We present a detailed study of the optical properties of short-period ZnS-ZnSe strained-layer superlattices. These superlattices have been grown by metalorganic vapor-phase epitaxy. We show that the photoluminescence exhibits a low-energy tail due to localization of the exciton to interfacial potential fluctuations. A detailed analysis of the electronic structure has been performed using the envelope-function approach to obtain the valence-band and conduction-band envelope functions and band lineups. This was completed by a self-consistent calculation of the exciton binding energies. In this calculation the marginal conduction-band offset deduced from the standard envelope-function calculation is corrected by the electrostatic deformation produced by the presence of a localized hole wave function.

We show that a low-intensity femtosecond pulse is severely distorted while propagating through a relatively thin (≊6300 Å) GaAs multiple-quantum-well sample near the exciton resonance at low temperatures. This pulse distortion depends critically on the dephasing time ${\mathit{T}}_2$, the total thickness l, detuning, and inhomogeneous broadening. In thinner, high-quality samples (l<3000 Å, exciton linewidth <1 meV), the distortion is smaller and free-induction-decay-like. An interferometric measurement reveals the existence of well-defined nodes at which the envelope function changes its sign, demonstrating that the source of the pulse distortion is the reradiation of the induced dipoles. While the effects of this pulse distortion on pump-probe or four-wave-mixing experiments are relatively small for samples with l<3000 Å, samples with l>6000 Å are strongly affected.

The magnetostatic and electrostatic Aharonov-Bohm (AB) effects in multiply connected normal-conductor rings are studied. A previously developed model of a single mesoscopic ring is generalized to include an arbitrary number of rings, and the oscillatory behavior of the total transmission coefficients for the serially connected N (N is equal to integer) rings are derived as a function of the magnetic flux threading each ring and as a function of the electrostatic potential applied to the rings. It is shown that quantum oscillation of multiple rings exhibits greater variety of behavior than in periodic superlattices. We investigate the influence of the scattering at a junction and the number of atoms in the ring in both magnetostatic and electrostatic oscillation of multiring systems. For the electrostatic AB effects, when scattering occurs at the junctions between the connecting wire and the ring, the conductance in the AB oscillation is modified to an N-1 peaked shape. It is shown that this oscillatory behavior is greatly influenced by the number of atoms in the ring and is controlled by the electrostatic potential or magnetic flux that is applied to the ring. We discuss the behavior of the quantum oscillations upon varying the number of connected rings and the number of minibands.

Binding energies and radiative lifetimes of excitons in crystallites are calculated with respect to size and temperature by using a tight-binding configuration-interaction technique. We discuss the recent proposal that, for porous silicon, the exchange splitting could be at the origin of the peculiar behavior of the lifetime and luminescence intensity with temperature, as well as the existence of an onset in selectively excited luminescence. We show that the exchange splitting has no influence on the luminescence lifetime of spherical- or cubic-silicon crystallites because of the spin-orbit and valley-orbit couplings. In contrast, in asymmetrical crystallites, we predict that the spin-orbit and valley-orbit couplings are quenched so that the influence of the exchange splitting can be detected. However, we calculate exchange splittings smaller than the onsets observed in the spectra of the selectively excited luminescence of porous silicon. We show that the Stokes shift induced by the lattice relaxation in the excitonic state is significant and may explain at least partially this decrepancy. We confirm that the direct radiative recombinations are not responsible for the observed decay of the luminescence in porous silicon.

We have investigated the formation of antiphase boundaries on the Ag/Si(111)-(√3 × √3 ) surface using scanning tunneling microscopy (STM). Analysis reveals that the atomic structure across all boundaries is identical, and corresponds to a shift of the silicon trimers between adjacent ${\mathit{T}}_4$ sites. We can distinguish two types of boundary that although structurally similar exhibit different voltage dependence in STM imaging. It is also shown that the interface between (√3 × √3 ) domains, which are separated vertically by the equivalent of a monatomic step, exhibits unusual contrast in STM images.

We perform a detailed numerical study of energy-level and wave-function statistics of a deformable quantum billiard focusing on properties relevant to semiconductor quantum dots. We consider the family of Robnik billiards generated by simple conformal maps of the unit disk; the shape of this family of billiards may be varied continuously at fixed area by tuning the parameters of the map. The classical dynamics of these billiards is well understood and this allows us to study the quantum properties of subfamilies, which span the transition from integrability to chaos, as well as families at approximately a constant degree of chaoticity (Kolmogorov entropy). In the regime of hard chaos we find that the statistical properties of interest are well described by random-matrix theory and are completely insensitive to the particular shape of the dot. However, in the nearly integrable regime nonuniversal behavior is found. Specifically, the level-width distribution is well described by the predicted ${\mathrm{\chi }}^2$ distribution both in the presence and absence of magnetic flux when the system is fully chaotic; however, it departs substantially from this behavior in the mixed regime. The chaotic behavior corroborates the previously predicted behavior of the peak-height distribution for deformed quantum dots. We also investigate the energy-level correlation functions, which are found to agree well with the behavior calculated for quasi-zero-dimensional disordered systems.

