We show the effect of an electric field normal to the quantum-well (QW) plane on the shape of a hole wave function in the vicinity of the type-I–type-II transition point, which leads to (i) the strong decrease of the oscillator strength of e1h1, e1h3 exciton states, and (ii) an increase in the strength of the e1h2 transition, which is forbidden in the absence of the field. The variational calculation of the hole wave function in a type-II QW reveals the existence of some critical electric field which initiates a shift of the hole center of mass to one of the interfaces.

We extend to finite frequency the calculation of the conductance correlation function for a mesoscopic device, using linear-response theory for noninteracting electrons. Because the conductance is complex, two functions are required to fully specify correlations in the real and imaginary parts. The two functions have different frequency dependences due to the placement of the diffusion propagators in the simplest diagram. We apply our results to the case of h/e conductance oscillations in a ring.

The interaction of electrons with GaAs optical phonons is investigated for a single GaAs/AlAs quantum well using the hybrid model of optical phonons, which incorporates bulk-mode dispersion. We predict resonances in the individual hybrid scattering rates as a function of well width corresponding to wave vectors where the modes anticross. Nevertheless, although the physics is quite different, we find that the total scattering rate is insensitive to the value of the bulk dispersion, and is given to an excellent approximation by the dielectric continuum model, for both intrasubband and intersubband scattering processes. We conclude, in accord with recent results of lattice dynamics, that for the evaluation of scattering rates, the dielectric continuum model provides a very good approximation.

An extended theoretical approach for calculating the 1s-1s electronic transition in spherically layered semiconductor quantum dots is presented. The extension over the common effective-mass approximation includes the implementation of the Coulomb interaction and finite potential wells at the particle boundaries. The calculations are carried out for the quantum-dot quantum well CdS/HgS/CdS and compared to recently available experimental results. The wave functions of electrons and holes spreading over the entire structure and the probabilities of presence in the different layers, as well as outside the structure in the surrounding dielectric water, are presented.

The linear and nonlinear reflection spectra from a molecular quantum well with N layers are calculated using Green-function techniques. Exciton quantization in the direction normal to the lattice plane results in the appearance of discrete resonances in the linear reflection, with the same N-independent peak heights, and different superradiant linewidths. Nonlinear reflection measurements are ideal for probing states with a weak oscillator strength, provided their nonradiative damping is much smaller than the radiative decay rate. Size enhancement of ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ due to dipole-dipole interaction and radiative coupling between layers is discussed.

We report ab initio calculations of the surface electronic structure of the hexagonal wurtzite semiconductors ZnO and CdS. The calculations are carried out self-consistently in the local-density approximation employing separable norm-conserving pseudopotentials. Localized Gaussian orbitals are used in the basis set for an efficient description of the strongly localized wave functions. The Zn 3d and Cd 4d electrons are explicitly taken into account as valence electrons since they are important for a quantitative description of the structural as well as the electronic properties, as our investigations of bulk ZnO and CdS have shown. Cationic d-d repulsion, p-d and s-d interactions, and orthogonalization effects are, therefore, included in our calculations of the ZnO and CdS (101¯0) surfaces. The nonpolar cleavage face is described in a supercell geometry with eight atomic and four vacuum layers in each cell. The surface atomic structure is determined by elimination of the forces. For ZnO, we find a rotation relaxation, in which both the Zn and O surface atoms move inward towards the substrate. The calculated surface-perpendicular displacements of the Zn atoms relative to the O atoms in the top layer turn out to be slightly smaller than those determined experimentally by low-energy electron diffraction. The surface electronic structure exhibits an oxygen-derived dangling-bond band in the fundamental gap which is essentially unaffected by the calculated surface relaxation.

This is in marked contrast to the results of previous studies using an empirical tight-binding approach which either find no surface states in the gap at all or one band of surface states that is shifted down into the projected bulk valence bands by the surface relaxation. For CdS(101¯0) we find a surface structure that nicely agrees in general with the relaxation model experimentally determined for the related CdSe(101¯0) surface. The surface electronic structure is compared with the results of polarization- and angle-resolved photoemission experiments. With the help of selection rules we are able to identify two occupied surface states at the top of the projected bulk valence bands in very good agreement with experiment.

The overall features of the energies and atomic structures of the twist boundaries in Si have been examined by energy-minimization calculations using the transferable semiempirical tight-binding method, and the results have been compared with those of the tilt boundaries in Si. First, the {122}Σ=9 tilt boundary has been dealt with as a typical tilt boundary observed in polycrystalline Si. It has been shown that the configuration consisting of atomic rings without any coordination defects can exist stably with a relatively small interfacial energy, 0.32 J/${\mathrm{m}}^2$, and with small bond-length and bond-angle distortions within about ±2% and about ±20°, similarly to the other tilt boundaries. The twist boundaries with different rotation axes and boundary planes, the 〈111〉 Σ=7, 〈011〉 Σ=3, and 〈001〉 Σ=5 twist boundaries, have been examined against various rigid-body translations parallel to the interface. It has been found that the twist boundaries contain larger bond distortions or more coordination defects, and much larger interfacial energies than those of the tilt boundaries, at least when they are constructed by ideal surfaces. It seems that there do not exist deep or sharp energy minima against the rigid-body translations, differently from the tilt boundaries. About the 〈111〉 Σ=7 and 〈011〉 Σ=3 boundaries, configurations without any coordination defects can be constructed for proper translations.

