Molecular dynamic simulations of uniaxial tension at nanoscale of semiconductor materials for micro-electro-mechanical systems (MEMS) applications

Molecular dynamic (MD) simulations of uniaxial tension at nanoscale were conducted on two semiconductor materials, namely, silicon (Si) and germanium (Ge) to determine their mechanical properties and investigate the nature of deformation under applied load at nanolevel. A general form of Tersoff-type, three-body potential was used for the interaction between the Si atoms and between the Ge atoms in the simulations. Both, Si and Ge were found to exhibit a linear elastic behavior followed by a nonlinear increase in stress in the plastic region up to the ultimate tensile stress (instead of catastrophic brittle fracture soon after the elastic limit, which is typical of most nominally brittle materials at macrolevel). Further loading beyond the ultimate tensile stress resulted in catastrophic failure of these materials by a ductile fracture mode, namely, slip at 45° to the loading direction. The strain at failure was found to be much higher than the corresponding values at macroscale possibly due to the higher loading rates used. Based on the simulation results, the Youngʹs modulii of Si and Ge in the [100] direction were determined to be ∼130 and ∼103 GPa, respectively, and the ultimate strengths, ∼25 and ∼20 GPa, respectively, at 500 m s−1. These results are in reasonable agreement with the experimental and simulation results reported in the literature. The effect of strain rate via the rate of loading (10–500 m s−1, where 1 m s−1 corresponds to 10−2 Å ps−1) on the nature of deformation and the measured properties were also investigated. As the rate of loading (or the strain rate) decreases, the stress–strain curves more or less overlap up to the ultimate strength with a slight decrease in the ultimate tensile stress but a significant decrease in the value of strain at failure or strain at ultimate tensile stress.