Room-temperature growth of ultrasmooth AlN epitaxial thin films on sapphire with NiO buffer layer
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Keisuke Saito Bruker AXS K.K., Kanagawa-ku, Yokohama 221-0022, Japan
Tokuo Yodo Electronic Information and Communication Engineering, Osaka Institute of Technology, Ohmiya, Osaka 535-8585, Japan
Mamoru Yoshimotoa) Materials and Structures Laboratory, Tokyo Institute of Technology, Midori, Yokohama 226-8503, Japan (Received 24 February 2004; accepted 11 June 2004)
Room-temperature epitaxy of AlN thin films on sapphire (0001) substrates was achieved by pulsed laser deposition using an epitaxial NiO ultrathin buffer layer (approximately 6 nm thick). Four-circle x-ray diffraction analysis indicates a double heteroepitaxial structure of AlN (0001)/NiO(111)/sapphire (0001) with the epitaxial relationship of AlN [10-10] 㛳 NiO [11-2] 㛳 sapphire [11-20]. The surface morphology of room-temperature grown AlN thin films was found to be atomically smooth and nanostepped, reflecting the surface of the ultrasmooth sapphire substrate with 0.2-nm-high steps. I. INTRODUCTION
In recent years, much interest has been focused on group III nitrides because of its promising applications as optical and microwave devices.1,2 Aluminum nitride (AlN), a direct wide-bandgap semiconductor (Eg ⳱ 6.2 eV), has high thermal conductivity and high surface acoustic-wave (SAW) velocity. These properties make AlN a useful material for high-power and high-frequency electronic devices as well as SAW devices. Until now, various methods for nitride film synthesis have been reported, e.g., reactive sputtering,3 metalorganic chemical vapor deposition,4 molecular-beam epitaxy (MBE),5 and pulsed laser deposition (PLD).6,7 Nitride films are usually grown epitaxially at substrate temperatures above 600 °C. Epitaxial growth at lower temperatures is favored to avoid a Volmer–Weber growth. PLD process using pulsed excimer laser is expected to reduce the temperature for epitaxial growth because the film precursors ablated from the target might impinge on the substrates with high kinetic energies.8 It is noted that PLD studies on epitaxial growth of oxide films at room temperature were reported by several
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0346 J. Mater. Res., Vol. 19, No. 9, Sep 2004
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groups, including ours, for systems such as CeO2 on Si (111), 9 CeO 2 on yttria-stabilized zirconia (YSZ)buffered Si (001),10 Sn-doped In2O3 on CeO2-buffered Si (111),11 BaO on SrTiO3 (100),12 ZnO on InP (100),13 NiO on MgO (100),14 NiO on ␣–Al2O3 (0001),15 Al2O3 on ␣–Al2O3 (10-12),16 and (Ni, Zn)Fe2O4 on ␣–Al2O3 (0001).17 There have also been some reports on the room-temperature epitaxial growth of nitride films by PLD, e.g., AlN on (Mn,Zn)Fe2O4 (111)18 and GaN on (Mn,Zn)Fe2O4 (111),19 but to our knowledge there have been no reports on the heteroepitaxial growth of AlN on ␣–Al2O3 substrates at room temperature by PLD. The artificial sapphire of single-crystalline ␣–Al2O3 with the corundum-type structure has been widely used as representative insulating subs
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