The microscopic nature of defects in ion-implanted crystalline silicon (c-Si) and amorphous silicon (a-Si) has been studied using Mössbauer spectroscopy. The evolution of the local structure around the probe atoms is followed during thermal annealing of ion-beam-created amorphous and ion-beam-damaged crystalline Si. Direct comparison of the Mössbauer parameters of ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sb}$ in c-Si and a-Si demonstrates that Sb occupies two distinct sites in each material with similar local environments in both materials. These sites are identified as fourfold-coordinated substitutional Sb and Sb-vacancy complexes. Annealing of ion-beam-damaged c-Si at 150 °C causes the Sb-vacancy complex to change from a substitutional Sb adjacent to a vacancy to a more complicated complex. Annealing of a-Si is shown to reduce distortions in the network, consistent with structural relaxation.

High-resolution Fourier spectroscopy combined with perturbation techniques such as Zeeman spectroscopy and uniaxial stress have been used to study the electronic structure of the interstitial magnesium double donor in silicon. In tetrahedral interstitial positions, the two charge states ${\mathrm{Mg}}^0$ and ${\mathrm{Mg}}^{+}$ have binding energies of 107.51 and 256.68 meV, respectively. Their electronic structure can be described as a solid-state analog of the helium atom in its neutral and singly positively charged states, respectively. Interesting deviations in the binding energies of the excited states from those calculated within the effective-mass approximation (EMA) were found which have not been observed previously, to the best of our knowledge, for any similar system. These deviations are explained by a perturbation in the central-cell region that does not only affect the localized 1s(${\mathit{A}}_1$) ground state but also the delocalized shallow-donor-like excited states. We focus especially on these deviations from EMA and the split ${\mathit{np}}_{\pm }$ states of ${\mathrm{Mg}}^{+}$ and find that the local potential has a radius comparable with that for the central cell.

While ordering in alloy crystals is well understood, short-range ordering in amorphous alloys remains controversial. Here, by studying computer-generated models of amorphous SiC, we show that there are two principal factors controlling the degree of chemical order in amorphous covalent alloys. One, the chemical preference for mixed bonds, is much the same in crystalline and amorphous materials. However, the other factor, the atomic size difference, is far less effective at driving ordering in amorphous material than in the crystal. As a result, the amorphous phase may show either strong ordering (as in GaAs), or weaker ordering (as in SiC), depending upon the relative importance of these two factors.

The structure of the nitrogen pair in crystalline silicon has been studied by the channeling technique and by infrared absorption spectroscopy. Silicon crystals have been implanted with ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ at room temperature and have subsequently been annealed at 725 °C to form pairs. The channeling measurements have been carried out at 136 K using the nuclear reaction ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$(p,α${)}^{12}$C to probe the ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ atoms, and the angular distribution of the α particle yield has been measured around all major crystal axes and planes. The major part of the implanted nitrogen atoms (70%) is found to occupy a unique type of site which is displaced by 1.1 Å from a perfect lattice site along a 〈100〉 direction. The population of this type of site has the same annealing properties as the local vibrational modes associated with the nitrogen pair in silicon. The experimental findings are consistent with a recently suggested model of the nitrogen pair.

Photothermal ionization spectroscopy of germanium, doped in the impurity-band conduction range with gallium acceptors and with thermal oxygen donors, reveals that the donors and acceptors compensate each other at temperatures higher than about 5 K, but that the impurities coexist as neutral donors and acceptors at lower temperatures. At the lowest temperatures, optical quenching is observed for wavelengths where both impurities have transitions. These effects are used to infer a weak coupling between the neutral acceptor and donor states. The small strength of this coupling sets the thermal oxygen donor apart from all other shallow donors in germanium and silicon.

A model for amorphization processes in ion-implanted Si has been proposed, in which a divacancy and di-interstitial (D-D) pair introduced in the crystalline Si lattice give rise to a local atomic arrangement including five- and seven-member rings. Stable atomic configurations were determined in supercells of 216 Si atoms, including randomly distributed D-D pairs, by employing the Tersoff potential for the interatomic force calculations. Based on the proposed model, radial distribution functions, bond-angle distribution, and phonon densities of states were calculated as functions of the number of the D-D pairs and it was found that complete amorphization starts to occur when the number of D-D pairs per atom exceeds ∼0.2 or approximately 2 D-D pairs are formed in the fcc cubic lattice.

