Exciton and biexciton binding energies, wave functions, and oscillator strengths are calculated variationally for rectangular GaAs quantum-well wires in an effective-mass approximation. The Coulomb interaction terms are treated exactly in their full three-dimensional form throughout the calculation, especially in the case of the biexciton, which is a more physically realistic procedure than was employed in previous calculations which used effective one-dimensional potentials. Our treatment is unique in the use of a two-dimensional Fourier expansion of the Coulomb potential, which removes the numerical difficulty with the 1/r singularity and considerably reduces the computational effort. As an intermediate step, we calculate the linear absorption coefficients at the exciton resonance as a function of the wire dimensions. The results compare favorably with those of Suemune and Coldren [IEEE J. Quantum Electron. QE-24, 1778 (1988)]. Using the results of the exciton and biexciton calculation, we then estimate the third-order nonlinear optical susceptibility for near-resonant excitonic absorption by using the approach of Ishihara [Phys. Status Solidi 159, 371 (1990)]. We obtain ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$’s on the order of ${10}^{\mathrm{-}1}$–1 esu for various wire dimensions.

The growth mode and the strain of InAs thin films grown on InP(001) were measured by x-ray-photoelectron diffraction (XPD). Since the effects of a (2×4) reconstruction at the InAs film surface may drastically overlap those of the growth mode, we also investigated a (2×4) reconstructed GaAs(001) surface as a standard for this kind of reconstruction. It appears that this type of reconstruction does not induce significant effects on the polar angular distributions recorded within the (110) and the (11¯0) planes. XPD measurements showed that the formation of thin InAs films on InP(001) by As stabilization is better described by a multilayer model of the InAs/${\mathrm{InAs}}_{\mathit{x}}$${\mathrm{P}}_{\left(1\mathrm{}\mathrm{-}\mathit{x}\right)}$/InP type than by a layer-by-layer InAs/InP model. For the strained 10-Å-thick InAs film grown on InP(001), the best agreement found between experiment and theory gives a vertical lattice expansion of 7.5%. This value is between 6.1%, the expansion obtained from the macroscopic linear elasticity theory, and 9.5%, the expansion obtained when assuming that the atomic bond length is retained.

The response of particle density to a dilation of a periodic potential in an insulator, with or without a fixed background potential or a magnetic field, is shown to be quantized. A similar phenomenon occurs in a quantum Hall system, where the derivative of the electron density with respect to the magnetic field is quantized in units of e/h, the inverse flux quantum. A number of other interesting results may be unified under the notion of quantized density response, including the quantization of adiabatic particle transport, the gap labeling theorem, and its generalization in the presence of a magnetic field.

We use luminescence with femtosecond resolution to study the transfer of electrons from the GaAs layers to the AlAs layers in type-II GaAs/AlAs superlattices. Transfer times are measured to vary from 140 fs to 25 ps depending on superlattice parameters and on the temperature. The temperature dependence, the absence of resonance effects, and the comparison with theory evidence the predominance of phonon-assisted intervalley scattering and rules out the contribution of state mixing as a transfer mechanism.

We report on an experimental study of Γ-X mixing effects on the optical transition of the no-phonon-assisted (pseudodirect) exciton consisting of the n=1 AlAs-${\mathit{X}}_{\mathit{z}}$ electron and the n=1 GaAs-Γ heavy hole in [001]-(GaAs${)}_{\mathit{m}}$/(AlAs${)}_{\mathit{m}}$ type-II superlattices (m=8–13 monolayers) by using cw- and time-resolved-photoluminescence spectroscopies and photoluminescence-excitation spectroscopy. We have estimated the relative oscillator strength of the pseudodirect transition to the direct transition between the n=1 Γ electron and Γ heavy-hole states from the relative photoluminescence-excitation intensity, the relative photoluminescence intensity, and the relative photoluminesence decay rate. The values of the relative oscillator strength estimated from these three kinds of experimental results, which are around 1×${10}^{\mathrm{-}4}$, are mutually consistent, and they are almost independent of the layer thickness. We evaluate the Γ-X mixing factor from the relative oscillator strength, using a first-order perturbation theory, as a function of the layer thickness and show that the mixing factor is mainly determined by the overlap of the envelope functions of the Γ and ${\mathit{X}}_{\mathit{z}}$ electrons.

