The tunneling of electrons from a semiconductor surface to a metal tip, across a vacuum gap, is influenced by two image interactions: an attractive image potential in the vacuum region, which lowers the apparent tunneling barrier, and a repulsive image potential in the semiconductor interior, which raises it for conduction-band electrons. We report on detailed calculations of tunneling currents and apparent barrier heights for a model metal-vacuum-semiconductor junction which utilize semiclassical dielectric functions to compute the image potential in all three regions. The effect of image forces is found to be small compared to that of either the vacuum barrier or tip-induced band bending. In particular, the image-induced barrier in the semiconductor has only a minor influence on either the apparent barrier height or the shape of current-voltage characteristics, both of which are routinely measured in scanning-tunneling-microscopy experiments. This finding is explained by a qualitative WKB analysis and several simple arguments.

In this paper we present a theory that describes the influence of the Coulomb interaction between electrons and holes on the optical spectra of flat quantum dots within the envelope-function formalism. Starting from a nonlocal Elliott-like formula, absorption and luminescence characteristics are traced back to properties of two-particle wave functions and energies, which are solutions of the corresponding Schrödinger equation for an electron-hole pair under the influence of the Coulomb attraction and confinement potentials, determined by the spatial variation of the band edges of the considered microstructure. We present a complete numerical solution of the two-particle problem for flat quantum dots, i.e., disks for which the size quantization in the growth direction is much stronger than that in the perpendicular plane. The resulting theoretical line shapes are compared with luminescence spectra obtained recently for quantum dots fabricated by laser-induced thermal cation interdiffusion in quantum-well structures.

Computational efficiency for the simulation of bulk crystals and surfaces is highly desirable. In an effort to study semiconductor crystals, we present a self-consistent treatment for the simulation of silicon crystals and surfaces based on the combination of a siligen model and a semiempirical Hamiltonian method. An artificial atom called siligen is introduced for the application of the semiempirical method to finite-size silicon clusters. The calculated average bond energies for the saturated silicon clusters are between 2.045 and 2.568 eV, compared to the measured value of 2.31 eV. A simulated bulk silicon surface using siligens is introduced in order to examine variation of the bond strength between fluorine atoms and the simulated silicon (111) surface. It is found that bond strength computed from the simulated surface, with siligens, rapidly converges to a saturated limit as the number of surface layers increases, while a pure silicon (111) surface without siligens yields no satisfactory convergence.

A quantum-mechanical calculation of the carrier densities, electron and hole quasi-Fermi-levels, and various radiative decay times in GaAs-(Ga,Al)As quantum wells is performed, under steady-state excitation conditions, as functions of the cw laser intensity, temperature, well widths, and acceptor distribution in the well. We consider the radiative recombination of electrons with free holes and with holes bound at neutral acceptors. Our calculations—which have no free parameters—are in quantitative agreement in the intermediate laser-intensity regime at T=300 K with the results by Ding et al. [Appl. Phys. Lett. 60, 2051 (1992)], who obtained the carrier density for multiple asymmetric coupled quantum wells through a fitting procedure that reproduced the total experimental photoluminescence intensity. Results for the carrier-dependent e-h recombination decay time are in good agreement with experimental data by Bongiovanni and Staehli [Phys. Rev. B 46, 9861 (1992)].

The quasi-three-dimensional electron gas found in wide n-type parabolic ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells has been studied through photoluminescence spectroscopy in perpendicular and in-plane magnetic fields. The conduction-band subband structure composed of several occupied closely spaced levels (δE≊1 meV) can be clearly identified from the spectra, aided by the sharpening of the luminescence emission into narrow peaks by Fermi-edge singularity effects (many-electron–one-hole exciton). The luminescence behavior in magnetic fields shows clearly the relative importance of the following energies in the problem: the Fermi energy, the subband spacing, the cyclotron energy, and the Coulomb perturbation by the photoholes. In the limit where the cyclotron energy dominates, an accurate measurement of the thickness of the electron slab is spectroscopically determined. A magnetic-field anomaly in the quantum limit near filling factor ν=2/3 has been observed.

