Growth and microstructural characterization of catalyst-free ZnO nanostructures grown on sapphire and GaN by thermal eva
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ZnO nanostructures were grown directly on sapphire substrates and GaN epilayers by thermal evaporation. Their morphologies and densities were found to be strongly dependent on the synthesis position and the kinds of substrate loaded into the reactor due to the different oxygen densities and the lattice mismatch, respectively. Scanning electron microscopy and transmission electron microscopy studies revealed that ZnO nanorods on sapphire substrates grew in four directions, one 〈0001〉Sapphire and three (101¯4)Sapphire directions. It was found that the in-plane lattice mismatch of inclined ZnO nanorods was remarkably reduced by forming the planar relationship of (0002)ZnO//(101¯4)Sapphire, compared to that of (112¯0)ZnO//(101¯0)Sapphire in the ZnO film. On the other hand, for the GaN epilayers, vertically well-aligned ZnO nanorods were grown after growing an epitaxial ZnO film due to reduced lattice mismatch. Electron energy-loss spectroscopy data showed that Zn-rich stoichiometry was responsible for the formation of ZnO nanostructures.
I. INTRODUCTION
ZnO with a wide band gap (3.37 eV) and a large exciton binding energy (60 meV) is a representative II-VI oxide semiconductor for ultraviolet (UV) optoelectronic applications.1,2 One-dimensional (1D) ZnO nanostructures including nanorods, nanowires, and nanotubes have been actively studied to utilize the advantages of low dimensional structure, including enhanced quantum efficiency by carrier confinement.1–5 ZnO nanorods are mainly grown at high temperatures using metal catalysts, which are detrimental to optoelectronic devices.6,7 However, synthesis without metal catalysts makes it difficult to control the growth direction artificially and show random nanostructure distributions.8–10 In a previous study, we reported that ZnO films on a Si substrate play the role of a template layer during the fabrication of vertically aligned ZnO nanorods produced without catalysts in vapor–solid (VS) mode.11 Because of poor crystal quality, high cost, and small size of ZnO substrate, c-sapphire, glass, and Si are more favored substrates for the growth of ZnO films and nanostructures. For heterostructures, a large lattice mismatch in the heterointerface greatly affects nanostructure
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0108 J. Mater. Res., Vol. 22, No. 4, Apr 2007
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morphology and growth direction. GaN, which is analogous to ZnO, shows the epitaxial relationship of [101¯0]GaN//[112¯0]Sapphire and [101¯0]GaN//[110]Si with c-sapphire and Si (111) substrate, respectively, to accommodate a large lattice mismatch.12,13 However, unlike ZnO and GaN films, few studies have been conducted on the effect of substrates on the formation of ZnO nanostructures.14,15 In this work, the growth and morphology of ZnO nanostructures produced by thermal evaporation on c-sapphire and GaN/c-sapphire substrates were investigated using field-emission scanning electron microscopy (FE-SEM), x-ray diffra
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