For the vibronic Koster-Slater impurity, a nonadiabatic model of deep levels in semiconductors, we use the Green-function approach to evaluate exactly the thermally stimulated electronic field-emission rates in a general electrostatic field and at a general temperature. It is demonstrated both analytically and numerically that the usual Oppenheimer formula is useful beyond its nominal validity range.

We have investigated the effect of hydrostatic pressure (P≤5 GPa) on optical absorption and photoluminescence spectra of Wöhler siloxene (${\mathrm{Si}}_6$${\mathrm{O}}_3$${\mathrm{H}}_6$, as prepared) at T=300 K. The absorption edge energy near 2.75 eV decreases linearly with pressure at a rate of -30±3 meV/GPa. For the position of the maximum of the green luminescence band we find a similar pressure coefficient (dE/dP=-24±3 meV/GPa). The intensity of the green luminescence shows a reversible decrease with increasing pressure. The pressure dependence of interlayer and intralayer lattice constants has been determined by x-ray powder diffraction. The observed changes in optical transition energies are discussed in the framework of a semiempirical quantum-chemical calculation of the electronic structure of siloxene.

We present a comprehensive numerical study of electroabsorption and electrorefraction in quantum-well modulators. Our analysis properly incorporates both the k⋅p subband matrix elements and the excitonic wave functions and further contains several alternative techniques for solving the exciton equation. Excellent agreement with experiment is achieved in both the discrete and continuum regions.

Quantum Hall devices have been investigated in the regime of the breakdown of dissipationless current flow by means of a photoresistance imaging technique. It is possible to distinguish three stages in development of the breakdown: (i) the initial rise of the longitudinal resistance due to a change with the Hall electric field of the percolation threshold leading to electron backscattering between the edges; (ii) at higher bias currents a strong response from the edges is observed in two-dimensional images, in agreement with the edge-state model; (iii) on increasing the bias current further, electron heating effects are seen to prevail.

The result of a combined study of the absorption and photoluminescence (PL) from high-optical-quality free-standing porous Si films is presented. These films, which have a scattering loss less than 10%, allow unambiguous, detailed study of the system’s absorption edge and, in addition enable us to correlate the absorption and emission behavior on the same sample. H-ion scattering and H forward recoil data are used to accurately determine the bulk equivalent thickness to allow extraction of the absorption coefficient. X-ray diffraction, in conjunction with transmission electron microscopy, are utilized to measure the lattice constant and to confirm the crystalline nature of the structure. The combined photoluminescence and absorption studies are used to examine the relative energy shift of the luminescence peak with respect to that of the absorption edge. It is shown that the absorption begins coincident in energy with the photoluminescence peak, rising smoothly over a range of 1–2 eV. More importantly, the absorption is very weak in the region of the PL in comparison to a direct-gap semiconductor, indicative of the intrinsically weak oscillator strength. An effective-mass-based model for an ensemble of Si nanocrystallites is compared to the experimental results. Phonon-assisted transitions dominate the optical processes. The results support the picture that the absorption derives primarily from the interior states of nanocrystalline Si particles of characteristic dimension below 100 Å. The dynamics of the photoluminescence are reexamined and found to be consistent either with recombination between interior electron and hole states, of the Si nanocrystallites, or involving shallow trap states.

We have studied silicon-nitrogen bonding states in amorphous ${\mathrm{SiN}}_{\mathit{x}}$:H alloy films and in crystalline α-${\mathrm{Si}}_3$${\mathrm{N}}_4$ and β-${\mathrm{Si}}_3$${\mathrm{N}}_4$ by applying x-ray-emission spectroscopy (XES). We present x-ray Si K, Si L, and N K emission bands of alloy films covering the concentration range 0≤x≤2.03 and identify spectral features on the base of ab initio calculations of all emission bands of β-${\mathrm{Si}}_3$${\mathrm{N}}_4$. The calculations delineate the role of Si 3s, 3p, and N 2s, 2p electrons in the bond and confirm the presence of silicon d-like electrons in the top region of the valence band. A comparison with available ultraviolet and x-ray photoelectron spectroscopy results is given. The position of the N 2s line observed in Si K and Si L emission bands of the alloys is analyzed in terms of the random-bond and random-mixture (RMM) models of nearest-neighbor structure of silicon atoms. The results unambiguously support the RMM structure of our samples and demonstrate that a detailed analysis of x-ray transitions observable in XES can yield unique and unconventional information about the local structure and the degree of phase separation in a-${\mathrm{SiN}}_{\mathit{x}}$:H alloy films.

