Ga droplets on SiO_x/Si(111) substrates: nucleation and etching

Liquid Ga droplets have been widely used in molecular beam epitaxy to induce the nucleation of nanostructures as well as to remove surface oxides from the substrate prior to epitaxial growth. In this work, we have employed Ga droplets to etch nanoholes into the native surface oxide of Si(111) substrates for the subsequent growth of III-V nanostructures. To that end, we studied the nucleation kinetics of Ga droplets on SiO_x and the interaction between Ga adatoms/droplets and SiO_x.
For given Ga flux and deposition time, we found that the number density of Ga droplets depends not only on the substrate temperature during deposition, but also on the annealing history of the substrate. This is attributed to the dependence of Ga adatom diffusivity on the surface morphology and/or the composition of the native oxide layer, both of which can be modified during the annealing. With an appropriate selection of substrate temperatures for the annealing and the Ga deposition steps, it was possible to vary deliberately the number density of Ga droplets within four orders of magnitude, i.e. 10⁷-10¹⁰ cm^-2. Interestingly, the droplet size and contact angle were found to be independent of the number density. This finding implies that low number densities of droplets are accompanied by an extensive loss of Ga atoms from the substrate surface. We discuss various explanations, such as desorption of Ga adatoms from the SiO_x surface or reaction of Ga adatoms with SiO_x and formation of volatile Ga₂O.
All Ga droplets induce local modifications in the SiO_x surface that evolve into nanoholes during a subsequent thermal annealing (which also results in complete evaporation of the Ga droplets). Depending on their size (which is a function of annealing temperature and duration), nanoholes can accommodate the nucleation of nanostructures on the underlying Si surface. As an example, we demonstrate the epitaxial growth of GaAs nanowires in the self-catalyzed mode. The number density of nanowires was controlled precisely and reproducibly within the range of 10⁷-10⁹ cm^-2, whereas their length distribution was exceptionally narrow.