The reaction of Al deposited on Sb(111) was studied by high-resolution electron-energy-loss spectroscopy. The formation of AlSb was confirmed by the observation of the surface optical phonon of the compound at 332 ${\mathrm{cm}}^{\mathrm{-}1}$ (41 meV). A quantitative interpretation of the AlSb/Sb interface formation is proposed, based on the dielectric theory of electron-energy-loss spectroscopy.

We have studied the vibrational and electronic structure and the elementary excitations of poly [1,6-di(N-carbazolyl)-2,4-hexadiyne] (polyDCHD), poly[1-(N-carbazolyl)penta-1,3-diyn-5-ol] (polyCPDO), and poly[1-(N-carbazolyl)penta-1,3-diyn-5-acetoxy] (polyCPDA) by electronic absorption, Raman scattering, and photoinduced absorption in the infrared region. The long-lived photoexcitations are charged bipolarons with associated infrared-active vibrational (IRAV) modes. The results are also discussed in relation to the degree of chain order of the different polydiacetylenes as evidenced by the electronic spectra. Only poly(DCHD) shows an absorption spectrum dominated by a single exciton, suggesting a one-dimensional ordered structure. Consequently, for this polymer only, triplet excitons with a lifetime of the order of ms are photogenerated in addition to bipolarons. Among polydiacetylenes, only poly(CPDA) shows photoinduced IRAV modes not affected by Fano coupling between electronic and vibrational states.

A scanning tunneling microscope has been used to extract atoms that are adjacent to native defects on the Si(001) surface. We found a spatially anisotropic extraction frequency for the different sites around a so-called C defect. The most common occurrence is the extraction of Si atoms from a C defect to form a double dimer vacancy. Extraction of double dimers from the neighboring dimer row on one side of the defect was also observed, but dimers in the neighboring dimer row on the other side of the defect and dimers adjacent to the defect in the same row were rarely extracted. In the case of native dimer vacancy defects, few neighboring Si atoms were found to be extracted. We discuss the relationship of these observations to the energetics of defects on the Si(001) surface.

Tunneling between two quantum dots is studied at low temperatures. The quantum dots are formed by the combined sidewall confinement and vertical confinement in an ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As-GaAs triple-barrier diode with a conducting diameter of 180 nm. The fine structure that is observed in the main resonance peaks of the current-voltage characteristics is related to lateral quantization effects. Electrons tunnel between zero-dimensional (0D) states in the two coupled quantum dots. A magnetic field applied perpendicular (transverse) to the tunneling direction shifts the main (2D) resonance peaks to higher bias and causes a substantial broadening. Within the fine structure we find that the resonance positions are virtually magnetic-field independent, whereas the resonance amplitudes show significant variations with increasing magnetic field; a simple model is developed to describe this behavior in terms of the magnetic-field dependence of the interdot transition probabilities.

We present a detailed analysis of a fully ab initio molecular-dynamics simulation of the surface reaction for ${\mathrm{Cl}}_2$ molecules incident on Si(111)-2×1 at an energy of 1 eV. We find that ${\mathrm{Cl}}_2$ adsorption on Si(111)-2×1 is dissociative with short-lived molecular precursor states involved in certain dissociation geometries. Dissociation is accompanied by an enormous surface response causing large, localized rehybridization effects in the π-bonded chains and excitation of vibrational modes in the substrate. Analysis of the charge density in the bond within the molecule during the breakup shows that the dissociation is chemically driven and proceeds by transferring charge to a more antibonding molecular orbital.

The mechanisms whereby 60° misfit dislocations are generated from dissociated threading dislocations in quantum-well heterostructures are considered. The two partial dislocations experience different misfit stresses, resulting in each partial having a different critical thickness. As a consequence, a number of different dislocation configurations are predicted, including the possibility of producing stacking faults of infinite width.