Using a micro-Raman setup, in-plane Raman scattering was performed on short-period (001)-Si/Ge superlattices, and the complete phonon spectrum (longitudinal and transverse, optical and acoustic modes) could be studied. In the relevant wave-vector range, the in-plane dispersion was found to affect the frequencies of folded-acoustic modes, while being negligible for confined optical modes. The comparison with theoretical spectra, calculated by means of first-principles interatomic force constants, shows that the observed deviations of ‘‘unfolded’’ confined optical modes from the bulk dispersions can be attributed to interface roughness and is well described by a simple alloy layer model (2–3 intermixed SiGe atomic layers at interfaces).

We present the results of an experimental and theoretical investigation of the initial oxidation of Si(100). Oxidation kinetics were measured in real time by x-ray-photoelectron spectroscopy. In the pressure and temperature regime studied, we observed the following kinetic phenomena: a decrease of the initial oxidation rate with increasing temperature, kinetic curves (i.e., coverage vs time) showing saturation with the saturation value increase for the higher temperatures, and a transition from ‘‘passive’’ to ‘‘active’’ oxidation. To account for the experimental results a phenomenological first-order phase-transition theory was used. On comparing the experimental and theoretical results, we suggested a physical mechanism for submonolayer silicon oxide formation.

GaAs(100) surfaces with adsorbed Se or Te have been studied using scanning tunneling microscopy (STM), x-ray-photoemission spectroscopy (XPS), and electron diffraction. STM reveals uniform dimerization which is consistent with the 2×1 symmetry of the electron diffraction. XPS shows an initial single-monolayer coverage of Se followed by a slow Se uptake which predominantly replaces As. Narrow single- and double-bilayer high islands, which are long in the [011] direction of dimer rows, are generally observed as well as terrace edges which show related kink anisotropy. Possible surface structures are discussed.

Using a transfer matrix method, we study the interface polariton modes in three-tile quasiperiodic semiconductor superlattices. It is shown that the distribution of the interface polariton modes depends obviously on the structure of the quasiperiodic superlattice.

Taking the inhomogenous broadening of the electron energy levels into account, a coherent model of the resonant tunneling (RT) of electrons in double quantum wells is presented. The validity of the model is confirmed with the experiments [M. Nido et al., Proc. SPIE 1268, 177 (1990)], and shows that the tunneling process can be explained by the simple coherent theory even in the presence of the carrier scattering. We have discussed the dependence of resonant tunneling on the barrier thickness ${\mathit{L}}_{\mathit{B}}$ by introducing the contrast ratio Λ and the full width at half depth of the RT valley, and found that Λ first increases with increasing barrier thickness, reaches a maximum, and then decreases with a further increase of ${\mathit{L}}_{\mathit{B}}$, in striking contrast to the Fabry-Pérot model where a monotonic increase of the peak-to-valley ratio is predicted. We attribute the reduction of Λ with large ${\mathit{L}}_{\mathit{B}}$ to the energy broadening resulting from the carrier scattering. A monotonic decrease of the full width at half depth of the RT valley with an increase of ${\mathit{L}}_{\mathit{B}}$ is also found.

Using a nonvariational method in k space, we study the effects of Γ-X valley mixing on the structure of excitons in an asymmetric ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/AlAs/GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum well. The formulation of excitons in this system involves two complications due to Γ-X mixing: the nonparabolic subband structures in the xy plane, and the Coulomb interaction between different electronic subbands. These effects lead to a set of two coupled integral equations. The eigenvalues and functions thus obtained are then used to compute the optical absorption spectra for various structures near the Γ-X crossing.

