Photothermal ionization spectroscopy on neutron-irradiated and subsequently hydrogen-plasma-treated silicon reveals the existence of new shallow donors. The binding energies of the observed effective-mass-like donors are between 34 and 53 meV. The optical dipole transitions of the different donors are shifted towards higher energies by ΔE=0.1–0.2 ${\mathrm{cm}}^{\mathrm{-}1}$, when deuterium is used in the plasma instead of hydrogen. This isotope shift of the optical dipole transitions between the electronic levels of the defects is direct proof of the incorporation of hydrogen in these defects.

Cathodoluminescence (CL) spectroscopy was used to study exciton recombination at temperatures from 80 to 280 K in undoped and boron-doped diamond particles grown by hot-filament chemical vapor deposition. Spectral lines due to free and bound excitons were observed in the near-band-gap region, 4.6–5.5 eV. (The band gap of diamond is at 5.49 eV.) In the undoped particles, free-exciton lines were observed at 5.27 and 5.12 eV. Another set of lines, not previously reported, was observed between 4.7 and 5.0 eV, with the most intense lines at 4.757, 4.832, and 4.950 eV. The latter set of lines is attributed to excitons bound to lattice defects, possibly dislocations. In the boron-doped particles, excitons bound to boron acceptors were found to dominate the near-band-gap CL spectrum at low temperature. The most intense acceptor-bound-exciton line is at 5.20 eV. In both boron-doped and undoped particles, the exciton lines were much less intense in {111} crystal-growth sectors than in {100} sectors. Quenching of the exciton luminescence due to nonradiative recombination is believed to be the cause of the reduced intensity in the {111} sectors. The temperature dependence of the intensities, peak positions, and peak widths of the exciton lines was examined. The free and acceptor-bound excitons in the boron-doped particles were found to be in thermal equilibrium with each other. In the undoped particles, the free and defect-bound excitons were not in thermal equilibrium; the higher-energy bound-exciton lines (at 4.832 and 4.950 eV) decreased more rapidly with increasing temperature than either the lowest-energy bound-exciton line (at 4.757 eV) or the free-exciton lines.

This behavior is attributed to thermally activated transitions from the higher-energy bound-exciton states to the lowest-energy state. The full width at half maximum (FWHM) of the free-exciton line, ${\mathit{W}}_{\mathrm{FE}}$, increased with temperature at about the rate predicted for excitations thermalized near the bottom of a parabolic energy band: ${\mathit{W}}_{\mathrm{FE}}$(T)=${\mathit{W}}_{\mathrm{FE}}$(0)+1.795kT. The FWHM’s of the defect-bound- and acceptor-bound-exciton lines also increased with temperature, but at slower rates than for the free exciton. The peak positions of the exciton lines increased slightly with increasing temperature.

A statistical-mechanical model of binary semiconductor alloys, consisting of a distortable diamond lattice whose sites may be occupied by A atoms, B atoms, or vacancies, is studied by Monte Carlo computer simulations. By extending a grand-canonical lattice gas, the model allows for atomic displacements governed by the Keating valence force field. Unphysical boundary conditions are avoided by keeping the pressure constant. This model is similar to a compressible Ising model, but differs from it by the occurrence of a bilinear coupling between spin field and displacement field. The interplay between the chemical and translational degrees of freedom shows up in the form of the unmixing phase diagram of a system whose parameters were chosen in an attempt to mimic a Si-Ge alloy. Methods of thermodynamic integration to obtain the free energies of different phases are discussed. The critical behavior of the unmixing transition is studied by a multihistogram data analysis. The finite-size scaling of the data is in better agreement with mean-field-like critical behavior than with an Ising transition or Fisher-renormalized exponents. Vegard’s law is verified, and it is shown that the Keating potential leads to a negative coefficient of thermal expansion.

A detailed investigation of the electron and hole occupancy of tail states in undoped amorphous silicon (a-Si:H) as well as changes in the dangling-bond occupancy as a function of excitation intensity was carried out using light-induced electron-spin-resonance (LESR) measurements. For very thick films the band-tail electron and hole densities are not proportional. Over a wide range of excitation conditions the excess hole density is constant, suggesting the presence of charged defects with a density that is 5–10 times larger than the neutral defect density in annealed or as-grown a-Si:H. Light soaking increases mainly the neutral defect density. The dependence of the excess hole density on film thickness and absorption profiles indicates that this effect is a bulk property, which may be masked in thinner films by the comparatively high interface defect density. Model calculations of nonequilibrium occupation statistics confirm the experimental results. For a defect distribution that includes charged defects, the calculations suggest a very small positive LESR signature of the dangling bond, in spite of the high density of charged defects in the material, as a necessary consequence of the asymmetries observed between electron and hole capture rates and tail-state distributions. The calculations demonstrate that the lack of this signature does not imply a defect structure that contains predominantly neutral defects.

