particular, we have given a complete description of the phonon modes of spherical structures embedded in another material, which includes a correct treatment of the mechanical and electrostatic matching conditions at the surface. We illustrate the theory for CdS microcrystallites in a glass matrix, which have interesting phonon properties since the TO phonon seems to have a positive dispersion in the bulk. We consider the Fröhlich interaction for resonance Raman scattering with laser energies between the first two confined electron-hole subband resonances. We also derive an electron-phonon interaction selection rule for spherical dots that only allows contributions from phonon states with angular momentum ${\mathit{l}}_{\mathit{p}}$=0 and ${\mathit{l}}_{\mathit{p}}$=1.

A detailed theoretical study of the one-electron density of states (DOS) for heterostructures in a strong and transverse magnetic field B is presented for low temperatures. The Green’s function for the Landau levels and the electron self-energy are calculated self-consistently including the electron interaction with background and remote impurities and with acoustic phonons through deformation and piezoelectric coupling. Transitions between Landau levels are taken into account in the self-energy. A semielliptic DOS is obtained and the Landau-level width shows a B dependence very different than ${\mathit{B}}^{1/2}$. The level shift shows considerable structure as a function of energy, magnetic field, and spin. A significant background DOS results from remote-impurity scattering for small background-impurity densities and intermediate spacer distances. The dependences on temperature, mobility, and the g factor are also investigated.

The effective spin splitting of the Landau bands produced by an external unidirectional periodic-modulation potential is studied in the thermodynamic Hartree-Fock (HF) approximation. The HF energies, which depend on three quantum numbers, the center coordinates of the wave functions, and the discrete Landau and spin quantum numbers, and also the thermodynamic density of states (TDOS), which is experimentally accessible by magnetocapacitance measurements, are calculated numerically for representative values of wavelength and strength of the modulation potential. For GaAs at low temperatures, the exchange enhancement can yield an effective spin gap comparable with the cyclotron energy. The spin gaps occur in strips, in which the center coordinates correspond to HF energies near the Fermi energy. The width of these strips depends on the steepness of the modulation potential and, in an intricate manner, on temperature and screening effects. Screening effects may lead to broad peaks in the TDOS, while van Hove singularities due to the one-dimensional Landau bands lead to sharp peaks in the TDOS. The conditions, under which such structures should be resolved in experiments, are discussed.

We study the statistics of level widths of a quantum dot with extended contacts in the absence of time-reversal symmetry. The widths are determined by the amplitude of the wave function averaged over the contact area. The distribution function of level widths for a two-point contact is evaluated exactly. The distribution resembles closely the result obtained when the wave function fluctuates independently at each point, but differs from the one-point contact case. Analytical calculations and numerical simulations show that the distribution for many-point contacts has a power-law behavior at small level widths. The exponent is given by the number of points in the contact and diverges in the continuous limit. The distribution of level widths is used to determine the distribution of conductance peaks in the resonance regime. At intermediate temperatures, we find that the distribution tends to normal and fluctuations in the height of the peaks are suppressed as the contact size is increased.

We report the rich variation in the evolution of step structures of vicinal Si(001) upon As deposition, studied by scanning tunneling microscopy. Clean 1° vicinal Si(001) exhibits a typical double-domain surface that consists of single-monolayer-height steps called ${\mathit{S}}_{\mathit{A}}$ and ${\mathit{S}}_{\mathit{B}}$. On the other hand, As-deposited surfaces show drastic changes in the step structures depending on the substrate temperature during the deposition and the As coverage: At relatively low temperatures (500 °C), the surface becomes nearly a single domain by the deposition of a half monolayer and two-dimensional islands appear after more As is deposited. At medium temperatures (600 °C), however, one-monolayer As deposition makes the surface nearly a single domain, consisting of double-monolayer-height steps called ${\mathit{D}}_{\mathit{B}}$. At high temperatures (700 °C), a double-domain surface is formed containing triple-monolayer-height steps where the dimer rows in the upper terrace are perpendicular to the step edge. These various surface step structures are explained by kinetic effects of atomic movement during deposition and desorption and by the energetically stable shapes of Si(100)-As in equilibrium.

