Decoupling the two roles of liquid Ga droplets in the self-catalyzed growth of GaAs nanowires on SiO_x/Si(111) substrates

Liquid Ga droplets play a double role in the self-catalyzed growth of GaAs nanowires on Si(111) substrates covered with a native SiO_x layer: they induce the formation of nano-sized holes in SiO_x and drive the nanowire growth directly onto the underlying Si. The independent control of the two mechanisms is a prerequisite for mastering the growth of nanowires, but it is challenging in a conventional growth procedure where they both take place under the same droplets. As a result, serious growth implications occur, i.e. an uncontrolled number density/yield of vertical nanowires, a broad nanowire length distribution, and growth reproducibility issues.
To resolve those issues, we have developed a three-step in-situ surface modification procedure (SMP) of the SiO_x/Si(111) substrates, which decouples the droplet-assisted hole formation in SiO_x from the droplet-assisted growth of GaAs nanowires. Step-1 concerns the first thermal annealing of the substrate, during which extremely small pinholes are created in the SiO_x layer. Ga deposition and droplet formation takes place at a lower substrate temperature in step-2. Depending on the substrate temperatures in both steps 1 and 2, the number density of Ga droplets can be varied deliberately from 10⁷ to 10¹⁰ cm^-2. Step-3 concerns the second thermal annealing of the substrate, during which all Ga droplets are evaporated completely from the substrate surface and an identical number of nano-sized holes of controlled size develop at their place. After the SMP, GaAs nanowires are always grown under identical conditions (T_gr=615°C, V/III=11) without pre-deposition of Ga. We found that different types of GaAs structures (vertical nanowires, inclined nanowires, or faceted islands) can nucleate inside the SMP-induced SiO_x holes depending on the size of the latter (e.g. the nucleation of vertical nanowires is favored in 8 nm wide holes).
After careful selection of the SMP parameters (substrate temperatures and annealing durations), it is possible (i) to precisely control of the number density of vertical GaAs nanowires from 10⁶ cm^-2 to 10⁹ cm^-2 without changing the growth temperature or beam fluxes, (ii) to control the size of the SiO_x holes in favor of high yields of vertical nanowires (up to 80 % of the total GaAs structures), (iii) to reproduce the wide range of nanowire number densities as well as the high yield values also on substrates from different batches, (iv) to obtain exceptionally narrow nanowire length distributions (below 1 % for 3 µm long nanowires), and (v) to control the nanowire diameter independent of the nanowire length and number density by changing only the V/III ratio.