The thermal mismatch strain in stacks containing PbSe, ${\mathrm{BaF}}_2$, and/or ${\mathrm{CaF}}_2$ on Si(111) substrates is relieved by the glide of dislocations in the principal 〈110〉{100} glide system. The strain in the ${\mathrm{BaF}}_2$(111) and ${\mathrm{CaF}}_2$(111) buffer layers is relaxed at room temperature regardless of whether they form the top layer in the stack or are overgrown by other layers. PbSe (as well as ${\mathrm{Pb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$Se and PbTe) top layers are capable of relieving the strain induced by the thermal-expansion mismatch even at 77 K, and after many temperature cycles between room temperature and 77 K. Even after 1400 such cycles, plastic relaxation still occurs on each cycle. The x-ray rocking curves, typically 150–190 arc sec wide, do not broaden on cycling. The total cumulative plastic deformation of the layer corresponds to as much as 400%. After the first few thermal cycles, no new thermal-strain-relieving dislocations are created, but the existing ones move back and forth on the same atomic glide planes with each cycle.

The introduction of a surfactant changes the growth in the Si(111)/Ge system from islanding to a continuous film. A Sb monolayer floats at the growth front without detectable incorporation in the growing film. The surfactant strongly influences the growth kinetics and prevents intermixing or indiffusion of Ge or Si. Up to 8 ML thickness the Ge film is completely strained and pseudomorphic; for thicker films the strain due to the 4.2% misfit is relieved by the generation of defects, which are finally all confined in a dislocation network at the interface. Low-defect, fully relaxed epitaxial Ge films of arbitrary thickness can be grown. Similarly, low-defect relaxed Si can be grown on Ge(111). Medium-energy ion scattering, high-resolution transmission electron microscopy, x-ray-photoelectron spectroscopy, and Raman scattering show that the crystal quality of these Ge films is excellent.

The transport properties of a quantum channel with a periodically modulated conducting width are investigated. A single constriction has a quantized conductance and its conduction plateaus have finite rises. For a short channel with just a few periods of modulation, conductance is no longer quantized. For a long channel with many periods of modulation, the quantized conductance characteristic of a simple quantum point contact is recovered, but the index of the quantization plateaus is no longer a monotonically increasing function. Rather, the conductance steps up and down with energy. We calculate the energy band structure of the corresponding infinite modulated channel. Comparison between the quantized conductance and band structure shows a one-to-one correspondence between the plateau index and the number of Bloch bands with positive group velocity at a given energy. The nonmonotonic conductance quantization for a long channel persists in the presence of an applied magnetic field, as does the direct correspondence between the plateau index and the number of energy bands. In the high-field regime this can be interpreted as selective resonant backscattering of edge states, and lends to nonmonotonic plateaus in the Hall resistance in the integer quantum Hall effect.

The behavior of Pb overlayers deposited on Si(100)-(2×1) at room temperature was investigated by scanning-tunneling microscopy and low-energy electron microscopy. Surface reconstructions of (2×2), c(4×8), (2×1), and c(4×4) were observed as Pb coverage increased from 0.5 to 1.5 monolayers. After the appearance of the c(4×4) phase, Pb islands with threefold-symmetric (111) orientation were observed on the twofold- or fourfold-symmetric Si(100) surface. The transformation of the island shape during growth can be broadly categorized as hexagonal→triangular→hexagonal. An internal structure of stepped-down depression was formed on all islands. As deposition continued, the depression was gradually filled by a step-flow growth mechanism. Our observations suggest that the initial growth of the islands is dominated by diffusion of Pb atoms on the substrate to the island edges; later, when the islands become sufficiently large, growth is determined mainly by atom diffusion on the Pb(111) islands.

In this paper we study the energy spectrum of a two-dimensional electron gas (2DEG) in a two-dimensional periodic magnetic field. Both a square magnetic lattice and a triangular one are considered. We consider the general case where the magnetic field in a cell can be of any shape. A general feature of the band structure is bandwidth oscillation as a function of the Landau index. A triangular magnetic lattice on a 2DEG can be realized by the vortex lattice of a superconductor film coated on top of a heterojunction. Our calculation indicates a way of relating the energy spectrum of the 2DEG to the vortex structure. We have also derived conditions under which the effects of a weak magnetic modulation, periodic or not, may be reproduced by an electric potential modulation, and vice versa.

