Table of contents

The electroluminescence (EL) at room temperature from Au - extra-thin silicon oxynitride (ETSON) - p-Si and Au - ETSON - n-Si structures is reported. The ETSON films were deposited by the rf magnetron sputtering technique, and composite targets were used (the area ratio of Si to was ). The EL spectra were measured under forward bias after the ETSON films had been annealed at 300, 600, 800, and in ambient for 30 min. The Au - ETSON - p-Si structure has a dominant EL band with peak wavelength around 680 nm, and its integrated EL efficiency is an order of magnitude higher than that from a Au - extra-thin Si-rich (ETSSO) film - p-Si structure. Distinguishing it from the Au - ETSSO - Si structure, which emits visible EL only when it is fabricated on p-Si, EL can also be observed in the Au - ETSON - n-Si structure under forward bias, and the two dominant EL bands peak one at around 700 nm and one at around 800 nm. Electron beam irradiation induces a new EL band peaked at 500 nm in the EL spectrum from an Au - ETSON - p-Si structure. From the experimental results it is suggested that electrons and holes from opposite sides tunnel into the ETSON layer and recombine radiatively at luminescence centres there.

Electron spectroscopic studies of monolayers adsorbed on a surface as a function of annealing temperature indicate three adsorption states. At room temperature interacts weakly with the surface and in consequence the electronic and vibrational excitations of the molecule are only slightly perturbed. Above changes in electronic and vibrational excitations indicate that, although the molecule remains intact, the nature of the surface - molecule bond changes, leading to what may be regarded as a strongly chemisorbed `precursor' state to cage opening and destruction. Beyond the fullerene molecules begin to break up, with some carbon atoms remaining on the substrate surface in the form of a carbide.

Results on the scattering and the dissociative sticking of molecules on the Ag(110) surface are presented. The dependence on the incidence energy and angle of the molecules is investigated by employing molecular beam techniques. Both the angle with respect to the surface normal and that with respect to the azimuthal direction on the surface are varied. The and azimuthal directions display a different corrugation as observed by the incident molecule. Dissociative sticking is observed to proceed via the molecular chemisorption state and is strongly enhanced by the incidence energy. An azimuthal dependence of the sticking is observed. Results on the desorption of from the Ag(110) surface are also given.

The Brillouin light scattering technique has been exploited for investigating the elastic properties of superlattices, with x = 0.5 and a total thickness of several hundreds of nanometres. These superlattices have been grown by chemical beam epitaxy and are close to being lattice matched to the InP substrates. The frequency position in the Brillouin spectra of the Rayleigh surface wave, and of the transverse and longitudinal thresholds for two different propagation geometries, enabled us to determine the three effective elastic constants of the superlattices. A comparison between these experimental values and those expected from calculation based on the properties of InP and InGaAs single films shows that interface effects do not play any significant role in modifying the elastic properties of InGaAs/InP heterostructures.

A detailed theoretical treatment is given of eigenfunctions and eigenenergies of a multilayer organic quantum well sandwiched between two different dielectric media. The abrupt change of dielectric constants at the interfaces distorts the wave function and results in there being possible surface states in addition to propagating states. The proper boundary conditions are accounted for by the method of image charges. Analytic criteria for the existence of surface states are established using the nearest-layer approximation; they depend not only on the intralayer and interlayer parameters but also on the number of layers. The size dependence together with the dependence on signs and relative magnitudes of the structure parameters fully determine the energy spectrum of propagating states as well as the number and the location of surface states.

