Porous silicon (PS) with its distribution of crystallite sizes is a highly disordered material. We present a theoretical formulation to explain the photoluminescence (PL) spectra of porous silicon. We base our formalism on the quantum confinement model using methods similar to those of Kane and Lifshitz. A minimal set of parameters is employed whose numerical values are obtained from independent experiments and/or microscopic theories. Our work demonstrates (i) a downshift in the PL peak due to the size distribution, thus facilitating the use of smaller and physically reasonable exciton binding energy; (ii) a PL spectrum with a line-shape asymmetry on the energy scale, having a full width at half maximum of ≃350 eV, in consonance with experiments; (iii) the presence of both columns and dots in PS; (iv) the presence of local inhomogeneities. Modifications of our model and extensions to related experimental phenomena are also discussed.

Intermittent-radiant-heating-induced manipulation of nucleation has been accomplished during molecular-beam epitaxy of Si on Si(111) and Ge on Ge(111). Features in the reflection high-energy electron-diffraction intensity were observed that could be related to the evolution of the surface-step density by comparing with Monte Carlo simulations. We report rapid phase shifts of up to 180° effectuated over the growth of one to two bilayers and distinct beat periodicities during layer-by-layer growth conditions. An anomalous phase shift was observed for ${\mathit{T}}_{\mathit{s}}$ close to the step-flow growth regime.

Raman scattering in amorphous GaAs is interpreted in the framework of a model taking into account multiple-order processes. This model describes, first, the background signal, which systematically underlies the first-order bands, and second, the temperature dependence of both Stokes and anti-Stokes Raman spectra. The coupling between the light and the vibrational modes is shown to be dependent on the extended or localized character of the implied modes. As a consequence of this analysis, a different method of achieving experimentally the density of vibrational states of amorphous systems is proposed. In GaAs, the data show remarkable agreement with theoretical calculations.

We present a theoretical study of the geometric and electronic structures of the Si(100) surface. The calculations are performed at the parametrized tight-binding molecular-dynamics level using a computation cell consisting of five surface layers with 32 atoms on each layer. The p(2×2) and c(4×2) surfaces are found to be equally likely to have ground-state structures. The surface states are determined based on the optimized geometric structure of p(2×2) and c(4×2). The results are compared with available experimental data and previous calculations.

We have developed a method for interpreting and classifying mechanisms from molecular-dynamics simulations. The reason for this effort is that molecular-dynamics simulations contain a wealth of information regarding the microscopic details of atomic motions. It is at best tedious, however, to extract all of the essential information in very complex processes. In particular, we are interested in particle ejection due to the keV-particle bombardment. Our method isolates the motion of the last two collisions, which we depict in a single frame showing the time sequence of the positions of the three atoms involved. The remainder of the atoms are shown at their initial positions. These atoms give the perspective of the entire system but small or irrelevant displacements do not distract from the key motions. This method is utilized to elucidate the structure-sensitive mechanisms of keV-particle bombardment of the {001} face of diamond-lattice crystals. We have observed that two mechanisms are responsible for the major features of the angular distributions, namely, the ${\mathrm{\Delta }}_1$ and the ${\mathrm{\Delta }}_3$ mechanisms. The two mechanisms are characterized by the difference in the number of layers of the atoms involved in the final momentum transfer.

The static second hyperpolarizability γ(N) is calculated for one-dimensional semiconductors consisting of N-coupled two-level systems. Coulomb interactions are treated exactly in the complete basis set of zero, one, and two electron-hole (e-h) pairs. Analytical solutions for the per-site response γ(N)/N are obtained in the polymer limit for both the strong- and weak-localization (free-particle) regimes. The double e-h pair states play a crucial role in determining the sign and magnitude of γ(N) near the strong-localization limit and prevent unphysical divergences in the per-site response as N increases. Applications to generic polysilanes are discussed.

Conductivity and diffusion coefficients of the holes in semiconductor superlattices are calculated within the Kubo formalism, taking into account interaction of holes with the δ-correlated disorder potential. An analytical approach using the strong difference between the heavy-hole (hh) and light-hole (lh) effective masses is applied in order to describe hole states in the lowest miniband. It is shown that lh-hh mixing leads to a multichannel description of hole transport, and lh-like channel (which is characterized by the lh effective mass in the tunneling exponent) is more important for superlattices with wider potential barriers at higher temperatures and hole concentrations. Calculated values of the diffusion coefficient are compared with the available experimental data.

Magneto-optical studies on a series of CdTe/${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te multiple-quantum-well structures with x≃0.08 have identified the 1S and 2S states of both the light- and heavy-hole n=1 excitons. This has allowed changes of the exciton binding energies to be studied as a function of the depth of the confining potentials, which were tuned through the ${\mathit{sp}}^3$-d exchange interaction in the barrier layers by application of a magnetic field. Calculations of these binding energies by a variational technique are in general agreement with the observations. The exchange-induced splitting of the heavy-hole exciton is found to be consistent with between 0.35 and 0.45 of the band offset being in the valence band, which accounts for the absence of any evidence that the valence-band structure changes from a type-I structure to a type-II structure above a certain value of a magnetic field. However, this offset is found to be too large to account for the exchange-induced splitting of the light-hole exciton, which appears to be anomalously large. Calculations have shown that this anomaly cannot be explained in terms of the diffusion of Mn ions from the barrier regions into the wells. An alternative explanation is given in terms of field-dependent interface potentials wherein the ${\mathit{sp}}^3$-d exchange interaction is considered to be different in the interface regions of the multiple quantum wells to that in the barrier regions.

