Selective molecular beam epitaxy of germanium on oxide-covered silicon.

摘要

This study demonstrates that Ge can be selectively grown on Si through openings in SiO2 nanotemplates by molecular beam epitaxy without applying selectivity-control agents. The SiO2 nanotemplates are created either by interferometric lithography or by "touchdown" process. The "touchdown" process takes advantage of the unique interaction between Ge and an ultra-thin layer of chemical SiO2. Due to the high concentration of OH groups in the chemical oxide layer, Ge readily diffuses through the oxide, segregates at the SiO2/Si interface, and creates dense nanoscale windows in the chemical oxide. Ge then selectively grows in the windows and coalesces into a high-quality relaxed Ge epilayer over the remaining SiO2. The high-quality and relaxation are attributed to three mechanisms: (1) the strain at the junction pad decays below the critical limit within 2 nm due to the nanoscale heterojunction; (2) the remaining SiO2 serves as artificially introduced 60° dislocations; and (3) the intermixing between Ge and Si at the heterojunction reduces the effective lattice mismatch.; To understand the surface phenomena governing the selectivity, we further experimentally measure the desorption activation energy (Edes = 42 +/- 3 kJ/mol) of Ge on SiO2 surface. The low Edes gives rise to a high Ge desorption flux from the SiO2 surface and a low diffusion barrier ( Edif ∼ 13 kJ/mol), which in turn leads to a long characteristic diffusion length. Based on these findings, we further demonstrate that hexagonally packed, single-crystal Ge rings can be grown at the contact region between self-assembled SiO2 spheres and chemical oxide covered Si substrates. These SiO2 spheres provide a surface diffusion path, which guides the Ge adspecies to the substrate. The Ge adspecies on SiO2 spheres undergo surface diffusion as well as desorption, and a fraction of Ge adspecies aggregate at the sphere/substrate contact region to form epitaxial rings by "touchdown" through the chemical SiO2.