Table of contents

This work describes the periodic poling of :1%Mg0 single-crystal fibres with an electric field applied during the laser-heated-pedestal growth process. The alternating electric field is induced by two electrodes located close to the Curie isotherm and parallel to the growth axis. In these first trials the obtained ferroelectric domains can be clearly visualized but do not lead to a single quasi-phase-matching condition for second-harmonic generation around 1060 nm.

In this work, results of detailed optical nuclear magnetic resonance (ONMR) studies of a series of five single GaAs/AlGaAs quantum wells centre-doped with beryllium are presented. Signals from and nuclei were clearly observed from single quantum wells with widths ranging from 42 to 143 Å. The resonance from the AlGaAs barrier layers was not observed, confirming that the ONMR signal originated within the individual quantum wells. De-polarization of excitonic luminescence from a 60 Å wide quantum well of up to 70% was observed under conditions of adiabatic fast passage through the resonance. The signal originated from approximately nuclei and represents an increase in sensitivity over conventional NMR of 5 - 6 orders of magnitude. Through studies of effects of optical power, RF power and external magnetic field on the ONMR signal, this work establishes and extends the range of experimental conditions suitable for ONMR in GaAs-based structures.

We observed that GaAs surface quantum wells on undoped AlGaAs substrates experienced a reverse surface photovoltage (RSPV) that caused the bands to bend upward when illuminated by femtosecond laser pulses for time-resolved two-photon photoemission spectroscopy (TRPES). The RSPV effect is created by trapping of photoexcited electrons in the quantum well. Secondary illumination creates a surface photovoltage opposite in magnitude that flattens the bands. The secondary illumination does not affect the width of the energy spectrum or the ultrafast dynamics with respect to the conduction band minimum. A simple model based on the recombination current from the AlGaAs fits the data excellently.

Molecular quantum chemical calculations were performed both at the ab initio and at the semi-empirical level to model the molecular conformations and electronic structure of oligomers of poly(phenylene vinylene) during the early stages of interface formation with Al, Ca and Mg. We found that the divalent metals, Mg and Ca, disrupt the conformation of the oligomers less than Al does. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) move into the energy gap both for Ca- and for Mg-doped systems, resulting in gap-state formation. This is consistent with the polaron/bipolaron picture. The electron density plots indicate that the de-localization of electrons is reduced more significantly by Al than it is by Ca and Mg. Our simulation results have been confirmed experimentally via XPS and NEXAFS.

MBE grown GaAs (100) surface quantum wells were studied using time-resolved two-photon photoemission spectroscopy where the electron distribution was directly determined. Relaxation lifetimes from 50 to 450 fs were measured and were found to be dependent on the energy of the excited electron distribution. Radiative lifetimes at the semiconductor - liquid interface were obtained using time-correlated single-photon counting. From the concentration dependence and other studies, we estimate that the electron transfer cross section is comparable to the molecular cross sections, i.e. electron transfer approaches the adiabatic limit and conditions are optimal for hot electron capture. This work provides parameters to make the hot electron transfer channel competitive with electron relaxation enabling a future generation of solar cells exploiting hot electrons.

The proper combination of sequential ion implantation and thermal annealing can successfully fabricate binary semiconductor quantum dots (QDs), such as InP, GaP and GaAs QDs. Far-infrared reflectance techniques can be effectively employed to identify the species formed before and after various thermal treatments via probing molecular vibrations and surface phonons of the QDs. This technique is very useful for the characterization of an ultra-thin layer of foreign species on or in the near surface of the bulk substrate materials. In addition, this technique is non-destructive, simple and versatile. The examples given in the present report are InP and GaP QDs formed in fused silica and GaAs QDs formed in fused silica and in [0001] substrates.

In consequence of the progress made in epitaxial techniques, strained layers are important parts of a few of today's optoelectronic devices. Much attention has thus been paid to the question of how the band line-up at a heterojunction is affected when one or both constituents are strained. This article reports experimental results on band offsets at heavily strained heterojunctions of arsenide compounds. The InAs/GaAs and InAs/AlAs heterojunction valence-band offsets are markedly strain dependent. Emphasis is placed on photoemission experiments and on the modifications of the classical core-level method due to the strain. The results are compared with theoretical data from both `model' theories and self consistent calculations.

Stimulated by the capability of photoinjection as a working principle for infrared detectors and solar cells we study the effect of scattering processes on the charge transfer from the metal (Au) into the semiconductor. The injection probability is strongly enhanced by elastic isotropic scattering events and multiple diffuse reflection at the boundaries of the photoemitting layer. Our predictions demonstrate that the magnitude of the quantum yield depends on the Au layer thickness with the former being up to 40 times larger than in the case neglecting all scattering processes (Fowler approach). Concerning the Au/n-Si structure our model calculations show that the external quantum yield passes through a maximum at Au layer thicknesses of 4 - 6 nm depending both on the mean free path and the exciting photon energy. Subsequently we determined experimentally the external and internal quantum yield at applied reverse bias voltage as a function of photon energy for Au/n-Si Schottky diodes of different Au layer thicknesses ranging from 3 nm to 50 nm. There is a reasonable agreement between our calculations and the experimental results referring to the Au thickness dependence. The experimental values of the internal quantum yield exceed the calculated values by more than one order of magnitude.