The tetrahedron model for the optical dielectric function of tetrahedrally coordinated amorphous compounds has been extended to materials made of four different atomic species. In particular, the case of hydrogen-rich silicon oxynitride is illustrated in detail with the general criteria used to extend the model. Some particularly meaningful examples are reported to illustrate the results that can be obtained applying the model.

Simple two-center tight-binding models have often been used in electronic-structure calculations, although uncertainty about the foundations of the tight-binding method has severely limited progress. This uncertainty has been much reduced following the recent successes of the non-self-consistent Harris scheme but several important questions still remain. In particular, the accuracy of the (almost ubiquitous) two-center approximation is still in doubt. It has often been argued that although this is a poor approximation when applied crudely, it can be justified within the more sophisticated context of chemical-pseudopotential theory. The argument looks sensible, but clear quantitative tests only became possible with the advent of the Harris scheme. This paper reports a careful series of tests of the accuracy of the two-center approximation for germanium, and shows that neither the crude version nor the more sophisticated chemical-pseudopotential version is accurate enough to be useful. The remarkable success of two-center tight binding in predicting the structures and surface reconstructions of semiconductors is, therefore, still mysterious.

A variational approach employing Gaussian-type trial wave functions and mixing of miniband states has been used to calculate the binding energies of the ground and first excited states of a hydrogenic donor in multiple-quantum-well structures consisting of a varying number of GaAs quantum wells separated by thin ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As barriers with periodicities of 89 Å (80-Å well and 9-Å barrier) and 49 Å (40-Å well and 9-Å barrier). The dependence of the binding energies on two different positions (on center and on edge of the central well) was investigated. It was found that the binding energies did not change in any significant way beyond 15 periods for either of the structures investigated at zero magnetic field. Calculations were also performed for superlattice structures with 15 periods in the presence of a magnetic field applied perpendicular to the interfaces. Results have been compared with recent experimental measurements for donor transition energies in GaAs/${\mathrm{Ga}}_{0.7}$${\mathrm{Al}}_{0.3}$As superlattices. Very good agreement is obtained.

We study transmission resonances associated with the n=1 quantum-well states of InAs/AlSb resonant-tunneling diodes using an empirical tight-binding model. We find that the transmission tends to be fairly sensitive to the wave vector in the plane of the interface, indicating that the usual expression for the tunneling current (which neglects this explicit dependence) is most likely not a good approximation. We show how this behavior is related to the E(k) relation for the InAs conduction band. Finally, we examine the envelope functions associated with the quasibound states, and discuss how their appearance relates to the orbitals of which they are composed.

We investigate the structure, energetics, and electronic properties of (Si${)}_5$/(Ge${)}_5$ and (Si${)}_3$/(Ge${)}_4$ superlattices. Our analysis, based on the empirical-potential approach, shows that the thermodynamic stability of these superlattices decreases with increasing substrate lattice constant. Electronic properties are inferred from tight-binding calculations. Use of the interatomic distances obtained here and of those given by elastic theory leads to similar results for the band structure and transition probabilities.

Small increments of indium were evaporated at 300 and 100 K onto the van der Waals (0001) surface of p-type ${\mathrm{WSe}}_2$ crystals. The interface formation was investigated in vacuo with x-ray photoemission spectroscopy, ultraviolet photoemisson spectroscopy, soft-x-ray photoemission spectroscopy, and low-energy electron diffraction. Additional scanning tunneling microscopy (STM), scanning electron microscopy (SEM), and microprobe measurements were performed ex situ. For deposition at 300 K a nonreactive interface is formed and the indium layer grows in the Volmer-Weber growth mode. The size and distribution of the In clusters for specific coverages were determined ex situ by STM and SEM. The band bending of 0.55 eV, as determined from binding-energy shifts of the substrate emissions, is far below the expected Schottky-limit value of 1.1 eV. The observed surface-photovoltage (SPV) shifts of the substrate emission lines are smaller (up to 0.2 eV) than those from the adsorbate lines. The maximum adsorbate SPV shift of 0.6 eV at 150 K exceeds the measured band bending, indicating that the band bending beneath the In clusters must be larger than between them. At a sample temperature of 100 K, In forms atomically flat layers (Frank–van der Merwe growth) allowing the determination of the actual band bending of 0.9–1.0 eV below the In-covered surface. For these conditions, the SPV is only 0.1 eV due to an electrical leakage current. During warmup to 300 K, a transition to the clustered interface occurs. For this interface, the band bending below the indium clusters could also be determined from temperature-dependent SPV measurements. The determined barrier height of 1.04 eV is in good agreement with the value measured at the unclustered interface.

