An analytic model is proposed for the valence-subband structure in a compressively strained quantum well which includes the effects of a finite barrier potential, valence-band anisotropy, and subband nonparabolicity. The solutions obtained for the zone-center effective mass and nonparabolicity factor are valid for both compressive and tensile strain, but the model gives a good approximation to the true band structure only in the case of compressive strain, where the nonparabolicity effects are relatively weak. For moderate compressive strain, the model provides a very fast, accurate representation of the subband structure within a few kT of the valence-band edge, which will be very useful in optoelectronic device design and simulation. The most important limitation to the model is its neglect of the strain-induced coupling to the spin-orbit split-off band, which becomes significant for large strain. Calculations performed on ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs quantum wells predict that both the effective mass and nonparabolicity decrease with increasing quantum-well width or compressive strain.

We present a variational calculation of the envelope wave function of an exciton inside a cube. We show that all sixfold integrals can be reduced to threefold integrals with a change of variables which shortens tremendously the computer calculations. Then (i) we calculate numerically the Bohr radius, the correlation energy (the difference between the energy of the exciton and of the uncorrelated electron-hole pair), the exchange energy, and the oscillator strength of the exciton for any value of the cube side and (ii) we obtain asymptotic expression of these four quantities for a large cube. This allows one to discern the difference between a large but finite cubic semiconductor and an infinite semiconductor. We show that the correlation energy tends to its bulk limit value in infinite volume as the inverse of the cube side while the effective Bohr radius and the exchange energy tend to their limiting value as the inverse of the square of the side. Finally our results are applied to porous silicon: the experimental exchange energy 10 meV corresponds to a cube of side 28 Å which is quite reasonable in this compound.

Four-terminal resistances in parallel electron-waveguide structures coupled by a window region are calculated using classical- and quantum-mechanical methods. An electron incident through one of the waveguides can travel into the other in the quantum ballistic regime. The transmission coefficients into the ends of the second waveguide oscillate as one of the sample parameters, such as the separation between the waveguides or the Fermi energy, is varied, leading to quasiregular modulations in the transfer resistance. Several different mechanisms are responsible for the oscillations including (i) classical specular reflections from the side wall of the window region, (ii) depopulation of quasi-one-dimensional subbands in the waveguides, and (iii) Fabry-Perot-type quantum interference in the window region. We propose that these scenarios of the oscillations can be distinguished experimentally by their peculiar dependence on the system parameters.

The electron yield from ${\mathrm{SiO}}_2$ films on Si was measured as a function of oxide thickness and ion energy for 150–300 keV ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{+}$, ${\mathrm{N}}^{+}$, ${\mathrm{Ne}}^{+}$, ${\mathrm{Ar}}^{+}$, ${\mathrm{Kr}}^{+}$, and ${\mathrm{Xe}}^{+}$ ions. For oxide films thicker than 200 Å, the electron yield was found to be approximately independent of oxide thickness. The electron yield from the thick oxide increased as a function of the energy deposited D in electronic excitations with approximately the form ${\mathit{D}}^{0.65}$ and not D as expected from theory. Close to the ${\mathrm{SiO}}_2$/Si interface, unexpected variations in the electron yield with oxide thickness, dependent on ion mass and energy, were found. Experiments were performed to investigate whether or not the oxide surface was charged by the ion beam, but such effects could not be observed. However, a satisfactory interpretation of the data could be obtained with a model previously suggested for explaining the dependence of the electron yield on the angle of ion incidence. In this model, the positive charge left behind in the oxide by the liberated electrons within the electron cascades of individual ions, causes the probability of escape of electrons to decrease with increasing electron yield.

The properties of a confined electron interacting with both the confined longitudinal optical (LO) and interface optical (IO) phonons in a quantum well (QW) are investigated. By using a modified Lee-Low-Pines variational method, an analytical expression for the polaron ground-state energy is obtained. Numerical calculation is performed for a GaAs/AlAs QW as a typical case. The results show that only two symmetric IO-phonon modes as well as the confined LO-phonon mode contribute to the energies of the polaron. It is also found that one of the symmetric IO-phonon modes is more important, its contribution to the energies is much larger than the others when the QW width is smaller than the polaron radius.

