The composition dependence of the electron energy spectrum in the ${\mathrm{Hg}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se solid solution investigated assuming the invariability of the position of the Fe d resonant level in the absolute energy scale. In the linear-in-x approximation, the individual shifts of the extrema of the ${\mathrm{\Gamma }}_6$ and ${\mathrm{\Gamma }}_8$ bands are determined from analysis of the x-dependent concentration of the conduction electrons n as ${\mathit{dE}}_{\mathrm{\Gamma }6}$/dx=2.10±0.05 eV and ${\mathit{dE}}_{\mathrm{\Gamma }8}$/dx=-2.45±0.05 eV. It is shown that a quantitative description of the experimentally observed n(x) and μ(x) dependences (where μ is the mobility of the conduction electrons) is achieved when taking into account changes of only two parameters of the theory corresponding to the positions of the extrema of the ${\mathrm{\Gamma }}_6$ and ${\mathrm{\Gamma }}_8$ bands. The energy of the conduction-band bottom is found to vary in a wide range (∼100–330 meV) with respect to the iron level. This explains, in particular, the absence of saturation of the contribution from scattering by ionized donors to μ(x), calculated in terms of the short-range correlation model. The transition from the semimetal to the semiconductor state is found to be at x≊0.06.

As an attempt to establish a unified treatise of the structure of amorphous germanium, silicon, and (tetrahedral) carbon, a series of reverse Monte Carlo simulations were carried out for all of the three materials. A number of constraints have been applied during the calculations in order to test how many (and how much) different assumptions can be consistent with given experimental data sets. Solely on the basis of diffraction data, the results of the unconstrained calculations seem to be the models of the above systems that are the most consistent with the experiment, but these models are not satisfactory from the electronic structure point of view. With slightly worse agreement with the data, it was possible to generate dominant tetrahedrally coordinated models (with coordination numbers equal to exactly 4) in each case. The most extreme model proposed is the one where no fourfold coordinated atom could be found. However, the proportion of tetrahedral angles in this latter case is still high. It is therefore concluded that the most characteristic common feature of these amorphous semiconductors is not the fourfold coordination but the always apparent presence of closely tetrahedral bond angles.

We report on the magnetic properties of the Cr-based diluted magnetic semiconductors ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Cr}}_{\mathit{x}}$Se and ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Cr}}_{\mathit{x}}$S (x<0.01). The specific heat was measured in the temperature range 1.5<T<10 K and in magnetic fields up to 3 T, while the magnetization was measured at 2<T<40 K and B<6 T. The magnetic behavior is neither that of Brillouin paramagnets [Mn, Co diluted magnetic semiconductors (DMS’s)] nor of Van Vleck systems (Fe DMS’s). It results from a strong, static Jahn-Teller (JT) distortion for the Cr ions. A simple crystal-field model, which includes the JT effect, provides an overall description of the experimental data, but some discrepancies remain.

The effects of the Ga 3d semicore levels on the electronic structure of GaN are discussed. While band-structure theory using the local-density approximation predicts these states to overlap with the N 2s band and to have important effects on the total energy, x-ray photoelectron spectroscopy (XPS) shows that they occur ∼3 eV below the N 2s band. This apparent discrepancy is resolved by means of a so-called Δ SCF or difference of self-consistent-fields calculation, in which the binding energy is calculated as a total-energy difference including solid state screening effects by means of the excited-atom model. The calculated valence-band densities of states are found to be in good agreement with the XPS spectrum. The differences between zinc blende and wurtzite GaN are discussed.

We report an inelastic light-scattering study of the intrasubband plasmons of a single two-dimensional electron gas (2DEG) at a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterointerface under a grating Schottky gate with a period comparable to the plasmon wavelength. Gaps appear in the plasmon dispersion relation because of the spatially periodic screening by the gate, and these are successfully modeled using a scattering matrix calculation of the electromagnetic response of the system. Biasing the gate periodically modulates the 2DEG carrier density, and has a striking effect on the plasmon dispersion relation that cannot be fully explained within the existing theoretical framework.