A study of the Fermi-energy edge singularity (FEES) in the second (n=2) subband of asymmetric modulation-doped (AlGa)As-(InGa)As-GaAs quantum wells (AMDQW’s) is reported. In one of the AMDQW’s a Schottky gate is employed to vary the electron density in the n=2 subband (${\mathit{n}}_{\mathit{s},2\mathrm{}}$) from 0 to 1×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. Temperature-dependent photoluminescence excitation (PLE) measurements clearly show that the n=2 PLE feature has FEES character for ${\mathit{n}}_{\mathit{s},2\mathrm{}}$≳0.4×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. In contrast to PLE, photoluminescence (PL) intensity is not a true measure of oscillator strength, since PL intensity can be affected by competing recombination pathways. Temperature-dependent PL measurements have been performed on two types of AMDQW. One type has ${\mathit{n}}_{\mathit{s},2\mathrm{}}$∼0, with the Fermi energy close to the n=2 subband energy. The other type has ${\mathit{n}}_{\mathit{s},2\mathrm{}}$=1×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ and a FEES associated with n=2 observed in PLE. We demonstrate that the very similar broadening and reduction in peak height of the n=2 PL peak with temperature for the two types of samples can be accounted for in terms of spreading of the electron or exciton populations near the n=2 subband edge. Therefore, we conclude that temperature-dependent PL does not provide unequivocal evidence for a many-body enhancement of the n=2 PL transition, in contrast to that reported by Chen et al. [Phys. Rev. Lett. 64, 2434 (1990)].

An ${\mathrm{Al}}_{0.36}$${\mathrm{Ga}}_{0.64}$As/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As inverse parabolic quantum-well structure was grown by molecular-beam epitaxy using both digital and analog compositional grading techniques. Photoluminescence and photocurrent measurements showed distinct exciton peaks for both types of wells. A large Stark shift was found for the digital well, in agreement with the calculations. An observed deviation for the analog well was ascribed to fluctuation in quantum-well parameters. Finally, advantages and disadvantages of the two growth techniques are discussed.

The roughness of monatomic A- and B-type step edges on 0.5° misoriented Si(001) has been analyzed on an atomic scale with scanning tunneling microscopy. On small length scales, measured along the step edge (<200 Å), one-dimensional random-walk behavior is observed for both types of step edges. For the rough B-type step edge we also found evidence for waviness of the edge. The period of this wave is about 100 dimer-row spacings (≊750–800 Å). The energetic step-step interaction and entropic repulsion, which both scale as 1/${\mathit{L}}^2$ (L is the average terrace length), are estimated to be about 0.03–0.06 and 0.2 meV per dimer-row spacing, respectively, for a 0.5° misoriented surface. These interactions are approximately three orders of magnitude smaller compared to the kink formation energies which are 0.1–0.2 eV. Despite the weak strength of energetic and entropic step-step interactions, these long-range interactions have a profound effect on the step-edge morphology, e.g., the distribution of terrace lengths and the long-range waving of the rough B-step edge.

A systematic theoretical study of the electronic properties of pseudomorphic (100)-strained ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ alloys grown on unstrained ${\mathrm{Si}}_{1\mathrm{-}\mathit{y}}$${\mathrm{Ge}}_{\mathit{y}}$ substrates is presented. Based on nonlocal empirical pseudopotential calculations with spin-orbit interactions, realistic estimates of the conduction- and valence-band-edge energies, higher-energy-band minima, effective masses, deformation potentials, and heterostructure band offsets for the whole range of alloy compositions x and y and strain are presented. The theory predicts that the band edges of weakly stressed Ge fall within the wider gap of the ${\mathrm{Si}}_{1\mathrm{-}\mathit{y}}$${\mathrm{Ge}}_{\mathit{y}}$ substrate for 0.7<y<1 (type-I alignment), in contrast to any Si-rich combination of active layer and substrate.