The optical properties of HgTe/CdTe superlattices (SL’s) in the three principal regimes are summarized. Experimental results from the photoluminescence and the cyclotron resonance of electrons and holes both parallel and perpendicular to the SL layer plane in the SL’s in the normal semiconducting regime are presented. These results, together with the previously reported optical and magneto-optical spectra of a series of SL’s in the inverted-band regime, are analyzed systematically under the framework of a six-band k⋅p model. Using a single value of 550 meV for the valence-band offset (VBO) the model correctly predicts all the major experimental results. These include (i) the energy gap between the electron subband and the heavy-hole subband and the energy gap between the electron subband and the light-hole subband of the SL’s in the normal regime over the temperature range from 4.5 to 150 K, (ii) the in-plane masses of the electrons and holes and the Voigt geometry mass of electrons of the SL’s in the normal semiconducting regime, (iii) the in-plane electron and hole masses in a SL in the semimetallic regime, (iv) the energy gap between the second electron subband and the second heavy-hole subband in the SL’s in the inverted-band regime, and (v) the electron in-plane masses of the SL’s in the inverted-band regime. The value of VBO that fits the experimental results is 550±50 meV at 5 K and it reduces, probably linearly, to 500±50 meV at 120 K. The origin of the discrepancy between the value of VBO found in this work and the ones reported in earlier works is discussed.

For the calculation of the ground-state energy in thin cylindrical quantum dots with an arbitrary radial potential, a theoretical approach has been developed. The results of the calculations are compared to the experimental observations of the photoluminescence energy of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum dots realized by the implantation-induced intermixing technique. For these structures an error-function-like radial potential is assumed and its steepness was estimated from the comparison with the calculations. It is concluded that the observed diameter-dependent luminescence blueshift is caused by the confinement in the radial potential, rather than by intermixing in the center of the quantum dots.

Recently, an optical emission process, termed the H band, has been observed in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructures which has been related to the regions surrounding the heterointerfaces. We show here that the mechanism responsible for this H-band photoluminescence (PL) in our structures is the recombination of quasi-two-dimensional (2D) excitons–and is not due to either the recombination of carriers bound at impurities and/or defects, or the recombination of 2D carriers with 3D free carriers. We have used such PL in high-purity, virtually ‘‘interface-free’’ GaAs(${\mathit{n}}^{\mathrm{-}}$)/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As(p) double heterostructures to study H-band decay dynamics, and thus prove that these excitations in our structures are ‘‘intrinsic’’ and arise from quantum-mechanically bound quasi-2D excitons. Time-resolved PL spectra show the emission to be spectrally nonstationary, with lifetimes across the band varying from a few nanoseconds to more than 50 μs. Further, we find that large interfacial recombination velocities, in inferior samples, may mask the truly intrinsic H-band recombination dynamics. Our accompanying quantum-mechanical numerical modeling of such 2D excitons allows us to interpret and reproduce virtually all of our experimental observations. Indeed, they demonstrate that H-band PL cannot be impurity induced, but instead arises from the recombination of intrinsic excitonic states bound at both heterointerfaces. Hence we find that in thinner structures, these excitonic states may be simultaneously associated with both interfaces, with a critical GaAs thickness at which this ‘‘exciton sharing’’ between heterointerfaces becomes significant of <0.5 μm. We also discuss the time evolution of the initially photoexcited 3D bulk excitons as they acquire this subsequent 2D character, through a mechanism at the interfaces analogous to the quantum confined Stark effect in quantum wells.

Our combined experimental and theoretical modeling, in these virtually interface-free samples, therefore, provide a direct measure and full quantum-mechanical explanation of the temporal evolution of these intrinsic 2D excitons–from 3D formation to 2D recombination–as undistorted by the deleterious influence of carrier trapping and nonradiative decay (in both the bulk and at interfaces), which may otherwise dominate the usually more-imperfect typical heterostructure. The results presented here are for specific, high-quality, interface-free GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As heterostructures; however, these measurement and analysis techniques may also be applicable to other types of structures.

The persistent current in isolated mesoscopic rings is studied using the continuum and tight-binding models of independent electrons. The calculation is performed with disorder and also at finite temperature. In the absence of disorder and at zero temperature agreement is obtained with earlier results by Loss and Goldbart in that there is half-quantum flux periodicity for a large and odd number of electrons, but full-quantum periodicity for any even number of electrons in the ring. Strong disorder converts the period into full-quantum periodicity. Finite temperature reduces the magnitude of the current, but preserves the quantum flux periodicity at zero temperature. However, the sign of the current may change as disorder or temperature is increased. A generalization of the parity effect, previously discussed by Leggett, Loss, and Kusmartsev is described for the case where there are electrons with spin, influenced by finite temperature and disorder.