We have observed strong peak shifts in the magnetotunneling I(V,${\mathit{B}}_{\mathrm{\perp }}$) characteristics of strained p-Si/${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ double-barrier resonant tunneling structures as the transverse field ${\mathit{B}}_{\mathrm{\perp }}$ orientation is rotated in the sample plane. These peak shifts map out the in-plane anisotropy of the light- and heavy-hole subbands in the Si-Ge well. At large in-plane wave vectors, the heavy- and light-hole E(${\mathit{k}}_{\mathrm{\perp }}$) contours are strongly crimped: the heavy-hole E(${\mathit{k}}_{\mathrm{\perp }}$) is dilated in the 〈100〉 and compressed in the 〈110〉 directions, while the light-hole anisotropy is rotated by 45° with respect to that of the heavy hole. The heavy-hole peak shifts are well described by a simple nonparabolic band model, from which we extract an anisotropic nonparabolicity factor that varies by more than a factor of 2 as a function of crystallographic direction.

This paper presents a solution to the complete Hartree-Fock equation for n-type semiconductor heterojunction quantum wells which includes the exchange term of the Coulomb interaction. It presents calculations that show the contributions of the direct and exchange terms to the linear absorption. It demonstrates that including the exchange term can improve theoretical predictions of experimentally measured intersubband absorption peaks.

We develop a formalism to study the role of local defects in tight-binding systems in the presence of a strong external ac electric field. It is found that the appearance and disappearance of localized states, as well as their localization lengths, can be controlled by the amplitude or frequency of the driving field. The theory is illustrated by numerical model calculations for imperfect semiconductor superlattices in far-infrared laser fields.

The ratio of the phonon peak current to the main peak current in double-barrier resonant-tunneling structures is significantly enhanced by barrier asymmetry. Previously, using the Keldysh formalism, we derived analytical expressions, valid in the zero-temperature, high-bias regime, which explained this effect. We now provide analytical expressions valid for finite temperature and bias obtained from (i) an intuitive derivation using a rate equation approach and (ii) a more general derivation using the Keldysh formalism. The results of the two different approaches are shown to be essentially identical for the experimental device parameters. The finite temperature expressions shed light on the effect of the Pauli exclusion factors in the contacts on the current. In particular, we show that in a transmission formulation, the transmission coefficients, T(ɛ,ɛ’), are themselves functions of the Fermi factors in the contacts.

High-resolution ultraviolet photoemission spectra portray substantial variations in Ga 3d core-level line shapes, which cannot be resolved by standard x-ray-photoemission-spectroscopy techniques. They indicate that the GaAs(100)/AlAs heterointerface grown by molecular-beam epitaxy possesses a complex microstructure that is severely influenced by Ga segregation into the AlAs overlayer. The reported segregation process is highly temperature dependent and is characterized by a segregation energy of ${\mathit{E}}_{\mathit{s}}$=0.15±0.1 eV at a substrate temperature of 630 °C. Si and Ge interlayers not only induce significant changes in the interfacial electronic structure but also effectively suppress the observed Ga segregation. Temperature-dependent measurements indicate that these structural changes do not cause the observed variations of core-level binding-energy differences and, thus, Δ${\mathit{E}}_{\mathit{v}}$ variations induced by group-IV interlayers.

We report electron-spin-resonance measurements on zinc-blende GaN. The observed resonance has an isotropic g value of 1.9533±0.0008 independent of temperature, a Lorentzian line shape, and a linewidth (18 G at 10 K) which depends on temperature. The spin-lattice relaxation time at 10 K was estimated to be ${\mathit{T}}_{1\mathit{e}}$=(6±2)×${10}^{\mathrm{-}5}$ sec. Using a five-band model a g value consistent with the experimental results was obtained and a conduction-electron effective mass ${\mathit{m}}^{\mathrm{*}}$/${\mathit{m}}_0$=0.15±0.01 was calculated. The observed signal, together with conductivity data, was attributed to nonlocalized electrons in a band of autodoping centers and in the conduction band.

The Aharonov-Bohm (AB) interference patterns in ring-shaped conductors are usually dominated by random features. The amplitude of the oscillations is random from sample to sample and from point to point on the magnetic field axis owing to random scattering of the electron trajectories by impurities within the wires. We report experiments on devices made with wet etching and global gates, which have shown major progress towards removing the random features. In loops that exhibit ballistic conductance plateaus and cylotron orbit trapping at 4.2 K, the random pattern of AB oscillations (observed for T<0.1 K) can be replaced by a much more ordered one—especially if only a few transverse modes are populated in the ring. The amplitude and shape of the oscillation envelope function change systematically as subbands are populated in the wires forming the loops. Mechanisms governing the AB effect in the ballistic regime are discussed. Correlation has been found between the G(${\mathit{V}}_{\mathit{g}}$,B=0) staircase and the beating period of the envelope functions. Quantum oscillations in G(${\mathit{V}}_{\mathit{g}}$,B=0) are consistent with direct interference of paths of unequal length. Both the correlations and the quantum oscillations in gate voltage are signatures of ballistic transport.