The influence of the surface-potential shape on exciton optical spectra for thin semiconductor films is investigated theoretically. Using a realistic model with both intrinsic and extrinsic contributions, we calculate the polarization vector and the electric field in the case of s-polarized incident light at one of the thin-film surfaces. Our analytic results are employed to study the relation between the surface potential parameters and the spectra of reflectivity and transmissivity. The generation of near-surface localized excitons and their optical manifestation are also analyzed.

Planar quantum-dot nanostructures have been shown to exhibit resonant features in their near-threshold conductance characteristics. However, the data often display strong nonuniformity in peak amplitude and a lack of reproducibility between devices, which suggests the presence of an influential disorder mechanism. In the present investigation, we perform a three-dimensional numerical analysis of structural disorder in a thin-gated quantum-dot nanostructure. The experimental conductance data for this device exhibit robust resonant features above T=4.2 K despite the presence of extremely wide tunneling barriers. A self-consistent simulation of the device in the virtual-crystal approximation shows no resonant features in the I-V characteristic, thereby excluding any direct influence of the quantum dot. Activating structural disorder in the simulation via interface and surface roughness, however, gives rise to resonant features that strongly resemble the experimental data. We thereby demonstrate that disorder mechanisms in planar nanostructures have a profound influence on near-threshold carrier transport to the extent that they may dominate the effects of intentionally fabricated confinement features.

We studied low-energy effects of impurities in several chiral one-dimensional (1D) electron systems that contain only excitations moving in one direction. We first considered single-impurity scattering between two branches of 1D chiral Luttinger liquids. The general form of the impurity-scattering matrix was found which, sometimes, allows the chemical potentials on the outgoing branches to be higher (or lower) than the maximum (or minimum) chemical potentials on the incoming branches. We also studied the effects of many impurities (with finite density) on two branches of 1D chiral Fermi liquids (that appear on the edge of the ν=2 integral quantum Hall liquid). We found that the impurities drive the system to a fix point at low energies which has an SU(2) symmetry even when the two branches originally have different velocities. [Note that a clean system is SU(2) symmetric only when the two branches have the same velocity.] The probability distribution and correlation of scattering matrices at different energies are described by universal functions which are calculated exactly for this fixed point.

We have augmented the effective-bond-orbital model to include second-nearest-neighbor overlap integrals. With these additional parameters we can fit the band energies at the Γ and X points, in addition to the zero-center masses. This model provides a more accurate bulk band structure with a negligible increase in computation time. The improved model is applied to short-period type-II InAs/${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$Sb superlattices, and we observe a significant departure from the nearest-neighbor results.

The carrier dynamics in photoexcited semiconductors is studied in a quantum kinetic approach based on the density-matrix formalism. Besides the memory effects related to the energy-time uncertainty, we discuss interference effects between different types of interactions describing the fact that a transition due to one interaction occurs between states, which are renormalized by other interactions. We first analyze the relaxation process in a one-band model, which allows us to concentrate on memory effects in the electron-phonon interaction. We then extend the model to a two-band semiconductor interacting with a short laser pulse, which is more realistic due to the explicit treatment of the carrier generation process. Here we discuss, in particular, the role of renormalization effects. It turns out that these effects reduce the broadening due to the non-Markovian dynamics and lead to distribution functions, which are more similar to the semiclassical case; the positions of the peaks, however, exhibit slight time-dependent shifts. On the other hand, phonon quantum beats in the decay of the interband polarization are increased by these renormalization effects.

We report on a study of the adsorption and decomposition of arsine molecules on clean Si(100) 2×1 surfaces using photoemission and scanning tunneling microscopy (STM). Arsine decomposition depends on the substrate temperature and occurs in five regimes: (i) Below 100 °C the arsine dissociatively adsorbs most likely into As-H and Si-H. At these temperatures there is negligible surface diffusion and the saturation arsenic coverage is 20% because the remaining Si sites do not have enough unoccupied near neighbors to provide four bonds for ${\mathrm{AsH}}_3$ adsorption. (ii) Between 100 °C and 400 °C the onset of surface diffusion increases the saturation coverage of As to 25% (i.e., one arsenic per four silicon atoms), As-As dimers form and coalesce into short chains. (iii) Above 400 °C the hydrogen starts to desorb significantly thereby allowing larger As coverages. (iv) This process reaches an extremum at about 575 °C where all hydrogen is desorbed resulting in nearly 100% As coverage. (v) Near 650 °C the As desorption rate becomes significant. The STM images recorded in these regimes reveal a wide variety of surface structures which are due to different relative rates of ${\mathrm{AsH}}_3$ adsorption, H and As desorption, and H, As, and Si surface diffusion. The ${\mathrm{AsH}}_3$ adsorption and H and As desorption processes are discussed in the context of a kinetic model.

