The electronic properties of amorphous carbon structures with varying microscopic mass densities, ranging from 2.0 to 3.52 g/${\mathrm{cm}}^3$, are analyzed. Using a semiempirical density-functional approach the model structures were generated by molecular dynamics performing a simulated cooling of liquid carbon clusters, containing 128 atoms, within periodically arranged cubic supercells. By investigation of properties related to chemical bonding we find a strong control of the band gap by the balance between π bonding and electronic defect generation. As important sources that influence the π-${\mathrm{\pi }}^{\mathrm{*}}$ band gap the distance distribution between non-σ bonded hybrid orbitals as well as the topological properties of clustering ${\mathit{sp}}^2$ (or sp) units are discussed. A number of local densities of states are calculated at representative atoms to validate the classification of π clusters and electronic defect states used.

The full-potential linear muffin-tin orbital method is used to study the electronic and structural properties of zinc-blende GaN. The total energy and electronic band structures under uniaxial strain and zone-center TO phonon distortions were calculated. This yields the cubic elastic constants, the zone-center TO phonon frequency, and the internal-strain parameter ζ. The elastic constants of hexagonal GaN are obtained using the transformation method of Martin [Phys. Rev. B 6, 4546 (1972)]. The corresponding deformation potentials are calculated for some of the important eigenvalues.

The bond lengths in ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{A}}_{1\mathrm{-}\mathit{y}}$${\mathrm{P}}_{\mathit{y}}$ quaternary-alloy semiconductors are calculated using a bond-orbital model. The bond lengths tend to be the same as those in binary compounds. Theoretical results agree well with extended x-ray-absorption fine-structure data. A small negative deviation of the lattice constant from the Vegard law is obtained. All of the results are explained successfully by the dimensionless relaxation parameters.

A series of electric-dipole spin-resonance (EDSR) lines, termed Si-SC1 lines, are found to develop in Czochralski-grown Si crystals due to annealing at 650 °C. Some of these lines are very close to Si-2K and Si-3K reported in a previous work. The experimental data are self-consistently explained by use of a model that shows that the EDSR signals are caused by additional electrons trapped by long quasi-one-dimensional defects lying along the 〈110〉 directions. The localization length of the trapped electrons is determined to be of the order of 100 nm and their mobility to be rather high along the defects, suggesting that a quasi-one-dimensional energy band is associated with the straight part of the defect. Si-SC1 centers are attributed to the so-called rodlike defects that are developed in the Czochralski-grown Si crystals due to the above heat treatment.

We use a quasichemical formalism to make quantitative predictions of the native point defect densities in ${\mathrm{Hg}}_{0.8}$${\mathrm{Cd}}_{0.2}$Te. The electronic contribution to the defect-formation free energy is calculated using the self-consistent first-principles full-potential linearized muffin-tin orbital method and the local-density approximation (LDA). A gradient correction is added to the LDA result so that absolute reference to the chemical potential of the mercury vapor phase can be made. A Green’s function approach based on a valence force field plus a point Coulomb model is used to calculate the vibrational contributions to the defect free energy (both energy and entropy). We find the double acceptor mercury vacancy is the dominant defect, in agreement with previous interpretations of experiments. The tellurium antisite is also found to be an important defect in this material. Predictions of the low-temperature hole concentrations are made as a function of annealing temperature and compared with available experiments. The order of magnitude of our predictions agrees well with experimental results, and discrepancies can be attributed to contributions to the free energy that we have neglected or to inaccuracies in the intrinsic reaction constant used. Suggestions for further experimental work are made.

We discuss the mathematical formalism of physically allowed lattice-invariant strains. When coupled with first-principles calculations of the energy as a function of strain, this approach becomes a powerful tool for understanding large lattice strains in solids. We also present an application of this tool to explore deformations of silicon, which predicts, to our knowledge, a previously unknown metastable structure. The physical properties of this structure are described and a method for obtaining it experimentally is explicitly provided.

Quantum kinetic equations with memory are formulated and solved for the femtosecond Coulomb kinetics of an optically excited electron-hole plasma. In a time interval smaller than the inverse plasma frequency screening and energy conservation are still absent. The resulting decay of the optical interband polarization is shown to be nonexponential and is in qualitative agreement with the results of the quasiclassical theory of Gurevich et al.

We report on the effects of intense light soaking at room temperature (RT) and at 77 K on the defect density in hydrogenated amorphous silicon (a-Si:H). It is found that at short light-soaking times, light soaking at RT is more efficient in creating metastable defects than at 77 K. With increasing light-soaking time, however, 77-K light soaking causes the defect density to increase at a higher rate than does RT light soaking. There are signs that the saturated value of the defect density for 77-K light soaking is larger than that for RT light soaking. Qualitatively, a correlation exists between the increase in the defect density and the decrease in the photoconductivity; however, an inverse proportionality is not observed between the photoconductivity and defect density. For a given defect density, the photoconductivity is smaller for 77-K light soaking than for RT light soaking. The defects generated by 77-K light soaking are found to be stable at 77 K. However, significant annealing of defects occurs after raising the sample temperature to RT. Light-induced annealing of defects is also observed. We explain our results by adopting the views that there is a broad distribution of defect-annealing activation energies and that the defects with small annealing activation energies are more effective recombination centers than those with large annealing activation energies. We show that many other related experimental results can also be accounted for by the above views.