The magnetocapacitive response of a double-barrier structure (DBS), biased beyond resonances, has been employed to determine the density of states (DOS) of the two-dimensional electron gas residing in the accumulation layer on the incident side of the DBS. An adequate procedure is developed to compare the model calculation of the magnetocapacitance with the experimental C vs B curves measured at different temperatures and biases. The results show that the fitting is not only self-consistent but also remarkably good even in well-defined quantum Hall regimes. As a result, information about the DOS in strong magnetic fields could reliably be extracted.

Band-edge optical transitions in GaAs/AlAs superlattices with different miniband widths are investigated by low-temperature-photocurrent, photoluminescence-excitation, and reflectance spectroscopy. Direct spectroscopic evidence is given for excitonic effects at saddle-type critical points which are related to ${\mathit{M}}_1$ Van Hove singularities at the upper miniband edges, for both heavy-hole and light-hole excitons. Excellent agreement is obtained between the observed transition energies and the miniband widths deduced from Kronig-Penney model calculations.

Optical-absorption spectra have been measured near the indirect-gap edge for quasi-one-dimensional ${\mathrm{ZrS}}_3$ single crystals with chains parallel to the b axis. The temperature evolution of polarized spectra allows the identification of two indirect parallel gaps (E∥b) at 2.165 and 2.062 eV with an exciton binding energy of 10 meV and one indirect perpendicular gap (E⊥b) at 2.091 eV with an exciton binding energy of 31 meV. While the band-structure model based on the molecular-orbital scheme of the (${\mathrm{S}}_2$${)}^{2\mathrm{-}}$ molecule is confirmed, the shape analysis of different structures of the absorption coefficient indicates clearly a two-dimensional indirect-band-gap character.

Different possible adsorption sites of sodium atoms on a gallium arsenide surface have been investigated using ab initio self-consistent unrestricted Hartree-Fock total-energy cluster calculations with Hay-Wadt effective core potentials. The effects of electron correlation have been included by invoking the concepts of many-body perturbation theory and are found to be highly significant. We find that the Na-atom adsorption at a site modeled with an ${\mathrm{NaGa}}_5$${\mathrm{As}}_4$${\mathrm{H}}_{12}$ cluster is most favored energetically followed by Na adsorption at the site modeled with the ${\mathrm{NaGa}}_4$${\mathrm{As}}_5$${\mathrm{H}}_{12}$ cluster. The effects of charge transfer from Na to the GaAs surface as also possibilities of metallization are also analyzed and discussed.

The magnetoresistance of two-dimensional electrons in a periodic potential is studied. The Landau levels are broadened by one-dimensional modulation, and are further split into a Hofstadter-like spectrum by two-dimensional modulation. The recently observed Weiss oscillation manifests the oscillatory bandwidth of broadened Landau levels, where the minima appear at the flat-band condition (zero bandwidth) for one-dimensional modulation. However the flat bands seem to give the maxima for two-dimensional modulation. In this paper, the conductivity for both one- and two-dimensional modulations has been obtained analytically. It is shown that the decrease of the scattering conductivity is enhanced by the band splitting, while the band conductivity is reduced significantly. Consequently, the scattering conductivity decreases faster than the increase of the band conductivity as the Landau level is broadened from the flat band. Therefore the total conductivity shows maximum at flat bands, in contrast to the one-dimensional case. Moreover a numerical study has been made by using the Thouless number method. A dramatic difference is observed between one- and two-dimensional modulations. This can be explained in the same way as the result of band splitting.

Room-temperature adsorption of disilane (${\mathrm{Si}}_2$${\mathrm{H}}_6$) on Ge(111)-c(2×8) and subsequent thermal reactions and desorption at elevated temperatures were studied using scanning tunneling microscopy and core-level photoemission. The initial adsorption results in the formation of various surface radicals, and the reacted areas on the surface grow laterally for increasing exposures. The sticking coefficient is rather low, and an exposure greater than about 30 000 langmuirs is needed to saturate the surface. The net amount of Si deposited for the saturated surface is about one-half of an atomic layer. Thermal annealing causes the hydrogen atoms to desorb and the Si atoms to move below the surface. For annealing temperatures beyond about 630 K, the desorption of hydrogen becomes complete, all of the Si atoms move below the surface, and the resulting surface resembles the starting clean Ge(111)-c(2×8) surface except that the c(2×8) long-range order is partially destroyed. Step flow and island coarsening, similar to growth by molecular-beam epitaxy, are observed.

