The results of transport measurements on p-type β-${\mathrm{FeSi}}_2$ single crystals in the temperature range of 30–300 K are presented and explained assuming the existence of an impurity band and an additional deep acceptor level. The values of the activation energies of the shallow and deep acceptors, their concentrations, as well as the concentration of the compensating donors and the density-of-states effective hole mass were calculated.

Positron-lifetime measurements indicate that the effect of hydrogen on the annealing behavior of defects in Si irradiated with 3.6×${10}^{17}$ neutrons/${\mathrm{cm}}^2$ is very obvious. Neutron-induced monovacancy-type defects in hydrogen-containing Si disappear at 400 °C, 200 °C lower than in neutron-irradiated Si without hydrogen because of hydrogen passivation and the formation of hydrogen-defect shallow donors; an annealing valley at 150 °C is mainly due to the annealing out of V-H pairs and passivation of acceptor centers by hydrogen. Hydrogen promotes annealing out of ${\mathit{V}}_2$-type defects above 400 °C. The reappearance of V-type defects in neutron-irradiated Si with and without hydrogen at 600 °C is proposed to be due to a completely different mechanism.

The tunneling transparency of a disordered superlattice with impurity centers located in the potential barriers is evaluated using the transfer-matrix technique in the effective-mass approximation. The widths of the quantum wells are distributed randomly along the superlattice chain. Strong enhancement of the transmission coefficient T due to impurities is observed. Energy intervals where T=1 or T≊1 appear in tunneling spectra.

We identify the binding sites for adsorption of a single Ge atom on the Si(100) surface using ab initio total-energy calculations. The theoretical diffusion barriers are in excellent agreement with experimental estimates. Using a large supercell we resolve the controversy regarding the binding geometry and migration path for the adatom, and investigate its influence on the buckling of Si dimers. We find that the adatom induces a buckling defect that is frequently observed using scanning tunneling microscopy, indicating that the study of a single adatom may be experimentally accessible.

The effect of on-site Coulomb repulsion on the process of resonant tunneling is studied in the tight-binding model of a quasi-one-dimensional system. The conductance through a small region where the Coulomb interaction is strong is calculated as a function of chemical potential with use of the Kubo formula. The Green’s function and the number of electrons in the region are obtained self-consistently using the recursion method in the mean-field approximation. It is found that the conductance shows various doublet structures depending on the broadening of the virtual bound state and the Coulomb interaction.

Several strained structures of two ideal ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As compounds have been investigated by performing ab initio total-energy and atomic-force calculations in order to give a microscopic interpretation of extended x-ray-absorption fine-structure results on strained ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterostructures. The achieved results show that the strain is accommodated by significant bond-angle distortions and small In-As bond contractions. A striking, stretched Ga-As distance (close to the In-As one) observed in the strained heterostructures should not be related therefore to the strain accommodation occurring in the ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As epilayer. Anisotropic strain effects are found, which lead to two different next-nearest-neighbor distances for atomic pairs on and out of the planes parallel to the ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs interfaces.

Successive hydrogen-elimination reactions with low activation energies during the formation of a-Si:H by silane plasma chemical-vapor deposition proposed by us were studied by using a larger cluster model on the basis of an ab initio molecular-orbital method. The activation energy of the first step, the reaction of a dangling-bond site with an adjacent tetrahedrally coordinated silicon, was found to be 18.2 kcal/mol (0.79 eV) by employing a larger cluster model. The total process was also shown to be thermodynamically more favorable by using larger cluster models. Thus, the successive process is considered to play an important role in a-Si:H formation processes.

The simultaneous occurrence of Aharonov-Bohm (AB) and Aharonov-Casher (AC) effects due to edge states in double-barrier two-dimensional wires formed by an electrostatic confinement potential, in the quantum Hall effect regime, is discussed. The AC effect is manifested via a shift of the AB conductance oscillations, and a method for measurement of the effect is proposed.