Self-sustained current oscillations in high-purity n-type GaAs epitaxial layers were investigated for intermittent behavior in the control parameter plane constituted by the constant bias voltage and an external magnetic field. Intermittent windows were observed in a sequence of Hopf bifurcations. Quasiperiodic and frequency-locked oscillations were found to follow an intermittency type-II or type-III route to chaos in the Pomeau-Manneville classification scheme. Additionally, intermittent bursts due to noise were observed at voltages well below the threshold of bifurcation.

The bound-electronic optical nonlinearities in highly excited semiconductors (i.e., semiconductor lasers) have been calculated using a two-parabolic-band model. The nonlinear absorption spectrum is first obtained using a dressed-state formalism taking into account the contributions from two-photon absorption, electronic Raman, and optical Stark effects. The nonlinear refractive index (${\mathit{n}}_2$) is then found by performing a Kramers-Kronig transformation on the nonlinear absorption spectrum. It is also shown that the quadratic Stark splitting of the bands leads to a shift in the quasi-Fermi levels, which introduces additional absorptive and refractive nonlinearities. The sign, magnitude, and the current-density dependence of the calculated ${\mathit{n}}_2$ agree well with some recently published experimental results for Al-Ga-As and In-Ga-As-P diode lasers.

Using a model cubic anharmonic crystal potential and Fermi’s golden rule, an expression for the anharmonic relaxation time for a nonequilibrium long-wavelength LO phonon is rigorously derived. Decay times for various bulk, alloy, and quantum-well semiconductor systems are calculated. Comparison with time-resolved Raman measurements indicates that a relatively high value of the Grüneisen constant (1.8 for GaAs) gives the best agreement between experiment and theory.

Pseudomorphic crescent-shaped quantum wires are shown to exhibit size-dependent symmetry breaking due to an anisotropic strain distribution. We predict level degeneracies and an increased carrier lifetime. As a model system ${\mathrm{In}}_{0.2}$${\mathrm{Ga}}_{0.8}$As/${\mathrm{Al}}_{0.2}$${\mathrm{Ga}}_{0.8}$As quantum wires are simulated with finite-element analysis to obtain the strain tensor. Within the confinement potentials, which are strongly modified by the strain-induced potentials and the piezoelectric field, the single-particle Schrödinger equation is solved for electrons and holes.

Measurements are reported of the temperature dependence of the conductance through a small region of electron gas separated from its leads by tunnel junctions. The quantization of charge and energy in the small region gives rise to sharp, nearly periodic peaks in the conductance as a function of electron density, one for each electron added to the isolated region. Because the charge and energy are quantized, we call this an artificial atom. At low temperature, the conductance is limited by resonant tunneling through a single quantum level of the artificial atom, but at high temperatures several levels participate. Changes in the temperature dependence of the width and height of conductance peaks display clear evidence for this crossover from single-level to multilevel transport.

Measurements of the electrical resistivity, Hall effect, and magnetoresistance are reported for metallic iron disilicide layers, fabricated by ion-beam synthesis (IBS) and solid-state reaction (SSR) on (100)Si. The analysis of the data is consistent with three-dimensional weak-localization theories, including a strong spin-orbit interaction, and electron-electron interaction. The spin-orbit scattering and electron wave-function dephasing rates are extracted from nonlinear least-squares fits to the magnetoresistance and the low-temperature electrical resistivity. From both the magnetic field and temperature dependence it is found that the electron-electron interaction due to spin-splitting effects also contributes to the scattering in this material. For IBS and SSR fabricated iron disilicide layers the dephasing rate saturates below 4.2 K indicating a scattering contribution from magnetic impurities, and varies as ${\mathit{T}}^{\mathit{p}}$ above 4.2 K. From the magnetoresistance measurements in the temperature range between 20 and 50 K, the exponent of the dephasing rate is found to be about 3 in agreement with electron-phonon interband scattering being dominant, consistent with the overall temperature dependence of the resistivity at zero magnetic field.

