We present a theoretical investigation of weak localization in a two-dimensional electron gas with unidirectional modulation by a cosine potential. By lowering the electron density the system gradually turns from a weakly modulated electron gas into an array of almost isolated quantum wires of finite width. A dimensional transition occurs in such a system due to the decreasing widths of the resulting minibands. The reduction of the dimensionality of interference effects takes place when coherent tunneling of electrons between adjacent wires is suppressed. In this case we find a pronounced enhancement of the weak-localization correction to the conductivity. Its functional dependence on the phase coherence length develops from a logarithmic behavior to a linear one, typical for one dimension. Close to the transition to strong localization the correction due to interference effects becomes additive in the resistance. The localization length deduced from the calculations exhibits a continuous change from an exponential to a linear dependence on the scattering length at the dimensional transition. Numerical results are discussed for GaAs structures with different electron densities.

It is shown that at sufficiently low temperatures, one-dimensional arrays of ultrasmall tunnel junctions (for example, semiconductor superlattices) may exhibit an unusual type of electron transfer. This process can be considered as fast ‘‘Bloch’’ oscillations with frequency ${\mathit{f}}_{\mathit{B}}$=ɛ/h (where ɛ is the electron energy change due to tunneling through one tunnel barrier), modulated with lower single-electron-tunneling frequency ${\mathit{f}}_{\mathit{S}}$=I/e (where I is the dc electric current through the array).

We present a statistical model of step meandering on small-miscut vicinal Si(100) surfaces, generalized to treat arbitrary azimuth. The model is solved using a transfer-matrix technique and the free energy of the vicinal Si(100) surface is calculated. From the free energy the equilibrium crystal shape is derived at various temperatures. The results are compared with existing experimental data.

We studied ballistic transport of electrons through quantum structures in a magnetic field. A numerical scheme based on the finite-element method for solving the two-dimensional single-electron Schrödinger equation has been extended to the case with a magnetic field. The method is applicable to transmission calculations of two-dimensional quantum structures of arbitrary geometry and any number of leads. We have applied this method to study the magnetoconductance of a stadium-shaped quantum structure.

We study the density of states (DOS) for a periodically modulated two-dimensional (2D) electronic system in a perpendicular magnetic field in the presence of a smooth random potential. We find an exact analytical solution for the DOS which is valid under the conditions that both the period of modulation and the correlation length of random potential are larger than the magnetic length. The magnetocapacitance, calculated with the use of this DOS, fits nicely the experimental data by Weiss et al. for the case of one-dimensional periodic potential. The fit of experimental data by Ismail et al. for magnetocapacitance of the samples with two-dimensional periodic modulation is also presented.

Exciton radiative decay and polaritons in single and double quantum wells are compared and contrasted. Two types of eigenmodes exist in double quantum wells, with the dipole moment in the quantum wells parallel or antiparallel. Energy splittings $\Delta \left({\mathbf{k}}_{\parallel }\right)$ between the two types of modes at small ${\mathbf{k}}_{\parallel })$ are on the order of 0.1 meV for $\frac{\mathrm{GaAs}}{{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}}$-based structures of well width ∼100 Å, which accounts for part of the increased luminescence linewidth in double compared with single quantum wells. Thus the polariton effect gives rise to an intrinsic source of inhomogeneous broadening through $\Delta \left({\mathbf{k}}_{\parallel }\right)$ as well as homogeneous broadening through the radiative width, and both effects are of the same magnitude.

A comparison of stresses among Si, C, and Ge dimers on the Si(100) surface has been performed by using first-principles calculations. The C dimer has been found to induce both tensile and compressive stresses depending on its azimuthal direction, while the Ge dimer has been found to induce almost identical stress to that of the Si dimer in the asymmetric form. Not only the atomic-radius difference among these group-IV atoms but also differences in the flexibility of their bonds determines the nature of the surface stresses.

Quasiparticle band structures of short-period superlattices and ordered alloys of AlN and GaN are calculated using the GW approximation. The calculated gaps range from 3.1 eV to 5.8 eV. Two types of structural materials are studied: zinc-blende superlattices in the (111) direction and hexagonal wurtzite superlattices and ordered alloys in the (001) direction. Comparisons between band structures of these materials using folding of Brillouin zone points and the transition from an indirect to direct gap semiconductor in the zinc-blende compounds are discussed. All of the wurtzite structures studied are found to have direct band gaps with values between 3.5 and 5.8 eV. The 1×1 and 1×2 AlN/GaN zinc-blende superlattices have direct gaps of 4.4 eV and 4.3 eV, respectively, whereas the 2×1 AlN/GaN superlattice gap is 4.6 eV and is indirect. Several fundamental gaps are predicted and remain to be tested by experiment.

