The design of bottom mirrors for resonant cavity enhanced (RCE) GaAs homojunction far-infrared (FIR) detectors has been investigated in the framework of three possible projects. The results show that neither the bottom contact layer of the detector itself nor the distributed Bragg reflector, which has been widely employed in near- and mid-infrared detectors, can perform well as bottom mirrors for FIR detectors. A new kind of bottom mirrors based on undoped/doped GaAs layers is proposed and optimized. Though the absorption cannot be neglected, the bottom mirror shows a satisfactory effect. The resulting absorption probability (as well as the quantum efficiency) in the detector cavity for this kind of RCE p-GaAs FIR detector is three times of the normal p-GaAs homojunction FIR detector demonstrated experimentally. This kind of bottom mirror design opens a new way to increase the quantum efficiency in homojunction FIR detectors.

The high power RF device performance decreases as operation temperature increases (e.g. fall of electron mobility impacting the cut-off frequencies and degradation of device reliability). Therefore the determination of device temperature is a key issue for device topology optimisation. This work presents the comparison between pulsed I-V at different temperature and DC measurements of AlGaN/GaN HEMTs grown on two different substrates: sapphire and silicon. This technique allows the determination of mean channel temperature and the device thermal resistance. The thermal resistance is a classical way to define the average channel temperature as a function of the dissipated power. In this work the thermal resistance ratio of the HEMT grown on sapphire compared to the one grown on silicon is found to be 1.7 instead of 3 as expected from straightforward thermal conductivity ratio. This lower difference is clearly attributed to the contribution of the GaN buffer layer.

The objective of this work is to determine the origin of a decohesion problem that occurs during the fabrication process of certain integrated circuits. This decohesion takes place at the Ti/dielectric interface with the dielectric being either SiO2 or borophosphosilicate glass (BPSG). The frequency of the decohesion increased when the dielectric is BPSG. In order to understand the reason for the difference in decohesion rates for the two dielectrics, a comparative study was performed before and after annealing. X-ray Diffraction was used to determine the microstructure of the different layers and Transmission Electron Microscopy coupled to Electron Energy Loss Spectroscopy and X-ray Energy Dispersive Spectroscopy was mainly used to characterize the interfaces and nanophases that had formed during annealing. A fundamental difference observed was the Ti/dielectric interface reactivity: in the case of BPSG, an amorphous layer rich in P is formed before the Ti5Si3 silicide. Two hypothesis are put forward in order to explain adhesion failures: a decrease in the rate of Ti5Si3 formation kinetics and/or a decrease of the glass transition temperature induced by P enrichment.

Magnetooptical and optical experiments are performed on size sorted maghemite ($\gamma-{\rm Fe}_2{\rm O}_3$) ferrofluids. Water based ferrofluid samples are synthesised by the coprecipitation method followed by a size sorting process. All the samples have the same particles volume fraction: 0.2%. Transmittance and Faraday rotation are found to be size dependent. The saturation of the Faraday rotation is decreasing when particles mean diameter increases. Faraday rotation curves are described by orientation Langevin theory and the size dependence of their magnitude is discussed using nanoparticles internal field.

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to powder formation, a new type of material, consisting of an amorphous matrix in which silicon nanocrystallites are embedded is obtained. This material, named hydrogenated polymorphous silicon (pm-Si:H), exhibits enhanced transport properties with respect to state-of-the-art hydrogenated amorphous silicon (a-Si:H). In order to understand the origin of such improved properties, we investigated structural characterization of pm-Si:H films. High resolution transmission electron microscopy (HRTEM) micrograph, micro-Raman and infrared spectra of the films are presented. The crystallite sizes deduced from the Raman spectra are supported by the HRTEM measurements. The infrared stretching modes of pm-Si:H films present a band at ~2030 cm−1 attributed to hydrogen platelets. Two approaches are then given in order to explain the enhanced photoconductivity properties. The first one, qualitative, appeals to low density of states at the Fermi level $N(E_{\rm F})$ and low capture cross-section of electrons $\sigma_{\rm c}$ due to the improved amorphous matrix. The second, more quantitative, suggests that recombination mainly occur at dangling bonds at the surface of the crystallites. Considering the dangling bond density at silicon surface passivated by hydrogen and the description of multiphonon carrier capture, both given in the literature, we derive $N(E_{\rm F})$ and $\sigma_{\rm c}$ values in good agreement with experiments.