The transient third-order polarization at the band gap of undoped and p-doped GaAs is investigated by spectrally and temporally resolved four-wave mixing. Excitonic and free-carrier contributions simultaneously excited within the bandwidth of the 100-fs pulses are clearly distinguished by their different spectral envelopes. The excitonic part dominates at carrier densities below ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and shows a time evolution governed by exciton–free-carrier scattering and by many-body effects. At higher density, the free-carrier polarization has a strength similar to the exciton contribution and exhibits a spectrum resonant to the femtosecond pulses with a photon-echo-like temporal behavior. The data are analyzed by a numerical solution of the semiconductor Bloch equations including an ensemble Monte Carlo simulation of the scattering dynamics of the carriers. The theoretical model is in good agreement with the experimental results.

High-resolution Fourier-transform photoluminescence spectroscopy combined with the resonant photoexcitation technique was used to study in detail the Zeeman effect on excitons bound to the neutral donors in high purity epitaxial GaAs. The neutral donor g-factor depends on the magnetic-field intensity and orientation in agreement with the predictions of the five-level k⋅p theory. The neutral-donor excited-state energies measured in a range of magnetic field from 0 to 12 T are in an excellent agreement with appropriately scaled calculations for the hydrogen atom. The correspondence between transitions having the same initial state but having as a final state either the nuetral-donor ground state (principal transitions) or a neutral donor excited state (two electron satellites) was established using resonant excitation and was verified by the angular dependences of the peak energies. Linear and circular polarizations of the 2${\mathit{p}}_{\mathrm{-}}$, ${2\mathrm{p}}_0$, and 2${\mathit{p}}_{+}$ two-electron satellites are consistent with the assignment of zero angular momentum to the ground state of the donor-bound exciton and we show that the transition energies for these components can be calculated with a 0.03-meV accuracy over the range from 0 to 12 T.

We report the photoenhancement and photoquenching of the 0.68-eV EL2 photoluminescence emission in low-temperature GaAs grown at 200–350 °C. EL2 exists in both a quenchable and unquenchable configuration. For the layer grown at 200 °C, EL2 is exclusively in the unquenchable configuration. The concentration of EL2 in the unquenchable configuration decreases with increasing growth temperature. The quenchable and unquenchable configurations can be explained by a model of the isolated ${\mathrm{As}}_{\mathrm{Ga}}$ under relaxed and strained conditions, respectively. The stress present in low-temperature layers is responsible for the unrelaxed condition and consequently for the lack of quenchability. The photoenhancement is attributed to the hole photoionization of (${\mathrm{As}}_{\mathrm{Ga}}$${)}^{+}$ and to the presence of a high concentration of the compensating ${\mathrm{V}}_{\mathrm{Ga}}$.

We measured the time evolution of the real and imaginary parts of the dielectric constant of GaAs following femtosecond laser pulse excitation. The data show a collapse of the average optical gap, or average bonding-antibonding energy-level separation. The rate of collapse increases with pump fluence. The decrease in the gap indicates that the pump beam induces a structural transformation from a covalent, tetrahedrally coordinated crystal to a phase with metallic cohesive properties.

Enhancements in Raman-scattering cross sections of phonon modes in ${\mathrm{AgGaSe}}_2$ have been observed both at room temperature and at liquid-nitrogen temperature. At low temperature the observed enhancements can be explained by outgoing resonances of the scattered photons at the lowest-energy free exciton. At resonance, multiphonon modes involving as many as three longitudinal-optical (LO) phonons have been observed. Exciton energy and damping constant and relative strengths of exciton–LO-phonon (Fröhlich) interactions in ${\mathrm{AgGaSe}}_2$ have been determined.

First-principles methods are used to study diamondlike amorphous carbon. Several structural models are introduced, and the topological, electronic, and vibrational properties are discussed. We explore the nature of defects in the amorphous network, and particularly the atomistic origin of the virtually defect-free optical gap. We observe that threefold-coordinated defects pair, and that this is important for obtaining a gap free of localized defect states as seen in experiments.

A traveling-wave instability at finite critical wave number is found in extrinsic semiconductors at the onset of impact ionization. A condition for this instability is a strong increase of the carrier-generation rate as a function of the electric field, e.g., due to a sufficiently low ionization energy of the impurities. The linear instability of the uniform steady state is discussed for a simple model, and traveling waves are shown to exist by solving numerically the basic partial differential equations.

We have developed a time- and space-resolved optical detector of nonequilibrium phonons utilizing excitonic photoluminescence at low temperatures. An epilayer of GaAs is ‘‘sensitized’’ by excitation with a low-power, focused ‘‘probe’’ beam, which creates free excitons (FE’s) and bound excitons (BE’s) in the layer. The photoluminescence intensity from these species is reduced by a flux of nonequilibrium phonons, created on the opposite side of the GaAs substrate by optical excitation of a metal film with a ‘‘pump’’ beam. A simple model is proposed for the phonon-induced change in photoluminescence intensity of the FE’s; previous work (e.g., by Blank et al. [Sov. Phys. Semicond. 25, 39 (1991)]) has treated the case of BE’s. Both time-resolved heat pulses and space-resolved heat flux associated with phonon focusing are observed. The heat pulses are characterized by a broad temporal distribution, the exact origin of which is not determined. If the sensitivity of this detection technique could be raised sufficiently, space and time resolutions of 1 μm and 1 ns, respectively, would be feasible.

