Table of contents

Thin films of La_0.65R_0.05Ca_0.3MnO₃ (R = Ho and Y) grown using pulsed laser deposition (PLD) technique on (100) NdGaO₃ (NGO) single-crystal substrates have shown a decrease in T_P farther from the corresponding values obtained on bulk samples. This decrease could be attributed to an additional substrate strain effect. The uniqueness of the present result is that Ho-doped La_0.7Ca_0.3MnO₃ (LCMO) films have shown drastic reduction in resistance (ρ_max) compared to that of Y-doping similar to what has been observed in the bulk samples. We have also fabricated a novel FM/I/FM (FM = ferromagnet (La_2/3Ba_1/3MnO₃ (LBMO) in this case); I = insulator) tunnel junction by replacing the widely used SrTiO₃ (STO) insulating layer by an insulating phase of Ho-doped LCMO composition (La_0.55Ho_0.15Ca_0.3MnO₃) to study the MR and electronic correlation effects. High resistance values have been realized across junctions made up of the LCMO-based insulating layer. This result could open up new avenues in TMR research.

Electrical properties of Mg-doped GaN epilayers grown by metal organic chemical vapour deposition (MOCVD) were investigated using photocapacitance measurements and deep level transient spectroscopy (DLTS). Annealing at different temperatures and different time durations gave gradual activation of Mg acceptors in samples taken from the same as-grown wafer. The samples exhibit a clear persistent photocapacitance (PPC) at low temperature. The photocapacitance versus illumination energy measurements shows the presence of energy levels labelled O1 and O2 at 1.1 and 1.9 eV, respectively, from the valence band. Their concentrations increase with the annealing duration and the temperature and thus they can be inferred to originate from Mg. They are found to be metastable. Our results support the hypothesis that the PPC observed in GaN : Mg originates from these metastable deep centres related to Mg. DLTS measurements show the presence of a deep level T1 at 0.35 eV from the valence band. When the sample has been illuminated at 80 K with photon energy of 1 eV or higher, the DLTS signal from this centre decreases with the illumination time. The energy 0.35 eV is therefore likely to be the thermal activation energy of the O1 centre.

Visible luminescence from erbium oxide layers grown on crystalline and amorphous silicon (c-Si and a-Si) has been investigated. The results show strong red and green cathodoluminescence bands due to intraionic Er³⁺ radiative transitions at room temperature. The use of c-Si or a-Si as substrate led to a red or green dominant emission, respectively, which has been explained in terms of the oxygen content in the substrate. The results obtained from samples grown in different atmospheres also support this assumption.

Indium tin oxide (ITO)-covered waveguides incorporating a grating and immersed in a fluid have been used to study the incoupling of light as a function of a voltage applied between the ITO and an electrode parallel to the waveguide and also immersed in the fluid. The optical set-up is typical for adsorption and binding studies of (bio)chemical material, but the voltage dependence is a new feature. Applied voltages, positive at the ITO surface, shifted the incoupling angle of linearly polarized laser light. Voltages below 1 V are sufficient; above, the effect saturates. dc and ac protocols have been applied. Weak dependence on solution composition and strong asymmetry with respect to the polarity of the applied potential imply that the ITO itself is involved in the effect. It is proposed to use it as a reference or compensation method for standard biosensor applications. It could considerably simplify them, avoiding high-precision goniometers or high-resolution position-sensitive devices for determining waveguide-effective refractive index shifts, and using a simple voltage sweep instead.

The floating potential is investigated for a planar probe/electrode in a DC/RF plasma with a bi-Maxwellian electron distribution. A form of Bohm velocity is adopted appropriate to a two-temperature (T₁, T₂) plasma with corresponding electron densities n₁, n₂. Taking hydrogen as an example, a numerical study shows that for given values of T₁/T₂, n₁/n₂ and RF amplitude (normalized to T₁), the floating potential (normalized to T₁) is unique. On the other hand, using the same normalizations for prescribed values of floating potential and RF amplitude, and given n₁/n₂, there are two values of T₁/T₂. Conversely for given values of floating potential, RF amplitude and T₁/T₂, n₁/n₂ may be single or double valued. Under conditions of sheath inversion RF plasmas again show uniqueness or two valuedness, according to the prescription.

