Papers by Author: Bernard Pichaud

Abstract: High temperature annealing effects on Oxygen-induced defects formation has been studied by IR-LST, FTIR and TEM techniques. The results show that most defects are amorphous oxygen precipitates and/or dislocations. Ham’s theory has been modified in order to take into account the variations of interstitial oxygen concentration during the formation of precipitates. Comparison between experimental data and simulation shows that the specificity of annealing cycle is to combine both nucleation and growth stages. The morphology and stoechiometry of SiOx precipitates are also studied.

Abstract: The nucleation of dislocation in semiconductors is still a matter of debate and especially in heteroepitaxial films. To understand this nucleation process the classical models of dislocation nucleation are presented and discussed. Two main points are then developed: emission of dislocations from surface steps and the role of point defects agglomeration on dislocation nucleation. Recent atomic simulation of half loops emission from surface steps and experimental evidences of anisotropic relaxation of GaInAs films deposited on vicinal (111) GaAs substrates strongly support surface steps as preferential sites for nucleation. In low temperature buffer layer structures (SiGe/Si) an original dislocation structure is observed which corresponds to the dislocation emission in different glide systems by a unique nucleation centre.

Abstract: First relaxation stages in Si1-x Gex layers on Si substrates are induced by annealing of metastable, low-temperature buffer layer samples and observed by X-ray topography (XRT). This method allows observing large area (several square millimetres) of a sample and reveals very low densities of defects, located in the layer as well as in the substrate. It allow to follow the evolution of the very first steps of the relaxation, starting with dislocation crosses which were characterized and evolving to misfit dislocation network by very low increases of thermal budget. It is proposed a nucleation mechanism of these crosses based on Frank loops due to point defects condensation which can transform locally in glide dislocations under the influence of the biaxial stress in the film.

Abstract: The knowledge and control of the structural and morphological properties of nanocrystalline silicon is a fundamental requisite for its proper application in photovoltaics. To this purpose, nanocrystalline silicon films grown by Low Energy Plasma Enhanced Chemical Vapour Deposition (LEPECVD) technique on different kinds of substrates were submitted to a systematic characterization using Raman spectroscopy, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The results showed that the nature of the film substrate induces deep changes in the structural properties of the deposited films. The importance of a Raman in–depth analysis for an accurate determination of the sample structure has been also demonstrated.

Abstract: Defects are introduced into (11-20) highly N-doped 4H-SiC by one surface scratch followed by annealing at 550°C or 700°C with or without an additional compressive stress. The defects are planar and always consist of double stacking faults dragged by a pair of partial dislocations. In a pair, the partial dislocations have the same line direction, Burgers vector and core composition. All the identified gliding dislocations have a silicon core. An analysis of their expansion during annealing proves that C(g) partial segments can be created but that C(g) partial dislocations are immobile.

Abstract: 4H-SiC samples were bent in compression mode at temperature ranging from 400°C to 700°C. The introduced-defects were identified by Weak Beam (WB) and High Resolution Transmission Electron Microscopy (HRTEM) techniques. They consist of double stacking faults bound by 30° Si(g) partial dislocations whose glide locally transforms the material in its cubic phase. The velocity of partial dislocations was measured after chemical etching of the sample surface. The formation and the expansion of the double stacking faults are discussed.