A detailed examination of the physics underlying femtosecond relaxation of optically excited carriers in the near-band-gap regime of compound semiconductors with a special emphasis on band renormalization and Coulomb enhancement of the optical matrix elements is presented. This is done using a Monte Carlo formulation including Coulomb enhancement, band renormalization, and dynamic screening. The accuracy of the simulation has been verified through correlation with a series of experiments performed over a wide range of near-band-gap photon energies and pulse intensities. The results are found to differ greatly from those obtained for energy excitations far from the band edge. The observed carrier relaxation is found to be very insensitive to all relative scattering rates in contrast to excitation at high energies. Coulomb enhancement and band renormalization together are found to be important factors at both low and high excitation energies and should be important considerations in all efforts in the field. For a cold distribution, the effect of these processes is to accelerate the observed relaxation while for a hot distribution the opposite is found. These two processes are unimportant if the carriers are excited near thermal energies. The insensitivity of the simulation to relative contributions of the scattering processes, in combination with the strong distortions introduced by band renormalization and Coulomb enhancement makes the extraction of scattering-rate information difficult. Thus, near-band-gap relaxation should rather be considered a probe into band renormalization, Coulomb enhancement, and screening. The good correlation between measured and simulated data provides justification for extending the present semiclassical formulation to the computation of macroscopic physical observables dependent on near-band-gap femtosecond carrier relaxation.

We present and discuss the room-temperature structural, vibrational, and electronic properties of a-GaAs, as obtained by a tight-binding molecular-dynamics quenching of the liquid from 1600 K. An interesting comparison is also made with the overcooled liquid at 800 K, where solidlike and liquidlike features do coexist. At variance with previous ab initio calculations, we have been able to produce a good amorphous sample without introducing ad hoc moves of atoms, favoring a preselected distribution of point defects. Our results are in rather good agreement with the overall structural properties of a-GaAs, as deduced from experiments, and provide a detailed microscopic characterization of such a material.

Several nitrogen-related centers have been introduced by ion implantation of nitrogen into germanium and studied by infrared-absorption spectroscopy. Two local vibrational modes at 825.3 and 658.6 ${\mathrm{cm}}^{\mathrm{-}1}$ were especially prominent. Measurements on annealed samples implanted with either pure ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ or ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$, or implanted with both isotopes showed these modes to arise from a nitrogen pair defect analogous to one previously suggested to occur in silicon. Based on this and ab initio pseudopotential cluster theory, a model of the pair is proposed which is consistent with the observed infrared absorption. This model is similar to that of the nitrogen pair in silicon and offers an explanation of the low donor efficiency of nitrogen in germanium. Several other nitrogen-related local vibrational modes are also observed in the implanted material.

High-pressure photoluminescence (PL) is reported for bulk and thin-film ZnSe containing ${10}^{18}$–${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$ P and As acceptors using a cryogenic (4–300 K) diamond-anvil cell to 55 kbar. We focus on the deep PL bands in the red and green regions of the visible spectrum; the red emission bands (1-atm peaks at ∼6000 A in ZnSe:P and ∼7000 A in ZnSe:As) are thought to stem from ${\mathit{C}}_{3\mathit{v}}$-distorted acceptor sites. With increasing pressure in the range studied, the peak energies of all the deep bands approach but do not cross the ZnSe band gap. The average deformation potential of the valence edge relative to the P or As deep levels attributed to ${\mathit{C}}_{3\mathit{v}}$ sites is -1.2±0.3 eV. The negative sign is confirmed by thermal quenching data at different pressures in ZnSe:P. These results imply that with pressure the valence edge shifts to higher energy, causing deep acceptor states to become less stable. We also observe under pressure strong transfer of oscillator strength from red to green emission, and several deep bands. However, since the shallow acceptor-bound exciton PL is not enhanced, we conclude that a deep-to-shallow transition has not occurred within the pressure range studied.