The phonon-drag contribution to the thermopower of a two-dimensional point contact is analyzed. The situation when the contact is wide enough in its narrowest point in the sense that the number of occupied one-dimensional subbands n is much greater than unity, as well as the situation of a quantum point contact (n∼1) is considered. It is shown that the phonon-drag contribution to the thermopower may appear important and may even prevail over the diffusive contribution, while its quantization picture is shown to differ drastically from that for the diffusive thermopower.

Quantum oscillation in serially connected one-dimensional rings is studied and the transmission coefficient of these multiply connected rings is derived. The transmission probability is calculated as a function of incident-electron energy. The relations between the number of connected rings and resonance peaks (minibands) in the quantum oscillation are predicted and demonstrated analytically and numerically. The temperature dependence of conductance in the system is also studied. With the magnetostatic Aharonov-Bohm (AB) effect of multiply connected rings, the oscillation amplitude is almost constant when the temperature is kept constant, regardless of the number of connected rings. In contrast, we found that with electrostatic AB effects, the oscillation amplitude decays as a function of electrostatic potential at a finite temperature. We predict that the degree of decay will continue to decrease as the number of connected rings increases.

We present in this paper a variational calculation for the ground state of the donor and light- and heavy-hole excitons in triangular quantum wells. The binding energy and the lateral and vertical extensions of the wave function are calculated for several well-height values, all as functions of the well width. Compared with a square-well structure or a δ-doped well, the triangular quantum well presents a greater restriction to the donor and exciton. As a result, the lateral and vertical extensions of the ground state are smaller, while the binding energy and the probability inside the well are larger. We conclude in this paper that a triangular quantum-well structure may be significant for theoretical research and practical application.

The conductance fluctuations of mesoscopic Au films are measured as a function of time while the film is irradiated with photons. The fluctuation amplitude caused by the displacement of Au atoms first increases with power and then saturates. This saturation amplitude of the conductance is the same as if all the scattering centers are moved. The magnetoconductance curves become progressively uncorrelated upon irradiation. The experimental effect of the sample irradiation is equivalent to a theoretical impurity averaging. In the saturation regime the fluctuation amplitude of the time trace and the magnetoconductance become equivalent.

A metal-insulator transition was observed in the layered cuprate ${\mathrm{Bi}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{0.3}$${\mathrm{Y}}_{0.7}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathit{d}}$, where the carrier concentration is not changed but the structural modulation was relaxed with increasing lead content x. Resistivity measurements and x-ray-photoelectron-spectroscopy (XPS) valence-band spectra manifested the occurrence of the metal-insulator transition; XPS core-level spectra, together with the results of a chemical analysis, confirmed that in this system the degree of the electron filling is constant. This metal-insulator transition is believed to arise from the change in the Cu-O-Cu bonding angle, as well as those in some nonlayered transition-metal perovskites.

We consider the slave-boson method within the coherent-state functional integral representation of the partition function, and show how to deal with the continuum imaginary-time limit required by the very definition of the functional integral. We find that a correct treatment of the continuum limit modifies the free energy when fluctuation (1/N) corrections beyond the mean-field solution are considered. Numerical results are presented for a two-level single-site model system (with an infinite Hubbard repulsion), for which the additional terms in the free energy introduced by the correct continuum limit act to validate the 1/N expansion. Our analysis calls for a revision of several outcomes of the slave-boson method with the inclusion of fluctuation corrections.

Raman-scattering spectra were investigated for strongly correlated electron systems, ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{TiO}}_3$ and ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_3$, which undergo a metal-to-Mott-insulator transition under variation of the band filling. The phonon spectra decrease in their intensity as the system approaches the metal-insulator phase boundary, accompanied by a critical enhancement of the value of the effective mass, and almost disappear in the insulating phase. We found that the phonon spectral intensity in the metallic region approximately scales with (n/m*${)}^2$, n and m* being the density and effective mass of conduction carriers.