Quantum oscillations in a mesoscopic ring with two leads are studied where a magnetic flux and an electrostatic potential are applied simultaneously. Ballistic transport is investigated where electrons pass through the entire system without being scattered except at the junction of the ring and the lead. The effects of magnetic flux threading the ring can be considered as a phase shift in an Aharonov-Bohm (AB) sense. In the electrostatic AB effects, the effects of electrostatic potential applied to either of the two electron paths has so far been considered in terms of the electrostatic phase shift (model A). In a mesoscopic system (or in a rigorous theoretical treatment), we must consider the additional effects of scattering due to potential discontinuities at the edges of the electrode to which the electrostatic potential is applied (model B). In this paper, models A and B are considered on an equal footing. By comparing these two cases, we find that the conventional treatment of AB effects is possible at low electrostatic potential in a mesoscopic system. In model B, where the scattering effects at the gate edges are considered, the amplitudes of the quantum oscillation slowly decay as a function of increasing electrostatic potential. At higher electrostatic potential, we predict that conductance oscillations exhibit strong resonance structures at the maximum plateau regions. The relation between the transmission probability and electron energy is also calculated for fixed θ and φ, and discussed.

We investigate the exact Kohn-Sham exchange-correlation potential at semiconductor interfaces, including Schottky barriers, heterojunctions, and semiconductor surfaces. By considering the electron density at the interface, we deduce the way in which the exact exchange-correlation potential differs from its bulk counterpart. The potential has a slow spatial variation related to the discontinuity, Δ, that occurs on addition of an electron to the bulk semiconductor. This variation, which corresponds to an ultra-non-local ‘‘vertex correction’’ in the Kohn-Sham formulation of the dielectric response of the semiconductor, results in correction terms for Schottky barrier heights and band offsets calculated using Kohn-Sham orbital energies. The effect is exhibited numerically for a model semiconductor.

We show how high-order quantum peturbative corrections to transport properties of disordered conductors can be systematically evaluated in the metallic regime by means of random transfer-matrix theory. Effects of strong spin-orbit coupling and applied magnetic field are taken into acount. We obtain explicit expressions for the first, second, and third cumulants of the conductance and shot-noise power. Formulas for the variance and third cumulant of an arbitrary linear statistic are obtained.

To study the effects of Coulomb interactions on the properties of fractional quantum Hall edge states, we introduce a realistic model of a two-dimensional electron gas at a mesa-etched sample edge and solve it within electrostatic and Hartree-Fock approximations. We discuss the physics of the fractional quantum Hall strips separating the conducting edge channels and ways of estimating the widths of these strips. We relate our results to the measurements of nonlocal resistance by estimating the quasiparticle scattering rate and the corresponding equilibration length between the edge states and the conducting bulk.

We provide a theoretical study of hot-electron energy loss via LO-phonon emission in GaAs quantum wires. Calculations are done for a model in which the hot-electron gas is described by a temperature T which is much higher than the lattice temperature. We take into account several relevant physical mechanisms such as quantum degeneracy, dynamic screening, quantum confinement, and hot-phonon bottleneck effect. We include the effects of quantum-wire phonon confinement using two prevailing boundary conditions, namely, the so-called electrostatic and the mechanical macroscopic models. The energy-loss rate calculated for the bulk LO-phonon emission is comparable to, but somewhat higher than, that due to slab (electrostatic) phonon emission, whereas the guided (mechanical) modes produce more than an order of magnitude slower energy relaxation than that due to the slab (electrostatic) or the bulk modes. Our results show that in the experimentally interesting electron-temperature range of 50–300 K, the hot-phonon bottleneck is the single most important quantitative effect provided the hot-phonon lifetime in GaAs quantum wires is comparable to that (∼5–7 ps) in the bulk GaAs. The numerical magnitudes of the quantum-wire hot-electron energy-loss rate due to LO-phonon emission are comparable to those in quantum-well structures under comparable conditions.