An unusual longitudinal-magnetic-field dependence of the muon polarization for anomalous muonium (${\mathrm{Mu}}^{\mathrm{*}}$) in polycrystalline semiconductor targets is predicted, and has been observed for Si. As a function of field, the experimentally measured muon polarization at 53 K exhibits a cusp, i.e., a discontinuous jump in the slope from negative to positive, at 340(5) mT, in good agreement with theory. This occurs because the effective field on the ${\mathrm{\mu }}^{+}$ vanishes at this external field as the angle between it and the ${\mathrm{Mu}}^{\mathrm{*}}$ symmetry axis approaches 90°. At 200 K no cusp is seen because ${\mathrm{Mu}}^{\mathrm{*}}$ has been thermally annihilated. Fractional populations of the different muonium species are derived from the 53-K data. All ${\mathrm{Mu}}^{\mathrm{*}}$ and other anisotropic centers, including molecular radicals, should exhibit such a cusp, allowing the possibility of observing such species even in disordered or amorphous solids where spectroscopic detection might be impossible. Integral counting (and therefore very high data rates) can be used for these observations.

Self-consistent linear muffin-tin-orbital band-structure calculations are used to investigate the optical and structural properties of III-V semiconducting nitrides under hydrostatic pressure. The pressure behavior of the energy band structures is discussed in the context of the postulated chemical trends in III-V semiconductors. The regions in k space of dominant interband contributions to the elements of structure in the dielectric functions are identified. The total-energy calculations suggest that all the nitrides under pressure transform to the semiconducting rocksalt phase. The calculated transition pressures are 21.6 GPA (InN), 51.8 GPa (GaN), 16.6 GPa (AlN), and 850 GPa (BN). Experimental values that agree well with this have been found for the first three compounds. The fact that GaN and AlN have such different transition pressures in spite of their very similar ionicities is explained by the presence of 3d states on Ga.

Significant atomic rearrangement at the interface was found to take place after post-growth annealing treatments of epitaxial ZnSe on As-passivated Si(001) substrates which were tilted by 4° towards the [11¯0] direction. The thermal stability of the ZnSe/As:Si interface was studied by rapid thermal annealing at temperatures up to 960 °C after growing an epitaxial GaAs cap layer to prevent evaporation of the ZnSe during the anneals. The ZnSe/As:Si interface was examined by high-resolution electron microscopy. After an anneal at 900 °C the ZnSe/As:Si interface transformed from an atomically smooth interface found in the as-grown films to a facetted structure with {111}-oriented sidewalls that extended preferentially in the [11¯0] direction. The 60° dislocations that were previously observed along this direc- tion combined into closely spaced pairs or into Lomer dislocations which were associated with the facets. We present a model for the atomic structure of the facetted interface which is consistent with the experimental data and satisfies electron-counting considerations. Total-energy calculations of the ZnSe/As:Si(001) interface were compared with those for the {111} interfaces seen after facetting.

To study the changes in structure and morphology of a Ge thin film as a function of annealing temperature, we implemented complementary surface analysis techniques such as angle-resolved ultraviolet-photoelectron spectroscopy, x-ray-photoelectron diffraction (XPD), and low-energy electron diffraction. The Ge thin film (≊15 Å) was obtained by exposing a Si(111)7×7 surface to catalytically decomposed germane at room temperature. The amorphous-crystalline transition temperature was found to be close to 400 °C. A 5×5 reconstruction of the Ge surface occurred for a 500 °C annealing temperature and remained stable to 600 °C. This 5×5 surface reconstruction appeared to have an effect on the XPD curve recorded along the Γ¯-M¯ line in the 1×1 surface Brillouin zone. A 2.5° shift of the peak located near 55° was observed along the [1¯21¯] azimuthal direction. Annealing at 700 °C led to the indiffusion of Ge into Si or to Ge islanding: the 5×5 surface reconstruction turned into a 7×7 surface reconstruction.

