Asymmetric surface effect on the configuration of bilayer Si/SiGe nanosprings

Abstract

This study investigates the asymmetric surface effect on nanosprings composed of Si/SiGe bilayer thin films. The misfit strain between Si and SiGe layers is known to be the driving force whereby the deformation into the nanospring shape occurs. The crystalline orientation and width-to-thickness ratio are the main factors that determine the deformed equilibrium configuration. In addition, as the thickness decreases to dozens of nanometers or less, the effect of the surface on the equilibrium configuration of the thin film is magnified. The diamond cubic crystal structure, unlike the face-centered or body-centered cubic structures, has asymmetric surface properties. Owing to the asymmetry, Si/SiGe bilayers with odd numbers of atomic layers have different surface configurations than those with even numbers of atomic layers. Finite element analysis with the surface effect has been performed to investigate the surface effect on the equilibrium configuration. It is observed that both size and surface configuration affect the equilibrium configuration of bilayer Si/SiGe nanosprings. An unexpected spring shape was observed when the film aligned in the < 100 > direction, which is unlikely if the surface effect is neglected.