A Monte Carlo approach is used to study the electron mobility in the Si/${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ system at low temperatures. The diffusion constant is evaluated in near thermal equilibrium simulations and is converted to the mobility by use of the Einstein relation for a degenerate two-dimensional electron gas. A modulation-doped structure is considered, where the doped ${\mathrm{Si}}_{0.7}$${\mathrm{Ge}}_{0.3}$ layer provides channel electrons and is separated from the channel by an undoped ${\mathrm{Si}}_{0.7}$${\mathrm{Ge}}_{0.3}$ spacer layer. The electron density is evaluated as a function of spacer width and doping concentration. Electrons are assumed to be only in the lowest subband. Acoustic-phonon scattering and remote impurity scattering determine the possible mobility that can be reached. We find mobility values of 2.5×${10}^5$ ${\mathrm{cm}}^2$/V s at 4.2 K and 3.1×${10}^5$ ${\mathrm{cm}}^2$/s at 1.5 K for an electron density of 7.5×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ (for typical choices of parameters: 10-nm spacer and 2×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$ doping). Peak mobility values of 5.0×${10}^5$ ${\mathrm{cm}}^2$/V s at 4.2 K and 7.6×${10}^5$ ${\mathrm{cm}}^2$/s at 1.5 K are possible for wider spacer layer widths, with subsequently lower channel electron densities. The effects of surface roughness scattering, as well as other scatterers are discussed. These processes can be a mechanism to explain the difference between the above ideal mobility and the reported experimental data.

Quantum wells are used to terminate the periodicity of a tightly coupled superlattice. When an electric field is applied, the quantum wells can interact with either the conduction or valence bands of the superlattice, depending upon the direction of the field, and whether the quantum wells are right-end or left-end terminations of the superlattice. We monitor the quantum-well–superlattice interactions with photocurrent spectroscopy at 77 K. Quantitative analysis of the conduction- or valence-band quantum-well–superlattice interactions depends sensitively on the band-offset ratio. The conduction-band and valence-band offsets are determined separately and used to test the validity and utility of an empirical two-band effective-mass model for analysis of complex heterostructures with both thick (∼100 Å) and thin (∼20 Å) layers.

Scanning tunneling microscopy has been used to study the initial stages of Al growth on decapped (2×4)/c(2×8) GaAs(001). Three orientations have been observed for the clusters of Al that form. On nominally flat GaAs(001) surfaces, Al(001)- and Al(011)-oriented clusters are dominant. These two types of clusters nucleate with the same probabilities for Al depositions at 60, 200, and 300 °C, indicating the importance of the intrinsic properties of the growth system rather than temperature in determining the Al crystal orientation. A model that considers energy arguments is used to rationalize the observed population of Al orientations. In contrast, Al(011)- and Al(011)R-oriented clusters dominate for growth on stepped GaAs(001), an effect related to an increase in the interface free energy for Al(001)-oriented clusters at steps. Previous results of this growth system are discussed in the context of this model.

A numerical method for simulating transient through steady-state electron transport in multidimensional mesoscopic structures in the presence of weak electron-phonon interactions is presented. This method allows both visualization and quantitative analysis of such electron-phonon coupling processes in these structures. To allow simulation of quantum interference effects within multidimensional structures, a previously developed method for simulating dissipationless transport based on the time-dependent Schrödinger equation is used as a starting point. Then, to allow simulation of the effects of electron-phonon coupling yet retain numerical tractability, a limited number of discretized harmonic oscillator degrees of freedom are added to those of the electron. Coupling between the electron and oscillator degrees of freedom is via Monte Carlo sampled potential functions that are obtained from the true electron-phonon coupling potentials. The method is exact to first order in the coupling and models some higher order coupling processes as well. Example simulations are performed for both real emission of polar-optical phonons and self-energy processes in prototypical mesoscopic structures.