Temperature-dependent surface morphologies resulting from spontaneous Br etching of Si(100)-2×1 in the range 600–1100 K have been studied using scanning tunneling microscopy. The etch pits and Si structures on the exposed surfaces exhibit temperature-dependent shape, size, and distribution characteristics. Although the morphology depends on temperature, the steady-state removal of Si is dominated by layer-by-layer etching that produces bounded surface roughness. Temperature-dependent kinetics, surface reactivities, and product evolution are responsible for the different morphologies.

Satellite peaks have been observed in undoped and p-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells (QW’s) in selective photoluminescence spectra upon excitation resonant with the free-exciton states. The satellites are interpreted as being due to shake-up transitions involving QW hole subbands, in which valence-band holes are shaken up from the lowest heavy-hole (hh) band to the higher light-hole or the n=2 hh band; thus these shake-up satellites are of intrinsic origin. The prerequisite for the shake-up process is sufficient overlap of the wave function of a free exciton and an adjacent weakly localized hole. This localization is proposed to be due to interface roughness as evidenced by the Stokes’s shift for the free exciton. Calculations have shown that interface roughness can give rise to the required localization by producing localized states that are weakly bound to each subband state. The measured intersubband transition energies agree with independent experimental results and theoretical predictions. A striking enhancement of the shake-up satellite intensity is observed with increasing applied magnetic field. This fact is consistent with an increasing localization of the exciton due to the compression of the wave function. The localization becomes increasingly important when the wave function of the exciton is comparable with or smaller than the lateral size of the island characterizing the interface. From the magnetic-field dependence of the shake-up satellite intensity we can roughly estimate the island size. In fact, the magnetic-field dependence can be utilized for probing the interface quality.

We examine the interaction between the Wannier-Stark ladders derived from two minibands in a semiconductor superlattice. We show that it leads to anticrossings whose magnitude and electric-field position can be calculated in a simple analytical model.

We have measured the low-temperature magnetoresistance of long (>50 μm), narrow (∼1 μm) quantum wires fabricated from a high-mobility two-dimensional electron gas. In the quantum Hall regime at high magnetic fields we observe that the Shubnikov–de Haas oscillations split into a series of aperiodic sharp peaks. Both the amplitude and sharpness of the peaks are strongly dependent on temperature below 1 K. We attribute the peaks to resonant tunneling between edge states through localized states in the bulk. The results are in quantitative agreement with the theoretical model of Jain and Kivelson.

We have investigated the nonequilibrium edge-state transport when a current is injected into inner edge states in the spin-resolved integer quantum Hall regime. The coupling between the spin-split states is observed generally to be stronger than that between the Landau levels. When the Fermi energy lies between the spin-split levels, the decoupling of the innermost state from the remaining Landau levels is remarkably improved even in the presence of metallic contacts placed on mesa boundaries, indicating that the wave function of the innermost state is substantially shifted toward the bulk. The magnetic-field dependence of the scattering between the spin-split states is found to exhibit a peculiar behavior. When more than two edge states are occupied in the bulk, the mixing is suppressed as the magnetic field is increased. On the other hand, it is enhanced when only a single Landau level is occupied. This result is ascribed to the fact that the scattering rate in the latter case is determined by spin flip due to spin-orbit interaction.

Photoluminescence spectra were measured as a function of temperature and excitation intensity in two periodic multiple narrow asymmetric coupled quantum-well structures. The peaks of the spectra have been identified and the temperature-dependent natures of the recombination processes have been determined. The half-widths at half maximum of the line shapes were deduced by fitting the low-energy sides to Gaussian functions. The excellent fits indicate that inhomogeneous broadening is dominant. The rate at which the linewidth broadens with temperature is more dramatic below 140 K due to the sharp changes of the ionized impurity and of the longitudinal-optical-phonon scattering mechanisms. Above 140 K, the linewidth has a linear dependence on temperature due to the thermal excitation of the longitudinal optical phonons. By using the model proposed by Lee et al. we have determined the scattering rates of the longitudinal acoustic and optical phonons, of the impurities, and of the inhomogeneous broadening. Our results show that unless the growth is interrupted at each interface, the inhomogeneous broadening remains dominant up to room temperature. Measurement of the temperature dependence of the photoluminescence linewidth proves to be a simple and effective technique to investigate the scattering mechanisms of the photogenerated carriers and the origins of the emission peak’s linewidth broadening in these structures.