However, in the 〈111〉 Σ=7 boundaries, large bond stretchings are inevitably introduced except at the sites of good coincidence, and the 〈011〉 Σ=3 boundary frequently contains four-membered rings with large bond-angle distortions. Thus the most stable configurations of these boundaries contain interfacial energies over three times larger than the value of the Σ=9 tilt boundary, and contain shallow states in the band gap. On the other hand, the 〈001〉 Σ=5 boundaries have very complex structures as compared with the other twist boundaries. The interfacial energies are much larger than the other twist boundaries, the configurations frequently contain coordination defects and deep states in the band gap, and there seem to exist many metastable configurations with different bonding networks in a similar energy range, similarly to the results of the same boundaries in Ge by Tarnow et al. [Phys. Rev. B 42, 3644 (1990)]. The present different features of the respective twist boundaries can be explained by the morphology of the respective ideal surfaces. It can be said that stable configurations of the tilt boundaries and other extended defects in Si or Ge are constructed by arranging the structural units consisting of atomic rings without any large bond distortions or coordination defects. For twist boundaries, such stable structural units cannot be easily constructed, which causes the present greater structural disorder and larger interfacial energies. This is the reason why twist boundaries are seldom found in polycrystalline Si as compared with the frequently observed tilt boundaries.

Single strained layers of GaAs grown in between barriers of InP have been studied by absorption and photoluminescence-excitation spectroscopy. This structure is believed to be weakly type II, with the GaAs layer forming a quantum well for holes but a barrier for electrons. The absorption increases slowly close to the threshold and is similar to the photoluminescence-excitation spectrum. The absorption has been calculated using a single-band effective-mass model as well as using an eight-band effective-mass model, which include strain- and confinement-induced warping of the valence band. We find agreement between experiment and the results of both calculations with respct to the shape and magnitude of the absorption. No sharp excitonic peaks have been found.

We study the conductance of a tunneling junction with two resonant impurities. We show that when the distance between the impurities is of the order of the de Broglie wavelength of an electron in the contacts, the coupling of the impurity states via the continuum of states in the contacts becomes important. If two identical impurities are located in the middle of the barrier, the resonant peak in the conductance, as a function of the Fermi energy, is dramatically narrowed. The resonant peak transforms into a deep minimum if impurity states have different energies or if impurities are shifted from the middle of the barrier. With nonvanishing overlap of the impurity wave functions, the position of the minimum is shifted from the resonance. We also study two-channel resonant tunneling between the edge states in the quantum Hall geometry. We show that for perfect resonance the coupling between the edge states is nonzero only within a narrow interval of magnetic fields.

We calculate the Auger recombination rate of carriers in a quantum well taking into account that due to the presence of the heteroboundaries the momentum-conservation law is violated. As a result, nonthreshold Auger recombination becomes possible, i.e., low-energy carriers may recombine, the excess momentum being transferred to the well boundaries. This greatly enhances the Auger rate at low temperatures. We develop a method for calculating the overlap integrals in the limiting case which is of practical interest and obtain an explicit analytical formula for the nonthreshold Auger rate. The calculated recombination rate weakly depends on temperature but increases drastically with decreasing well width and may become more important for narrow wells.

The scanning tunneling microscope has been used to identify and distinguish between various stages of reactive growth of Ag on Si(111)-(7×7) at 450 °C. Continuous investigation of the reaction trend was made possible by producing a concentration gradient of Ag on the substrate. The surface features observed, such as hole-island pairs, are dependent on the sample coverage. Mechanisms for the formation of Ag/Si(111)-(√3 × √3 ) are proposed, and experimental confirmation of their validity is presented. Antiphase boundaries between neighboring (√3 × √3 ) domains have been observed, and a model for the atomic structure across such a boundary is shown. At near monolayer coverages small regions that exhibit the Si(111)-(1×1) structure have been identified.

A description of the surface structures consisting of hexagonal (2×2)-reconstructed domains of ad- atoms, which tile the (111) face of diamondlike crystals and have domain boundaries building up a continuous net of c(4×2)-antiphase domain boundaries, is presented. Calculations of the structure factors of such honeycomb reconstructions show that not all of them account for the splitting of the half-order reflections observed in the diffraction pattern of the moderate-temperature phase of Ge(111). The relation between the splitting and the average domain radius of the honeycomb structures is deduced. Using the temperature dependence of the splitting measured by Phaneuf and Webb [Surf. Sci. 164, 167 (1985)], the average domain radius is found to vary almost linearly with temperature. The effect on the diffraction pattern of either the deformation compatible with a perfect honeycomb net of antiphase domain boundaries or the breaking of this net is discussed. Deformed structures are similar to those real-space images obtained with the scanning tunneling microscope on the stabilization at room temperature of the moderate-temperature phase of Ge(111) by means of Ga.