We have studied the temperature dependence of the band overlap in InAs/GaSb structures by performing magnetotransport measurements on both semiconducting and semimetallic superlattices at temperatures between 110 and 340 K and in fields up to 45 T. Semiconducting samples have very few carriers at 4 K, but at these higher temperatures intrinsic carriers are thermally excited across the energy gap. By measuring the variation of the electron and hole densities with temperature it is possible to determine the energy gap between the electron and hole confinement energies and thus the band overlap between the InAs conduction band and the GaSb valence band. We find that this overlap is 30 meV larger at 300 K than at 4 K, which agrees with similar measurements on a semimetallic sample and also the model-solid theory of Van de Walle.

Quantum confined GaAs/AlAs resonant tunneling diodes with submicron lateral dimensions are studied theoretically using the transfer Hamiltonian formalism. It is shown that a high magnetic field normal to the current and parallel to the quantum wire cavity can be used to unambiguously identify the lateral quantum confinement. The current-voltage characteristics for other magnetic field orientations are also discussed.

We study the equilibrium properties of a quantum dot connected to a bulk lead by a single-mode quantum point contact. The ground-state energy and other thermodynamic characteristics of the grain show periodic dependence on the gate voltage (Coulomb blockade). We consider the case of almost perfect transmission, and show that the oscillations exist as long as the transmission coefficient of the contact is less than unity. Near the points where the dot charge is half-integer, the thermodynamic characteristics show a nonanalytic behavior identical to that of the two-channel spin-1/2 Kondo model. In particular, at any transmission coefficient the capacitance measured between the gate and the lead shows periodic logarithmic singularities as a function of the gate voltage.

According to the composite-fermion theory, the interacting electron system at filling factor ν is equivalent to the noninteracting composite-fermion system at ${\nu }^{\mathrm{*}}$=ν/(1-2mν), which in turn is related to the noninteracting electron system at ${\nu }^{\mathrm{*}}$. We show that several eigenstates of noninteracting electrons at ${\nu }^{\mathrm{*}}$ do not have any corresponding states for interacting electrons at ν, but, upon composite-fermion transformation, these states are eliminated, and the remaining states provide a good description of the spectrum at ν. We also show that the collective mode branches of incompressible states are well described as the collective modes of composite fermions. Our results suggest that, at small wave vectors, there is a single well-defined collective mode for all fractional quantum Hall states. Implications for the Chern-Simons treatment of composite fermions will be discussed.

We examine the band structure and the dielectric response of confined Si systems such as quantum wells and wires through first-principles pseudopotential calculations and find the direct band gaps. With the surfaces of the quantum systems saturated by hydrogen atoms, the optical excitation peaks in the range of visible light are prominent, as compared to those with unsaturated Si dangling bonds. In quantum wires, we find luminescence excitation peaks near the band-gap energy, in good agreement with measured absorption spectra. At energies higher by about 1 eV than the band-edge excitation spectra, we also find strong wire-related absorption peaks, while these were previously interpreted as bulklike excitations. We analyze optical absorption spectra and find strong transitions between relatively localized states that are caused by strong Si-H bonds and quantum confinement in the near-surface region. In correlating to the photoluminescence in porous Si, the importance of quantum confinement and surface effects is stressed.

The electronic properties of semiconductor, vertical, double-quantum-dot systems with few electrons are investigated by means of analytic, configuration-interaction, and mean-field methods. The combined effect of a high magnetic field, electrostatic confinement, and interdot coupling, induces a class of few-electron ground states absent in single quantum dots. In particular, the role played by the isospin (or quantum-dot index) in determining the appearance of ground states is analyzed and compared with the role played by the standard spin.