GaN films were grown using metal-organic chemical vapor deposition (MOCVD) on intentionally roughened c-sapphire in order to study the effect of the substrate roughness. Controlled substrate damage was achieved by exposing the substrate surfaces to controlled amounts of trimethylgallium followed by exposure to hydrogen at elevated temperature. The bulk film quality was studied by high-resolution X-ray diffraction and the surface topography was examined by atomic force microscopy. It was observed that mild substrate roughness (surface pit depth up to 30 nm) does not have a significant effect on the bulk film quality nor the GaN surface topography, and that recovery of the surface roughness is observed in very early stages of film growth. Cross sectional transmission electron microscopy confirmed that MOCVD GaN grown on inclined sidewalls etched in c-sapphire maintained the $\{0001\}_{\rm sapphire}\parallel\{0001\}_{\rm GaN}$ epitaxial relationship despite the substrate miscut of ca. 15° with respect to the c-axis.

A numerical study of a nitrogen cold plasma created by a pulsed discharge in a point-to-plane geometry at 4 torr is presented. The relative model is based on fluid description of the cold plasma, on Poisson's equation for the electric field and on balance equations for the excited population concerning only some vibrational band of molecular nitrogen (C3∏u, B3∏g and A$^3\sum_{\rm u}^+$). Secondary ionization phenomena and longitudinal diffusion effects are included in the model and their contribution to the glow-discharge establishment is studied. Results for space and time variations of the charged particles, electric field, potential and electronic current densities are reported. Particular attention was paid to the physical factors forming the cathodic layer and influencing its evolution. Results concerning the influence of the electronic current density on the creation of excited (radiative and metastables) particles are presented. According to these results, the plasma of the glow discharge occurs by means of three successive sequences with different characteristics. During the first sequence the gap is slowly filled of ions formed close to the anode, then in the second one an ionizing front formed close to the anode is propagating towards the cathode with an average velocity of 2 × 106 cm/s. It forms the cathodic layer zone and starts the participation of the secondary ionization effects leading to the third sequence which deals with the glow discharge establishment. The first significant production of excited states occurs within the ionizing front but the most important one is obtained when the glow discharge is established. It has been shown that at 4 torr and a gap of 1 cm, a mean electronic current density of 5 mA/cm2 is sufficient to create 109 cm−3 of (B3∏g, v = 0), 108 cm−3 of (C3∏u, v = 0) and 1011 cm−3 of (A$^3\sum_{\rm u}^+$, v = 0 ) excited particles.

Atomic force microscopy observations of the fine structure of cracks have been carried out for nickel thin films on polyimide substrates under tensile test. For strains of the order of 10%, the cracking of the metallic thin film induces notable damaging of the polymeric substrate. Post-mortem observations of the substrate and finite element simulations of the stress field at the cracks head have been performed to characterize this unexpected damaging: the crack propagation at the film/substrate interface and inside the substrate.

In this work, a low-frequency dynamic hysteresis model is proposed and applied to nonoriented Fe-Si steel. The model based on Preisach theory can be used when skin effect is negligible over the lamination thickness. In our model, the Preisach engine that usually takes the magnetic field as input variable must be inverted. Some B(H) symmetric loops are calculated and compared with measurements in the rolling direction both in quasistatic and dynamic operations under sinusoidal induction.

Here is reported a new scheme to accurately determine the vapor pressure of undercooled, liquid, and high temperature solid materials. The method relies on an imaging technique that measures the time variation of the radius of an electrostatically levitated sample. This scheme, compared to other techniques, offers unique opportunity to explore not only the liquid above the melting point but also the undercooled states of highly reactive materials in a contamination free environment. This was exemplified in this paper with titanium. For the first time, we report the vapor pressure $(V_{\rm p})$ of its liquid phase over a large temperature range, covering the undercooled region. Over the 1700 to 2050 K temperature range, it was measured as Log $V_{\rm p}(T) = 9.154 - 17978\ T^{-1}$ (3%). Similarly, for high temperature solid titanium, the vapor pressure could be expressed as Log $V_{\rm p}(T) = 16.634 - 32960\ T^{-1}$ (6%) over the 1770 to 1940 K temperature interval. From these data, the average latent heats of vaporization and sublimation were calculated respectively as 344.8 kJ/kg (8%) and 632.1 kJ/kg (6%) respectively.

