Raman Scattering and Efficient UV Photoluminescence from Well-Aligned ZnO Nanowires Epitaxially Grown on GaN Buffer Layer Hsin-Ming Cheng, ²,‡ Hsu-Cheng Hsu, ² Yung-Kuan Tseng, ‡ Li-Jiaun Lin, ‡ and Wen-Feng Hsieh* ,² Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung UniVersity, 1001 Tahsueh Road, Hsinchu 30050, Taiwan, Republic of China, and Material Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, Republic of China ReceiVed: December 15, 2004; In Final Form: February 24, 2005 Optical phonon confinement and efficient UV emission of ZnO nanowires were investigated in use of resonant Raman scattering (RRS) and photoluminescence (PL). The high-quality ZnO nanowires with diameters of 80-100 nm and lengths of several micrometers were epitaxially grown through a simple low-pressure vapor- phase deposition method at temperature 550 °C on the precoated GaN(0001) buffer layer. The increasing intensity ratio of n-order longitudinal optical (LO) phonon (A 1 (nLO)/E 1 (nLO)) with increasing scattering order in RRS reveals the phonon quantum confinement as shrinking the diameter of ZnO nanowires. The exciton- related recombination near the band-edge transition dominate the UV emissions at room temperature as well as at low temperature that exhibits almost no other nonstoichiometric defects in the ZnO nanowires. I. Introduction ZnO is a promising material for photonic devices in the wavelength range of ultraviolet to blue since it has a wide band gap energy of 3.37 eV and a large exciton binding energy of 60 meV at room temperature (RT). It is expected that exciton as well as polariton lasers will be fabricated using the ZnO- related materials and that they will overwhelm the GaN-related materials soon because excitonic-stimulated emission and opti- cally pumped laser action in high-quality ZnO epitaxial films were observed 1,2 at room temperature. Because of the optical losses, including not only nonradiative recombination centers but also traps of excitons, the high quality of ZnO becomes even more imperative in the excitonic lasing process. Recently, one-dimensional (1D) ZnO nanowire has attracted considerable attention owing to its good crystal quality and unique photonic properties. 3,4 ZnO nanowire is also expected to play an important role as crystal quality and photonic properties interconnect as functional units in the fabrication of electronic, optoelectronic, electrochemical, and electromechani- cal devices with nanoscale dimensions. 4-20 However, the synthesis of an array of well-aligned ZnO nanowires is of great interest because it is an imperative step to realize nanophotonic devices, which include light-emitting diodes and laser diodes. Different methods have been reported for fabricating arrays of well-aligned ZnO nanowires that include template methods, 20,21 vapor transports and condensation methods, 3,4 metal-organic source vapor deposition methods, 22,23 and buffer layer precoated methods. 18,24 The conductive ZnO buffer layer behaves as the nucleus for growth of wires in the meantime as the electrode for field-emission measurement. 18 Park et al. 19 also reported vertically aligned n-ZnO nanorods grown on the p-GaN surface. GaN has the same wurtzite-type structure as ZnO with a low lattice mismatch (-1.9%). Therefore, ZnO nanowires on an epitaxial GaN buffer were expected to improve the crystal quality and to reduce the existence of structural defects. The p-n heterojunction of n-ZnO and p-GaN showed high current density and strong electroluminescence even at a low reversed bias voltage of 3 V. Accordingly, the buffer layer precoated method is a promising candidate for photonic device applica- tions. This method has also been extensively used in epitaxial ZnO films, 25-28 but it was seldom used to grow ZnO nanowires. In this letter, we report that vertical arrays of well-aligned ZnO nanowires were epitaxially fabricated on the GaN-buffered sapphire substrate by employing catalyst-free metal vapor deposition. We will discuss the interesting optical features of phonon quantum confinement and efficient UV emission in the use of Raman and photoluminescence (PL) spectroscopy. We also confirm a very small strain in the arrays of single-crystal ZnO nanowires by transmission electron microscopy (TEM) and high-resolution X-ray diffraction (HRXRD). II. Experiments Synthesis of the ZnO nanowire arrays was carried out in a low-pressure vapor-phase deposition system, which included a horizontal alumina tube in a conventional furnace. The zinc vapor source is 99.9% pure Zn metal powder from Strem Chemicals. High-quality epitaxial GaN(0001) buffer layers of up to 2-μm thick were grown on (0001) sapphire substrates by metal-organic vapor-phase epitaxy (MOVPE). The GaN/ sapphire substrates and zinc vapor source in an alumina boat were inserted into the quartz tube and placed closely to the middle of the furnace. Afterward, the chamber was heated to 550 °C at a rate of 20 °C/min under a constant flow of 55 sccm Ar and 1 sccm oxygen for 1 h. After cooling to ambient temperature, a white deposition layer was found over the substrate. More detailed growth conditions and the description of the reaction chamber are presented elsewhere. 24 The as-synthesized products were characterized by field emission scanning electron microscopy (FESEM) [LEO-1530], * To whom correspondence should be addressed. Tel: +886-3-5712121 ext. 56316; fax: +886-3-5716631; e-mail: wfhsieh@mail.nctu.edu.tw. ² National Chiao Tung University. ‡ Industrial Technology Research Institute. 8749 J. Phys. Chem. B 2005, 109, 8749-8754 10.1021/jp0442908 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/06/2005