We have considered the influence of electromagnetic fluctuations on electron tunneling via one nondegenerate resonant level, the problem that is relevant for electron transport through quantum dots in the Coulomb blockade regime. We show that the overall effect of the fluctuations depends on whether the electron bands in external electrodes are empty or filled. In the empty band case, depending on the relation between the tunneling rate Γ and characteristic frequency Ω of the fluctuations, the field either simply shifts the conductance peak (for rapid tunneling, Γ≫Ω) or broadens it (for Γ≪Ω). In the latter case, the system can be in three different regimes for different values of the coupling g between electrons and the field. Increasing interaction strength in the region g<1 leads to gradual suppression of the conductance peak at the bare energy of the resonant level ${\mathrm{\varepsilon }}_0$, while at g≫1 it leads to the formation of a new peak of width ${\mathit{E}}_{\mathit{c}}$/${\mathit{g}}^{1/2}$ at the energy ${\mathrm{\varepsilon }}_0$+${\mathit{E}}_{\mathit{c}}$, where ${\mathit{E}}_{\mathit{c}}$ is a charging energy. For intermediate values of g the conductance is nonvanishing in the entire energy range from ${\mathrm{\varepsilon }}_0$ to ${\mathrm{\varepsilon }}_0$+${\mathit{E}}_{\mathit{c}}$. These results provide a possible explanation for the experimentally observed extra width of the conductance resonances at low temperatures. For filled bands the problem is essentially multielectron in character. One consequence of this is that, in contrast to the situation with the empty band, the fluctuations of the resonant level do not suppress conductance at resonance for g<1. At g>1 the Coulomb gap appears in the position of the resonant level as a function of its bare energy, which leads to suppression of conductance.

We investigate the influence of local defects on the dc transport in mesoscopic quantum wires of finite width. Using the Anderson Hamiltonian for the description of the wires, we solve the Schrödinger equation for scattering boundary conditions. The conductance is then calculated within the Landauer approach. We present a detailed study of the defect-induced fluctuations in the conductance spectra, which are of the order of ${\mathit{e}}^2$/h. These fluctuations can be related with Fano resonances in the electron transmission spectra, which are due to the coupling between localized defect states and the propagating states of the perfect waveguide. Different kinds of defects are investigated. The case of the double Anderson chain is treated with particular emphasis, since in spite of its simplicity this system possesses all the characteristic features of more complex systems. Our analytical and numerical results reveal the intimate relation between conduction spectra and localized impurity states and provide a basis for the understanding of conductance spectroscopy experiments in mesoscopic systems. The importance of the interaction between localized states and propagating states is also demonstrated by our results for the local current distribution, which becomes strongly inhomogeneous and even vortexlike for Fermi energies near the Fano resonances.

The electric conductivity σ of δ-doping superlattices (SL’s) along the growth direction is investigated theoretically at low electric fields and zero temperature. A plateau behavior is predicted for σ as a function of the Fermi energy ${\mathit{E}}_{\mathit{F}}$. If ${\mathit{E}}_{\mathit{F}}$ crosses a miniband σ increases, and if ${\mathit{E}}_{\mathit{F}}$ crosses a minigap σ stays constant. Under certain circumstances this causes an insulator-metal transition. The competition of this transition with the Mott transition is discussed, and the Mott criterion is adapted to SL’s. The general results are applied to GaAs and Si n-type δ-doping SL’s. From miniband structure calculations for various SL periods d and sheet-doping concentrations ${\mathit{N}}_{\mathit{D}}$ we derive conductivity plots σ(${\mathit{N}}_{\mathit{D}}$) and insulator-metal phase diagrams in the ${\mathit{N}}_{\mathit{D}}$-d plane.

Exciton energies and eigenfunctions in asymmetric double quantum wells are calculated as functions of an applied electric field. From these data, absorption spectra are calculated and the localization behavior of the excitonic states is investigated. The coherent dynamics in asymmetric double quantum wells, which is strongly influenced by excitonic effects, is analyzed by a numerical solution of the multisubband semiconductor Bloch equations. Under resonance conditions the oscillating dipole moment leads to a coherent terahertz emission. In general, the spectrum of this radiation has contributions due to both, the free-carrier and exciton energies, the details being determined by the excitation and dephasing conditions. The calculated absorption spectra and the frequency of the terahertz radiation are in good agreement with recent experiments.