Magnetic field effects in a semiconductor quantum dot (QD) are studied theoretically. Magneto-optical effects originating from electron-hole pairs in the lowest and the higher excited states are discussed. The theory is based on the effective-mass approximation with the following effects taken into account: the direct Coulomb interaction, the electron-hole exchange interaction, and the valence-band mixing effect. A calculation is performed with a numerical diagonalization method. The transition from the quantum confined Zeeman effect for a weak magnetic field to the quantum confined Paschen-Back effect for a strong magnetic field is discussed. Special attention is paid to a magnetic field dependence of the optical transition probabilities which is found to be a pronounced effect for a CdSe QD, where the confinement by a potential and a magnetic field have competing contributions.

A theory of quasi-two-dimensional (2D) polarons in semiconductor quantum wells is presented. We have derived analytical expressions for the energy-gap shift and effective polaron mass in the small-well-width limit. The analytical results agree exactly with those obtained for an ideal 2D system in the limit of zero well width. For larger quantum-well widths, we have numerically evaluated the values for the energy-gap shift and effective mass of a polaron.

A theory of the amplification of the polar-optical-phonon population in a quantum well of polar material under an intense laser field is presented by taking into account the discontinuity of the electronic effective mass crossing the interface of the quantum-well materials. We have found that for the confined-polar-optical phonons, the number of phonons will grow with time if the laser field strength ${\mathrm{E}}_{\mathrm{d}}$ is greater than a threshold value ${\mathrm{E}}_{\mathrm{dL}}^{\mathrm{c}}$ and the phonon wave vector q<${2\mathrm{k}}_{\mathrm{\parallel }}$. For the interfacial-polar-optical (IPO) phonons, the conditions of the amplification of the phonon population are ${\mathrm{E}}_{\mathrm{d}}$>${\mathrm{E}}_{\mathrm{dsi}}^{\mathrm{c}}$ (i=1,2) and q<${2\mathrm{k}}_{\mathrm{\parallel }}$ in the well region and q${2\mathrm{k}}_{\mathrm{\parallel }}$,${\mathrm{k}}_{2\mathrm{l}}$+${\mathrm{k}}_{2\mathrm{l}\prime }$) in the barrier regions. We have also found that the amplification of the IPO-phonon population is easier in material i than in material j if the electronic effective mass ${\mathrm{m}}_{\mathrm{i}}^{\mathrm{*}}$<${\mathrm{m}}_{\mathrm{j}}^{\mathrm{*}}$. Under the conditions of the small phonon wave vector q and large quantum-well potential ${\mathrm{V}}_0$, the number of the (α-q) IPO phonons is not amplified under an intense laser field and the amplification of the (α+q) IPO-phonon population is mainly determined by the amplification of the (α+q) IPO-phonon population inside the well region. The change regulation of the rates of the phonon excitations with the quantum-well width are studied.

Photoemission spectroscopy using synchrotron radiation and reflection high-energy electron diffraction have been used to examine sublayer and multilayer deposits of Ge onto Si(111)-(7×7) substrates. The decomposed Si 2p and Ge 3d core levels as well as angle-integrated valence-band spectra are analyzed as functions of Ge coverage and annealing temperature. The results reveal details concerning the ordering of the surface atoms and the incorporation of Ge into the surface reconstruction. The modification of the surface atomic structure and local chemical composition is correlated with the observed changes in the surface electronic structure.

The spectrum of the collective excitations of a magnetoplasma, confined in a quasi-one-dimensional quantum-well wire is analyzed. The dispersion relations of the intrasubband and intersubband magnetoplasmons are calculated with excellent agreement to experimental results. The calculations are done in the random-phase approximation with no further simplifying approximation. We find analytical results for the dispersion relations of the intrasubband and intersubband magnetoplasmons.

The phenomena of nonvolatile switching in amorphous-hydrogenated-silicon thin-film devices are explained by a theory based on the presence of charged inclusions in the film, having originated from the initial forming treatments. The inclusions almost penetrate the film, and are able to move under applied fields of sufficient magnitude, and in a direction determined by the applied polarity. The differences between analog and digital switching are ascribed to differences in the homogeneity of the inclusions, being affected by the nature of the top contact.