A known electrodynamic approach is generalized to guided-wave structures using a hypothetical space-dispersive medium with drifting charge carriers and simultaneously possessing elastic, piezoelectric, and magnetic properties. On the basis of Maxwell's equations along with the appropriate equations of medium motion, the power-energy theorem is derived in the general form involving additional contributions from specific properties of the medium. The general power-energy relation yields particular expressions obtained previously for specific cases of the elastic piezo-dielectrics, magnetized ferrites, and drifting charge carriers in nondegenerate plasmas. Substantial features of our electrodynamic study are the allowance for medium losses and the separation of potential fields peculiar to the slow quasi-static waves which propagate in such active media independently of the fast electromagnetic waves of curl nature. Separating the potential fields gives a certain modification of electromagnetic terms in the power-energy theorem.

Differential scanning calorimetry (DSC) and X-ray scattering measurements on water confined in silica gel is studied for several levels of hydration at room temperature and down to 77 K. The DSC analysis of water confined in silica gel in function of the level of hydration shows that the solidification of water inside the pore is sensitive to the pore opening but the fusion is closely related to the curve of the pore. The X-ray pair correlation function showed the presence of a distorted tetrahedral-like hydrogen-bonded network of water characterized by the peaks at ~2.8, ~4.1 and ~4.9 Å. This distortion is caused by the competition between confinement effect and silica-water interaction. To eliminate the contribution of the water-silica gel interaction we have determined the structure of confined water by subtraction of the feebly hydrated silica gel from that strongly hydrated sample. The pair correlation function of extracted water show an oxygen-oxygen correlation at 3.8 Å. A similar peak is observed in the case of water confined in polyHEMA, in charcoal and at vicinity of myoglobin.

A vectorial formalism for analysis and design of polyphase synchronous machines without reluctance and saturation effects is described. We prove the equivalence of such a machine with a set of magnetically independent machines, which are electrically and mechanically coupled. Specific problems of polyphase machines can thus be favorably analyzed with this concept. Rules of conception and constraints on electric supply can be deduced. Moreover the vectorial approach, which generalizes the complex phasor method, can also be used to control n-leg voltage source inverters. This methodology is applied to 3-phase and 6-phase synchronous machines.

In this paper we demonstrate that the presence of thermoelectric anisotropy, no matter how small, can be exploited to unmask the presence of cracks and holes in metals. This is in direct contrast with situations involving isotropic metals where such detection is not possible. Although such a phenomenon will occur for all types and shapes of cracks and holes, for simplicity we shall demonstrate our theoretical and numerical modeling on the more mathematically tractable case of a cylindrical hole aligned along an axis of symmetry of an anisotropic metal medium.

This article presents a particle-in-cell (PIC)/test-particle Monte Carlo (TPMC) simulation of the dynamics of Ar+ ions in the region (sheath and presheath) surrounding an attracting cylindrical Langmuir probe at medium He buffer gas pressure. The numerical model is based on a combination of PIC simulation for the ion component and fluid description for the electrons using the Boltzmann relation. The electric field is self-consistently computed via the Poisson equation. The neutrals are modeled directly by an analytical method. The elastic collisions between Ar+ ions and He atoms are accounted for using a TPMC method based on the polarization potential theory. The results of the present simulation are compared with measurements, with other probe theories and the results of other authors simulations.

The energies and charge-state fractions of carbon plasma ions produced with the pulsed arc-discharge system have been measured using the Doppler-shifts in the line spectra emitted by carbon ions. The arc-discharge voltages ranged from 3 to 4 kV and the corresponding currents from 7.5 to 10 kA. Pulse duration times were about 30 μs. Strong populations of doubly and triply ionized carbon ions were observed. The maximum ion energy for the triply ionised carbon ions was 250 eV. The Doppler-shift measurements were performed with a crossed-dispersion spectrograph (theoretical spectral resolution 0.01 nm) and the lines were photographed.

