Quantitative study of GaAs nanowires catalyzed by Au film of different thicknesses
© Xu et al.; licensee Springer. 2012
Received: 3 September 2012
Accepted: 14 October 2012
Published: 24 October 2012
In this letter, we quantitatively investigated epitaxial GaAs nanowires catalyzed by thin Au films of different thicknesses on GaAs (111)B substrates in a metal-organic chemical vapor deposition reactor. Prior to nanowire growth, the de-wetting of Au thin films to form Au nanoparticles on GaAs (111)B in AsH3 ambient at different temperatures is investigated. It is found that with increasing film thickness, the size of the Au nanoparticles increases while the density of the nanoparticles reduces. Furthermore, higher annealing temperature produces larger Au nanoparticles for a fixed film thickness. As expected, the diameters and densities of the as-grown GaAs nanowires catalyzed by these thin Au films reflect these trends.
III-V semiconductor nanowires have shown superior electrical and optoelectronic properties due to the reduction of dimension, tunability of their direct bandgaps, and the capability of the bottom-up assembly. These properties make nanowires promising building blocks for future devices and systems. Various nanowire-based applications have already been developed including lasers [1, 2], solar cells [3, 4], biological and chemical sensors , and integrated electronic devices . In order to realize the exceptional properties promised by III-V nanowires, the precise control over the morphologies and structures of the nanowires is the key, which has been devoted with much effort by tuning the growth parameters [7–9]. In general, Au colloidal particles , aerosols, and/or thin films  can be used as catalysts to induce the epitaxial growth of nanowires in various growth apparatus, including molecular beam epitaxy and metal-organic chemical vapor deposition (MOCVD), which are the most advantageous ones in terms of the precise control of the bottom-up growth of epitaxial III-V nanowires. Among the various catalyst materials and growth apparatus, the use of annealed thin Au films as catalysts in a MOCVD system is a cost-effective and simple approach to induce the growth of III-V nanowires. However, there are only a handful of studies demonstrating the III-V nanowires catalyzed by Au films [11–14]. A number of studies have been focused on the Au film annealing and de-wetting on Si and SiO2 substrates [15–17], where the thickness of the film, the annealing temperature, and the morphology of the substrate surface play an important role in controlling the shape and size of the Au particles. In this study, we explore the morphologies of Au thin films deposited on the GaAs (111)B substrates and, more importantly, the effects of the thickness of Au films and the annealing temperature in the formation of Au nanoparticles, when using Au films to grow epitaxial GaAs nanowires. Furthermore, the morphological and structural characteristics of the GaAs nanowires catalyzed by these different thin Au films have also been studied in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This quantitative study systematically examines the process of film de-wetting to form Au nanoparticles on GaAs(111)B substrates under different conditions and the GaAs nanowires catalyzed by these resulting Au nanoparticles.
The GaAs nanowires were epitaxially grown by MOCVD (AIXTRON 200/4, AIXTRON, Herzogenrath, Germany) using trimethylgallium (TMGa) as the group III precursor and arsine (AsH3) as the group V precursor. Before growths, Au thin films with different thicknesses (namely, 0.5, 1, 2, 3, and 5 nm, respectively) were deposited onto three batches of GaAs (111)B substrates using an electron beam evaporator under high vacuum (<4×10−6 Torr). The thickness of the Au film is measured by a carefully calibrated single-crystal sensor in the evaporator chamber. For each batch, substrates with different thicknesses of Au films were then loaded into a MOCVD reactor simultaneously to ensure the identical growth conditions for the purpose of direct comparison. Firstly, two annealing experiments were carried out at 600°C and 650°C for 10 min in AsH3 ambient for two batches of Au-coated (111)B substrates, respectively. For the third batch, the growth of GaAs nanowires was carried out under a pressure of 100 mbar and a total input gas flow rate of 15 slm. Prior to the nanowire growth, the substrates of the third batch were annealed in the growth chamber at 600°C for 10 min under AsH3 ambient. The annealing temperature of 600°C was chosen based on the results from the first two annealing batches. The growth temperature of the nanowires was set at 450°C. The flow rates of TMGa and AsH3 were set at 1.16 × 10−5 mol/min and 5.36 × 10−4 mol/min, respectively, resulting in an overall V/III ratio of 46. The nanowire growth parameters were chosen based on the optimal growth condition for growing near defect-free zinc-blende GaAs nanowires catalyzed by Au colloidal particles .
The as-grown nanowires were analyzed by SEM (JEOL 7800F, operated at 1.5 kV with an in-lens electron detector setup and at 15kV with a conventional setup; JEOL Ltd., Akishima, Tokyo, Japan) and TEM (Philips Tecnai F20, operated at 200 kV and equipped with energy dispersive spectroscopy (EDS) for compositional analysis; Philips & Co., Eindhoven, The Netherlands). SEM was used to explore the morphologies of the Au thin films of different thicknesses deposited on the GaAs (111)B substrates before and after annealing at different temperatures. Furthermore, morphological characteristics of the as-grown GaAs nanowires, such as their diameters, heights, and densities, are determined by SEM. TEM was used to understand the structural and chemical characteristics of the as-grown nanowires. For TEM investigations, individual nanowires were deposited on holey carbon supporting films. The composition of the post-growth Au catalysts was analyzed by EDS in the scanning TEM mode.
Results and discussions
In summary, by the quantitative study of GaAs nanowire growth using Au films of different thicknesses, we found that the Ostwald ripening plays an important role at the pre-growth annealing before the nanowire growth. In the growth of GaAs nanowires with Au film as catalysts, the Ostwald ripening during the annealing process can be enhanced with the increased thickness of the Au film and the annealing temperature. By fine-tuning those two parameters, Au nanoparticles with moderate sizes and narrow distribution in sizes can be produced from Au thin films, which can be used to induce the vapor–liquid-solid growth of high-quality epitaxial GaAs nanowires. This approach provides a promising alternative in the controlled synthesis of III-V nanowires to the approaches using Au nanoparticles.
This research is supported by the Australian Research Council. The Australian National Fabrication Facility and Australian Microscopy & Microanalysis Research Facility, both established under the Australian Government's National Collaborative Research Infrastructure Strategy, are gratefully acknowledged for providing access to the facilities used in this work.
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