The electronic structure of buried α-${\mathrm{FeSi}}_2$ and β-${\mathrm{FeSi}}_2$ layers produced by ion-beam synthesis has been studied. Using fluorescence spectroscopies, we were able to investigate the silicide layers, which are buried below a 600-Å Si cap layer, in situ. The occupied local Fe d density of states was determined by Fe ${\mathit{L}}_3$ soft-x-ray-emission spectra, excited both with high-energy electrons and monochromatized synchrotron radiation. Using soft-x-ray-absorption spectra recorded in the fluorescence-yield mode at the same Fe ${\mathit{L}}_3$ edge, the unoccupied Fe d density of states was determined. The formation of a band gap of 0.8±0.2 eV in the electronic states of d symmetry at the Fe atoms of β-${\mathrm{FeSi}}_2$ could be observed in the experiments. We present ab initio band-structure calculations for α-${\mathrm{FeSi}}_2$ and β-${\mathrm{FeSi}}_2$ and compare the results to the spectroscopic data. The influence of a core hole on the soft-x-ray-absorption spectrum is studied by calculating the electronic structure of a substitutional impurity in the form of a core 2${\mathit{p}}_{3/2}$ ionized Fe atom using a supercell calculation.

We develop a theory of electronic transport through three-dimensional ballistic microwires in longitudinal magnetic fields at low temperatures. In weak fields (${\mathit{scrl}}_{\mathit{c}}$≫a, ${\mathit{scrl}}_{\mathit{c}}$ is the cyclotron, and a is the wire radius) we find steplike fluctuations in the magnetoconductance and sharp spikes in the thermopower coefficient. These mesoscopic fluctuations, which appear on a new scale corresponding to a small fraction ${\mathrm{\Phi }}_0$/(${\mathit{k}}_{\mathit{F}}$a) of the quantum unit of magnetic flux ${\mathrm{\Phi }}_0$=hc/e, are caused by the shift of electronic levels through the Fermi level in a changing magnetic field. Increased temperature or surface scattering transforms the discrete structure into a smooth, oscillatory variation with period ${\mathrm{\Phi }}_0$. The energy scale for smearing the fine structure is given by the average distance between transverse energy levels in the wire, ${\mathit{E}}_{\mathit{F}}$/(${\mathit{k}}_{\mathit{F}}$a${)}^2$. The smooth Aharanov-Bohm-type oscillations vanish at the larger temperature scale ${\mathit{E}}_{\mathit{F}}$/(${\mathit{k}}_{\mathit{F}}$a). The flux-induced structure should be observable in, e.g., submicrometer Bi whiskers at temperatures of order 0.1 K. Sample-characteristic fluctuations are predicted to appear in fields as low as a few gauss. In strong magnetic fields (${\mathit{scrl}}_{\mathit{c}}$≪a) bulk Landau levels in the wire gives rise to an oscillatory magnetoconductance of the Shubnikov–de Haas form.

We investigate the lateral tunneling of electrons in a weakly coupled system of parallel quantum wires considered recently by Kawamura et al. Instead of discretizing the problem, we solve perturbatively the coupled system of Schrödinger equations coming from the one-band approximation for both the symmetric and asymmetric cases. Our results are in agreement with the principal conclusion of the authors mentioned that there is a robust change in conductivity if the wire system is set out of balance, and allow us to explain the effect as a consequence of the difference between the degenerate and nondegenerate perturbation theory.

A systematic study of structural, electronic, and optical properties of strained ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ alloys, coherently grown on a Si(001) surface, is presented. We find that for bulk alloys the lattice constant deviates from Vegard’s law, while for the strained alloys it deviates from the results of elasticity theory. The strained alloys are indirect gap materials, with the minimum of the conduction band appearing in the [100] direction. We present results for the indirect as well as the direct gaps. Finally, we investigate the optical properties of strained alloys and present results for the critical energies as a function of the concentration of Ge in them.

