Phosphorus-doped, hydrogenated amorphous Si films [a-Si:H(P)] deposited on crystalline Si substrates can be modified locally on a nanometer scale (<10 nm) by means of scanning tunneling microscopy. Upon increasing tunnel voltage and current beyond imaging conditions, the kinetic energy of the electrons and high current densities provided by the tip are sufficient to induce changes in the bonding configurations of atomic sites. By accurately controlling the modifying conditions, these changes can either be solely induced in topmost film regions or both in surface near- and deeper-layer regions. While in the first case the local conductivity or, more precisely, the tunneling probability is enhanced, modifications penetrating deeper into the amorphous coating lead to a local reduction in the film conductivity (electrically active structures).

With the use of the variational method to solve the effective-mass equation, we have studied the magnetopolaron bound to a Coulomb impurity in quantum-well wires. The magnetopolaron binding energy in the ground state and in the excited state, as well as the resonance energy of the magnetopolaron are calculated. Their dependence on the size of quantum-well wires and the magnetic field is depicted.

We consider a finite type-II superlattice with different dielectric media on either side of the surfaces, which consist of unit cells with alternating electron and hole layers in the unit cell. Under some assumptions, we have studied the density-density correlation function including overlap effects between electron and hole wave functions of adjacent layers using the random-phase approximation. We have obtained the dispersion relation for surface and bulk plasmons in a few cases. Furthermore, we have calculated the Raman intensity in order to compare the result with those of others.

We study the Laughlin wave function on the cylinder. We find it only describes an incompressible fluid when the two lengths of the cylinder are comparable. As the radius is made smaller at fixed area, we observe a continuous transition to the charge-density-wave Tao-Thouless state. We also present some exact properties of the wave function in its polynomial form. We then study the edge excitations of the quantum Hall incompressible fluid modeled by the Laughlin wave function. The exponent describing the fluctuation of the edge predicted by recent theories is shown to be identical with numerical calculations. In particular, we obtain the occupation amplitudes of edge state n(k) for four to ten electron-size systems. When plotted as a function of the scaled wave vector they become essentially free of finite-size effects. The resulting curve obtains a very good agreement with the appropriate infinite-size Calogero-Sutherland model occupation numbers. Finally, we numerically obtain n(k) of the edge excitations for some pairing states which may be relevant to the ν=5/2 incompressible Hall state.

We report measurements of optically induced density depletion of the two-dimensional electron system formed at the interface of a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterojunction with a δ-doped layer of Be acceptors in the GaAs 250 Å from the interface. Our measurements show that at low laser power the depletion effect is controlled by the recombination of two-dimensional electrons with photoexcited holes that have become bound to the Be acceptors. The point at which all the Be acceptors in the sample have been neutralized by photoexcited holes is indicated by the sudden appearance of free holes in the GaAs, which then control the density depletion in the high-power regime. We present a comprehensive dynamic model of the depletion effect that includes both regimes, and calculate the densities and mobilities of the carriers involved in the process.

We have used a classical hydrodynamic approach to study the magnetoplasma excitations in electron dots. The electrons are assumed to be confined within a plane by an anharmonic potential having circular symmetry. As a necessary first step, we develop a general procedure for determining the equilibrium electron distribution within the dot. This information is then used in the hydrodynamic analysis to yield a generalized eigenvalue problem for the magnetoplasma frequencies. Detailed applications are made to dots defined by a parabolic confining potential with a quartic radial perturbation. Contact with experiment is achieved by means of the calculated power absorption, which is found to be in good agreement with the observed far-infrared transmission spectra. In particular, the hydrodynamic model provides an explanation of the weak satellite, observed to track the high-frequency center-of-mass mode, in terms of the coupling of parabolic modes induced by the potential anharmonicity.

