Cubic (Mg,Zn)O nanowire growth using catalyst-driven molecular beam epitaxy
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L-C. Tien and D.P. Nortona) University of Florida, Department of Materials Science and Engineering, Gainesville, Florida 32611 (Received 6 June 2005; accepted 2 August 2005)
We report on the growth of Mg-rich cubic (Mg,Zn)O nanowires using a catalysisdriven molecular-beam-epitaxy method. Nanowires were grown on both Si and Al2O3 substrates coated with a nominally 2-nm-thick layer of Ag. The (Mg,Zn)O nanowires were grown with a Zn and Mg cation flux, with an O2/O3 mixture serving as the oxidizing species. The growth temperature was 400 °C. Under these conditions, nanowires were observed to grow on the Ag sites. The nanowire diameter was on the order of 90 nm. (Mg,Zn)O nanowires as long as 2 m were realized. High-resolution transmission electron microscope imagery shows the nanowires had single-phase cubic rock salt structure (Mg,Zn)O with a growth direction along the [100]. The presence and compositional distribution of Mg and Zn in the single nanowire were confirmed using a compositional line-scan, profiled across the nanowire, by energy dispersive spectrometry with scanning transmission electron microscopy.
I. INTRODUCTION
In recent years, significant interest has emerged in the synthesis of metal oxides, nanowires, and nanorod materials.1–13 Oxide nanowires provide interesting systems for investigating fundamental properties of low dimensional electronic and mechanical effects. They can also be useful for exploring novel device concepts via single prototype device construction. In some cases, oxide nanowires may serve as nanoscale substrates for subsequent film growth. One material system of significant interest for nanowire formation is Zn–Mg–O. At one end of the solid solution, ZnO is a direct band gap (Eg ⳱ 3.37 eV)semiconductor, possessing the wurtzite crystal structure, which has significant potential in wide band gap electronics. At the other end of the solid solution, MgO is a binary dielectric (Eg ⳱ 7.5 eV) oxide possessing the cubic rock salt structure. From the phase diagram of the ZnO–MgO binary system, the thermodynamic solubility limit of MgO in wurtzite ZnO is less than 4 at.%. However, several studies involving epitaxial film growth have shown that single-phase wurtzite (Mg, Zn)O thin films can be realized with a Mg concentration
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0388 3028
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 11, Nov 2005 Downloaded: 26 Mar 2015
as high as 33 at.%. For larger Mg/Zn ratios, the structure reverts to cubic rock salt. It should be noted that the wide gap cubic (Mg,Zn)O alloy is also of interest for ultraviolet (UV) optoelectronics due to its wide gap and UV photoconductivity response.14,15 MgO nanowires have been investigated as effective flux pinning centers in high-temperature superconductors.16 As noted earlier, the end compound (MgO) is routinely used as a substrate material for epitaxial oxide film growth. Cubic MgO nanowires have been explored as nanoscale crystalline substrates. For exampl
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