thin films were obtained by solid state reaction (induced by annealing under S pressure) between the W and S constituents in thin film form. The tungsten layers were deposited by RF sputtering, while S layers were evaporated. The thickness of the layers varied between 50 and 100 nm. The films have been investigated using x-ray photoelectron spectroscopy, scanning electron microscopy, x-ray diffraction, optical absorption and conductivity measurements. The effects of annealing conditions (duration and temperature) on the crystallization and composition of the films have been investigated. The films were crystallized in the hexagonal structure. To complete the reaction between W and S, annealing under S pressure at a temperature of at least 820 K for 24 h is required. Under these conditions the films were nearly stoichiometric. The best films present a c-axis length of 1.2500 nm and an a-axis length of 0.3185 nm. All the films were found to be preferentially oriented with the c axis perpendicular to the plane of the substrate. The degree of preferential orientation was found to increase slightly with increasing annealing time and/or temperature. The nearly stoichiometric films present good optical properties, similar to those of single crystals. The conductivity of the films depends strongly on the annealing conditions, i.e. on their composition and on their crystallization state.

The quasiatomic approach is applied for the first time to the consideration and computation of angular distributions of (x-ray) photoelectrons scattered from surfaces. The anisotropic region surrounding an ionized-core atom in a specimen is regarded as acting as an electron-optical lens focusing or defocusing the initial (prescattering) photoelectron flux along various directions. The quasiatomic equations describing the photoelectron angular distributions as functions of the electron-optical properties of the surroundings are obtained, and stereographic projections of the photoelectron yield from model octahedral clusters (such as and are computed with their help. Calculations of the low- and medium-energy photoelectron diffraction patterns show that the splitting of atomic transitions and the -hybridization of the atomic continuous states dominate the focusing effect. It is revealed that: (1) a central role is played by the collective action of all of the atoms in the environment, whose symmetry compels the major directions of photoelectron yield to coincide with fourfold and threefold rotation axes rather than with directions to the nearest neighbours; (2) the low- and medium-energy diffraction patterns are highly sensitive to the electronic structure of the ionized atom and the integral electron-optical properties of the surroundings; and (3) the photoelectron angular distributions change dramatically when resonant inner-shell photoemission is achieved, and correlate with the symmetry of the highly excited molecular orbitals. The relationship between the description in terms of quasiatomic waves and that in terms of multiply scattered waves is discussed.

The series of spinels was prepared by ceramic technology. The crystal structures were refined from the x-ray diffraction data using the Rietveld profile method. The high-temperature magnetic susceptibility was measured by the Faraday method in the temperature range 290 - 600 K and the data were fitted to the Curie - Weiss law. The cation distribution in these diluted magnetics is obtained from x-ray diffraction and high-temperature magnetic susceptibility measurements. It was found that, for , ions occupy exclusively the octahedral position in the spinel type of structure. In the low-concentration region a strong clustering of magnetic ions is observed. In the range , ions are distributed in both tetrahedral and octahedral cationic sites. The Curie - Weiss paramagnetic temperature is negative in the whole range of concentrations, as the consequence of a predominant antiferromagnetic interaction. The appearance of the ions in tetrahedral sites (around x = 1.33) leads to a drop in the absolute -value. The effective magnetic moments of ions are about for and depend on the concentration for x>1.33.

By means of high-resolution x-ray scattering, we have studied the temperature dependence of the critical scattering associated with the Peierls transition in . From a careful intensity and line shape analysis the critical exponents , and were derived. , and are the critical exponents of the temperature dependence of the order parameter, the susceptibility and the correlation length of the order parameter fluctuations, respectively. Though this incommensurate structural phase transition is reported to be continuous, an unusual exponent value is obtained: . The small value of suggests a blurred first-order transition. We also report on the anisotropy of the correlation in the basal plane perpendicular to the -chain direction.