The effects of a short-range impurity potential V of radius ${\mathit{a}}_0$ on a model two-dimensional (2D) quantum-point contact (QPC) are studied. In this model the 2D electron gas is confined between two symmetrical hyperbolas. ‖Ṽ‖=2${\mathit{ma}}_0^2$‖V‖/${\mathrm{ħ}}^2$ is assumed to be small but ‖V‖ can be high on the scale of Fermi energy. A bound state at an attractive short-range impurity exists, at finite widths w of the constriction, only if the potential or w exceed some threshold values. The dependence of the binding energy on the width of the constriction and on the parameters of the impurity is found. An explicit expression for the conductance G(E) of the QPC with an impurity (attractive or repulsive) as a function of the Fermi energy E is derived. At small ‖Ṽ‖ the sign of the impurity effect on the conductance, ΔG, is opposite to the sign of the potential. As the depth of the attractive potential increases, ΔG(E) becomes negative in some regions of Fermi energy. For stronger potentials ΔG is negative for all Fermi energies both for attractive and repulsive impurities. For attractive impurities the dependence of ΔG(E) on V is nonmonotonic: the ‖ΔG(E)‖ peaks and minima are especially high, and the total conductance G(E) is a nonmonotonic function of Fermi energy, when the depth of the potential is close to the threshold value at which a bound state appears. At even higher ‖V‖ the ‖ΔG(E)‖ peaks and minima decrease but are on the order of conductance quantum ${\mathit{e}}^2$/2πħ, and G(E) is monotonic.

The ‖ΔG(E)‖ peaks and minima correspond, respectively, to the conductance steps, i.e., regions of transition from one plateau to the next one, and to the feet of these steps. The modulation resistance noise generated by random abrupt changes of the impurity potential is discussed. At high-impurity potentials the deviation of a conductance plateau from its quantized value grows with its number, i.e., the plateaus become progressively less perfect and more noisy. The results are compared with those found earlier for quasi-one-dimensional QPC models.

A theoretical analysis of the valence-band discontinuity (Δ${\mathit{E}}_{\mathit{v}}$) at the high-index (311)A GaAs/AlAs interface is reported together with a possibility of Δ${\mathit{E}}_{\mathit{v}}$ control by Si-insertion layers. The calculations are performed by using a self-consistent ${\mathit{sp}}^3$${\mathit{s}}^{\mathrm{*}}$ tight-binding method in a (GaAs${)}_6$/(AlAs${)}_5$ (311) superlattice. The Δ${\mathit{E}}_{\mathit{v}}$ at the (311)A interface is calculated to be 0.50 eV, which is practically equal to the Δ${\mathit{E}}_{\mathit{v}}$ of 0.51 eV at the (100) and (110) low-index interfaces. The orientation independence of the Δ${\mathit{E}}_{\mathit{v}}$ holds for the (311)A high-index interface. The result of the calculations is consistent with our experimental determination by x-ray photoemission spectroscopy. At the (311)A interface, the inserted Si double layers can have two possible layer spacings; one is a/(4 √11 ) on As-terminated GaAs and the other 3a/(4 √11 ) on Ga-terminated GaAs, where a is the lattice constant. In the former case, Δ${\mathit{E}}_{\mathit{v}}$ is calculated to be -0.12 eV (reduced by 0.62 eV), while in the latter it is 1.67 eV (increased by 1.17 eV). Δ${\mathit{E}}_{\mathit{v}}$ depends almost linearly on the Si layer thickness (0–2 ML) on an As-terminated GaAs. The result predicts a possibility of Δ${\mathit{E}}_{\mathit{v}}$ control at the (311)A GaAs/AlAs interface.