The spherical version of Laughlin’s quasiexciton wave functions at ν=1/m, which were originally constructed in planar geometry, is formulated, and extended to other filling factors. It is shown that the quasiexciton wave functions are rather accurate in describing the low-lying excitations. At zero angular momentum, the quasiexciton state reduces to the ground state.

A theory of phonon-assisted intervalley Γ-X transfer in periodic heterostructures GaAs/AlAs is developed on the basis of the envelope-function approximation. Matrix elements of intervalley transitions are expressed through the overlap integrals of Γ and X envelope functions with use of Γ-X intervalley deformation potential constants of bulk GaAs and AlAs. The phonon spectrum of an ideal GaAs/AlAs periodic heterostructure is studied in a microscopic approach. It is shown that phonons responsible for intervalley transitions are GaAs-like and AlAs-like X-point LO phonons, confined within GaAs and AlAs layers, respectively. Γ-X transfer times due to phonon-assisted scattering are calculated for the problem of photoexcited-electron relaxation in the type-II GaAs/AlAs superlattices and compared with the transfer times due to the phononless transition mechanism (Γ-X mixing). Both phonon-assisted Γ-X scattering and Γ-X mixing are important for the determination of Γ-X transfer times. The calculated values of Γ-X transfer times and their dependence on GaAs and AlAs layer thicknesses are in agreement with the experimental data.

I reduce the eigenstates of the Schrödinger equation in a magentic field B and in an arbitrary set of scatterers to the eigenstates of an Hermitian matrix, which is presented by the explicit analytical formula. Its eigenstates prove that thermodynamics and transport in a random system in a magentic field qualitatively depend on the nature of scatterers. If they are short-range ones and there is less than one magnetic-flux quantum per scatterer, then a finite fraction of states condenses into narrow high-density-of-states bands in the vicinity of the Landau levels in vacuum. The states in these bands have high mobility in two dimensions and are extended in three dimensions. Magnetoresistance oscillates with magnetic field and may be nonmonotonic with temperature and impurity concentration.

The frequency dependence of the absorption constant α(ω) and the enhancement of the refractive index n(ω) in the region of interband transitions between confined quasi-two-dimensional states of the valence and conduction bands of PbTe/${\mathrm{Pb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Eu}}_{\mathit{x}}$Te multiple-quantum-well (MQW) structures is studied experimentally as well as theoretically. Transmission, reflectivity, and photoconductivity spectra of several MQW’s with PbTe well widths from 62 to 118 Å and ${\mathrm{Pb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Eu}}_{\mathit{x}}$Te barrier widths from 486 to 621 Å (x≊3%) are compared with calculated spectra. The experimental data are compared to calculations of the transmission and reflectivity of the multilayer structures based on frequency-dependent dielectric functions ε(ω) of the buffer and MQW layers. For the buffer layer experimentally determined values for ε(ω) are used, whereas for the MQW system the sum of a background dielectric function and an additional term, which takes into account the electronic contribution due to interband transitions between electric subbands in the valence band (VB) and conduction band (CB), are used.

For calculating the second term two approaches were followed. The conventional approach is based on treating these interband transitions as originating from confined states within the PbTe wells only. The second, more realistic, approach is based on a complete calculation of the band structure of the MQW systems using a k⋅p envelope function approximation which yields proper energy eigenstates, eigenfunctions, and oscillator strengths. The absorption constant α(ω) is then obtained from an integration in k space. The electronic contribution δ${\mathrm{\epsilon }}^{\mathrm{VB}\mathrm{-}\mathrm{CB}}$(ω) to the total dielectric function is independent of the position z along the growth direction in the MQW structure, i.e., in this respect the MQW is treated as being optically homogeneous. The calculated transmission and reflectivity data fit the experimental data very well. The steplike changes in α(ω) result in cusplike changes of the refractive index n(ω). From a comparison of the experimentally determined interband transition energies with the calculated values on several samples, the conduction-band offset is determined to be Δ${\mathit{E}}_{\mathit{c}}$/Δ${\mathit{E}}_{\mathit{g}}$=0.55±0.2.