A structure model for the Ge(111)-Au(√3 × √3 )R30° surface reconstruction is proposed based on scanning tunneling microscopy (STM) and photoelectron spectroscopy on the Ge 3d and Au 4f core lines. The basic unit is a ${\mathrm{Au}}_3$Ge molecule binding in one-third of the ${\mathit{T}}_1$ substrate sites with a gold trimer pointing out of the surface. This leaves two-thirds of a monolayer of unoccupied ${\mathit{T}}_1$ sites which make up a hexagonal honeycomb pattern. Two types of STM images have been obtained which are explained within this model, where either the trimers or the substrate Ge atoms are probed depending on the specific tip conditions in combination with the sample bias voltage. Furthermore, small insets of a metallic (1×1) structure are found at low gold coverage together with a distorted (2×2) surface structure.

A simple method for calculating the free-exciton binding energies in the fractional-dimensional-space model for single-quantum-well (SQW) structures has been extended to symmetric coupled double quantum wells (CDQW). We also apply these results to estimate the diamagnetic shift of the free excitons for both CDQW and SQW systems. Corresponding experimental photoluminescence data are obtained on CDQW and SQW samples grown by molecular-beam epitaxy. The calculated binding energies and diamagnetic shifts show good agreement with the experimental data.

We present an approach for modeling nonequilibrium steady-state transport in quantum dots with interacting electrons, which employs real-time many-body Green’s functions on a tight-binding basis and accounts for dot-lead coupling exactly. This real-time Green’s-function description of transport is used to investigate the steady-state charge-density and current-voltage characteristics of one- and two-dimensional quantum dots having interacting electrons. The quantum dots are composed of a rectangular lattice of tight-binding sites, allowing for multiple degrees of freedom and a spatially varying external electric field. We investigate the effects of both the dot-lead coupling strength and finite current flowing in the dot. Our results indicate that as dot-lead coupling is increased, the Coulomb blockade disappears smoothly; furthermore, the Coulomb blockade may persist even in dots which have tunnel-junction resistances less than h/${\mathit{e}}^2$. We also find that in the Coulomb-blockade regime under conditions of current flow, the assumption of integer steady-state charge on the dot does not necessarily hold true, although the effect of single-electron tunneling does.

The energy levels and binding energies of an off-center donor in a GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As spherical quantum dot are calculated by a linear variational method. The results clearly show the quantum size effect. The location effects of a donor ion on splitting, level ordering, and binding of the donor states are studied. It is found that the alteration of the position of a single Coulomb center can largely change the single-electron spectrum in a quantum dot with a larger radius.

We have solved the problem of dynamical localization in a one-dimensional system with an impurity under the action of a dc field along the single-band tight-binding model. By fixing the impurity potential, we show that characteristic field values can make two on-site energies coincide, enhancing in this way the hopping of carriers between the two sites. The distance between the degenerate sites decreases with increasing field intensity. So for high-field values the sites can be nearest neighbors producing strong oscillations. On the other hand, for weak-field values the two degenerate sites are far apart from each other and, as a result, there is an almost complete localization at the impurity site. We propose a model which allows us to determine the impurity level in a superlattice.

We have studied the polarization anisotropy of the radiative recombination of a two-dimensional hole gas (2DHG) confined at the interface of a modulation-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterostructure. The observed ‘‘cleaved-side’’ luminescence from the heavy-hole ground subband is due to indirect optical transitions in real space and is found to be polarized perpendicular to the plane of the 2DHG. This unexpected behavior is ascribed to the admixture of light-hole states to the heavy-hole ground subband. For increasing in-plane wave vector, the degree of linear polarization decreases due to an enhanced penetration of the heavy-hole states into the bulk GaAs layer. These results can be qualitatively explained by our calculations.