In this paper a rigorous three-dimensional theoretical model describing the temperature distribution in multi-layered samples, heated by an intensity-modulated Gaussian laser beam, is presented. The thermal interface resistances and the thermal conductivity anisotropy of one of the layers are taken into consideration. The three-dimensional thermal diffusion model is used to investigate the effect of thermal interface resistances on thermal conduction in three-layered samples, which is monitored in terms of the dependences of the amplitude and phase of the surface temperature rise of the sample on the modulation frequency of the excitation beam. The theoretical results can be used as a means of quantitative interpretation for the determination of interfacial thermal resistances in layered materials with modulated photothermal techniques. A direct comparison between the one- and three-dimensional theoretical models for thermal conduction in layered materials is also presented.

The (PZT 53/47) and (PZT 20/80) films deposited on substrates by RF magnetron sputtering are single-phase epitaxial films. In order to prevent large thermal stresses being induced in the films during the preparation process, the PZT films were cooled slowly through the Curie temperature. Scanning electron microscopy (SEM) showed that the PZT 53/47 film had a pyramid structure on its surface, whereas the PZT 20/80 film had a smooth surface without any observable features. Transmission electron microscopy (TEM) observations showed similar features: the PZT 53/47 film had a coarse columnar structure, whereas the PZT 20/80 film had a fine crystalline structure; this is due to the better lattice matching in the latter.

The gas-sensitive properties of nanostructured 94, 85, 71 and 7 mol% materials prepared by high-energy ball milling have been investigated in atmospheres containing alcohol, CO and gases. It has been found that the materials show a high sensitivity with a short response time to the alcohol gas and almost no sensitivity to the other two gases, CO and in the concentration ranges studied. Furthermore, the gas-sensitive behaviour of these materials in response to the alcohol gas has been discussed on the basis of the microstructure of the materials. It is found that the tin ion content in and tin ions on the surface of particles may play an important role in the enhancement of the gas sensitivity.

Magnetic relaxation measurements have been performed in the temperature range 300 - 820 K on a cobalt single crystal with [0001] orientation. The relaxation spectrum may be conveniently divided into three regions. Two of these, region I and region II , lie in the hexagonal phase; the third region lies in the cubic phase. In region I we have found a broad relaxation peak, whereas region II is characterized by the complete absence of any relaxation process. In the cubic phase, region III, we have observed a plateau-like magnetic relaxation process having an approximately logarithmic time dependence. An analysis of these relaxation processes is given and they are interpreted in terms of the defects present in the sample.

Rare-earth permanent magnet motors are being used increasingly in electrical machines and modern production methods frequently require in situ magnetization of the magnets. Magnetic measurements have been performed on magnetically anisotropic powder (Magnequench MQA-T), sintered and sintered and were compared in order to study the influence of the magnetizing field strength on the measured values of the remanence and intrinsic coercivity. For thermally de-magnetized material, the internal magnetic field necessary to obtain limiting values of the remanence and intrinsic coercivity varies with magnet type. For example, the and magnets studied have similar values of remanence and intrinsic coercivity , but the latter requires an internal magnetizing field three times larger than does the former in order for one to attain limiting values of the remanence and coercivity. Further insight into the magnetization processes in these three materials has been gained from the determination of the magnetic viscosity parameter, S, and the irreversible susceptibility, .

Nonlinearities in square wave recording on three rigid disks have been studied experimentally. Measurements of the roll-off curves and of the noise power spectra as functions of packing density both show that in square wave recording nonlinearities arise from transition width broadening at high densities. Transition width parameters measured by the two methods show excellent agreement over the whole range of packing densities studied.

Argon up to pressures of 3 MPa was excited by intense electron beam pumping. At the maximum an energy of 20 meV per atom was deposited into the gas. The population of the transient argon species , , and was investigated for different pumping conditions by absorption and emission spectrometry. A kinetic model is proposed that reproduces the experimental data. The spin-flip reaction with a rate constant of is responsible for the rapid destruction of the metastable excimer and for the population of the lasing state. The rapid dissociation of vibrationally excited by plasma electrons leads to a saturation in the population of with rising pumping density.

