High-Density, Defect-Free, and Taper-Restrained Epitaxial GaAs Nanowires Induced from Annealed Au Thin Films Hongyi Xu, † Yong Wang, † Yanan Guo, † Zhiming Liao, † Qiang Gao, § Nian Jiang, § Hoe H. Tan, § Chennupati Jagadish, § and Jin Zou* ,†,‡ † Materials Engineering and ‡ Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane St. Lucia, Queensland 4072, Australia § Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 0200, Australia ABSTRACT: In this study, we demonstrated that by using annealed Au thin films as catalysts, high-density, defect-free, and taper-restrained epitaxial GaAs nanowires were grown on GaAs (111)B substrates. The as-grown nanowires were compared with low-density Au colloidal nanoparticle catalyzed GaAs nanowires grown under identical conditions in the same metal-organic chemical vapor deposition reactor. Through detailed morphological and structural characterizations using advanced electron microscopy, we discovered that GaAs epitaxial nanowire tapering can be efficiently restrained by increasing the density of Au catalysts. By increasing the density of the Au catalysts, the axial growth rate of nanowires is reduced, which, in turn, limits the formation of lattice defects. Furthermore, the comprehensive investigation of GaAs nanowires catalyzed by Au thin film of different thicknesses (1 nm, 2 nm, 3 nm, and 5 nm) and Au colloidal particles of different densities indicates that the density of the Au catalysts play an important role in GaAs nanowire growth. This comprehensive study provides an opportunity to explore the effects of the catalysts and the growth mechanisms of III-V epitaxial semiconductor nanowires. ■ INTRODUCTION In the past few decades, nanoscale III-V semiconductor-based technologies and devices have drawn extensive attention. 1-3 As one of the most promising one-dimensional (1-D) nanostruc- tures, III-V semiconductor nanowires exhibit outstanding potential as building blocks of future electronic 4 and optoelectronic devices, 5-7 such as chemical and biological sensors. 8 In order to explore their newly demonstrated physicochemical properties, much effort has been devoted to fabricating high-quality semiconductor nanowires using differ- ent techniques. 9-11 One of the most precise growth techniques for epitaxial III-V nanowires is metal-organic chemical vapor deposition (MOCVD), 12 which often uses Au nanoparticles to catalyze the nanowire growth via the vapor-liquid-solid mechanism. 1,13 It is well documented that the morphologies, position, and distribution of the Au catalysts on the substrate are crucial factors for controlling the nanowire growth. Many deposition techniques of Au catalysts have been used including position control by using SiO 2 masks, 14 depositing Au particles through nanochannel alumina templates, 15 manipulating Au particles using atomic force microscopy, 16 and depositing well- positioned Au particles using electron beam lithography. 17 These techniques offer precise control over the distribution of nanowires on the substrates; however, they are expensive and the processes are rather complicated. Other deposition methods include the Au aerosol method, 18 colloidal Au nano- particles deposition with the poly L-lysine (PLL) treatment, 19 and Au thin film deposition method. 20 These techniques are simple and cost efficient but have less control over the position of the nanowires on the substrates. Nevertheless, in order to make the nanowires ready for devices, considerable efforts have been devoted to understanding the precise control over the nanowire morphologies and crystal structures by changing the nanowire growth conditions. Although significant progress has been made and fine-tuning of the nanowire properties is possible, engineering device-quality (defect/taper-free) nano- wires in a single growth step is still challenging. For example, nanowires can be grown with low defect density but tapered morphology, 9,19 or with near taper-free morphology but with planar defects. 21 Even though a two-step growth method was adopted to achieve simultaneous defect-free and near taper-free epitaxial GaAs nanowires, the nanowire density was low in such a case. 10 In many device applications, high-density, defect/ taper-free nanowires are desired to achieve high-performance. In this study, we demonstrate that, by using an annealed Au thin film as a catalyst (to generate a high-density of Au nanoparticles), defect-free and taper-restrained epitaxial GaAs nanowires with a high density can be realized in a one-step growth. Through carefully designed comparative nanowire growth and detailed morphological and structural character- ization, the fundamental reasons for why such high-quality Received: December 31, 2011 Revised: February 21, 2012 Published: February 22, 2012 Article pubs.acs.org/crystal © 2012 American Chemical Society 2018 dx.doi.org/10.1021/cg201725g | Cryst. Growth Des. 2012, 12, 2018-2022