The conductance G and the thermopower S of a saddle-point-potential quantum point contact in the presence of a δ-potential impurity are calculated. For the case when an attractive impurity is located inside the quantum point contact, there are dips, peaks, and kinks in G. These structures can be found below and near the band bottom of some transverse subbands, respectively. The peaks in G give rise to double peaks in S and the dips in G give rise to a shift in the peak positions of S, towards a smaller chemical potential value. In addition, a broad dip in G is found to give rise to negative thermopower in regions between the peaks of S. For the case when an attractive impurity is located in the classical forbidden region for the electrons, we find in G the structures that correspond to resonance tunneling and resonance reflection. The corresponding S is found to show large and negative spikes. Our study shows that structures not so transparent in G manifest unequivocally in S, rendering S a very informative physical quantity to be measured.

Valence- and conduction-band-edge energies and effective masses of hydrogen-terminated silicon wires are calculated using a first-principles pseudopotential method, and the results are compared with effective-mass-theory (EMT) calculations. The first-principles result for the ordering of states at the valence-band maximum is different from the prediction of EMT. The magnitudes of the valence- and conduction-band effective masses for motion along the wire axis increase from their bulk values as the wire thickness decreases, but by much less than predicted by other calculations.

A rigorous calculation of the intensity dependence of the photoreflectance (PR) amplitude is presented, and the derived relation is compared with published experimental results. The method utilizes a Taylor-series expansion to determine the change in reflectance in terms of the modulation of the surface electric field, and the Fourier-series technique is employed to explicitly develop the harmonic components of the photoreflectance amplitude for square-wave excitation. In particular, it is shown that if the photoreflectance amplitude depends upon the optical excitation intensity I as ln(γI+1), which is normally the case experimentally, then the small-modulation PR signal should have a line shape proportional to the first derivative of the sample reflectance with respect to the surface electric field. In the high-field limit, the nature of the Franz-Keldysh oscillations is explained for both small- and large-modulation conditions, and the theoretical predictions are correlated with recent experimental data. Overall, this theoretical study of the photoreflectance effect clarifies certain issues regarding the connection between the observed intensity dependence of the PR amplitude and the PR line shape, and it also illustrates how the nature of the PR line shape changes as one moves from low- to high-field conditions, in both the small- and large-modulation limits.

A study of the adsorption of sulfur on the GaAs(100) surface after in situ thermal desorption of a protective As capping layer is presented. The sulfur flux was generated by the decomposition of silver sulfide in an UHV-compatible electrochemical cell. Use of As-capped samples provided a means to study the interaction of sulfur with both the c(2×8) and (4×1) surface reconstructions. Scanning-tunneling-microscopy images of the sulfur-covered surface indicated the formation of disordered surface layers which display a diffuse (1×1) low-energy-electron-diffraction pattern. This (1×1) phase is attributed to the symmetry of the bulk structure visible through the disordered surface overlayer, caused by the adsorbed sulfur breaking the surface dimer bonds. Synchrotron-radiation core-level photoemission spectra indicate evidence of sulfur bonding to both gallium and arsenic at room temperature, but that the relative magnitude of these bonding interactions depends on the Ga/As ratio of the clean surface. Sulfur 2p photoemission spectra from the annealed surfaces show that sulfur diffuses into the topmost atomic layers as well as bonding to the surface. Annealing the sulfur-covered surface above 400 °C results in the formation of a (2×1) low-energy-electron-diffraction pattern with a dimer-row structure clearly visible in scanning-tunneling-microscopy images. Our results would suggest that the adsorption of sulfur on the c(2×8) clean surface results in dimer rows consisting of both arsenic and sulfur dimers, while only sulfur dimers are observed after adsorption on the (4×1) surface. The degree to which the clean surface band bending is altered on these respective surfaces appears to be related to the precise chemical composition of the dimer rows.

