Strain in Si/Ge Heterojunction Nanowires
Tzu-Hsien Shen1*, Chia-Hao Yu1, Cheng-Yen Wen1
1Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan
* presenting author:TZU-HSIEN SHEN,
The formation of abrupt Si/Ge heterointerfaces in nanowires presents possibilities for bandgap engineering by tuning lattice mismatch-induced strain at the interface; the coherent strain at the heterointerface is expected to effectively modify the electronic properties of the materials. The key to fabricate such heterojunction nanowire structures and hence get the optimal strain state is to form compositionally abrupt and defect-free heterojunction interfaces. In this study, we use solid catalysts, such as AgAu or AlAu2, with low Si and Ge solubility via the vapor-solid-solid growth mechanism to fabricate abrupt Si/Ge heterojunction nanowires. Transmission electron microscopy (TEM) analysis of the Si/Ge heterojunction nanowires shows that no misfit dislocations are formed at the interface and the width of compositional transition at the interface is less than 1 nm. On the other hand, the distribution of strain near the interface, measured by the geometrical phase analysis (GPA) of high-resolution TEM images, extends to more than 6 nm from the interface on either the Si side or the Ge side. Despite the compositional intermixing in the 1 nm wide region at the interface, the Si lattice at 1 nm away from the interface should be dilated parallel to the (111) interface, with a strain larger than 1%. Electron energy loss spectroscopy analysis of the strained Si lattice shows no shift of the L-edge relative to the unstrained Si, but the near-edge fine structure slightly differs, showing the effects of strain on the bondings of the distorted Si lattice.

Keywords: Si/Ge heterojunction nanowire, vapor-solid-solid mechanism, transmission electron microscopy, geometrical phase analysis, electron energy loss spectroscopy