The crystal structure of (KTP) has been determined as a function of pressure to 8 GPa at ambient temperature, using single-crystal x-ray diffraction techniques. The first-order isosymmetric phase transition, at 5.8 GPa, appears to have a strong effect on the compression mechanism of the structure: in the low-pressure phase the structure compresses by polyhedral tilting within the cross-linking chains while in the high-pressure phase relative translations and rotations of the chains themselves account for a significant proportion of the unit-cell volume reduction both at the transition and on further pressure increase. The distortion of the framework at the phase transition is accompanied by relatively large displacements of the potassium atoms to positions more closely related to the Pnan symmetry of the high-temperature phase. A comparison of the pressure-induced changes of the highly distorted octahedra with recent high-pressure optical measurements indicates that neither the lengths nor the orientations of the Ti - O bonds correlate, at least directly, with the observed variation of the second-harmonic-generating (SHG) power of the crystal. This strongly suggests that other regions of the structure may be important for the SHG properties.

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The crystal structure of the title compounds has been characterized by quantitative room-temperature X-ray powder diffraction to be isotypic with the type with partial disorder among the p-metal atoms. The low-temperature behaviour of these alloys is characterized by a decrease of the ordering temperatures as the Au content increases. Weak Kondo interaction for alloys richest in Au is observed from various transport and specific heat measurements.

The temperature variations of the refractive index and of the linear birefringence have been studied in flux-grown nominally pure and (SBT) single crystals (x = 0.02, 0.08 and 0.17) in the temperature range from 5 to 300 K. Polar domain structures were studied by polarizing microscopy. In SBT (x = 0.17) two phase transitions (PTs) at K (cubic, paraelectric-to-tetragonal, ferroelectric) and K (to orthorhombic or rhombohedral) were detected. SBT (x = 0.08) undergoes a PT at (cubic, paraelectric-to-orthorhombic or rhombohedral, ferroelectric). In the low-x limit (x = 0.02) the cubic-to-tetragonal PT of pure was observed at . Below a polar state with symmetry and weak anisotropy was observed. Contributions of the spontaneous electro-optic effect to the temperature variation of the refractive index in SBT (x = 0.08 and 0.17) allow us to estimate the spontaneous polarization and its temperature variation.

We have reinterpreted the metal 2p core-level x-ray photoemission spectra of the 3d transition-metal pyrites , and using a new version of the single-impurity cluster model which includes the effects of charge transfer to the conduction band. By accounting for the effective screening of the strong core-hole potential involving the unfilled S 3p orbitals on the ligand sites, we can successfully model the highly asymmetric lineshapes and weak satellite structures found for and . Reduced screening in leads to the appearance of strong satellite structures and the spectrum is best interpreted using the screening channel to the narrow upper Hubbard band. These results indicate the importance of the empty ligand orbitals when interpreting the physical properties of the pyrite-type chalcogenides.

In this paper, the behaviour of the electron - positron momentum density of non-interacting particles, , in a periodic lattice potential is investigated. On the basis of general considerations, we show that in each metal there are some components of which are only weakly sensitive to approximations used in the calculation of the positron wave function . This means that in these regions of p, the two-particle density can be evaluated with a high accuracy, largely independently of uncertainties arising from the different approximations used in calculating the function . This property of can be useful for the verification of various approaches used for describing the electron - positron interaction in real metals.

We have determined the three-dimensional electron - positron momentum density for the ordered binary alloy using two-dimensional angular correlation of positron annihilation radiation (2D-ACAR). The Fermi surface obtained from the momentum density is compared with the results from de Haas - van Alphen measurements and theoretical calculations. We have also observed the high-momentum components, and the enhancement effect on the momentum density.

A single-crystal synchrotron radiation study of gadolinium selenide has been made in the temperature range 15 - 100 K. GdSe has the rocksalt structure and becomes antiferromagnetic below a reported Néel temperature of 65 K. At 15 K, magnetic reflections are observed at with modulation wavevector propagating from reciprocal lattice point G. This is achieved by exploiting the resonant enhancement in the vicinity of the and edges. Similar enhancements are observed at the two edges, with the maximum effect occurring approximately 3 eV above the absorption edge. The temperature dependence of the intensity of the magnetic reflections indicates a Néel temperature of 63(1) K. These measurements, together with high-resolution studies of the fundamental reflections , contribute further evidence of magnetic or structural changes in the sample at 37(1) K. Our observations are discussed and compared with previous x-ray diffraction and magnetic susceptibility measurements.

