Formation of Epitaxial Ge Nanorings on Si by Self-assembled SiO² Particles and Touchdown of Ge Through a Thin Layer of SiO2

Abstract

We demonstrate that hexagonally packed single-crystalline Ge rings can be grown around the contact region between self-assembled SiO 2 spheres and 1.2-nm-thick chemical SiO 2 on Si. When the oxide-covered Si substrate is pulled from a colloidal suspension of SiO 2 spheres, the SiO 2 spheres self-assemble into a hexagonally packed monolayer on the substrate. These SiO 2 spheres provide a surface diffusion path to guide the Ge adspecies to reach the substrate. We have previously determined that the Ge adspecies readily desorb from the bulk SiO 2 surface with a desorption activation energy of 42±3 kJ/mol. This low desorption activation energy gives rise to a low surface diffusion barrier, which in turn leads to a high diffusion length on the order of several micrometers, exceeding the dimension of the SiO 2 spheres. With a flux of Ge impinging at 45° from the surface normal, the Ge beam cannot directly impinge on the underlying substrate through the openings between SiO 2 spheres. The Ge adspecies diffuse around the SiO 2 spheres and "touchdown"[1] through the chemical SiO 2 , forming epitaxial ring structures. The touchdown process anchors nanoscale Ge seed pads to the underlying Si substrate. The ring formation uniquely takes advantage of the SiO 2 sphere self-assembly; the weak interaction between Ge adspecies and SiO 2 ; and the touchdown where Ge densely nucleates on Si surface through the 1.2-nm-thick chemical oxide.

A transition of three to two dimensional Si growth on Ge (100) substrate

“…The ring average diameter and depth is $131 nm and $0.9 nm, respectively. For Ge growth on Si, the ring could be due to the SiO 2 particles from other's work 15 or surface diffusion of adatoms to relatively strain-free regions from our early work. 16 For Si growth on Ge, since the strain is almost relaxed at Si dot facet on Ge substrate, 17 the Si adatoms can diffuse to relatively strain-free facet regions and aggregate on the facet of the Si dot to form the ring-like structure in this work.…”