In this paper, the one-dimensional effective-mass equation is solved analytically by means of Hermitian functions as envelope functions under crossed magnetic and electric fields. By using the transfer-matrix method, we calculate the transmission coefficient for electrons tunneling through several different double-barrier resonant-tunneling structures (DBRTS’s). It is found that, for DBRTS’s with higher barriers and thinner width, the peak transmission coefficient ${\mathit{P}}_{\mathit{m}}$ is nearly equal to unity and is not off unity until the magnetic field B becomes much stronger. However, for DBRTS’s with much lower barriers and thicker width, ${\mathit{P}}_{\mathit{m}}$ is far off unity even when B is small. With the bias increasing, for a constant B, the resonant-tunneling peaks will shift to lower energy, and after reaching the unity value, the value of ${\mathit{P}}_{\mathit{m}}$ will decrease up to the disappearance of the peak. The J-V characteristics of DBRTS’s are also presented in which the peak markedly decreases and shifts to higher bias when B is increased.

When the barrier potential is high enough and/or its thickness large enough resonance tunneling becomes negligible in multiple-quantum-well structures. In this case a possible mechanism to connect the quantum wells is a Förster-type interwell coupling that originates from the long-range electron-hole exchange interaction. We theoretically investigate excitons in such a coupled multiple-quantum-well structure subject to an external light field, and show that the resulting polaritons behave in a qualitatively different way than those in uncoupled quantum-well structures. Of particular interest are interbranch polariton transitions and the redistribution of the radiative damping rate between different polariton branches. Numerical calculations are performed for a CdS-coupled double-quantum-well structure with dephasing by phonons taken into account.

The collective plasmon excitation spectrum of a one-dimensional electron gas confined in a GaAs quantum wire is calculated within the two-subband random-phase approximation. We calculate the collective-charge-density-excitation (CDE) dispersion assuming that both the ground and the first excited subband states are occupied by electrons. We find four different branches of CDE modes, two of which are intrasubband CDE’s and the other two intersubband CDE’s. The mode-coupling effect between them is investigated by introducing a parity-breaking asymmetry in the confinement potential. Our calculated mode dispersion is in excellent quantitative agreement with a recent inelastic-light-scattering experiment.

Acoustic phonon scattering of electrons and excitons in fully quantized systems based on GaAs/${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$As quantum wells is studied theoretically. We compare spatial quantization by lateral potentials (quantum dots) with Landau quantization by a magnetic field. In the Born approximation, the rate of electron scattering from the first excited state to the ground state of a parabolic quantum dot is one-half of the corresponding transition rate between Landau levels. When an increasing magnetic field is applied to a dot of sizable lateral confinement, the scattering rate first increases strongly, then exhibits pronounced oscillations, and finally decreases at high fields. Exciton relaxation by phonon emission is enhanced by a magnetic field in this system. The relaxation dynamics of quantum dot excitons strongly differs from that of magnetoexcitons in quantum wells.

The molecular-beam deposition of selenium onto ultrahigh-vacuum cleaved GaAs(110) surfaces and subsequent annealing leads to the formation of a thin, stable, Se-rich reacted layer with distinct structural and electronic properties. We perform ab initio calculations based on density-functional theory of the Se-deposited GaAs(110) surface for different coverages, FTHETA=1/4, 1/2, 1, and 3/2, and various structural models. In the FTHETA=1/4 case we give a detailed analysis of the total-energy surface of the Se/GaAs(110) system. The highest adsorption energy is found for Se atoms bound in an interchain bridging position. For higher coverages we discuss a variety of adsorption models and compare these with exchange reactions in terms of the thermodynamic stability. We observe a tendency for an exchange of As and Se atoms in the uppermost layers and subsequent segregation of As at the surface. For the energetically most favored structures, we compare the calculated band structures with data from soft-X-ray photoemission spectroscopy.

The tensorial characteristic of optical second-harmonic generation (SHG) has been used to study Si-O bonding arrangements at kinks on vicinal Si(111) surfaces with a polar angle of 4° from [111] along an azimuth rotated away from the high-symmetry [1¯ 1¯2] or [112¯] directions by different degrees. The onefold contribution to the SHG rotational anisotropy exhibits a component arising from Si-O bonds with a net orientation parallel to the step edges. This specific component reflects the kinked-step structure of the vicinal Si(111) surface. For a remote plasma-enhanced chemical-vapor-deposited oxide film, the kink-induced onefold component changes after a rapid thermal annealing at 1000 °C indicating a relaxation of Si-O bonds at the corners of steps. The results are discussed in comparison with a microscopic model of the oxidized misoriented surface.