We present a systematic study on the infrared absorption of S-, Se-, and Te-doped metal-organic chemical vapor deposition GaAs layers which were subjected to a prolonged hydrogen plasma treatment. For all donor species, two new vibrational modes are detected at 780.58 and 1512.30 ${\mathrm{cm}}^{\mathrm{-}1}$ (GaAs:S); 777.95 and 1507.46 ${\mathrm{cm}}^{\mathrm{-}1}$ (GaAs:Se); 771.81 and 1499.93 ${\mathrm{cm}}^{\mathrm{-}1}$ (GaAs:Te). In samples treated in a deuterium plasma, the modes attributed to hydrogen are shifted by a characteristic amount. We identify the modes as due to hydrogen group-VI donor stretching and wagging vibrations. The proposed model for the complexes, where the hydrogen is bound in the antibonding position of the nearest Ga atom, accounts for the observed vibrational properties.

The antisite and interstitial phosphorus defects in ZnSe have been studied by pseudopotential total-energy calculations to investigate their roles in the p-type doping problem of ZnSe. Our calculations suggest that the difficulty in p-type doping of ZnSe with phosphorus is caused by the compensation of shallow acceptors by the antisite defects ${\mathrm{P}}_{\mathrm{Zn}}$, which act as triple donors. A microscopic model for two ionization levels of phosphorus acceptors in ZnSe (one shallow at 84±4 meV and one deep at 0.6–0.7 eV) is also proposed. The shallow acceptor level is attributed to a P substituted at a Se site, while the deep acceptor is assigned to the 2+ charge state of an interstitial P atom near the hexagonal interstitial site.

While the direct band gaps of wurtzite (W) and zinc-blende (ZB) structures are rather similar, the W and ZB gaps can differ enormously (e.g., ∼1 eV in SiC) in indirect gap materials. This large difference is surprising given that the structural difference between wurtzite and zinc blende starts only in the third neighbor and that total energy differences are only ∼0.01 eV/atom. We show that zinc-blende compounds can be divided into five types (I–V) in terms of the order of their ${\mathrm{\Gamma }}_{1\mathit{c}}$, ${\mathit{X}}_{1\mathit{c}}$, and ${\mathit{L}}_{1\mathit{c}}$ levels and that this decides the character (direct, indirect, pseudodirect) of the wurtzite band gap. The observation of small ${\mathit{E}}_{\mathit{g}}^{\mathrm{W}}$-${\mathit{E}}_{\mathit{g}}^{\mathrm{ZB}}$ differences in direct band-gap systems (‘‘type II,’’ e.g., ZnS), and large differences in indirect gap systems (‘‘type IV,’’ e.g., SiC) are explained. We further show that while both type-III systems (e.g., AIN) and type-V systems (e.g., GaP) have an indirect gap in the zinc-blende form, their wurtzite form will have direct and pseudodirect band gaps, respectively. Furthermore, a direct-to-pseudodirect transition is predicted to occur in type-I (e.g., GaSb) systems.

The effect of thermal annealing on the conduction- and valence-band edges in porous silicon is studied by photoemission, Auger, and x-ray-absorption spectroscopies. The quantum shifts in the conduction- and valence-band edges of porous Si relative to bulk Si are found to decrease with progressively higher annealing temperatures, up to 550 °C, at which point the band-edge energies revert to the values for bulk Si. The ratio of the shift in the valence-band edge to the shift in the conduction-band edge remains approximately constant with annealing and equal to 1.5, in agreement with an effective-mass model for the quantum shifts.

We have examined the electron mobility μ as a function of the concentration of a two-dimensional electron gas n, in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructures. Depending on the method used to vary n, different values of μ can be obtained for the same n and in the same heterostructure. We suggest the origin of this effect lies in the interdonor interactions leading to spatial correlations of remote impurity charges. It results in a significant mobility enhancement at low temperatures. This finding demonstrates the necessity of taking into account many-body effects within the impurity system in different phenomena occurring in highly doped semiconductors.