An axial plasma jet emitted from the dense plasma focus (DPF) was detected using a Faraday cup. The jet was registered into a cone with a half angle of . The angular measurements of the D - D fusion neutron flux, performed with CR-39 nuclear track detectors, allowed us to see that there is a high anisotropy of the neutron flux just in a narrow cone, practically coincident with that of the plasma jet. This phenomenon is consistent with a DPF thermonuclear model based on the hypothesis of axial inertial plasma confinement. The same Faraday cup measurements showed the existence of very collimated and brief (about 10 ns) ion beams. Image-converter pinch photographs showed that these beams could be produced by spatial charge-separation effects due to m = 0 instabilities. The beam energy spectrum, obtained with track detectors, had the range from 0.7 - 5 MeV. Both plasma jets and high energy beams can explain the high angular anisotropy of the neutron flux, in terms of non-thermal fusion mechanisms. On the basis of these experimental results we discuss the coexistence of thermal and non-thermal mechanisms for neutron production in a DPF.

Energy dissipation in the electrodes of CuBr laser pulsed discharges has been studied in a gaseous environment relevant to a copper vapour laser and with discharge length scaling. The power lost in the electrodes significantly increases not only with a decrease in electrode separation but also more compounds are added to the medium. The total energy losses in both electrodes range from to of the storage capacitor energy. This contributes much to the low efficiency of a copper vapour laser. The result cannot be explained by a simple redistribution of electrical power between the discharge plasma and the electrodes proportionally to their impedance - pre-electrode processes must also be considered.

Positive streamers in atmospheric pressure air in strong uniform external fields (40, 50 and 60 kV ) are simulated within the scope of the diffusion-drift model. Photo-ionization was taken into account. A simple model which describes streamer acceleration and expansion is proposed on the basis of the simulation results. It is shown that photo-ionization is essential only during the early stage of streamer formation. In strong external field further streamer evolution is governed by collisional multiplication of seed photoelectrons in a gap. The detailed structure of the streamer head is presented. It is shown that streamer advancement occurs due to avalanche ionization in a thin ionization domain (ID) with a width of the order of the space-charge layer's width. The ID can be considered as a region where some initial level of electron density is rapidly converted to the density in the streamer channel . This allows one to assume that the shape of the streamer head coincides with the instant position of the contour line on the seed electron density surface. This assumption leads to a simple relation for the streamer velocity which is consistent with the simulation results.

A method is developed to treat the effective thermal conductivity of periodic composites whose inclusions are of different sizes or of different thermal conductivities. Based on the Green's function formalism, the effects of induced heat sources on inclusion boundaries are explicitly taken into account, and a set of Rayleigh identities are established. Hence the temperature distribution in a polydisperse composite and its effective thermal conductivity can be evaluated with very high accuracy. The method is applied to two-dimensional polydisperse systems with cylindrical inclusions as well as to three-dimensional ones with spherical inclusions. Illustrative calculations of the effective thermal conductivity are presented for both isotropic and anisotropic systems. The application of the method is further exemplified by an analytical formula for the effective thermal conductivity of a composite which possesses a square symmetry and contains two different kinds of inclusions. The numerical calculations have been carried out for two-dimensional systems.

The conductivity of -doped emeraldine polymers is studied as a function of temperature in the range . The dopant concentration of the samples varies between 0.1 M and 1.0 M. The temperature dependence of the dc electrical conductivity gives evidence for a transport mechanism based on variable-range hopping in three dimensions. Using Mott's formula for the dc conductivity, physically meaningful values of the density of states at the Fermi energy, the hopping energy and hopping distance are calculated.

The contribution of the photoelastic effect to quadratic electrostriction is studied from a macroscopic point of view. The predicted values are found to be an order of magnitude or more smaller than the total quadratic electrostrictive effect of typical alkali halides.

This issue of Journal of Physics D includes a number of papers deriving from a special symposium of the Materials Research Society (MRS). The symposium took place at the end of 1995, and was dedicated to the spectroscopy of heterojunctions.

For decades this has been one of the most active fields in applied physics and materials science. Heterojunctions are, in fact, basic building blocks of many semiconductor devices. Furthermore, they are intrinsically flexible systems; their understanding and subsequent control can lead to better devices tailored to specific needs, and to entirely new devices. Such is the spirit, for example, of Capasso's bandgap engineering approach (see, for example, F Capasso and G Margaritondo (eds) 1987 Heterojunction Band Discontinuities: Physics and Device Applications (Amsterdam: North Holland)). Since the late 1970s, spectroscopic techniques have emerged as a basic tool for understanding heterojunctions on a microscopic scale. This stimulated Norman Tolk and myself to propose a symposium to assess the status of this active field in the late 1990s. The proposal was cheerfully accepted by the MRS, and we were lucky - besides finding a number of outstanding speakers, we also found a co-chair, Enrique Viturro, who worked so hard that our own task was amazingly light.

The papers published here are only a fraction of those presented at the conference. Nevertheless they represent the spirit and spectrum of the meeting, and its quality, well. They clearly demonstrate that the field is active and full of new ideas, with many potential applications.

Giorgio Margaritondo Ecole Polytechnique Federal Lausanne, Switzerland

18 April 1997