Spin-flip scattering of oriented carriers between Zeeman-split quantum-confined states is observed directly by femtosecond-resolved photoluminescence spectroscopy in magnetically coupled double quantum wells. Polarization measurements reveal that the spin dynamics are determined by the initial carrier energy and spin orientation with respect to the Zeeman separation of the levels. Studies as a function of applied field and barrier thickness show that spin equilibration is remarkably sensitive to the magnetic environment.

A 300-nm-diameter gate is used to introduce an antidot or artificial impurity into a quantum wire defined in an ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs two-dimensional electron gas. At low magnetic fields, geometry-induced quantum interference effects are observed, while at higher fields adiabatic edge-state transport is established. In the transitional regime, conductance resonances due to magnetically bound impurity states exhibit distinct characteristics including beating, sharp period changes, and spin splitting. An asymmetry is observed between the resonances observed as a function of magnetic field and gate voltage. The results are explained by a model based on an interedge-state coupling mechanism.

The one-dimensional (1D) density of states (DOS) is calculated self-consistently for double-barrier quantum-well structures in the presence of a perpendicular magnetic field. With the application of a magnetic field, there is a redistribution of the 1D DOS along energy and distance. Contrary to this, an applied electric field results in redistribution mainly along the energy axis. The inclusion of the phase-breaking scattering mechanism diminishes the first peak of the 1D DOS along the energy axis. The total number of states is constant, irrespective of any external (electric or magnetic) fields or phase-breaking scattering.

We present a comprehensive investigation of the dynamics of resonantly excited nonthermal excitons in high-quality GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple-quantum-well structures on picosecond time scales. The dynamics was investigated using the luminescence upconversion technique with two independently tunable, synchronized dye lasers, which allowed measurements of the time evolution of polarized resonant luminescence with 4-ps time resolution. We show that the evolution of excitons from the initial nonthermal distribution to the thermal regime is determined by three different physical processes: (1) the enhanced radiative recombination of the metastable two-dimensional exciton polaritons, (2) the spin relaxation of excitons, and (3) the momentum relaxation of excitons. We also show that these three processes have comparable rates, so that a unified model accounting for all important processes is essential for a correct analysis of the experimental results. Using such a unified model, we have determined the rates of these processes contributing to the initial relaxation of excitons as a function of quantum-well width, temperature, and applied electric field. Quantum confinement strongly influences the radiative recombination and spin relaxation of excitons, and our study provides significant insights into these processes in quantum wells. The measured radiative recombination rate is about a factor of 2 smaller than calculated theoretically. The electric field reduces the electron-hole overlap and hence reduces the spin-relaxation rate of excitons between the optically allowed ‖±1〉 states. The measured variation is in good qualitative agreement with a recent theory, but somewhat slower than predicted by the theory.

An empirical tight-binding (ETB) approach combined with a Green’s-function (GF) method has been employed to investigate the electronic structure of the Si(111) and arsenic-passivated Si(111) surfaces. An accurate silicon band structure has first been developed keeping up to second-nearest-neighbor terms in the Hamiltonian. The ETB-GF approach has allowed the calculation of the local density of states (LDOS) at various atomic layers of the semi-infinite samples. It has also been possible to calculate the contribution of the individual atomic orbitals to the LDOS. The results obtained for the arsenic-passivated Si surface provide the position and strength of localized surface states as well as the density of the bulk states in the valence band, the band gap, and the conduction band. The calculated localized states reproduce the existing experimental data and other partial calculations. Furthermore, this calculation predicts a localized state in the conduction band, which has so far not been studied experimentally.