The optical properties of pseudomorphic (defect-free) (Si${)}_{\mathit{n}}$/(Ge${)}_{\mathit{n}}$ strained-layer superlattices grown on ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ (001) substrates are studied within the framework of the empirical tight-binding method. Attention is focused on the nature (direct/indirect) of the fundamental energy gap and on the effects of the superlattice periodicity on the optical absorption near the band edge. The band-edge absorption is the main distinguishing feature among different superlattices. It is found that the behavior of the absorption is strongly dependent on light polarization (anisotropy) and substrate lattice constant. The contribution of the lowest interband transitions to absorption spectra is also analyzed. Theoretical results are compared with recent experimental data.

The effects of an electric field along the [110] growth axis on the polarization properties of the interband transitions in a (110) quantum well are studied within a multiband effective-mass approximation. The transfer-matrix method is applied to coupled effective-mass equations in order to obtain the eigenenergies and eigenstates of holes under an electric field within a steplike approximation to the potential. The calculated results are shown for a ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As/${\mathrm{Al}}_{\mathit{y}}$${\mathrm{In}}_{1\mathrm{-}\mathit{y}}$As quantum well grown on a (110) InP substrate in which the ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As well layer is strained due to a lattice mismatch to the substrate, while the lattice constant of the ${\mathrm{Al}}_{\mathit{y}}$${\mathrm{In}}_{1\mathrm{-}\mathit{y}}$As barrier layer is matched to that of the substrate. In the absence of an electric field, the first hole level (ν1) in a quantum well having a Ga content of x=0.58 and a well width of 80 Å is proved to have a light-hole character, while the second level (ν2) has a heavy-hole character, due to the effect of tensile strain. The calculated result of the electric-field dependence of the hole energy levels for this quantum well shows an anticrossing behavior between the ν1 and ν2 levels. The optical matrix element of the dipole transition between the first electron state (c1) and the ν1 or ν2 state shows an anomalous behavior in the electric-field dependence, related to the anticrossing: For linear polarization along the [110] growth axis, the optical matrix element for the c1-ν1 transition suddenly decreases with increasing field, while that for the c1-ν2 transition rises in the anticrossing region, representing character changes from light- to heavy-hole-like and vice versa; for polarization parallel to the (110) quantum well, the in-plane optical anisotropy is extremely enhanced to as much as 100% near the anticrossing.

A hydrodynamic description of the collective excitations of an inhomogeneous electronic system is developed on the basis of the Thomas–Fermi–Dirac–von Weizsäcker approximation to the equilibrium ground state. This approximation allows one to define realistic equilibrium densities which are then used to obtain a consistent description of the dynamical behavior. An application to a parabolically confined electron gas is presented and the magnetoplasmon modes are obtained from a solution of the linearized hydrodynamic equations. The wave-vector dispersion of the modes is determined, as well as the detailed dependence on the orientation of the applied magnetic field. The power absorption in the long-wavelength limit is also calculated to illustrate the center-of-mass mode excitations probed by transmission experiments.

We have studied the electronic properties of anisotropic quantum dots in a magnetic field. The evolution of single-electron energy levels as a function of magnetic field and the corresponding chemical potentials are investigated for various cases of anisotropy. We have also analyzed the effect of interelectron interaction on the isotropic as well as anisotropic quantum dots by calculating the energy spectra and the pair-correlation function of a two-electron system.

A model for a layered electron system is developed in which the electrons are treated completely quantum mechanically. In this model the ionic cores are replaced by parallel sheets of uniform positive charge. These charged sheets provide the neutralizing charge and the external potential which confines the interacting electrons in the local-density-functional calculation of the ground-state system. The plasma excitations of this model are obtained in a self-consistent calculation using the time-dependent local-density approximation.

We investigate electronic states and the far-infrared absorption spectrum of a two-dimensional (2D) hydrogenic impurity in a parabolic quantum wire in a magnetic field. The problem is mapped into the problem of interacting nonlinear harmonic oscillators. The evolution of the energy levels and level statistics from a 2D to a 1D effective hydrogen atom is investigated in the quantum and classically chaotic regimes. The ground-state energy reflects a delicate balance between a blueshift due to confinement and a redshift due to an increase in binding energy. In the absence of a magnetic field, the model reduces to the well-known problem of quantum chaos in a 3D hydrogen atom in a magnetic field in a zero angular-momentum channel. The presence of a magnetic field in the wire breaks the scaling behavior inherent in the 3D hydrogen problem.