The Urbach tail in crystalline GaAs and Si due to thermal vibration is evaluated in the temperature range 0–810 K. Recent experiments suggest that there is no thermally induced Urbach tail in undoped GaAs. This is most unexpected since thermally induced tails at the absorption edge having the Urbach form and widths ${\mathit{E}}_0$(T) linear in T are observed in a wide range of crystalline and amorphous semiconductors. Clear understanding of the tail is important in device applications. In our calculations, we obtain an absorption tail having a simple exponential energy dependence (Urbach form) in GaAs at all temperatures with ${\mathit{E}}_0$(T) proportional to T. However, due to a very weak electron-phonon interaction, the magnitude of ${\mathit{E}}_0$(T) is unusually small, much smaller than the temperature independent, structurally induced component ${\mathit{E}}_0$≃10 meV. As a test of the method, the tail is similarly evaluated in Si providing an ${\mathit{E}}_0$(T) value in agreement with experiment.

We study the effects of screening on polaronic corrections to the effective band edge in a quasi-one-dimensional GaAs quantum wire. We find that the screening effects and finite well width considerably reduce the polaron energy and oppose the polaronic band-gap renormalization. We calculate the polaronic effective mass as a function of the carrier density and temperature. Effects of the vertex corrections to the conduction- and valence-band edges are also discussed.

Photoelectron-spectroscopy methods combined with electrical-resistance measurements were employed to study the effects of intermixing at Au/Sb interfaces at low temperatures. For the purpose of characterizing the growth processes of the intermixed phase on a ML scale, Au/Sb bilayers (layer thicknesses ${\mathit{D}}_{\mathrm{Au}}$=0.5–75 ML and ${\mathit{D}}_{\mathrm{Sb}}$=150 ML) were evaporated at 77 K and the different in situ techniques allowed a comparison to vapor-quenched amorphous ${\mathrm{Au}}_{\mathit{x}}$${\mathrm{Sb}}_{100\mathrm{-}\mathit{x}}$ alloys. For Au thicknesses between 0.5 and 0.9 ML, a change from a semiconducting to a metallic behavior of the samples has been detected, as indicated by the development of a steplike photoelectron intensity at the Fermi level. Evidence has been found that for Au coverages ≤ 6 ML chemical reactions at the Au/Sb interface occur, leading to the formation of a homogeneously intermixed amorphous layer with a maximum thickness of about 2.3 nm and Au concentrations as high as x≊80 at. %. This latter value corresponds to the limiting Au content where amorphous alloys can be prepared at low temperature (0 at. % ≤x≤ 80 at. %). For nominal coverages beyond 6 ML polycrystalline Au films were formed. Consequently, Au/Sb multilayers with sufficiently small modulation lengths, which were prepared at 130 K by ion-beam sputtering, were observed to grow as a homogeneous amorphous phase over a broad range of compositions, as evidenced by in situ resistance measurements and by comparing the obtained crystallization temperatures to those of vapor-quenched amorphous alloys. Variation of the deposition temperature ${\mathit{T}}_{\mathit{s}}$ revealed that an amorphous interface layer is only formed for ${\mathit{T}}_{\mathit{s}}$≤ 220 K. This is consistent with the fact that for multilayers with large modulation lengths containing unreacted polycrystalline Au and Sb layers, long-range interdiffusion is found to set in at temperatures above 230 K. This interdiffusion, however, results in the formation of polycrystalline Au-Sb alloys.

Scandium adsorbed on the (001) face of tungsten has been studied using Auger-electron spectroscopy, low-energy electron diffraction, k-resolved inverse photoelectron spectroscopy, work-function measurements, and relativistic-electronic-structure calculations. We find that the work function of W(001) does not decrease monotonically as a function of scandium adsorption but reaches a minimum value of φ=3.25 eV at 50% coverage. For a complete monolayer, an order 1×1 Sc overlayer is formed. Inverse photoemission in the isochromat mode was used to map the unoccupied energy levels of this overlayer along the Γ¯ M¯ symmetry direction of the surface Brillouin zone. Local-density-functional calculations using the muffin-tin orbital method were performed for a 1×1 Sc overlayer on W and are compared with the experimental two-dimensional band structure.