E. Constant

Combination of Local Heating and Radiometry by Microwaves

New methods for combination of microwave heating and microwave radiometry are presented. The processes experimented with here make it possible to collect the thermal signal emitted in the very volume in which the power dissipation is achieved. They can be used for medical (atraumatic control of local hyperthermia) or industrial applications (regulation of microwave heating).

Simultaneous microwave local heating and microwave thermography. Possible clinical applications.

A basic experiment showing the possibility of combining microwave local heating of subcutaneous living tissue, and microwave radiometry by the same system was carried out. The combined process may be used - in hyperthermia therapy, for measurement and control of the local temperature, - in some diagnoses using radiometry to detect diseased tissue.

Strong isotope effects in the dissociation kinetics of Si–H and Si–D complexes in GaAs under ultraviolet illumination

Under ultraviolet (UV) illumination of GaAs with photon energies above 3.5 eV, Si–H complexes are known to be efficiently dissociated at room temperature. Studying the dissociation kinetics of Si–H and Si–D complexes in GaAs, we have observed that, for a given incident UV photon density, the concentration of dissociated Si–D complexes is 10–20 times below the concentration of dissociated Si–H complexes. This strong isotope effect is discussed under the light of recent excitation models of Si–H(D) bonds at the surface of silicon and at the Si/SiO2 interface.

Electron-beam-induced reactivation of Si dopants in hydrogenated GaAs: A minority carrier generation effect or an energetic electron excitation effect?

We study the changes in electronic properties of Si-doped GaAs epilayers exposed to a hydrogen or a deuterium plasma, and then submitted to electron-beams of different injection energies (between 10 and 50 keV). Using Hall effect measurements, we have observed the formation of Si–H or Si–D complexes and their dissociation induced by the electron-beam irradiation. A strong isotope effect is observed in the dissociation rate of these complexes. Moreover, increasing injection energy reduces the dopant reactivation efficiency. Both effects are quite difficult to explain assuming minority carrier induced dissociation and could be understood under the light of recent works on electronic excitation of Si–H complexes in GaAs, and also of Si–H (D) bonds at the surface of silicon.

Isotope effect on the reactivation of neutralized Si dopants in hydrogenated or deuterated GaAs: The role of hot electrons

Room-temperature bias stress and annealing experiments have been performed on hydrogenated and deuterated Si doped n-type GaAs Schottky diodes. From a careful analysis of capacitance–voltage characteristics, we have studied the variation of the active doping concentration and the reactivation of neutralized dopants. In thermal annealing experiments at 250 °C, the Si–H complex dissociation rate does not vary significantly by incorporating deuterium rather than hydrogen. On the contrary, by applying high reverse bias voltages to the Schottky diodes at room temperature, a strong isotope effect is observed on the dissociation rate. In this case, the dopant reactivation should be due to hot electron excitations.

Computer Simulation of Molecular Dynamics of Anisotropic Fluids

Abstract Molecular dynamics calculations have been performed for fluid systems containing ellipsoidal particles which interact pair-wise with a modified Lennard-Jones potential. Assuming molecular parameters close to those of the CB7 compound, we have calculated both static and dynamic properties such as the internal energy, the entropy and specific heat changes at T NI, the compressibility factor, the temperature dependence of the orientational order parameters, the reorientational angular momentum and velocity time-dependent self-correlation functions, and the diffusion coefficients in both the nematic and isotropic phases. The thermodynamical data show a transition between isotropic and nematic phases and the results obtained are in qualitative agreement with the experimental data. The influence of a decenterd dipole along the molecular axis has been studied and partially bilayered smectic phases have been obtained.

Prediction of negative diffusivity under transient regime conditions

We report on the possibility of a negative diffusivity under transient regime conditions. This effect is predicted by Monte Carlo calculations in polar semiconductors undergoing Gunn effect.

Fabricating conductive microstructures by direct electron-beam writing on hydrogenated n-type Si-doped GaAs

We study the changes in electronic properties of Si-doped GaAs epilayers exposed to a hydrogen plasma and then submitted to electron beams of 20 keV injection energy. Using Hall-effect measurements, we have observed the formation of Si–H complexes and their dissociation due to the electron-beam irradiation. As this last effect increases the conductivity of the epilayer, we have been able to fabricate, with an electron-beam lithography system, various conductive microstructures. Characterizations have been achieved by cathodoluminescence(CL) imaging. Actually, due to the CL reading method which has been used, we have only fabricated micronic-size structures. However, taking into account the high spatial resolution of electron-beam writing, such a process could, possibly, be used as a way of fabrication of mesoscopic structures.

Effect of saturation current on noise in IMPATT diodes under large‐signal conditions

A large‐signal noise theory for IMPATT diodes, not neglecting the effect of the parasitic injected current (saturation and/or tunnel current) is presented. Under large‐signal conditions, it is found that the noise increase is much less for high effective saturation current than for a low one. The theoretical conclusions are experimentally evidenced. The true physical mechanism behind the effective saturation is briefly explained.

Electron-beam-induced reactivation of Si dopants in hydrogenated two-dimensional AlGaAs heterostructures: a possible new route for III?V nanostructure fabrication

Hydrogen incorporation in n-type Si-doped GaAs epilayers is a well-known process which leads to the neutralization of the active Si impurities with the formation of SiH complexes. Recently, we have shown that SiH complex dissociation and, consequently, Si-dopant reactivation could occur when the epilayers are exposed to an electron beam. Two epilayers have been studied: the first is a 0.35 μm thick hydrogenated Si-doped GaAs epilayer and the second is Si planar-doped AlGaAs/GaAs/InGaAs heterostructures. Firstly, Hall effect measurements have been carried out on the epilayers exposed, after RF hydrogen plasma exposition, to increasing electron doses with different injection energies. For the 2D heterostructures, we have observed that the free carrier density N s does not vary significantly for weak electron densities. This reactivation presents a threshold value, contrary to the 0.35 μm epilayer in which N s varies quite linearly. It will be shown that such phenomena might be attributed to the filling of surface states as the dopants are progressively reactivated. Then, using a high spatial resolution electron beam lithography system, nanometric conductive patterns have been fabricated starting from hydrogenated epilayers. Electric measurements have been performed and the results obtained show that about 15 nm spatial resolution could be expected. In conclusion, taking into account this spatial resolution, the high spatial contrast of conductivity which could be expected due to the existence of an electron dose threshold, and the high mobility of the AlGaAs/GaAs/InGaAs heterostructure, the effects described in this paper could open a new way for the fabrication of III-V 1D or 2D mesoscopic structures for electronic or optoelectronic applications.