Change in polarity of zinc oxide films grown on sapphire substrates without insertion of any buffer layer
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Change in polarity of zinc oxide films grown on sapphire substrates without insertion of any buffer layer Yutaka Adachi,a) Naoki Ohashi, Tsuyoshi Ohnishi, Takeshi Ohgaki, Isao Sakaguchi, and Hajime Haneda National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan
Mikk Lippmaa Institute of Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan (Received 4 April 2008; accepted 28 August 2008)
We have investigated the polarity of zinc oxide (ZnO) and Al-doped ZnO films grown on (112¯0) and (0001) sapphire substrates, using coaxial impact collision ion scattering spectroscopy. The films grown by pulsed laser deposition with a nominally undoped ZnO ceramic target had a (0001¯) surface, whereas the films prepared with a 1 mol% Al-doped ZnO ceramic target had a (0001) surface. The usage of Al-doped and undoped targets caused no difference in the in-plane lattice orientation. Electron microscope observations revealed that polarity change due to doping occurred without the formation of any interfacial phase between ZnO and sapphire.
Zinc oxide (ZnO) has been extensively investigated for applications, particularly in light-emitting diodes (LEDs), because of its direct wide-band gap energy (e.g., ∼3.3 eV) and large exciton binding energy (∼60 meV) at room temperature.1 Because ZnO crystallizes as a wurtzite-type structure, the crystal shows spontaneous electrical polarization along its c-axis and thereby has polar surfaces, i.e., (0001¯) face [c(−)-face] and (0001) face [c(+)-face]. Consequently, various properties of ZnO depend on its polarity, for example, surface electronic structure,2 chemical stability of the surface,3 interfacial properties,4,5 and impurity incorporation.6,7 Therefore, for designing devices using ZnO, it is important to examine and control the polarity of ZnO films. Nominally undoped zinc oxide films grown on the (0001) face of sapphire tend to have a c(−)-face.8 Because film orientation is determined by the energy balance between the surface and the interface energy, this trend suggests that the film growth having c(−)-face gains in free energy to yield a global minimum. Polarityselective growth of ZnO on a (0001) sapphire substrate was achieved by insertion of a buffer layer, such as MgO,9 AlN,10 GaN,11 and Cr compounds,12 between the ZnO film and the sapphire substrate. Undoped ZnO films
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0404 J. Mater. Res., Vol. 23, No. 12, Dec 2008
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with a c(+)-face could be deposited on (0001) sapphire substrates by pulsed laser deposition (PLD) using very specific growth conditions, e.g., low-growth temperature (450 °C) with high-growth rate on an atomically flat sapphire substrate.13 A sapphire substrate with a (112¯0) face has also been used as a substrate for ZnO film growth, because the (112¯0) sapphire substrate suppresses the formation of in-plane rotation domains or twins, which are often observed in Zn
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