We report experimental results on charge transport in a low-disorder and low-density one-dimensional electron system. At zero magnetic field (B=0), we observe quantized conductance steps in our sample, which is a channel longer than 1.2 μm. At B≠0, we observe the integer and the fractional quantum Hall effect. In the extremely high-B limit, we find a gate tunable transition from the fractional quantum Hall effect to an insulating phase and observe conductance oscillations as a function of density in this B-induced insulating phase. A careful study of the temperature dependence and the current-voltage characteristics of these conductance oscillations is made and the results are consistent with the recent theoretical model of a one-dimensional Wigner solid.

The theory of phonon-assisted cyclotron resonance in quantum wells is given; we consider cases where electrons are scattered by confined LO phonons described by the Huang and Zhu model, Fuchs-Kliewer slab modes, and Ridley’s guided mode model. The effect of interface phonon modes on cyclotron resonance is also studied. Extra peaks due to transitions between Landau levels accompanied by emission of confined and interface phonons in the absorption spectrum are predicted. Numerical results for frequency, field, and well-width dependence are given for parameters characteristic of GaAs/AlAs quantum wells.

We examine the reconstructions and surface energies of ZnSe(100) surfaces by first-principles total-energy calculations. The surface energies for the Zn- and Se-terminated (1×1), (1×2), (2×1), (2×2), and Zn-terminated (4×2) reconstructed surfaces consisting of various combinations of dimers and vacancies are determined as a function of coverage and atomic chemical potential. For the Zn- and Se-terminated surfaces, dimerization lowers the energies by 2.12 and 1.08 eV per dimer, respectively. When exchange with bulk Zn or Se reservoirs is considered, a c(2×2) Zn-vacancy structure is found to be energetically more favorable than a dimer structure for the Zn-terminated surface, while a (2×1) dimer structure is the most favorable one for the Se-terminated surface.

Nanometer-resolved lateral variations in the Schottky barrier height (SBH) formed at a chemically prepared Au/n-type GaAs interface were measured using ballistic-electron-emission microscopy (BEEM). The spatial profile and the statistical distribution of the SBH’s thus obtained were compared to current-voltage (IV) and capacitance-voltage (CV) characteristics of the same metal-semiconductor contact. This comparison showed that the macroscopic SBH obtained from the IV measurements can be successfully interpreted using the parallel conduction model applied to the BEEM-derived distribution, if the effect of thermionic field emission is included. The SBH obtained from the CV measurements is greater than the mean value obtained from BEEM measurements by nearly the image-force lowering expected for a Au/GaAs diode.

The density of states of cylindrical quantum wires is calculated in the presence of charged impurities located in the center of the wire. A multiple-scattering approach (Klauder’s fifth approximation), which represents a self-consistent t-matrix approximation, is used. For small impurity densities and in the weak screening limit the ground-state impurity band and four excited-state impurity bands are obtained within our approach. We find good agreement between the numerically obtained spectral densities with the corresponding analytical spectral densities calculated with the single-impurity wave functions. The merging of impurity bands is studied. For large impurity densities we obtain a band tail. We present an analytical expression for the disorder-induced renormalized band-edge energy in the band-tail regime.

We study tunneling in semiconductor heterostructures where the constituent materials can have a direct or an indirect band gap. In order to have a good description of the lowest conduction band, we have used the nearest-neighbor ${\mathit{sp}}^3$${\mathit{s}}^{\mathrm{*}}$ tight-binding model put forward by P. Vogl et al. A recursive Green-function method yields transmission coefficients from which an expression for the current density may be written down. The method is applied to GaAs/AlAs heterostructures. Electrons may traverse the AlAs barriers via different tunneling states ${\mathrm{\psi }}_{\mathrm{\Gamma }}$ and ${\mathrm{\psi }}_{\mathit{X}}$ (ΓX mixing). With an applied bias V≳0.5 V electrons may enter the GaAs collector contact in both the Γ and the X valleys (ΓX transfer). We have studied a number of GaAs/AlAs structures. For very narrow barriers there is little ΓX transfer, but AlAs barriers wider than about 25 Å act as ‘‘ΓX filters,’’ i.e., most transmitted electrons have been transferred to the X valley.