The Schottky-barrier and interface formation of Cs/GaSb(110) and Rb/GaSb(110) at room temperature are investigated by soft-x-ray core level photoemission spectroscopy using synchrotron radiation. A large splitting (0.9 eV) is found at the Cs 4d core level and indicates two different adsorption sites (having a large difference in ionicity) for the alkali-metal atoms on the GaSb(110) surface with bonding to both cation and anion. The large Fermi-level overshootings observed at very low alkali-metal coverages [0.03 monolayer (ML) Cs or Rb] for the p-type GaSb(110) samples are directly related to the specific donor character of alkali-metal atoms with charge transfer into the empty Ga dangling bond. The unique room-temperature growth properties of alkali metals are also likely to be of relevance. Further Cs or Rb deposition is found to be highly disruptive with large reactive components at Sb 4d but also Ga 3d core levels and a higher reactivity for the smaller alkali metal (Rb). Interestingly, this interface reactivity, which results from surface defects, is also significantly higher for p-type than for n-type samples, suggesting that the nature of the doping might play some role in the interface chemistry. The final Fermi-level pinning position for both p- and n-type GaSb(110) is already achieved at an alkali-metal coverage of about 0.3 ML, corresponding to significant changes in the mode of growth and to the threshold of surface disruption. This pinning position is located at about 0.1 eV above the valence-band maximum, in agreement with a defect acceptor state model.

We propose a method of exploring the random potential in the vicinity of a quantum point contact (QPC). The position and parameters of the saddle point of the smooth potential in the QPC are varied by four gates above the two-dimensional electron gas. Two of them form the QPC and shift the saddle point across the channel. The other two gates are placed in the source and drain areas to drive the saddle point along the channel. A resonant tunneling peak appears in the conductance when the saddle point and a local minimum of the random potential coincide. The most pronounced peaks appear in the pinch-off regime when the QPC is classically closed and the Fermi energy of the two-dimensional electron gas equals the binding energy in a local minimum. For a quantitative description, a simple realistic model of the heterostructure has been considered. We have assumed that the free surface of the heterostructure has a pinned potential and that no charge moves in the doped layer in response to a gate voltage. A three-dimensional electrostatic problem has been solved for the above situation. The position and other parameters of the saddle point are calculated analytically and a practical procedure for determining the parameters of the bound states is proposed. We also discuss the influence of Coulomb blockade phenomena on the proposed experiment.

In a previous study an unconventional species of Jahn-Teller system was presented, where electronic excitations of the mixed-valent host crystal adopt the role, which is usually played by nuclear displacements in the conventional coupling mechanism. In the present study the molecular model is extended to a doped-semiconductor model including an electronic transfer coupling. An electron capture in the defect states appropriate to describe electron spin resonance measurements of ${\mathrm{SmB}}_6$:Gd can be explained within this model by the formation of a band-gap state in the coupled system. The energetic lowering of a defect state as observed for ${\mathrm{SmB}}_6$:Er is examined, and is revealed to be most effective for a direct coupling to excitations of the localized 4f states of the Sm surrounding.

We report density-functional theory calculations of the electronic structure, total energy, and forces for the Na adsorption on GaAs(110) using the local-density approximation of the exchange-correlation functional and ab initio pseudopotentials. Results are presented for coverages ranging from one adatom per substrate surface cell up to the thick overlayer limit. The atomic and electronic structure of the substrate is locally changed by the sodium adsorption on GaAs(110), depending on the coverage. In particular, we analyze the wave-function character of the states at the Fermi level, how it changes with sodium coverage, and we identify the formation of metal induced gap states (MIGS) at the interface. These MIGS are found to have mostly Ga dangling-bond character for all coverages. The calculated values of the p-type Schottky barrier and of the variation of photothreshold as a function of coverage are in good agreement with experimental data.