The circular polarization of cw photoluminescence as a function of applied magnetic field has been measured at 1.8 K for excitons in a series of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As and GaAs/AlAs type-I quantum wells having well widths between 2.5 and 8.0 nm. The results show evidence of level crossings, which have been analyzed to give the short-range, spin-dependent exchange interaction. The associated experimental exchange splittings between optically allowed and nonallowed exciton states are of the order of 0.15 meV for the narrowest wells and fall monotonically with increasing width. Simulations of the data using solutions of rate equations for the exciton-level populations show that the optically nonallowed exciton states lie below the allowed states and give insight into the field dependence of the spin-relaxation rates of excitons and of holes and electrons in an exciton. Calculations of the enhancement of exchange interaction due to carrier confinement relative to the bulk value give agreement between the quantum-well and bulk-exchange values, and confirm that exchange is particularly sensitive to barrier height and other details of the structure. The data also show evidence for eigenstate-polarization changes, which indicate a much smaller zero-field splitting due to departure of the quantum-well symmetry from the ideal ${\mathit{D}}_{2\mathit{d}}$. The simulation indicates that only a fraction (up to 20%) of excitons in these samples actually experiences a nonideal, distorted environment.

Different possible adsorption sites of cesium atoms on a gallium arsenide (110) 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 Cs atom adsorption at a site modeled with a ${\mathrm{CsGa}}_5$${\mathrm{As}}_4$${\mathrm{H}}_{12}$ cluster is most favored energetically followed by Cs adsorption at a site modeled with the ${\mathrm{CsGa}}_4$${\mathrm{As}}_5$${\mathrm{H}}_{12}$ cluster. For molecular cesium, a site modeled by a ${\mathrm{Cs}}_2$${\mathrm{Ga}}_6$${\mathrm{As}}_9$${\mathrm{H}}_{21}$ cluster is most favored energetically. However, here all four sites considered remain competitive energetically at the correlated levels of theory. The effects of charge transfer from Cs and ${\mathrm{Cs}}_2$ to the GaAs surface and the possibilities of metallization are also analyzed and discussed.

GaAs(110) and (100) wafers are reacted with ${\mathrm{XeF}}_2$ at room temperature and studied with soft-x-ray photoelectron spectroscopy (SXPS), photon-stimulated desorption (PSD) and electron-energy-loss spectroscopy (EELS). The reaction between GaAs and ${\mathrm{XeF}}_2$ results in the homogeneous growth of a ${\mathrm{GaF}}_3$ film and an interface region consisting of intermediate products. The band gap of ${\mathrm{GaF}}_3$ and its band lineup with the GaAs substrate are determined via EELS and SXPS. ${\mathrm{F}}^{+}$ PSD spectra collected after the initial ${\mathrm{XeF}}_2$ exposures indicate a single desorption onset at ∼28 eV, due to the excitation of a F 2s electron to the GaAs conduction-band minimum. PSD spectra collected after larger exposures contain a number of features due to transitions within the ${\mathrm{GaF}}_3$ band structure. These features appear when the film thickness exceeds ∼10 Å, indicating that the bulk electronic structure has developed. Annealing the film to ∼250 °C results in the inhomogeneous removal of ${\mathrm{GaF}}_3$.

Extremely well resolved Si 2p core-level spectra from the clean Si(111)7×7 surface have been obtained at ∼100 K. Surface-shifted components become visible to the naked eye. Contributions from the constitutive entities of the complex 7×7 reconstruction, i.e., rest atoms, adatoms, pedestal atoms, and dimer atoms are clearly identified. This puts into new perspective the spectroscopy and physics (structure and electronics) of this prototypical surface.

We present realistic effective-mass calculations of electron and exciton states in [311] GaAs/AlAs corrugated quantum wells (QW’s) taking into consideration a finite barrier potential and flux-conserving boundary conditions. One-dimensional minibands are formed in the periodic lateral corrugation potential. Light- and heavy-hole exciton binding energies are calculated as a function of QW effective width and discussed in connection with existing experimental data.