We have used in situ x-ray-scattering techniques to analyze surface reconstructions and ordered steps at various stages in the growth of GaAs by organometallic vapor-phase epitaxy. In contrast to an earlier report of a c(4×4) reconstruction, our current measurements reveal a related structure with 1×2 symmetry. The 1×2 structure is a disordered form of the c(4×4) reconstruction in which long-range correlations between dimer rows are preserved but dimer vacancies are located at random. Aside from reconstructions, ordered step structures were studied on samples with both high (2.2°) and low (0.07°) levels of miscut. Using Gaussian step-spacing distributions with widths of order the mean terrace size, a numerical model is used to reproduce measured truncation rod profiles in scans perpendicular to steps. We also use the model to follow the narrowing of the step-spacing distribution which occurs during growth. After the growth of several hundred angstroms of GaAs, truncation-rod profiles sharpen, accompanied by a shift in peak positions. The shift is due to an asymmetric weighting of the scattering from terraces of different widths: wide terraces make a greater contribution to the structure factor. For samples with a broadened distribution of step spacings, this effect results in a smaller peak splitting than that which would occur for an ideally miscut sample.

Scattering from a localized potential of arbitrary strength is found for weakly interacting electrons. Poor man’s renormalization group procedure allows us to calculate the renormalized transmission amplitudes at any energies. Simple formulas that describe the conductance at any temperature are derived. In the presence of the electron-electron backscattering, the low-temperature conductance deviates from the results of Luttinger liquid theory. In particular, the temperature dependence of the conductance may become nonmonotonic. In the presence of a magnetic field, backscattering gives rise to a peak in the differential conductance at bias equal to the Zeeman splitting.

Epitaxial Ce layers have been obtained by evaporation on a V(110) surface. The Ce phase reported some years ago for this system is not confirmed in our experiments: the analysis of reflection high-energy electron-diffraction and x-ray-photoemission data leads us to identify the epitaxial Ce phase with a Ce-γ phase. Under several circumstances, epitaxy of Ce oxides has been observed on V(110): in these cases, the epitaxial surface lattice is found to be hexagonal with a parameter ${\mathit{a}}_{\mathit{s}}$=3.9±0.1 Å, and does not depend (in the limit of experimental errors) on the amount of oxygen.

It is generally accepted that scanning tunneling microscopy and scanning tunneling spectroscopy (STS) get their information from the sample local density of states. At present, however, there is little theoretical ground on which to explore this assumption. We contribute here with a theory of STS for semiconductors, treated within the tight-binding approach. In the context of this theory, we demonstrate that the current-voltage (I-V), and the conductance-voltage (σ-V) curves are related to the density of states. In particular, the current is a sum over tip states of semiconductor local density of states modulated by tip-dependent coefficients. This result is consistent with those found in previous works. The σ-V curve has nondifferentiable points which are a direct manifestation of the Van Hove singularities in the global density of states of the semiconductor.

The dielectric constant and ac conductivity of rf-sputtered insulating zinc oxide films has been measured in the metal-insulator-metal configuration over a wide temperature (90–500 K) and frequency (0.1–100 KHz) range. The films sputtered from oxide targets in argon were c-axis oriented having a resistivity of ≃${10}^{12}$ Ω cm. The dielectric constant shows a strong frequency dispersion at temperatures above 330 K which is the intrinsic behavior of the films. The dielectric dispersion data has been fitted to (i) Debye-type relaxation (with a distribution of relaxation times), (ii) a random-free-energy-barrier model (which assumes that for hopping conduction, the dielectric dispersion is correlated to the dc conductivity), and (iii) the dielectric modulus (which assumes that microscopic inhomogeneities and the distribution of conductivity relaxation times is responsible for dielectric dispersion). The origin of dielectric dispersion has been discussed in the light of these models. It has been found that hopping conduction dominates in ZnO films up to 450 K.

Auger-electron spectroscopy and electron-energy-loss spectroscopy methods have been employed to study the growth process and possible intermixing of Au and Pd films on top of polycrystalline In at 90 K. To accomplish this, In films (thickness 30 nm) were deposited by ion-beam sputtering onto fused-quartz substrates and then stepwise covered by Au or Pd up to a total coverage of 5 nm. During this process, the intensities of the Auger emission lines and the accompanying plasmon loss lines of In were measured as a function of the increasing Au or Pd coverage. Additional information was extracted by insitu comparison of energy-loss spectra of Pd/In interfaces obtained with 120-eV electrons to those of bulk ${\mathrm{In}}_3$Pd and ${\mathrm{In}}_3$${\mathrm{Pd}}_5$ alloys as well as of pure In and Pd. The results indicate that during the deposition of Au or Pd onto In intermixed phases are formed even at 90 K. This intermixing is restricted to a coverage of 1.5 nm for Au and 0.4 nm for Pd corresponding to mixed layer thicknesses of 6.0 and 2.8 nm, respectively. For higher coverages, growth of the pure metals is observed.

