The Role of Native Oxide on the Mechanical Behavior of Silicon Nanowires

Molecular dynamics simulations are employed to study the effect of native oxide on the size-dependent mechanical properties of silicon nanowires. Despite their immense potential as essential building blocks in nanoelectromechanical systems, mechanical behavior of silicon nanowires still needs further attention for a full understanding. The leading source of ambiguity can be traced back to the fact that the presence of native oxide on silicon nanowire surfaces is ignored when interpreting nanomechanical test data, when it comes, for example, to converting force and deflection measurements to stress and strain. This problem needs immediate attention, because, first, nanowires have a significant surface area, and second, native oxide is the dominant surface state. With prior work reporting conflicting dimensional and computational viewpoints regarding the effect of native oxide on silicon nanowires properties, size dependence of nanowire mechanical properties is investigated here with great attention placed on critical size and atomistic simulation perspectives. For this purpose, Tersoff-Munetoh and modified Stillinger-Weber potentials are employed in this intensive computational study to address the influence of size and crystal orientation on nanowire elastic behavior and tensile strength. As a result, a striking set of differences is obtained. First, the presence of native oxide layer is observed to decrease both the modulus of elasticity and the ultimate strength. The reduction in the modulus of elasticity is observed to be as much as 30% and 40% for < 100 > and < 110 >-oriented nanowires, respectively. Similarly, the reduction in the ultimate strength is estimated to be as much as 20% using the modified Stillinger-Weber potential, which proved to be more suitable for strength analysis compared to Tersoff-Munetoh potential. Finally, the failure behavior is studied through the ductile failure probability calculations, where a higher size-dependent failure probability is observed for decreasing nanowire width upon oxidation. These results shed light on the background of existing inconsistencies between experimental and numerical findings in the literature, as opposing trends for silicon nanowire stiffness and strength were reported with decreasing size. The study provides a guideline to quantify the scale effect in silicon nanowire mechanical behavior as a combined outcome of oxide thickness, nanowire size and crystal orientation and thus to reduce the extent of uncertainties originating from inadequate interpretation of nanomechanical test data.