Photoreflectance (PR) and optical absorption (OA) have been used to study the energy-gap dependence on temperature in ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As thin layers, metalorganic vapor-phase epitaxy grown on InP substrates under nearly lattice-matched conditions (‖Δa/${\mathit{a}}_0$‖≤${10}^{\mathrm{-}3}$). Both PR and OA spectra show a split structure in the band-gap energy region. Moreover, the energy-gap values at all temperatures, obtained by Franz-Keldysh-oscillations analysis of PR line shapes, are blueshifted with respect to the values of the perfectly lattice-matched alloy (x=0.472). The results are related to the valence-band splitting at k=0 generated by the biaxial strain in ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As, due to the small lattice mismatch. We calculated the splitting and shift values using the orbital-strain Hamiltonian and accounting for compositional effects. The values are in agreement with the experimental results giving optical evidence of coherent growth of the alloy film.

An analytical approach based on the methods of disordered-systems theory and taking into account the strong difference between light-hole and heavy-hole effective masses is developed and applied to the problem of optical transitions in semiconductor quantum wells with nonideal interfaces. Two kinds of interface disorder are considered: inhomogeneity of the right- and left-interface positions and inhomogeneity of alloy composition in the potential barriers (wide-gap materials of the quantum-well structure are assumed to be alloys). Calculation of the photoluminescence and photoluminescence-excitation spectra are carried out for the cases of long-scale and short-scale correlated disorder. Both nondoped and heavily n- and p-doped heterostructures are considered. Theoretical results are compared with the experimental data for the n-doped heterostrucures ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$${\mathrm{In}}_{0.52}$As.

Effects of the well width and an electric field on electron-phonon scattering are studied for two kinds of quantum-well structures. One is a symmetrical single quantum well (SSQW) ${\mathrm{Ga}}_{0.6}$${\mathrm{Al}}_{0.4}$As/GaAs/${\mathrm{Ga}}_{0.6}$${\mathrm{Al}}_{0.4}$As, and the other is an asymmetrical single quantum well (ASQW) ${\mathrm{Ga}}_{0.6}$${\mathrm{Al}}_{0.4}$As/GaAs-${\mathrm{Ga}}_{0.8}$${\mathrm{Al}}_{0.2}$As/${\mathrm{Ga}}_{0.6}$${\mathrm{Al}}_{0.4}$As. Calculated results reveal that for both structures without an electric field, the intrasubband and intersubband scattering rates due to interface phonons, respectively, decrease and increase with increasing well width while those due to confined phonons increase with increasing well width. It is found that the difference between SSQW and ASQW structures results in a significant change of phonon modes and electronic structure and consequently the dependence of scattering rates on an applied electric field is quite different for the two structures, and that the intersubband scattering rates can be considerably changed by applied electric field as the well width is large. The result would be useful for analyzing experimental results and designing some devices in the future.

The unpinned behavior of the Fermi level, photovoltage, and recombination velocity is demonstrated experimentally at room temperature for a p-type GaAs(Cs,O) interface by means of photoreflectance and photoluminescence techniques. This behavior manifests itself as the multiple reversible switching of surface band bending, the amplitude of photoreflectance, and photoluminescence intensity under alternate deposition of cesium and oxygen on a clean surface of epitaxial p-type GaAs layers. The results prove that cesium-induced donorlike surface states dominate over defect-induced states.

Starting from a rate-equation model proposed by Beenakker, we calculate current-voltage characteristics for symmetric and asymmetric vertical quantum dots. We include up to 26 electrons and show how single-electron charge interaction, inelastic scattering, and nonadiabatic subband mixing can enhance the valley current significantly by opening new conduction channels. Subband mixing is the dominant mechanism increasing the valley current in both symmetric and asymmetric quantum dots. Single-electron charging is important in asymmetric structures but relatively weak in symmetric structures. Effects of inelastic scattering within the quantum dot are dramatic for asymmetric structures, but negligible in symmetric structures.

The theory of exciton polaritons in multiple quantum wells with arbitrary interwell spacings between successive wells is approached from two alternative viewpoints. The material approach considers the Green’s function for the coupled exciton-electromagnetic modes of the structure viewed as the interband elementary excitations of the crystal. The optical approach considers the light transmitted and reflected given an incident electromagnetic wave in linear-dispersion theory or, equivalently, the behavior of the modes of the vacuum fluctuations in the context of quantum electrodynamics. The optical approach is formulated in terms of transfer matrices. The complementary nature of the two methods is discussed. The theory describes T-mode polaritons and also a type of LZ-mixed-mode polaritons [a special case of LT-mixed-mode polaritons, in the terminology of K. Cho, Solid State Commun. 33, 911 (1980)] in multiple quantum wells. It is shown, however, that the effects of LZ mixing are negligibly small for GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells in which the constituent wells are of width much less than the relevant optical wavelength.