Self-consistent calculations of the potential, wave functions, and subband structure of modulation-doped multiple-quantum wires are presented. The model parameters are chosen so as to descibe recently fabricated quantum wires where clear evidence of one-dimensional behavior has been found. Quite good agreement with experimental data is obtained when a distribution of negatively charged centers (presumably induced by the ion milling in the real samples) is included. Exchange and correlation corrections on the Hartree approximation are included within the local-density formalism and found to renormalize the intersubband splittings by a fraction of a meV.

We study a system consisting of magnetic tunneling barriers; in particular we have studied the following structures: magnetic quantum steps, barriers, and magnetic wells. The energy spectrum (bound and scattered states) for these systems is obtained and the nature of the states is discussed.

We present a detailed analysis of carrier-induced modifications on the spatial band diagrams of In-Ga-As/In-Ga-As-P separate-confinement multiple-quantum-well structures and its consequences on optical properties. In the high-carrier-density regime, we observe experimentally a second peak in the spectra of the optical gain. This can be understood by a buildup of a space charge and band bending which is caused by a spatial separation between electrons and holes due to the different density of states of the conduction and valence bands. This band bending causes a localization of barrier conduction band states in the quantum-well region and an enhancement of the overlap integral of those states with quantized heavy hole states giving rise to substantial modifications in the optical matrix elements. These modified matrix elements are the origin of the new class of optical transitions observed in the optical gain spectra.

The ab initio quantum chemistry program sckgnmol-89 was used to study the cleavage of silicon and germanium clusters in the diamond structure. The potential energy of stretching and shearing planes with three bonds per atom (glide planes) increases much faster than for planes with single bonds (shuffle planes). The cleavage process is discussed and it is shown how glide-plane cleavage can occur, with consequences for surface structure models.

Based on the Chern-Simons gauge theory of the planar electrons together with the lowest-Landau-level projection, we analyze the dynamical mechanism of a Josephson effect predicted recently in a double-layer quantum Hall (QH) system. It is shown that a Goldstone mode exists in a certain double-layer QH system and that the mode induces the Josephson effect through a weak interlayer tunneling.

We calculate the electrical and heat currents flowing through a narrow, mesoscopic, normal-metal–superconductor (NS) junction containing a single point impurity. The NS junction exhibits large subgap conduction steps in its current-voltage (I-V) relation when the impurity is located in the normal metal. Our new subgap step appears in the I-V characteristic whenever an additional quasiparticle is trapped between the impurity and NS interface. Locating the impurity inside the superconductor produces both an ‘‘excess current’’ and oscillations in the I-V characteristic analogous to the Tomasch effect. A maximum excess current of 2eΔ/h, slightly less than the 8eΔ/3h found for a ballistic NS junction obtains when the impurity is located a few coherence lengths inside the superconductor.

We utilize a geometric argument to determine the effects of boundary scattering on the carrier mean free path in samples of various cross sections. Analytic expressions for samples with rectangular and circular cross sections are obtained. We also outline a method for incorporating these results into calculations of the thermal conductivity.

Low-temperature transport in disordered conductors exhibits a variety of fascinating quantum-mechanical interference effects associated with the phenomenon of weak localization. Such effects are typically isolated and probed by virtue of their sensitivity to applied homogeneous magnetic fields, which introduce Aharonov-Bohm phase factors into quantum-mechanical amplitudes. Analogous interference effects have been proposed in the context of the quantum transport of (possibly electrically neutral) particles with spin in the presence of inhomogeneous magnetic fields, which have the effect of introducing Berry phases. Thus, the possibility is raised of isolating and probing quantum interference effects through their sensitivity to the inhomogeneity of applied magnetic fields. In this appear we develop an approach to the study of quantum transport in disordered conductors in the presence of almost arbitrarily inhomogeneous magnetic fields, which is based on diagrammatic and semiclassical path-integral techniques and a subsequent adiabatic approximation. We illustrate these ideas with applications to three examples: anomalous weak-field magnetoconductance, conductance oscillations in mesoscopic multiply connected structures, and sample-dependent mesoscopic conductance fluctuations. Among other things, we find that while in the context of the disorder-averaged conductance it is accurate to regard systems as being composed of two independent subsystems (having spins aligned or antialigned with the local external magnetic field) a more interesting and refined structure emerges in the context of conductance fluctuations.