The surface electronic structure of epitaxial √3 × √3 R30° ${\mathrm{ErSi}}_{1.7}$ layers on Si(111) has been studied by high-resolution angle-resolved ultraviolet photoemission spectroscopy. Typical surface states or resonances are unambiguously identified and their band dispersions mapped along the high-symmetry Γ¯ M¯, Γ¯ K¯, and K¯ M¯ lines of the (1×1) surface Brillouin zone. These data are compared to the band structure of two-dimensional p(1×1) Er silicide extensively studied in previous works. It is found that the prominent surface bands observed in the 0–3-eV binding-energy range can be readily derived from the p(1×1) surface-silicide bands folded back into the reduced √3 × √3 zone. This indicates that the bulk silicide is also terminated with a buckled Si layer without vacancies, quite similar to the surface-silicide termination. In particular, specific surface bands reflect the doubly (essentially) occupied dangling bonds and the back bonds of the buckled Si top layer.

We investigate a number of issues related to the application of the envelope-function method to calculate confined-state energies and subband structure in quantum-well structures. We first consider zone-center confined-state energies and show how the explicit elimination of spurious solutions from the envelope-function band structure leads to a slightly modified form of the standard result through which the conduction-band confined-state energies are calculated using a one-band model and an energy-dependent effective mass. We show that the effects of nonparabolicity can be predicted directly from the bulk band structure in an infinitely deep quantum well, and demonstrate how the bulk band structure can also be used to predict the errors in calculated confinement energies in wells of finite depth. The correct choice of boundary conditions still remains controversial for the calculation of valence-subband structure using the Luttinger-Kohn Hamiltonian. We compare the valence-band structure calculated with the lowest conduction band included either explicitly or treated as a remote band, using perturbation theory. We demonstrate that the boundary conditions recently derived by Burt and Foreman are correct. Finally, we compare the valence-band structure calculated using the 4×4 and 6×6 Luttinger-Kohn Hamiltonians. We show how the warping of the highest valence band is markedly different at both intermediate and large wave vectors when the spin-split-off band is included. The use of the axial model to calculate valence-band density of states is therefore questionable with the 6×6 Hamiltonian. The calculated warping is very sensitive to the values of the Luttinger γ parameters used, indicating the importance of investing more effort to determine these parameters accurately.

We report on ultrahigh-vacuum scanning tunneling microscopy of submonolayers of ${\mathrm{C}}_{60}$ molecules adsorbed on a Si(111)-(7×7) surface and the statistics of their bonding sites. The images reveal that as the submonolayer coverage increases, the adsorbate-adsorbate interaction becomes larger than that of the adsorbate-substrate interaction. For a full monolayer coverage of adsorbates, the adsorbate-adsorbate interaction becomes strong enough and they form two ordered structural phases registered with those pinned on the substrate corner holes. This result is in contrast to a previous report [Phys. Rev. Lett. 70, 1850 (1993)] stating that several monolayers of adsorbates are required for the formation of the ordered structural phases.

The development of metal character in the surface region of K-covered Si(100) is monitored via the appearance of a Fermi edge in photoemission spectra recorded at low photon energies (ħω<6.0 eV). The Fermi-edge intensity shows an onset at a coverage close to that required to obtain the work-function minimum typical for this adsorption system. For clean, cooled (∼70 K) Si(100) substrate (n-type, 2 Ω cm), the light source used for photoemission nearly eliminates the surface band bending, but upon K deposition the band bending reappears and then has nearly the same magnitude as for the uncovered Si(100) surface left in darkness. The changeover from flat bands to full band bending occurs over a narrow monolayer coverage interval at coverages somewhat higher than required for the observation of a Fermi edge. Upon continued deposition a photoemission peak is observed which is assigned to a quantum-well-type resonance formed by alkali-metal valence electrons propagating back and forth in the overlayer between the vacuum barrier and the overlayer-substrate interface.