We have found a resonance of an alternative type in the current-voltage curve of the non-one-dimensional resonant-tunneling structure with three quantum dots. It is an exponentially narrow dip with an exponentially small minimum that appears independently of the width of the energy spectrum of incident electrons. The structure considered is the simplest two-lead structure that has such a property.

The submonolayer regime of the thermal (600 °C) oxide growth process on Si(111)7×7 surfaces has been studied in comparison with room-temperature (RT) oxidation using scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and STM simulation based on the empirical tight-binding method. STM measurements show randomly distributed dark or depressed areas on surfaces oxidized at room temperature and at 600 °C in the background of unreacted 7×7 adatoms. The area ratio of the total dark area to the entire surface is found to increase with oxygen exposure and to be comparable to the oxygen coverage measured by XPS, indicating that the dark areas represent oxidized regions. STM images reveal differences in the distribution and pattern of the dark areas between room-temperature and 600 °C oxidations, even with the same oxygen coverage. The dark areas of the 600 °C oxidation are highly segregated like islands, while at room temperature they are as small as single atoms and uniformly distributed over the entire surface. This result provides direct evidence of island formation during high-temperature oxide growth, as previously suggested in photoemission studies. Furthermore, STM images reveal that the islands of 600 °C oxidation are darker than the oxides of RT oxidation. Tight-binding calculations ascribe the darker areas to regions where a larger number of Si layers are oxidized. This result indicates that 600 °C oxidation proceeds in a vertical direction (normal to the surface) even in the submonolayer regime.

Angle-resolved constant-initial-state spectroscopy (ARCIS) from the valence-band maximum (VBM) as initial state was used to determine conduction-band energies at the Γ point up to 66 eV above the VBM for GaSb and GaP. With the help of nonlocal-empirical-pseudopotential (EPM) calculations, structure in the ARCIS spectra up to 17 eV can be assigned to particular interband transitions. The experimental results are discussed in light of our previous work on the conduction-band states of GaAs, InP, and InAs. Above 40 eV there appears to be no correspondence between conduction-band states as calculated in the EPM framework and structure in the spectra. Autoionizing resonances due to surface core excitons have been observed in both materials. From their energies a surface core-exciton binding energy of 0.6 eV for GaSb and 1.4 eV for GaP has been determined.

Recently developed single-electron devices are based on the control of electron tunneling across each tunnel junction of the circuit. However, unwanted higher-order tunneling processes, referred to as cotunneling processes, modify this simple picture and reduce the accuracy of the devices. We calculate the cotunneling rate in a linear array of tunnel junctions beyond the lowest order of perturbation theory by partially resumming the infinite perturbation expansion for the energy of a metastable state. We apply this calculation to the transition between two different tunneling regimes in various single-electron circuits.

We present a calculation of persistent current in an ideal mesoscopic ring coupled to an energy and electron reservoir. In this system, elastic scattering and energy dissipation in the ring occur only at the junction coupled to the reservoir. We consider the case where a magnetic flux and an electrostatic potential are applied simultaneously. It is demonstrated that an electrostatic potential modulates the period, amplitude, and phase of the persistent current vs magnetic-flux oscillation. In some electrostatic potentials, we find an h/2e oscillation in the persistent current vs magnetic-flux relation. The current density and density of states in such a system is also derived. The electrostatic potential causes resonance peaks of the current density and density of states vs magnetic-flux oscillations if the magnetic phase shift approaches nπ. For ɛ=0.5 (maximum coupling between the ring and wire), we also derive an analytical expression for the peak height of the current density and density of states vs magnetic flux under constant electrostatic potential.

The Si(100) surface is known to be reconstructed due to formation of dimer rows. The recent temperature programmed desorption data for hydrogen desorption from this surface indicate that the adsorbate-adsorbate lateral interaction in dimers is attractive and very strong (${\mathrm{\epsilon }}_{\mathit{A}\mathit{A}\mathrm{\simeq }}$-6 kcal/mol). Employing the lattice-gas model, we study the effect of this interaction on the coverage dependence of the chemical diffusion coefficients for jumps of hydrogen atoms along and perpendicular to dimer rows. The adsorbate-adsorbate lateral interaction in the activated state is also taken into account. At reasonable values of the parameters employed, both diffusion coefficients are shown to decrease with increasing coverage. The anisotropy in the coverage dependence of the diffusion coefficients is expected to be rather weak.