We present a detailed analysis of the influence of the various phonon modes characteristic of the single heterostructure ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs on its electronic transport by using a Monte Carlo simulation. The electronic states of the system are calculated by solving self-consistently the coupled Schrödinger-Poisson equations for the system. LO-phonon states are treated within the dielectric continuum model by using two different dielectric functions to describe the two semiconductors, the usual Lyddane-Sachs-Teller expression for GaAs and a generalized two poles expression for ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As. Two sets of optical modes characterize the system, the half-space LO modes and the interface modes. The scattering rates for the interaction of these modes with the confined electrons are calculated from the Fermi golden rule. A Monte Carlo simulation is then used to study the effect of the electron-phonon interaction on the transport properties of a single ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterostructure in the presence of an electric field applied along the heterointerface. The results of simulations performed at 300 and 77 K compare favorably with available experimental data. Drag and heating effects related to nonequilibrium phonon effects are found and discussed.

In this work, the optical transmission and reflection of α-SiC:H thin films, grown by rf sputtering, are studied. The absorption coefficient is found. The real (${\mathrm{\varepsilon }}_1$) and the imaginary (${\mathrm{\varepsilon }}_2$) part of the dielectric function are determined by means of ellipsometric measurements, in the region 1.5–6.3 eV. The dependence of ${\mathrm{\varepsilon }}_1$ and ${\mathrm{\varepsilon }}_2$ on substrate temperature and hydrogen flow rate is obtained. The experimental results are modeled using a theoretical formula for the dielectric function. rf ion etching and an in situ ellipsometric technique are used to check the influence of possible overlayers and establish realistic values of the dielectric function. The energy gaps of the films are found to be controlled by their content in hydrogen, increasing with hydrogen incorporation. Evidence that the incorporation of hydrogen induces the formation of voids, resulting in a density deficit in the front surface layers, is given. The maximum values ${\mathrm{\varepsilon }}_{2\mathrm{m}\mathrm{a}\mathrm{x}}$, found at approximately 5 eV, are reduced with the increase of the hydrogen flow rate, while increased substrate temperatures result in increased optical response. The formation of α-Si or α-C clusters in the film-substrate interface of the rf sputtered α-SiC:H films is probed using back reflection techniques.

The collective excitations and the metallization of the Bi/GaAs(110) interface grown at room temperature, up to the completion of a few tens of a monolayer, have been studied by means of the high-resolution electron-energy-loss spectroscopy (HREELS). Through analysis of the HREELS data, also by means of an appropriate semiclassical dielectric model, the modifications experienced by the substrate-related loss structures (Fuchs-Kliewer phonon and dopant-induced free-carrier plasmon) and by the quasielastic peak are related to changes in the dielectric response of the overlayer and in the semiconductor space-charge region. The influence of bismuth is effective in enlarging the depletion layer thickness, leaving the interface semiconducting at the monolayer-coverage scale. A band bending value of 0.56 eV is obtained at the coverage of one monolayer on the highly n-type doped sample (n∼2.7×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$); a determination free from any possible surface photovoltaic effect. At coverages greater than two monolayers, which corresponds to a structural transition, the intermediate structural phase becomes metallic, thus marking a clear semiconductor-metal transition. This intermediate metallic stage further develops towards the formation of actual semimetallic crystalline bismuth layers oriented with the basal plane parallel to the substrate surface.

We study the zero-temperature energy and atomic structure of twist grain boundaries (GB’s) in diamond, using a semiempirical potential due to Tersoff. The (111) twist GB energies are similar to predictions of an empirical tight-binding potential. For the previously unstudied (001) twist boundaries in diamond, both the energy and the excess volume are insensitive to misorientation at large twist angles, although small energy cusps exist for the so-called Σ5 GB’s. The (001) twist GB energies are typically ≊ 6 J/${\mathrm{m}}^2$. Significant atomic relaxation occurs only in the two atomic layers nearest the grain boundary. Atoms in these contact layers are predominantly threefold coordinated. For (001) twist GB’s, in-plane translations significantly affect the Σ5(001) energies, but not the ${\mathrm{\Sigma }}_{\mathit{n}}$ energies, with n large.