Surface Stress Effect on Silicon Nanowire Mechanical Behavior: Size and Orientation Dependence
MN Esfahani and BE Alaca, MECHANICS OF MATERIALS, 127, 112-123 (2018).
As truly nanoscale building blocks, silicon nanowires find applications in electromechanical and electronic devices. A deeper understanding of their transport properties is of utmost importance for improving the performance of such devices or for introducing new functionalities. Due to this reason, the size dependence of properties has become a major field of study. The determination of mechanical properties of silicon nanowires has proved to be specifically challenging with contrasting conclusions drawn from experimental and computational studies regarding the surface stress effect. In this respect a comprehensive study on the crystal orientation and geometrical parameters can shed light on the role played by the surface stress in determining silicon nanowire mechanical behavior. This is accomplished by linking the local surface stress at the atomic scale to the overall behavior of the continuum system. The study starts by a primary surface analysis through atomistic simulations for silicon 100 and 110 surfaces. The implication of the surface stress on the mechanical behavior of a continuum system - nanowire in this work - is predicted through basic elasticity problems. Surface stress is then transferred to the continuum system using the Surface Cauchy-Bom approach. Deformation associated with the surface stress is computed for two different cantilevers with longitudinal orientations in < 100 > and < 110 > having 100 and 100/110 transverse surfaces, respectively. Results indicate a twist deformation at the free end of < 100 > silicon nanowires with rectangular cross- sections. Similarly, a bending deformation is observed for < 110 > silicon nanowires. A case study is carried out to emphasize the importance of the surface stress in the operation of a nanowire electromechanical switch. Due to associated deformations, the surface stress is observed to cause variations in the electrostatic force along the nanowire cross-section by as much as 50%. The presented approach indicates a proper pathway to analyze the surface stress effect on the mechanical behavior of nanowires. Addressing the existing controversy regarding the contribution of the surface stress in the mechanical behavior of silicon nanowires, results in this work can provide a guideline for the interpretation of existing and the design of future experimental investigations.
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