Papers by Author: Marie France Barthe

Abstract: Positron annihilation gamma energy distribution, lifetime spectroscopy and time-of-flight method were used to study surfactant-templated mesoporous silica films deposited on glass. The lifetime depth profiling was correlated to Doppler broadening and 3γ annihilation fraction measurements to determine the annihilation characteristics inside the films. A set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size was directly determined. The lifetime measurements were performed in reflection mode with a specially designed lifetime spectrometer mounted on a slow positron beam system. The intensity of the 142 ns vacuum lifetime component was recorded as a function of the energy of the positron beam. In a film with high porosity a reemission efficiency of as high as 40 % was found at low positron energy. Positron lifetime in samples capped by a thin silica layer was used to determine the pore size. The energy of the reemitted o-Ps fraction was measured by a time-of-flight detector, mounted on the same system, allowing determination of both o-Ps re-emission efficiency and energy in the same sample. We demonstrate the potential of the simultaneous use of different positron annihilation techniques in the study of thin porous films.

Abstract: In this work we used Positron Annihilation Spectroscopy (PAS) and Electron Paramagnetic Resonance (EPR) to investigate the properties of vacancy defects produced by low energy electron irradiation. N-doped 3C-SiC and 6H-SiC monocrystals have been irradiated with electrons at different energies from 240keV to 900keV. EPR measurements show that Frenkel pairs VSi 3-/Si are created in 6H-SiC when electron irradiation is performed at low energy (240-360 keV). EPR also indicates that the silicon displacement threshold energy is higher in 3C-SiC than in 6HSiC. Moreover, PAS results show that the size and concentration of the vacancy defects decrease when the electron energy decreases for both polytypes. PAS detects vacancy defects in 240keV electron irradiated 3C-SiC, and the detection of the carbon vacancy is proposed.

Abstract: This paper presents positron lifetime results which give information on the nature of vacancy defects induced by electron irradiation in bulk nitrogen doped (nD-nA= 2.3x1017 cm-3) Cree 6H-SiC. The electron irradiations have been performed at different energies and with different fluences from 5􀂗1017 e-cm-2 to 3􀂗1018 e-cm-2. Positron lifetime have been measured with a 22NaCl source as a function of temperature between 15 and 300 K. The lifetime spectra were analyzed as sums of two exponential lifetime components 􀁗i weighted by the intensities Ii, convoluted with the resolution function. From the temperature dependence of the lifetime spectra we can infer that several vacancy defects exist in the electron irradiated n-type 6H-SiC. The nature of detected vacancy defects depends on the electron energy.