Gallium arsenide grown by the metallorganic-chemical-vapor-deposition method was irradiated at room temperature with fast neutrons in the fluence range ${10}^{12}$ to 3×${10}^{15}$ ${\mathrm{cm}}^{\mathrm{-}2}$ (1.00 MeV equivalent fluence in gallium arsenide). The effects of neutron irradiation were studied by photoluminescence (PL) spectroscopy in the energy range 0.5–1.55 eV. In the samples irradiated to 3×${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}2}$ and higher fluences, we observe a transition at 1.4745±0.0003 eV in the PL spectrum recorded at 2 K. Its intensity increases with neutron fluence. Upon increasing the temperature at which the PL is recorded, the transition shifts to higher energies, and an additional higher-energy component appears. At 20 K, the energy of the doublet is 1.4750 and 1.4786±0.0003 eV. The temperature dependence suggests that the low-energy component is a donor-acceptor pair and the high-energy one is a transition from the conduction band to the acceptor. We identify the acceptor as the gallium vacancy (${\mathit{V}}_{\mathrm{Ga}}$). We deduce its energy to be at 42.2±0.3 meV above the valence band. Replicas involving the 37-meV LO phonon were observed. Isochronal annealing was performed on samples irradiated to 1×${10}^{13}$ and 3×${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}2}$. Samples which were irradiated and then annealed to 500 and 550 °C exhibited the same doublet and their phonon replicas in the PL spectrum. This suggests that ${\mathit{V}}_{\mathrm{Ga}}$ is dissociated at 500 °C from a complex formed during neutron irradiation. The effect of neutron irradiation and of isochronal annealing on other transitions observed by PL is presented. These transitions include the sharp transition at 0.702 eV, which has been associated with the arsenic antisite defect (${\mathrm{As}}_{\mathrm{Ga}}$), and the broad transition at approximately 1 eV. The modifications in the infrared PL spectrum are discussed in parallel with deep-level transient-spectroscopy data.

We used scanning tunneling microscopy and low-energy electron microscopy to study the etching of the Si(100)-(2×1) surface by oxygen at low pressures and elevated temperatures. At 5×${10}^{\mathrm{-}8}$ torr partial pressure of oxygen, the transition from random etching of terraces to step etching occurs at ∼580 °C, while at the higher partial pressure of 5×${10}^{\mathrm{-}7}$ torr, the transition temperature is ∼625 °C. The diffusing species is a dimer vacancy, formed by the desorption of two SiO molecules. Anisotropic diffusion of the vacancies is observed, with a preferred direction along the dimer rows. The reaction probability, defined as the number of desorbed SiO molecules to the number of incident ${\mathrm{O}}_2$ molecules, is estimated to be 0.010±0.005.

We have investigated the dependence on hydrostatic pressure of the photoluminescence of InAs monolayers embedded in bulklike GaAs at 10 K and for pressures up to 8 GPa. Below the pressure-induced Γ-X conduction-band crossover in GaAs (4.2 GPa) the optical emission is dominated by direct optical transitions between Γ-like excitonic states bound to the InAs monolayer. With increasing pressure this luminescence band shows a blueshift similar to the lowest direct band gap of bulk GaAs. At pressures above the band crossover two emission bands are observed. These bands, characterized by having negative pressure coefficients, are attributed to the type-I transition between conduction-band ${\mathit{X}}_{\mathit{x}\mathit{y}}$ and heavy-hole states of the InAs monolayer and the type-II transition from X states in GaAs to InAs heavy-hole states. The results are interpreted in terms of tight-binding band-structure calculations for the strained InAs-monolayer–bulk-GaAs system.

A detailed experimental study of damping and velocity of low-frequency edge magnetoplasmons in GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructures is presented. The damping is observed to be frequency dependent at filling factors close to integer values. The magnitude of the damping increases with frequency, the dependence being somewhere between linear and quadratic. This finding indicates that the damping of low-frequency edge magnetoplasmons cannot be described by the effective relaxation time. The experimental results are discussed in terms of existing models of low-frequency edge magnetoplasmons.

We show the resonance and the resonance splitting effect in the transmission probability (TP) for electron transport through superlattices (SL’s) of periodic potential structure with arbitrary profile in one dimension is inherent. The formalism presented provides an explicit description of the correlation between the resonance energy domains in TP and the miniband structure of the corresponding infinite SL. Examples demonstrating the dependence of the miniband structure and the resonance features on the SL material parameters are presented.

