Optoelectronic property of ZnO epitaxial layer grown by plasma-assisted epitaxy at temperature as low as 340°C using Ti2O3 buffer layer on a-sapphire were studied by low temperature photoluminescence at 10 K comparing to the layers on c-sapphire and a-sapphire without the buffer layer. The near band-edge emission consisting of free-exciton emissions and neutral-donor bound exciton emissions was significantly dependent on the buffer thickness and dominated by the free-exciton emissions in the layer grown on the very thin buffer layer about 0.8 nm, whereas the intense emissions by neutral-donor bound excitons were observed in the ZnO layer on c-sapphire. The structural behavior indicated the donor was originated from the three-dimensional growth of ZnO layer and details of the optoelectronic feature suggested the residual donors were Al and interstitial-Zn.
 M. A. L. Johnson, S. Fujita, W. H. Rowland, Jr., W. C. Hughes, J. W. Cook, Jr. and J. F. Schetzina, “MBE Growth and Properties of ZnO on Sapphire and SiC Substrates,” Journal of Electronic Materials, Vol. 25, No. 5, 1996, pp. 855-862. doi:10.1007/BF02666649
 A. B. M. A. Ashrafi, I. Suemune, H. Kumano and S. Tanaka, “Nitrogen-Doped p-Type ZnO Layers Prepared with H2O Vapor-Assisted Metalorganic Molecular-Beam Epitaxy,” Japanese Journal of Applied Physics, Vol. 41, 2002, pp. L1281-L1284. doi:10.1143/JJAP.41.L1281
 R. D. Vispute, V. Talyansky, Z. Trajanovic, S. Choopun, M. Downes, R. P. Sharma, T. Venkatesan, M. C. Woods, R. T. Lareau and K. A. Jones, “High Quality Crystalline ZnO Buffer Layers on Sapphire (001) by Pulsed Laser Deposition for III-V Nitrides,” Applied Physics Letters, Vol. 70, No. 20, 1997, pp. 2735-2737. doi:10.1063/1.119006
 S. Yamauchi, H. Handa, A. Nagayama and T. Hariu, “Low Temperature Epitaxial Growth of ZnO Layer by Plasma-Assisted Epitaxy,” Thin Solid Films, Vol. 345, No. 1, 1999, pp. 12-17. doi:10.1016/S0040-6090(99)00096-6
 S. Yamauchi, T. Ashiga, A. Nagayama and T. Hariu, “Plasma-Assisted Epitaxial Growth of ZnO Layer on Sapphire,” Journal of Crystal Growth, Vol. 214-215, 2000, pp. 63-67. doi:10.1016/S0022-0248(00)00060-9
 S. Yamauchi, Y. Goto and T. Hariu, “Photoluminescence Studies of Undoped and Nitrogen-Doped ZnO Layers Grown by Plasma-Assisted Epitaxt,” Journal of Crystal Growth, Vol. 260, No. 1-2, 2004, pp.1-6. doi:10.1016/j.jcrysgro.2003.08.002
 D. C. Look, D. C. Reynolds, C. W. Litton, R. L. Jones, D. B. Eason and G. Gantwell, “Characterization of Homoepitaxial p-Type ZnO Grown by Molecular Beam Epitaxy,” Applied Physics Letters, Vol. 81, No. 10, 2002, pp. 1830-1832. doi:10.1063/1.1504875
 B. K. Meyer, H. Alves, D. M. Hofmann, W. Kriegseis, D. Forster, F. Bertram, J. Christen, A. Hoffmann, M. Straßburg, M. Dworzak, U. Haboeck and A. V. Rodina, “Bound Exciton and Donor-Acceptor Pair Recombinations in ZnO,” Physica Status Solidi (B), Vol. 241, No. 2, 2004, pp. 231-260.
 L. S. Vlasenko and G. D. Watkins, “Intrinsic Defects in ZnO: A Study Using Optical Detection of Electron Paramagnetic Resonance,” Physica B, Vol. 376-377, 2006, pp. 677-681. doi:10.1016/j.physb.2005.12.170
 S. Yamauchi and Y. Imai, “ZnO Heteroepitaxy on Sapphire Using a Novel Buffer Layer of Titanium Oxide: Crystallographic Behavior,” Crystal Structure Theory and Applications, Vol. 2, No. 2, 2013, pp. 39-45.
 S. S. Kurbanov and T. W. Kang, “Spectral Behavior of the Emission Around 3.31 eV (A-Line) from ZnO Nanocrystals,” Journal of Luminescence, Vol. 130, No. 5, 2010, pp. 767-770. doi:10.1016/j.jlumin.2009.11.030
 A. Teke ü. Ozgür, S. Dogan, X. Gu, H. Morkoc, B. Nemeth, J. Nause, H. O. Everitt, “Excitonic fine Structure and Recombination Dynamics in Single-Crystalline ZnO,” Physical Review B, Vol. 70, No. 19, 2004, Article ID. 195207-1-10.
 D. C. Reynolds, D. C. Look, B. Jogai, C. W. Litton, T. C. Collins, W. Harsch and G. Cantwell, “Neutral-Donor-Bound-Exciton Complexes in ZnO Crystals,” Physical Review B, Vol. 57, 1998, pp. 12151-12155. doi:10.1103/PhysRevB.57.12151
 H. Alves, D. Pfisterer, A. Zeuner, T. Riemann, J. Christen, D. M. Hofmann and B. K. Meyer, “Optical Investigations on Excitons Bound to Impurities and Dislocations in ZnO,” Optical Materials, Vol. 23, No. 1-2, 2003, pp. 33-37. doi:10.1016/S0925-3467(03)00055-7
 B. K. Meyer, J. Sann, S. Lautenschlager, M. R. Wagner and A. Hoffmann, “Ionized and Neutral Donor-Bound Excitons in ZnO,” Physical Review B, Vol. 76, 2007, Article ID. 184120-1-10. doi:10.1103/PhysRevB.76.184120
 H. Shibata, M. Watanabe, M. Sakai, K. Oka, P. Fons, K. Iwata, A. Yamada, K. Matsubara, K. Sakurai, H. Tampo, K. Nakahara and S. Niki, “Characterization of ZnO Crystals by Photoluminescence Spectroscopy,” Physica Status Solidi (C), Vol. 1, No. 4, 2004, pp. 872-875.