A nonlocal analysis of the infrared second-harmonic generation associated with intersubband transitions in a symmetric semiconductor quantum-well structure is presented. Taking as a starting point a fundamental self-consistent integral equation for the local field, the p-polarized first-harmonic field inside the quantum well is studied. By using the infinite-barrier wave functions and taking into account only the two lowest subbands, analytical expressions are obtained for the local field. The result of the local-field calculation at the first-harmonic frequency, in turn, is used to calculate the p-polarized second-harmonic local field. The conversion efficiency of the second-harmonic generation from the quantum well is thus determined. Numerical calculations of the frequency and angular spectra of the second-harmonic intensities are presented for a symmetric GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum well. The numerical results show that strong second-harmonic generation occurs in the vicinity of the resonance frequencies for both the first- and second-harmonic local field inside the quantum well. The influence of the dynamic local-field interaction of the electrons on the optical second-harmonic generation is investigated. It is demonstrated that the dynamic screening can lead to a notable upward shift of the locations of the resonant peaks in the frequency spectra of the second-harmonic conversion coefficient.

Various single-particle and collective-mode scattering processes that contribute to the intrinsic dephasing time of photoexcited electron–valence-hole pairs in degenerate, modulation-doped quantum wells are discussed. Detailed calculations show that sharp discontinuities in the energy dependence of the electron dephasing time can give information about short-range correlations associated with large wave vectors and high-frequency excitations of the coupled electron–LO-phonon system. However, to extract dephasing times from coherent optical excitations the effects of the complex valence-band structure, i.e., mixing of light and heavy holes in GaAs quantum wells, must be considered.

In this paper we report on Schottky barrier height measurements of nickel on both diamond (111) and (100) surfaces, as a function of surface preparation. The Schottky barriers of thin (<5 Å) nickel films on natural type-IIb (p-type semiconducting) diamond (111) and (100) surfaces were determined with ultraviolet photoemission spectroscopy. Exposing the diamond (111) surfaces to an argon plasma while heated to 350 °C resulted in a change from a negative-electron-affinity surface to a positive-electron-affinity surface. This effect was used as an indication that a hydrogen-free surface had been obtained. Deposition of a monolayer of nickel on the hydrogen-free diamond (111) surface resulted in a Schottky barrier height of 0.5 eV. The nickel caused the surface to exhibit a negative-electron-affinity surface. Nickel deposited on a diamond (111) surface with a negative electron affinity, indicative of a monohydride-terminated surface, resulted in a ∼1.0-eV Schottky barrier height. Diamond (100) surfaces were prepared by vacuum annealing to temperatures ranging from 500 °C to 1070 °C. The various anneals resulted in a lowering of the electron affinity by up to ∼1 eV, which resulted in a negative electron affinity after the surface had been annealed to ∼1000 °C. Oxygen was initially present on the surface but could not be observed after the 1000 °C anneal. The removal of oxygen and the appearance of a negative electron affinity coincided with the appearance of a 2×1 surface reconstruction. Nickel was deposited after the various anneals, and Schottky barrier heights were found, ranging from 1.5 eV for the 545 °C-annealed surface to 0.7 eV for the 1070 °C-annealed surface. These measurements suggest that for both the (111) and the (100) diamond surfaces the presence of chemisorbed species, such as hydrogen and oxygen, results in an increase in the Schottky barrier height.

We present a Fourier-transform photoluminescence study of InAs/AlSb type-II heterostructures, spanning the midinfrared 1–12-μm wavelength range. The investigated samples consist of single and double quantum wells, grown on GaAs substrates by molecular-beam epitaxy, with different buffer-layer structures for strain accommodation. The photoluminescence intensity is found to be very sensitive to the detailed structure of the underlying buffer sequence. The use of a GaSb buffer or a GaSb/AlSb superlattice in the buffer gives rise to an additional radiative-recombination channel in the near infrared, which competes with radiative recombination in the InAs quantum wells at longer wavelengths. Variations in the spectra observed under visible and infrared excitation provide evidence that the recently observed persistent photo effect is governed by hole capture in the GaSb layers in the buffer sequence. Furthermore, very wide quantum wells (100 and 200 nm) show spatially direct as well as spatially indirect photoluminescence, providing insight into the nature of the transition from the bulklike semiconductor to the type-II heterostructure.