Our previous work on finite-frequency shot noise S(ω) in a correlated tunneling current has been extended to investigate the shot-noise behavior in a single-electron transistor (SET), with emphasis on asymmetric SET. We found that both the increase of the asymmetry and the increase of the gate voltage will broaden S(ω), and S(0) is further suppressed by the gate capacitance. At low bias voltage, a condition is obtained under which the noise spectrum S(ω) is white (frequency-independent). We have also demonstrated at the quantitative level how the signal-to-noise ratio in a SET can be controlled by adjusting the gate voltage and the degree of asymmetry.

We determined exciton binding energies in monolayer InAs-GaAs quantum wells by studying photoluminescence excitation spectra in a magnetic field up to 8 T. The effective-mass approximation was used to calculate the energy levels and determine the excitonic effects associated with Landau-level transitions and the exciton binding energy, which was also determined by extrapolation of higher-lying Landau-level transition energies to zero field. Both procedures lead to heavy-hole-exciton binding energies of the order of 10 meV, i.e., an enhancement of nearly 300% over bulk GaAs. From the diamagnetic shift of the exciton ground state, an estimate of the light-hole-exciton binding energy is made. In-plane effective mass reversal between heavy-hole- and light-hole-exciton states of submonolayer InAs was also observed. Furthermore, electron (exciton)-phonon coupling was also observed by level anticrossing, involving longitudinal as well as local vibrational phonon modes in ultrathin InAs.

Starting from a continuum theory of optical phonon modes in an assumed isotropic GaAs/AlAs quantum well, we have used perturbation theory to determine the effects of anisotropy on the mode frequencies. In particular, a [001]-grown quantum well with modes dispersing along [010] and [110] has been considered. We have derived analytical expressions for the frequency corrections and illustrate the differences for modes dispersing in these two directions.

Within the framework of the dispersion-equation treatment, the characteristics of the propagation of elastic waves along one-dimensional superlattices radiated by an infrared laser field are investigated. The Kubo formalism is generalized to evaluate the conductivity tensor in this system. When the condition ${\mathit{s}}_0$/Δd‖${\mathit{J}}_0$(eFd/Ω)‖≃1 is satisfied (2Δ and d are the miniband width and period of superlattice, F and Ω are amplitude and frequency of the laser field, respectively, and ${\mathit{s}}_0$ is the sound velocity in the absence of interaction), a strong renormalization of the sound velocity is predicted, along with intense absorption peaks indicating the localization of waves. The spectrum of plasma oscillations propagating along the superlattice axis is also calculated. The characteristic feature of these plasmons is the absence of Landau damping. In the dynamic localization regime the plasma oscillations along the superlattice axis disappear.

We investigate the electronic structure of a series of InAs-monolayer and submonolayer structures synthesized on both (100) and (311)A GaAs substrates by molecular-beam epitaxy. The chosen growth conditions ensure excellent structural and optical quality of the samples. Consequently, we have the opportunity to analyze the electronic properties of these ultrathin InAs layers which we explore by high-resolution x-ray-diffraction, photoluminescence (PL) and PL excitation (PLE) measurements. InAs submonolayer structures with an InAs coverage down to 0.78×${10}^{14}$ ${\mathrm{cm}}^{\mathrm{-}2}$ give characteristic x-ray-interference spectra that evolve continuously with increasing In content. In contrast, the optical spectroscopies reveal a clear crossover from electronically disconnected InAs clusters to quantum-well behavior at an InAs coverage of about 3×${10}^{14}$ ${\mathrm{cm}}^{\mathrm{-}2}$ for both orientations studied. The energies of the features observed in PL and LE spectra of these structures are modeled as a function of InAs layer thickness using an eight-band k⋅p-type effective-mass theory. The valence-band anisotropy of the bulk compounds is reflected in differences between both growth directions. In addition, our results indicate that the band offset ratio at the strained InAs/GaAs interface is orientation dependent.

A Wigner-crystal variational wave function accounting for both the Landau-level mixing and finite thickness of a two-dimensional (2D) system of particles is investigated. The energy is obtained analytically within an approximation scheme similar to that of Maki and Zotos and numerically using the variational Monte Carlo method. Comparison with the energy of the translationally invariant Laughlin liquid at ν=1 shows that our wave function has lower energy for ${\mathit{r}}_{\mathit{s}}$≳12 which is achieved easily in 2D hole systems.

We report a calculation of the electronic properties of GaAs/AlAs (311) superlattices through the Brillouin zone using realistic values of the parameters in a tight-binding Hamiltonian. The nature and the localization of the lowest conduction bands for these low-symmetry structures is investigated and its origin is studied with respect to the layer number. A direct-gap–to–indirect-gap transition is evidenced for L=3 nm which is equal to the thickness for which the crossing of Γ-X states occurs in the (100) superlattices. The in-plane anisotropy of the optical transitions and the relative intensity is found to be dependent on both the layer number and the nature of the states involved in the transition. Comparison with available experimental data is quite satisfactory.