Time- and wavelength-resolved circular dichroism of the excitonic saturation in GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells at room temperature is investigated using a degenerate excite-probe method. For moderate excitation, the effects of photogenerated carriers on the heavy-hole exciton resonance are separately identified as the contributions of phase-space filling and Coulombic effects (screening and line-shape broadening). These contributions are observed to be of a similar magnitude. The induced circular dichroism of the exciton saturation was observed to decay with a time constant of 50 ps, which is attributable to electron spin relaxation.

In the Landauer-Büttiker approach, the dc-transport properties of a system are expressed in terms of its scattering matrix. We propose a method to sum the multiple-scattering events for an arbitrary ensemble of interconnected scatterers, which leads to closed analytic expression for the matrix elements of the overall-scattering matrix. This approach is used to investigate the dc magnetotransport of a planar periodic lattice of interconnected quantum wires. The calculated conductance as well as the Hall voltage depend strongly on the Fermi wave vector and on the magnetic field. This behavior is a consequence of the interference between waves traveling along different scattering paths. It is closely related to the eigenvalue spectrum of electrons in a two-dimensional lattice in the presence of a perpendicular magnetic field, which is described by the Hofstadter butterfly. We present a detailed analysis of the observed features of the Hall voltage.

We reconsider the classical problem of the quantum-mechanical resistance due to a quantum-mechanical reflection off a heterostructure interface. In the presence of a current, the electron distribution in the vicinity of the interface is different from that in the bulk due to the angular dependence of the reflection coefficient. The interface also modifies the electron concentration, leading to a violation of the local neutrality. This creates a self-consistent electric field and affects the angular distribution. The interface resistance depends on the actual form of the electron distribution. Incorporating all these factors in a kinetic transport model, we have reduced evaluation of the resistance to an integral equation. The equation is solved analytically in the limits of either strong or weak reflection. A simple model shows an appreciable difference between our results and the conventional Landauer approach.

A number of ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$${\mathrm{Ge}}_{\mathit{x}}$${\mathrm{C}}_{\mathit{y}}$ layers with different concentrations of Ge and C were grown by molecular-beam epitaxy on a Si(001) substrate to investigate the possibility of strain compensation. The layers were characterized by transmission electron microscopy, x-ray diffraction, and Raman scattering and modeled using a valence-force field model. For a [Ge]/[C] ratio of approximately 10, the lattice constant in the growth direction is equal to that of the substrate, indicating the absence of macroscopic strain. Experimental and theoretical results are compatible with Vegard’s rule. The bond lengths in the alloy exhibit a significant relaxation away from the ideal ‘‘chemical’’ value as given by the sum of the corresponding covalent radii. The measured shifts of the Raman frequencies relative to the constituents cannot be understood in a straightforward description based purely on the softening or hardening of the interatomic bonds as deduced from the Grüneisen parameters.

We consider a single short-range impurity in a two-dimensional electron-gas channel in a magnetic field. The conductance scrG of the channel with impurity is calculated in terms of the Green’s function of the channel without impurity and the impurity scattering length. It is found that a series of downward dips in the graph of scrG versus Fermi energy E exist due to quasibound states with wave functions localized near the impurity. In a strong magnetic field such states exist also for a repulsive impurity and not only below the band bottom of each transverse mode, but also above it.

We present a theory that unifies the transport properties in the integer and in the fractional-quantum-Hall effect regime. We show that the current is carried by incompressible edge channels. However, to generate a net current, electrons have to be added in compressible regions so that the size of the channels is increased. This theory allows us to account for the experimental data when different fractional-quantum-Hall fluids coexist, and to explain the period of the fluctuations near the diagonal-resistance minima.

We study the collective excitation spectra of double-layer quantum Hall systems using the single-mode approximation. The double-layer in-phase density excitations are similar to those of a single-layer system. For out-of-phase density excitations, however, both inter-Landau-level and intra-Landau-level double-layer modes have finite dipole oscillator strengths. The oscillator strengths at long wavelengths for the latter transitions are shifted upward by interactions by identical amounts proprotional to the interlayer Coulomb coupling. The intra-Landau-level out-of-phase mode has a gap when the ground state is incompressible except in the presence of spontaneous interlayer coherence. We compare our results with predictions based on the Chern-Simons-Landau-Ginzburg theory for double-layer quantum Hall systems.

The formation of the two-dimensional subbands in uncompensated δ-doped semiconductors from single impurity bound states with increasing impurity concentration is calculated using multiple-scattering theory. Previous calculations treated the δ plane as a metallic sheet by solving a one-dimensional Schrödinger equation in a self-consistently screened one-dimensional potential and could not reproduce single impurity bound states in the limit c→0. In contrast to this approximation multiple-scattering theory can describe well both limits of small and large c. The cross over from impurity bands at low concentrations to subbands at large concentrations is discussed and a Mott transition within the δ plane (defined as the vanishing of the gaps in the density of states) is found at ${\mathit{c}}_{\mathrm{cr}}$=9.95×${10}^{10}$ ${\mathrm{cm}}^{\mathrm{-}2}$ in the case of GaAs:Si.