The electronic properties of the (111) Si/${\mathrm{SiO}}_2$ interface and, in particular, one of its most characteristic defects, known as the ${\mathit{P}}_{\mathit{b}}$ center, are studied. To model the interface, we consider ${\mathrm{SiO}}_2$ Bethe lattices attached to a semi-infinite silicon crystal. Our study includes changes in defect-state energies and charge transfer across the interface towards nearest-neighbor oxygen atoms. Green’s function techniques were used to consider electron-electron correlations within an extended Hubbard-like Hamiltonian and under an unrestricted spin Hartree-Fock approximation. We believe that the inclusion of the effect of the parallel correlations results in a satisfactory agreement between our results and the available experimental data.

We study the inter-edge-channel scattering length in two-dimensional electron systems in the presence of far-infrared radiation by solving the time-dependent Schrödinger equation exactly. A coupled model is proposed for treating quantum transport through edge and bulk currents. The calculated longitudinal resistances are compared with a recent experiment [(Phys. Rev. Lett. 70, 651 (1993)].

Low-temperature (T<150 K) reflection high-energy electron-diffraction (RHEED) oscillations during the growth of ultrathin films suggest the presence of nonzero adatom mobility. In most systems thermal diffusion cannot account for the observed oscillations, and the origin of the oscillations is still unclear. Experiments on Ag/Si(111) at 150 K demonstrate the absence of thermal diffusion based on the scaling of the RHEED intensity at different flux rates (i.e., the diffracted intensity is only a function of the total deposited amount independent of the flux rate). We have performed molecular-dynamics simulations to understand possible low-temperature mechanisms responsible for the oscillations. Classical two- and three-body Si potentials were used together with an adatom mass that is 3.84 times the Si mass to account for the Ag/Si mass ratio. Results indicate that the landing site of the adatom is important in predicting the subsequent lateral motion of the adatom. A majority (about 75%) of the deposited atoms adsorb within a lattice spacing of the landing site. However, a fraction (25%) of the deposited atoms eventually adsorb a few lattice spacings away from the landing site before equilibration to the substrate, but it is not clear if this is sufficient to account for the oscillations. The energy transfer to the substrate is much less efficient than in fcc/fcc systems, because of the stiffness of the Si-Si potentials.

The DMol first-principles molecular-cluster total-energy and atomic-force approach is used to study the Ge dimer on the Si(100) surface with clusters containing up to 85 atoms. It is shown that the Ge asymmetric dimer is energetically more stable than the symmetric dimer due to charge transfer from a down dimer atom to an up atom even though this accompanies a weakening of the covalent bonding between the dimer atoms. By calculating the forces on atoms in the first three Si layers and minimizing the total energy of the cluster, the optimized atomic geometry is obtained. The calculated dimer height displacement of this buckled dimer is shown to be 0.60 Å and the tilt angle is 14.2°. The magnitude of this buckling lies between the range that was calculated for clean Si(100)2×1 and Ge(100)2×1 using the same approach, which agrees with the experimental measurements for those surfaces.

We study the effect of a magnetic field applied along the growth axis on exciton binding energies in dielectric quantum-well structures, in which the dielectric constant of the confining barriers is significantly smaller than that of the well material. The anisotropic electron-hole Coulomb interaction potential is obtained analytically by solving the Poisson equation in the layered geometry of quantum wells. Confinement is provided by the image charge distribution arising from the mismatch of dielectric constants at the interfaces, in addition to that of the quantum-well potential and the applied magnetic field. Exciton binding energies are calculated using the Gaussian-type orbital expansion method. Significantly enhanced binding energies are obtained for the excitons in various dielectric quantum-well structures and their behavior in a magnetic field is discussed.

The infrared-absorption spectra of acceptors confined in the center of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells in the presence of an external magnetic field have been calculated. The calculations are based on a four-band effective-mass theory in which the valence-band mixing as well as the mismatch of the band parameters and the dielectric constants between well and barrier materials have been taken into account. The dipole transition rule is assumed for the infrared absorption. The oscillator strengths of allowed transitions between the acceptor ground states and excited states, corresponding to the transitions G, D, and C in bulk GaAs, are calculated for different well widths in the range 50–200 Å with an external magnetic field up to 10 T. The oscillator strength of these infrared acceptor transitions exhibits a strong polarization dependence.