Brittle to Ductile transition in silicon at low dimensions
role of : <br />1- the yield stress on the dislocation nucleation <br />2- the yield stress on the crack opening <br />3- the density of dislocation sources on the surface <br />4- the competition between the dislocation sources in volume and on surfaces.
simulations : deformation of nanopillars by classical simulation.
experiments : 2 ways are investigated. One is the analysis of the in situ compression of nanopillars by coherent x ray diffraction. the second is the deformation of nanopillars by a nanoindentor controlled in displacement.
First results : the compressions along  have shown an artifact of the stillinger weber potential that has not been corrected by the modified version of the stillinger weber potential fitted in our group.
1- the first samples will be ready in the middle of June. The first tests by x ray coherent diffraction on the ID01 beam line at ESRF are planed for the second week of July.
2- the comparison between the experimental and the simulation results are planned for September during the second main meeting of the ANR.
publications will be written as soon as possible.
Since it is possible to design and handle nano-objects such as nanopillars, nanowires…, the investigations of their physical properties have revealed amazing behaviors, different from those currently observed in their bulk counterparts. For example, an increase of the yield stress is almost always observed when the size of the system decreases, and it almost reaches the theoretical elastic limit of the perfect crystal for the smallest objects. These research areas are currently exciting the material science community under the trendy name ‘size effect’. Another size-related topic is the brittle-ductile transition (BDT) in semiconductors. In bulk, the BDT mainly depends on the temperature, with a transition temperature around 0.6 times the fusion one. However in nano-objects it has been found that semiconductors are ductile at room temperature when the system size is small enough, whereas the bulk is brittle at such temperature. Very large stresses are then required to observe plasticity in semiconductors nano-objects. The fact that semiconductor nanostructures can sustain very large stresses is already used in the strained silicon nanotechnology for modifying the electronic properties of transistors. However the stored stresses in the devices can lead to defects formation during the ageing, at the origin of the device failure. The understanding of the release of the stored energy through fractures or/and dislocations in the semiconductors is then crucial. In this scope, the project has for objectives to understand and describe in details the BDT in a model semiconductor, the silicon, at room temperature when the sample size decreases below few hundreds of nanometers. This fundamental work will be mainly focused on the deformation of nanopillars free of defect, using high level numerical and experimental approaches. As far as simulations are concerned, both classical and multiscale (coupling classical and ab initio) calculations will be performed in order to identify how the onset of plasticity occurs at low dimensions. Experimentally, in situ compression under X ray diffraction followed by post mortem analyses of the deformed Si nanopillars using geometrical-aberration corrected HRTEM (High-Resolution Transmission Electron Microscopy) will be carried out. It will allow for studying the very first stages of plastic deformation in silicon nanopillars. Finally, the comparison of the results obtained from both experiments and simulations will allow for identifying the realistic mechanisms occurring at the atomic scale.
Monsieur Julien GODET (Institut Poitevin de Recherche et Ingénierie en Matériaux Mécanique) – email@example.com
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
CNRS DR12 _ IM2NP Centre National de la Recherche Scientifique Délégation Provence et Corse _ Institut Matériaux Microélectronique Nanosciences de Provence
LPMCN Laboratoire de Physique de la Matière Condensée et Nanostructures
Institut Pprime Institut Poitevin de Recherche et Ingénierie en Matériaux Mécanique
Help of the ANR 488,593 euros
Beginning and duration of the scientific project: August 2012 - 48 Months