Systematic trends in the properties of a linear split-gate heterojunction are studied by solving iteratively the Poisson and Schrödinger equations for different gate potentials and temperatures. A two-dimensional approximation is presented that is much simpler in the numerical implementation and that accurately reproduces all significant trends. In deriving this approximation, we provide a rigorous and quantitative basis for the formulation of models that assumes a two-dimensional character for the electron gas at the junction.

Transport of electrons and holes through an asymmetric GaAs-AlAs triple-barrier structure is investigated using time-resolved photoluminescence spectroscopy. Carriers are excited in the GaAs contact regions of the diode only, by choosing the excitation energy below the absorption edge of the quantum-well states. The intensity, half width, and temporal development of the photoluminescence signal of the quantum-well ground-state transitions are investigated by sweeping the applied bias through the tunneling resonances. Resonant tunneling of photocreated electrons is found to occur on a time scale shorter than about 50 ps, whereas resonant transfer of holes takes about 200 ps. Nonresonant tunneling times of both types of carriers, however, are on the order of 1 ns.

We investigate the optical properties of a narrow-band GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattice, whose miniband width is of the order of the exciton binding energy. We show that the electron miniband is locally destroyed by the Coulomb interaction with the photoexcited hole. We observed a modified Stark ladder, with both the positive and negative orders apearing above the zero-order transition. We find field-dependent temporal oscillations in four-wave-mixing experiments. We show that they are quantum beats between electron states in neighboring sites.

Transport through quantum point contacts (QPC’s) with various geometries close to those used experimentally is studied in the ballistic regime. We neglect impurity and temperature effects in an effort to understand the detailed status, as a function of geometry, of various popular approximations in this field, namely, the local adiabatic, the global adiabatic, and the diagonal approximation. By appeal to a combination of two well-known procedures, we are in a position to study, by exact numerical solution, continuous but rapidly varying geometries of QPC’s. Our calculations show that the diagonal approximation is unreliable (as found previously by Castaño and Kirczenow). The local adiabatic approximation (Glazman et al.) can serve as a rough estimate, but significantly underestimates the sharpness of conductance steps. The global adiabatic approximation is remarkably successful, in spite of strong mode mixing. However, this approximation can also significantly underestimate the degree of conductance quantization. The physical reason for this is given. Resonance effects in continuous QPC’s are found that confirm our physical interpretation.

We report full-scale pseudopotential calculations concerning the difference frequency mixing in Si-SiGe quantum-well structures. We predict the line shape, the magnitude, and the microscopic origin of the second-order susceptibility. In particular, we establish a link between the strength of the spectra and the degree of asymmetry in the quantum-well system.

Excitonic superradiance has been predicted to induce a rapid decay rate of coherently excited excitons in thin quantum wells and microspheres. In this paper we revisit this problem and derive the enhancement factor of the radiative decay due to superradiant effects. In a GaAs quantum well, the excitonic superradiant radiative decay can be roughly 320 times faster than the decay of a free electron-hole pair. The exciton decay rate depends strongly on the lateral size of the wave function when the size is smaller than the inverse of the wave vector of the emitted light. In a GaAs quantum well this corresponds to a distance of ≊365 Å. Therefore, it is predicted that localization and elastic phonon scattering may lead to decay rates substantially smaller than that predicted for an ideal quantum well. It is also shown that if the quantum well is placed inside a microcavity, the decay rate can be enhanced even further since the cavity modifies the available density of radiation states in a way that matches the superradiant emission. It is predicted that in a matched dielectric cavity the decay rate can be enhanced by an additional factor of 100, but that inhomogeneous broadening may in practice limit the possible enhancement.