Nanostructured silicon for Ge nanoheteroepitaxy

Abstract

A fabrication process for Si nanopillars (NPs) as template for the Ge nanoheteroepitaxy (NHE) was developed. The NHE concept suggests that by fabricating three-dimensional free standing NP, the NP can be elastically deformed by deposited heteroepitaxial material on the top due to lattice mismatch strain. To enable the elastic deformation, small diameter NPs are required with a specific height. A fabrication process was developed to create Si NPs with <100 nm width and >100 nm height. The top of Si NPs must be opened for selective Ge growth, while the sidewall and bottom surface must be covered by SiO2. Spectroscopic ellipsometry was used to control Si NPs before selective Ge epitaxial overgrowth. The process was optimized based on transmission electron microscopy (TEM) and X-ray diffraction investigations. Ge on top of NPs is fully relaxed. The relaxation occurs however not elastically but plastically by misfit dislocation nucleation at the Ge/Si interface due to SiO2 passivation of the sidewall. The possible approaches to enable elastic deformation of Si NPs are to reduce the SiO2 thickness of the sidewall and lateral dimension of NPs. The Ge of high crystallinity grown on top of the NPs only is indicating that the developed technology guarantees a clean top Si surfaces and a sufficient SiO2 protection of the rest of the structure. No threading dislocations are observed by TEM. This shows that the small lateral dimension of the seed area allows the TDs to glided out to the surfaces with high probability.