We have used the scanning tunneling microscope (STM) to study the deposition of Se onto the GaAs(001)-(2×4)/c(2×8) surface and the formation of the GaAs(001):Se-(2×1) surface reconstruction. The experiments were carried out in a combined STM–molecular-beam-epitaxy (MBE) system, allowing the surface to be imaged after deposition without transferring through air. Se was deposited onto well-characterized GaAs buffer layers grown in situ by MBE. We have imaged the surface at several stages during the deposition of up to two monolayers of Se, with depositions carried out at 300 °C and 470 °C. We have also looked at the effect of annealing the Se covered surface at over 520 °C, observing smoothing of the surface and desorption of Se from the surface. Se reacts with the GaAs surface at 470 °C, displacing the surface As and causing significant disruption to the GaAs surface. After approximately 0.25 monolayer of Se is deposited, the original (2×4) reconstruction is replaced by a largely disordered phase. With further Se deposition the Se-terminated (2×1) reconstruction is formed. The general surface morphology is similar to the original GaAs surface prior to Se deposition except that there are many small (2×1) islands on the terraces. When Se is deposited onto the GaAs surface at 300 °C a disordered Se overlayer is formed. The (2×1) reconstruction is only formed by subsequent annealing at higher temperature.

The resulting surface morphology is similar to that formed by Se deposition at 470 °C. High-resolution STM images resolve the Se dimers that make up the (2×1) reconstruction; however, the individual atoms within one dimer have not been resolved. Small islands on the GaAs(001):Se-(2×1) surface can be removed by annealing at above 520 °C for up to 20 min. The resulting (2×1) surface has well-ordered flat planes several hundred angstroms across, which are comparable to those on the original GaAs buffer layer. The temperature at which the islands are removed is very close to the temperature at which Se is desorbed from the (2×1) terraces, suggesting that the removal of the islands involves desorption of Se and is not a simple diffusion process.

We propose a response-function formalism to investigate electronic transport in semiconductor devices. To this end, a closed hydrodynamic approach coupled with the Poisson equation for the self-consistent electric field is used to simulate the stationary and transient characteristics of near micrometer ${\mathit{n}}^{+}$-n-${\mathit{n}}^{+}$ InP diodes. The response functions of the local electric field and applied voltage are calculated by means of a procedure which is based on the response of the device to an impulsive current perturbation. The good agreement achieved with available experiments validates the present theoretical approach which suggests spatial negative differential mobility as the physical mechanism responsible for microwave generation. A significant increase of the cutoff frequency of the microwave power generation up to 600–700 GHz is predicted for submicrometer GaAs and InP diodes.

We study strained-layer growth of Ge on the Si(001) surface by means of an empirical potential that is further refined by fitting it to recent total-energy results for the stress and energies of various missing-dimer reconstructions of the Si(001) surface. The modified potential predicts the buckled dimer surface to be substantially lower in energy than the symmetric dimer surface. It is seen that the reconstruction of the GeSi surface, before the occurrence of islanding, has the missing-dimer–trench structure as observed in a number of scanning tunneling microscopy experiments. Our calculations provide direct support to the notion that islanding does indeed set in after layer-by-layer growth of three monolayers. We discuss the implications of our calculations for the growth of long thin islands of high aspect ratios as seen recently in Ag-on-Si(001) systems which has been attributed to elastic relaxation.

The three-band effective Hamiltonian of k⋅p perturbation theory, which is relevant for nonparabolic semiconductor conduction bands, is solved for the case of a uniform electric field along the heterostructure growth axis. The solutions are analytical and nearly exact, typically giving conduction-band eigenvalues to accuracies within a fraction of a percent. Good accuracy is obtained by considering the effects of the split-off hole band via an effective two-band interaction, which is obtained by linearizing the valence-band term in the exact two-band subspace of the three-band Hamiltonian. A straightforward extension provides an approximate analytical account of the remote band perturbations as well. The solutions are based on exact analytical results for the two-band model (electrons and light holes only), from which a universal family of curves characterizing the subband shift versus electric field has been derived. As a test case of the approximate three-band solution, the quantum-well Stark shift of the conduction electron is computed and compared with exact numerical results.

We have determined the atomic structure of the Ge-covered Si(100) surface in one-monolayer coverage using the pseudopotential density-functional total-energy calculation scheme. We find that the asymmetric Ge dimer structure is energetically favored over the symmetric one with a stabilization energy of 0.35 eV/dimer: at equilibrium, the dimer bond length is 2.39 Å and the dimer axis tilts 16.3°. The present result confirms the experimental observations of the asymmetric dimers on this surface, but the calculated dimer bond length differs from a recent x-ray standing-wave measurement.