A numerical model is developed to clarify three-dimensional effects of the radial injection of carrier gas and particles on the thermo-fluid fields of the plasma jet with and without swirl. The plasma-particle two-way interactions are modelled by coupling a Lagrangian approach for particle behaviour with an Eulerian approach for plasma flow under dense loading. The effect of radial injection of the carrier gas on the flow and temperature fields of the plasma jet with and without swirl is clarified by numerical simulation. The deformations of the plasma jet thermo-fluid fields caused by dense particle loading with and without turbulent dispersion of the particles are presented. It is shown that the high mass flow rate of the carrier gas affects plasma jet fields strongly and particle turbulent dispersion can control the deformation of the plasma jet fields. The given swirl flow reduces the momentum and energy transfers between plasma and particles.

This paper presents a numerical simulation of the first negative corona in SF₆ at atmospheric pressure, in a point-to-plane electrode system. A few experiments are first carried out in order to acquire a basic knowledge of the physical processes involved in the first negative corona. Since it does not seem convincing that photoemission would be the predominant secondary process at the cathode surface, field effect emission will be introduced into the numerical modelling of the ignition and development of the first negative corona. The axial and radial development of the discharge is also optically measured. Continuity equations of electron, positive- and negative-ion densities are numerically solved on a non-uniform mesh with a monotonic upwind scheme for conservation law (MUSCL) method. These equations are coupled with Poisson's equation by using the classical method of disks introduced by Davies. The novelty of this model bears upon the choice of the boundary conditions which take field effect emission into account. According to the numerical simulation, this secondary effect could indeed explain the fast rise time of the typical negative corona current pulse, and enables numerical simulation results to be in agreement with the experimental ones.

Global model (volume averaged) calculations are used to study the plasma chemistry in an O₂/F₂ low-pressure high-density discharge. The plasma parameters, electron temperature T_e, dc plasma potential V_pl, electronegativity n_-/n_e as well as the densities of the species, were calculated as a function of F₂ content and discharge pressure in the pressure range 1-100 mTorr. The fluorine discharge is highly dissociated. The density of the fluorine atom increases as F₂ is added to the discharge and for mixtures with ~35% F₂ atomic fluorine is the dominating specie in the discharge. At a fixed ratio of O₂ to F₂, <50% F₂ in the flow rate, the electron temperature decreases with increased pressure and the dominant positive ion species changes from being O⁺ at low pressure (<5 mTorr) to O⁺₂ at high pressure (>50 mTorr). At a fixed ratio of O₂ to F₂, >50% F₂ in the flow rate, the electron temperature decreases with increased pressure at low pressure (<5 mTorr), increases with further increase in pressure up to roughly 40 mTorr and decreases again with further increase in pressure. The dominant positive ions change from being O⁺ and F⁺ at low pressure (<5 mTorr) to O⁺₂ and F⁺₂at high pressure (>50 mTorr).

A one-dimensional LTE model of a microwave-driven sulfur lamp is presented to aid our understanding of the discharge. The energy balance of the lamp is determined by Ohmic input on one hand and transport due to conductive heat transfer and molecular radiation on the other. We discuss the origin of operational trends in the spectrum, present the model and discuss how the material properties of the plasma are determined. Not only are temperature profiles and electric field strengths simulated but also the spectrum of the lamp from 300 to 900 nm under various conditions of input power and lamp filling pressure. We show that simulated spectra demonstrate observed trends and that radiated power increases linearly with input power as is also found from experiment.

The present experiment for the first time compares in detail emission characteristics of laser-produced plasma pulses ablated from a binary material with those of its pure components. As an example system we have chosen a tungsten-copper alloy (W34Cu66). The plasma was produced by obliquely incident Q-switched pulses (E_L = 170 mJ, τ = 5 ns and λ = 1.06 µm) focused to an ablation area of 1.74 mm². The most remarkable results are: the angular emission distribution of the totally ablated mass of the binary system W34Cu66 is almost equal to that of pure copper, i.e. the lighter component of the composite, and no segregation effect is observed between W and Cu. These findings are in contrast to published experimental results and simulation models so far. Additional findings are that the angular distributions of the average kinetic energies are similar for corresponding ions W^{q +} and Cu^{q +} ablated from either the compound system or the single component targets Cu and W, respectively and that stoichiometry appears to be conserved in the composite at least for the ions during ablation and expansion.

An investigation of the growth mechanisms, electronical and structural properties, and field emissions of carbon films obtained by chemical vapour deposition showed that field emissions from films composed of spatially oriented carbon nanotubes and plate-like graphite nanocrystals exhibit non-metallic behaviour. The experimental evidence of work function local reduction for carbon film materials is reported here. A model of the emission site is proposed and the mechanism of field emission from nanostructured carbon materials is described. In agreement with the model proposed here, the electron emission in different carbon materials results from sp³-like defects in an sp² network of their graphite-like component.