First-principles and tight-binding electronic-structure calculations of hypothetical tubule forms of hexagonal ${\mathrm{BC}}_3$ have been performed. According to the total-energy and force calculations, stable atomic geometries of the ${\mathrm{BC}}_3$ tubules can be obtained by rolling hexagonal ${\mathrm{BC}}_3$ sheets keeping the C—C and C—B lengths at 1.42 and 1.55 Å, respectively. Since the energy increase upon tubule formation is smaller than that of carbon tubules, we conclude that the ${\mathrm{BC}}_3$ tubules are likely to form. The planar ${\mathrm{BC}}_3$ sheet has π and ${\mathrm{\pi }}^{\mathrm{*}}$ bands above the Fermi level, and this feature is retained in the electronic structures of the ${\mathrm{BC}}_3$ tubules. The influence of concentric formations of the ${\mathrm{BC}}_3$ tubules on their conductivities is also discussed.

The interaction of excitons with impurities in an intentionally doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum well has been studied using picosecond time-resolved photoluminescence employing resonant excitation. The dominance of exciton recombination in a quantum structure allows a detailed analysis of the interaction kinetics, not previously possible in bulk GaAs. The recombination kinetics on a picosecond time scale are found to be strongly dependent upon the doping level, temperature, and well width. The contribution of these terms to the balance of the population between free excitons and impurity-bound excitons is investigated. A numerical solution of the associated differential equations allows a quantitative evaluation of the parameters describing this interaction (for a 100-Å acceptor-doped QW at 5 K we obtain the exciton lifetime ${\mathrm{\tau }}_{\mathrm{FE}}$∼180 ps, the bound exciton lifetime ${\mathrm{\tau }}_{\mathrm{BE}}$∼450 ps, and the capture rate for free excitons to neutral acceptors ${\mathit{C}}_{\mathrm{FE}}$∼2 ${\mathrm{cm}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$). A consistent picture of the capture and thermal emission processes for excitons at neutral acceptors and their effect on the observed quantum-well recombination transient emerges from this analysis.

We present time-resolved differential-absorption data for GaAs quantum wells, in which temporal oscillations originating in a biexcitonic pairing appear. We interpret the oscillations within a four-level model of the exciton-biexciton system, and discuss the cases of homogeneously and inhomogeneously broadened exciton absorption lines. Our differential absorption measurements in high magnetic fields normal to the quantum wells’ layers show that both the oscillations’ frequency and phase depend on the strength and direction of the magnetic field. The frequency change corresponds to the exciton Zeeman splitting in our samples. These findings are in excellent agreement with the predictions of the model, and thus verify its validity.

We study RH1 ordering in Si-Ge superlattices using empirical potentials for the Si-Ge system, one of which incorporates long-range interactions and accurately reproduces the phonon spectra as well as equilibrium structures. Our simulations show that the energy difference between the RH1 and the disordered phase remains small over a wide range of situations, which cannot account for the high transition temperature and the reversibility of the former. We consider experimental evidence of further frustration in the disordered phase only; even so we show that it fails to stabilize the RH1 phase up to the observed temperature. Experimentally, ordering is observed by means of transmission-electron diffraction done on thin samples cut in the (110) plane. Our simulations show that the reconstructions on the (110) surface may provide the missing energy necessary to stabilize RH1 ordering.

By means of resonance Raman spectroscopy we have investigated intersubband transitions of quasi-two-dimensional (2D) hole gases in p-type modulation-doped GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum-well structures. The observed excitations have an essentially single-particle character due to Landau damping of collective excitations and due to single-particle scattering by energy-density fluctuations under conditions of extreme resonance. In samples with well widths of typically 100–200 Å and 2D hole densities ρ∼${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$, we observe a characteristic variation of intersubband-transition energies with laser frequency in depolarized and in polarized scattering configurations. This variation is caused by the nonparabolic subband dispersion of the 2D single-particle hole subbands. Experiments under variation of p, by illuminating the sample with photons that have energies above the band gap of the ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As barriers, allow an estimate of the relative strengths of direct and exchange Coulomb interactions. From these experiments a greater relative strength of the exchange interaction, in comparison to that found for 2D electron gases, can be deduced.

We present full pseudopotential band-structure calculations of the frequency dependence of the second-order susceptibility due to transitions between valence minibands in p-type asymmetric stepped-well superlattices. These theoretical predictions are supported by experimental observations of second-harmonic generation. From an analysis of the theoretical results the microscopic processes that contribute to the response are identified and the features of the response are related to the superlattice band structure over a range of temperatures. We demonstrate that simple particle-in-a-box models are unable to correctly describe the positions of the peaks, or the mechanisms involved, in the second-order susceptibility. Experimental results confirming the detection of second-harmonic generation in these structures are presented and compared with the theoretical predictions. An enhancement over the response in bulk of approximately 8 times is observed. The angular dependence observed is consistent with the directional properties of the second-order susceptibility predicted by the theoretical calculations.