Seeded epitaxial growth of ZnO thin films on MgAl 2 O 4 substrates using the chemical solution deposition method
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David Andeen and Fred F. Lange Materials Department, University of California at Santa Barbara, Santa Barbara, California 93106 (Received 26 April 2006; accepted 31 October 2006)
Epitaxial ZnO thin films were grown on (111) MgAl2O4 with a pre-seeded, two-step chemical solution deposition process. Isolated, epitaxial ZnO islands (seeds) were formed on the substrate in the first step by spin coating a very thin layer of the precursor solution and heat-treating to 950 °C/3 h. In the second step, the seeded substrate was coated with another layer of precursor to produce an epitaxial film. The result was compared with the case in which a MgAl2O4 substrate was not seeded. Both the seeded and unseeded ZnO films have out-of-plane and in-plane orientation relationships of (0001)ZnO 㛳 (111)MgAl2O4 and [112¯0]ZnO 㛳 [1¯1¯2]MgAl2O4, respectively. However, only the seeded ZnO films have very faceted surface morphology without grain boundaries, indicating epitaxy, whereas the unseeded ZnO films have deep grain boundaries indicative of polycrystalline nature. This result shows that the formation of seeds in the first step plays an instrumental role in the formation of an epitaxial ZnO film.
I. INTRODUCTION
Recently, Zinc oxide (ZnO) has been considered the most promising candidate material in a wide variety of applications, such as room-temperature ultraviolet lasing, transparent conduction electrodes, surface-acoustic-wave devices, varistors, and gas sensors, because of its unique properties of wide band gap energy (Eg ⳱ 3.37 eV)1 with large exciton binding energy (60 meV) and large piezoelectric constant.2 To fully utilize these properties, it is necessary to grow high-quality epitaxial ZnO thin films, which have been generally produced using vapor phase methods such as metalorganic chemical vapor deposition (MOCVD),3 pulsed laser deposition (PLD),4,5 molecularbeam epitaxy (MBE),6,7 magnetron sputtering,8–10 and the ion gas layer reaction technique.11 However, these growth methods have the disadvantage of the high cost of the processing equipment. The chemical route has been extensively used for preparing various kinds of epitaxial and polycrystalline oxide thin films because it offers a fast, simple, and low-cost method.12–14 There are several reports on the preparation of ZnO thin films using the chemical route
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1577/JMR.2007.0109 J. Mater. Res., Vol. 22, No. 4, Apr 2007
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both as polycrystalline phase15–17 and as single crystalline phase.18–20 Andeen et al. reported the growth of epitaxial ZnO thin films on MgAl2O4 spinel substrates using a hydrothermal method,20 and Wessler et al. reported the growth of epitaxial ZnO thin films on scandium magnesium aluminate (ScMgAlO4)18 and on basal plane sapphire19 using the chemical solution deposition (CSD) method, which are the only reports to date of epitaxial growth of ZnO thin films using chemical routes. MgAl2O4 is a relatively low-cost sub
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