Photoluminescence and cathodoluminescence properties of SrGa₂S₄ : Eu²⁺ thin films prepared by reactive RF magnetron sputtering are investigated. Luminescence performances of the phosphor in the thin film form are compared to those of powder samples: the brightness efficiency of thin films is found to be about 30% of the efficiency of powder at low current density. A ratio higher than 40% is expected at higher current density. Thin film screens for FEDs will become a positive alternative to powder screens provided that film quality and light extraction could be improved by optimization of thickness and deposition parameters.

Streamers initiated in point-plane electrode geometry under impulse voltage are studied. Characterization up to 30 kV in both polarities shows two streamer types. In negative polarity, only slow `bush-like' streamers with a velocity of about 100 m s<sup>-1</sup> are observed. In positive polarity, both `bush-like' and filamentary streamers appear. Positive `bush-like' streamers have similar velocity as in negative polarity, whereas filamentary streamers have a propagation velocity up to 30 km s^-1. This high velocity is correlated to large transient current and light emission. Spectroscopic study of the light emitted by filaments shows mainly the second positive system of nitrogen. This system allows us to estimate the gas temperature inside a filament by making a comparison with a numerically simulated spectrum. A temperature close to 500 K is found. Dilution of hydrogen gas in liquid nitrogen allows us to determine also the electron density by Stark broadening of the Balmer H_α line. An electron density of about 1.10¹⁸ cm^-3 is found.

Edge bonded bulk acoustic wave transducers may be used for surface acoustic wave generation on the surface of a non-piezoelectric substrate. An arbitrary (about the X axis) rotated Y-cut lithium niobate transducer may be used in place of a standard (36°, 90° or 163° rotated) cut. This may allow one to optimize the performances of such a transducer, whose design is presented in this paper. Numerical examples for lead molybdate substrates are given and discussed.

In the recently developed high-sensitive CaSO₄ : Dy phosphor, sieving before the high-temperature sintering treatment has successfully eliminated particle agglomeration during subsequent sintering, and has further enhanced its thermostimulated luminescence (TSL) sensitivity to γ-rays. The reduction in TSL sensitivity of higher sized grains observed earlier following the procedure of sieving after sintering has also more or less vanished. Maximum TSL sensitivity is seen after sintering around 700°C, whereas maximum photoluminescent (PL) sensitivity is seen after sintering around 325°C. While the observed increase in TSL sensitivity (by 30%) with increasing sintering temperature in the range 325-700°C is explained on the basis of diffusion of Dy³⁺ ions from the surface to the whole volume of the grains (0-75 µm), the drastic decrease (by a factor of 3) in PL sensitivity with increasing sintering temperature is explained on the basis of change in the Dy³⁺ environment on the grain surface perhaps due to oxygen incorporation. Washing with water and acetone, which affect mainly the surface traps, enhances the PL sensitivity of CaSO₄ : Dy slightly; however, it does not influence TSL sensitivity very significantly. Grinding reduces PL in general, but no such trend was noticed in TSL which supports the conclusion that PL originates mainly from surface traps since grinding affects mainly the grain surface. However, the sharp reduction in TSL and PL sensitivities observed at 400°C indicates that an unusual process takes place near that sintering temperature.

Samples of BaZr_0.95M_0.05O_3-α (where M = Al, Er, Ho, Tm, Yb and Y) were prepared by solid state reaction method. The bulk conductivity of the specimens under wet N₂ exposure was studied as a function of temperature, dopant ionic radius and the electronegativity of dopants. The correlation between the conductivity and electronegativity of the dopant, which has not been reported in any other studies, will be discussed.

The ability of modulated differential scanning calorimetry (MDSC) technique to perform a specific heat spectroscopy is examined by comparing MDSC results with those of alternating current calorimetry technique. The comparison is performed on three glass formers: glycerol, propylene glycol and salol. Both techniques give rise to similar activation energies, fragility index m and non-exponential parameters β for the different compounds. It shows the relevance of the MDSC technique in providing a convenient laboratory probe of the molecular mobility. MDSC data are also compared with available dielectric results. This allows a check on the consistency of the results with regard to the differences between calorimetric and dielectric analysis.

In this paper, high-order geometric and chromatic aberrations of magnetic electron lenses are dealt with together by using the differential algebraic method. High-order geometric-chromatic aberration analytic expressions are obtained and arbitrary order combined aberrations can be calculated. As an example, a Glaser's bell-shaped magnetic electron lens has been studied and the second- and third-order combined geometric-chromatic aberration coefficients have been calculated. The aberration figures of the combined second-order aberrations are also presented. This developed method can be of great utility in high-order combined aberration analysis and correction for high-resolution charged particle optical systems.