Autoionization from vibrationally resolved core excited states is demonstrated, using nitrogen physisorbed on graphite. The results are compared to photoemission and x-ray absorption, and reveal an adsorption induced elongation of the internuclear separation in the 3${\mathrm{\sigma }}_{\mathit{g}}^{\mathrm{-}1}$ final state. It is demonstrated that it is only in the case of vibrationally selective excitation that the autoionization final states coincide with the photoemission final states.

Previous work has described the growth mode of tin deposits on the Au(111) surface and the subsequent oxidation of the tin at room temperature. Because the gold does not oxidize under the conditions of this experiment, one can monitor the growth of the oxide layer in an incremental fashion. This paper extends the study to oxidation at higher temperatures. From 500 to 800 K at about ${10}^{\mathrm{-}6}$ Torr of oxygen, at least half the tin in-diffuses, with the rest forming an ordered ${\mathrm{SnO}}_2$ layer on the surface. This ordered oxide has no counterpart for bulk tin, which melts at 505 K. Heating above 800 K causes diffusion of the remaining tin and oxygen into the gold substrate. The formation of the oxide layer has been studied by Auger spectroscopy, electron-energy-loss spectroscopy, work-function measurements, and low-energy-electron diffraction.

The internal excitation of silver clusters sputtered from a polycrystalline silver surface by ${\mathrm{Ar}}^{+}$ ions of 5 keV was investigated by laser postionization of the ejected neutral species and time-of-flight mass spectrometry. For sputtered dimers, resonant two-photon ionization spectroscopy was employed using either a one- or a two-color scheme. From the resulting spectra, vibrational and rotational population temperatures around 2700 and 6700 K are determined which agree well with corresponding predictions obtained from molecular-dynamics simulations of the molecule sputtering process. For larger clusters, single-photon ionization by a fixed frequency laser was used for cluster detection and the spectra obtained with different laser wavelengths were compared. The results show that around 50% of the sputtered ${\mathrm{Ag}}_6$ clusters are formed with internal energies in excess of 0.75 eV.

The initial nucleation and subsequent growth of vapor-deposited Co on Pt(111) has been studied by scanning tunneling microscopy and Auger-electron spectroscopy. When deposited at room temperature, Co nucleates spatially in a homogeneous manner and then grows quasi-layer-by-layer up to a coverage of FTHETA=3 monolayers. For FTHETA>3 monolayers, a three-dimensional island growth mode is observed. From a detailed analysis of the observed island shapes we present direct evidence that Co grows in a hcp pattern on Pt(111) after the third layer. The nucleation of Co at a deposition temperature above 300 K is strongly influenced by the appearance of a Co-induced surface reconstruction of Pt(111). This reconstruction leads to a dendritic growth of Co deposited at 400 K. Fingers of Co 3–5 nm wide and up to 250 nm long can be observed. The influence of the Pt reconstruction on the growth becomes smaller at higher deposition temperatures, leading to compact, triangular Co islands at 600 K.

Using the thermodynamics of irreversible processes, we examine spontaneous interfacial reaction at room temperature in bimetallic Cu-Sn thin films. The reaction forms an intermetallic compound and is interfacial reaction limited. The volume change in forming the compound produces a biaxial compressive stress in the Sn film. The stress results in the growth of Sn whiskers to accompany the reaction. This is a case of an irreversible process of involving the interaction of chemical affinity and mechanical stress. However, the atomic mobility can significantly affect the reaction product and morphology. If the reaction temperature is raised above 100 °C, the compound formation becomes so fast that there is no time to grow whiskers, instead hillocks form. To model the whisker growth (or hillock growth), we assume that whiskers grow at weaker spots on the Sn surface where the surface oxide has been broken. These spots become localized stress relief centers, and they are surrounded by a long-range stress gradient needed for whisker growth.

Results from a high-resolution core-level-photoemission investigation of the (0001) surface of a WC single crystal using synchrotron radiation are reported. Surface-shifted components were identified in both the C 1s and the W 4f levels. A surface C 1s core-level shift of -0.66 eV and a surface W 4f core-level shift of -0.38 eV were determined using a curve-fitting procedure. When using a thermochemical model and accounting only for the loss of coordination at the surface a shift of -0.62 eV was predicted for the carbon level and -0.39 eV for the tungsten level. Oxygen exposures were made for aiding in the assignment of surface-shifted features but also for investigating the initial oxidation behavior. Two chemically shifted W 4f components, having shifts of +0.29 and +0.66 eV, could be identified after oxygen exposures in the 30–300-L range.