Multi-scale mechanics of amorphous silicon
Unlike the well-ordered lattice of crystalline silicon (Si), amorphous Si consists of atoms forming a continuous random network. Intense scientific interest in the applications of amorphous silica (silicon dioxide (SiO2))-based materials has highlighted the difficulty in describing their mechanical responses at atomic scale. EU funding of the project PLASTAMORPH supported the development of a unifying theoretical description of these materials at varying length scales. The multi-scale modelling provided insight into the role of physical mechanisms at the atomic scale in shaping macroscopic phenomena. An accurate description of behaviours at small length scales will help overcome problems of plasticity (deformation) and fatigue in Si-based devices.The project set four technical objectives. Enhanced understanding of the local mechanical response of Si-based amorphous materials should be linked to their macroscopic rheological behaviour. Researchers also sought to characterise the vibrational behaviour of the materials and to investigate effects of pressure on Si nanopillars that are gaining interest for applications in optoelectronics.Exploiting molecular dynamics simulations based on multi-body dynamics algorithms, researchers have analysed effects of shear (static conditions) and shear rate (time-varying conditions) as well as bond directionality on the small-scale mechanical response. The simulations provided evidence of two types of plastic rearrangements: nucleation of isolated events and avalanche-like rearrangements.Studies of the vibrational properties of the amorphous Si model shed light on the unusual properties of amorphous materials. In particular, scientists showed that the classical description in which any arbitrary vibration can be described as the superposition of elementary ones does not apply. Finally, numerical investigations studied the mechanical properties of nanopillar as a function of size, demonstrating a decrease in inner pressure as a function of the square of the nanopillar radius.PLASTAMORPH has provided important insight into the atomic-scale behaviour of amorphous Si-based materials, filling an important knowledge gap previously inhibiting their full exploitation in optoelectronic devices. Controlling deformation and fatigue are now within the realm of possibility. In addition to paving the way for future development of amorphous Si-based materials, the models will be integral to the study of amorphous solids for a variety of applications.
Keywords
Silicon, amorphous silicon, multi-scale modelling, mechanical response, plasticity, molecular dynamics, vibrational properties, optoelectronics