The effect of nitrogen insertion on the electronic properties of cerium and iron in has been studied by means of x-ray absorption spectroscopy (XAS) and magnetic circular x-ray dichroism (MCXD). The evolution of the electronic structure of cerium upon nitrogen insertion has been studied. It emerges from this MCXD study at the edges of cerium that the cerium atoms bear a 5d magnetic moment of about that is coupled antiferromagnetically to that of iron. The spin polarized EXAFS signal has been observed; an attempt to evidence the magnetic environment has been made in the light of previous neutron diffraction results. Comparison of EXAFS and spin polarized EXAFS is revealed to be an efficient new tool to distinguish between magnetic and non-magnetic atoms in the environment of the absorber. The analysis of the Fe K edge MCXD spectra shows that insertion of nitrogen induces a trend to strong ferromagnetism with an almost completely filled majority spin subband.

The morphotropic phase transition in the system has been studied by far-infrared reflectance spectroscopy. The reflectance measurements are performed on compositions with x = 0.0, 0.10, 0.20, 0.30, 0.35, 0.40 and 0.525 at room temperature in the frequency range . The spectrum is fitted with a model based on the factorized form of the dielectric function. The dependences of the mode frequencies, damping and oscillator strength on x are reported. The static dielectric constant is calculated from the frequencies obtained from the fitting. The observed experimental data show incomplete softening of the lowest E(1TO) mode towards the morphotropic phase boundary. The oscillator strength of the lowest mode carries nearly all the contribution to the lattice part of the dielectric constant.

Through this work it is shown that the luminescence observed in samples is related to the formation of microcrystalline precipitates. This conclusion is supported by the experimental emission and excitation spectra as well as by the temperature dependence displayed by the emission intensity. Also the EPR spectrum of most doped samples clearly shows the presence of a signal (superimposed on that coming from isolated ions) whose temperature dependence is similar to that displayed by crystals. The absence of luminescence associated with isolated ions in the host lattice is discussed. It is pointed out that luminescence quenching is probably related to a multiphonon non-radiative process rather than to a (G) excited state close to the bottom of the conduction band.

Although luminescence quenching appears for a Dexter - Klick - Russell parameter probably close to 0.1, this is qualitatively related to a high radiative lifetime ( ms). The analysis made also shows that firstly increases by a factor of about 1.7 on going from -doped fluoroperovskites to in fluoride-type crystals, and secondly increases on going from to . Both variations mainly reflect a parallel increase in the Stokes shift.

A study of the electronic states near the Fermi level in high- superconductors, which is so important to understanding the mechanism of superconductivity, has been carried out on well characterized single crystals of superconducting . A large number of polarized x-ray absorption spectra have been recorded with the E vector parallel to the a - b plane and perpendicular to it at the Dragon beam line at the Brookhaven Light Source using the fluorescence mode of detection. These bulk spectra of and O K are then analysed to obtain the density of the unoccupied and orbitals from the spectra and the density of the and orbitals from the O K spectra. The results clearly show the presence of orbitals in a substantial amount although the same cannot be said of the orbitals. Tight-binding calculations of the holes are also reported and compared with experimental results.

Positron annihilation spectroscopy in conjunction with a variable-energy beam has been used to investigate the variation of the vacancy-type defect density of titanium nitride films with varying nitrogen concentrations. Decreases in the Doppler-broadened lineshape parameter were observed as the nitrogen partial pressure used in the manufacture of the samples was increased from 0.01 Pa to 0.1 Pa indicating a decrease in the positron trapping at vacancy defects in this range. It is suggested that the positrons are trapped in Ti vacancies with the variation in trapping being due to increasing N-atom migration onto Ti sublattice vacancy sites as the nitrogen partial pressure is increased.