Decoupling the Two Roles of Ga Droplets in the Self-Catalyzed Growth of GaAs Nanowires on SiO_x/Si(111) Substrates

Abstract

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 nanosized holes in SiO x and then drive the uniaxial 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. To that end, we have developed an in situ procedure where the Ga droplets used for the formation of SiO x holes are removed before new Ga droplets drive the growth of GaAs nanowires. In that way, it was possible to study the interaction between Ga droplets and SiO x , to create holes in SiO x with controlled number density and size, and, finally, to grow GaAs nanowires only within those holes. Our results show unprecedented control of the nanowire nucleation with unique possibilities: (1) deliberate control of the number density of nanowires within 3 orders of magnitude (106–109 cm–2) without patterning the substrate and without changing the growth conditions, (2) highly synchronous nucleation events and, thus, exceptionally narrow nanowire length distributions (standard deviation <1% for 3 μm long nanowires), (3) high yield of vertical nanowires up to 80% (against GaAs islands), (4) highly reproducible results, and (5) independent control of the nanowire diameter from the number density. We anticipate that our methodology could be also exploited for different materials or other types of nanostructures.

Widely tunable GaAs bandgap via strain engineering in core/shell nanowires with large lattice mismatch

“…Vertical GaAs/In x Ga 1− x As and GaAs/In x Al 1− x As core/shell nanowires were grown on Si(111) substrates by molecular beam epitaxy (MBE). First, GaAs core nanowires with a diameter of 20–25 nm and a length of 2 μm were grown in self-catalysed mode 36 and then In x Ga 1− x As or In x Al 1− x As shells were grown around the core nanowires (see Methods). The shell thickness ( L S ) and composition ( x ) were varied independently according to the needs of our study.…”

“…All nanowire samples were grown by solid-source MBE. Si(111) substrates covered with a native oxide layer were subjected to an in situ surface modification process with Ga droplets to create nano-sized holes in the oxide layer for the subsequent nucleation of GaAs nanowires directly on Si 36 . The GaAs core nanowires were grown for 10 min at a substrate temperature of 615 °C using Ga and As 4 beam fluxes equal to 6 × 10 13 cm −2 s −1 and 2 × 10 15 cm −2 s −1 , respectively.…”

X-ray photoelectron spectroscopy study of Ga nanodroplet on silica-terminated silicon surface for nanowire growth

“…The last step is the NW growth itself, which is obtained by opening the Ga and As shutters of the Molecular Beam Epitaxy (MBE) reactor. It should be noted that if the separation into distinct steps is not necessary to obtain actual NW epitaxial growth, it offers a supplementary degree of control of the NW density and size as recently demonstrated 19 . In short, the control of GaAs NW growth on silicon is almost completely dependent on these early stages, especially the deposited Ga amount, the pre-deposition substrate temperature, and the post-deposition annealing temperature.…”

Low-Temperature In-Induced Holes Formation in Native-SiOx/Si(111) Substrates for Self-Catalyzed MBE Growth of GaAs Nanowires

“…Küpers et al [19] introduced a two-step growth method, where Ga droplets are pre-deposited on SiO x /Si substrates to form pinholes in SiO x for subsequent NWs synthesis. In order to further improve the homogeneity and controllability of GaAs NWs density and diameters, Tauchnitz et al [20] developed a three-step in-situ surface modification procedure for non-patterned native-SiO x /Si(111) substrates which decouples the Ga-induced hole formation in SiO x from the following Ga-assisted growth of GaAs NWs. Koivusalo et al [21] added two more steps to the technology described in [20]: crystallization of Ga droplets into GaAs by As 2 exposure and spontaneous oxidation of the Si surface by air exposure outside the MBE setup.…”

“…In order to further improve the homogeneity and controllability of GaAs NWs density and diameters, Tauchnitz et al [20] developed a three-step in-situ surface modification procedure for non-patterned native-SiO x /Si(111) substrates which decouples the Ga-induced hole formation in SiO x from the following Ga-assisted growth of GaAs NWs. Koivusalo et al [21] added two more steps to the technology described in [20]: crystallization of Ga droplets into GaAs by As 2 exposure and spontaneous oxidation of the Si surface by air exposure outside the MBE setup. But in all these cases, substrate temperature reached 660-780 • C during the procedure before the initiation of NWs MBE growth.…”