Transmission Line Transformers (TLT) based on the use of ferrite beads are presented. This work aims at optimising the performance of the voltage gain and the compactness of the TLT according to the position, the type and the number of ferrites used. The ferrites have been selected taking into account two main parameters: the maximum currents flowing through the outer conductor of the lines of a TLT without ferrite beads and the current frequencies. The function of these ferrites is to decrease the effect of parasitic short-circuit transmission lines between the outer conductors of the coaxial cables of the TLT. Voltages and secondary currents are measured on each cable. The voltage gain achieved reaches the optimum theoretical value thanks to the use of ferrites for a 2, 4 and 10-stage transmission line transformer. Each cable is shorter than 1.5 meter in length which provides an unquestionable advantage of compactness.

In point-to-plane corona discharges in air, the collisions of charged particles with neutral particles induce a gas movement between the point to the plane called electric wind. A one-dimensional model of the neutral particle velocity along the discharge axis and between the electrodes is first developed. Laser Doppler Anemometry is used to measure the axial velocity profile of the gas between the electrodes. Discrepancies between experimental results and predictions of the on-axis velocity are discussed. Finally, the measured velocity profile compares quite well with a cos5θ distribution. Significant differences between measured and cos5θ profiles are observed near the discharge axis which could be attributed to the presence of seeding particles.  

The aim of this paper is to model the printed connections in static power converters. An analytical technique, based on the transmission line theory has been proposed previously (Labarre et al., Eur. Phys. J. AP 3, 169 (1998)). This paper shows that this modeling technique, named the thin wire method (TWM) is well adapted to predict the behaviour of various configuration of printed circuit board with a ground plane: isolated microstrip lines exhibiting a constant or not width, parallel or non-parallel lines of any length. New analytical formulas are proposed in any layout configuration. However, the total circuit modeling needs to combine the features of different electromagnetic models: MTL, PEEC, CTL with a numerical technique: the two dimensional transmission line matrix method (TLM2D).

In the first part of this work the regime of the DC electrical discharge in nitrogen, in absence and in presence of polystyrene thin films in the cold plasma reactor, for a constant gap length d = 1 cm and for variable pressures ($1~{\rm~mbar} < p < 10~{\rm~mbar}$) is characterized, while the gas flow rate is maintained at Q = 1 sccm. V-I curves underline that in any case a “normal glow-like” discharge is established. Experimental results bring out that the presence of the films in the volume of the produced plasma seems to modify the structure and some of the discharge characteristics, like the cathode secondary emission effects. V-I curves ($I <200~\mu$A), discharge repetitive current impulses, discharge repetitive emitted light impulses along the discharge symmetry axis and photos of the discharge, in presence and in absence of the films, are presented. In the second part of this paper results of polystyrene thin films treatment under the above DC “normal glow-like” discharge regime in nitrogen for improving the wettability of the films are presented. The effect of discharge main parameters (pressure p and mean current I through the discharge) on the treatment is studied under certain experimental conditions. The wettability is characterized by contact angle measurements using deionized water drops and the evolution versus the treatment time ttr of the $\Delta \theta/\theta_{\rm i}=(\theta_{\rm i}-\theta_{\rm f})/\theta_{\rm i}$ ratio ($\theta_{\rm i}$: the contact angle before the treatment and $\theta_{\rm f}$: the contact angle after the treatment) is estimated. It is deduced that optimizing main parameters of the DC electrical discharge in nitrogen it is possible to obtain films with very good wettability (suggestively $\Delta\theta/\theta_{\rm i}=0.951$ corresponding to $\theta_{\rm f}=4^{\circ}$, at p = 4 mbar, $I=200~\mu$A, $t_{\rm tr}\cong 600$ s). A rapid change of the films surface nature ($0~{\rm s} < t_{\rm tr} < 60$ s, suggestively) seems to be followed by a slower one and finally by a saturation. The gas pressure and the discharge mean current seem to play a very important role for the treatment. The experimental results allow us to start a discussion about the possible role of some active particles produced in the plasma to the treatment of the films and especially the role of the neutrals and the metastables.