BiDuL

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.