ARTICLES Optical and Field Emission Properties of Thin Single-Crystalline GaN Nanowires Byeongchul Ha, ² Sung Ho Seo, ² Jung Hee Cho, ² Chong S. Yoon, ‡ Jinkyoung Yoo, § Gyu-Chul Yi, § Chong Yun Park, | and Cheol Jin Lee* ,² Department of Nanotechnology, Hanyang UniVersity, Seoul 133-791, Korea, DiVision of AdVanced Materials Science and Engineering, Hanyang UniVersity, Seoul 133-791, Korea, Department of Materials Science and Engineering, Pohang UniVersity of Science and Technology (POSTECH), San-31 Hyoja-dong, Pohang 790-784, Korea, and Department of Physics, Center for Nanotubes and Nanostructured Composites, Sungkyunkwan UniVersity, Suwon 440-746, Korea ReceiVed: December 12, 2004; In Final Form: March 21, 2005 Thin high-quality gallium nitride (GaN) nanowires were synthesized by a catalytic chemical vapor deposition method. The synthesized GaN nanowires with hexagonal single-crystalline structure had thin diameters of 10-50 nm and lengths of tens of micrometers. The thin GaN nanowires revealed UV bands at 3.481 and 3.285 eV in low-temperature PL measurements due to the recombination of donor-bound excitons and donor- acceptor pairs, respectively. The blue shifts of UV bands in the low-temperature PL measurement were observed, indicating quantum confinement effects in the thin GaN nanowires which have smaller diameters than the exciton Bohr radius, 11 nm. For field emission properties of GaN nanowires, the turn-on field of GaN nanowires was 8.5 V/µm and the current density was about 0.2 mA/cm 2 at 17.5 V/µm, which is sufficient for the applications of field emission displays and vacuum microelectronic devices. Moreover, the GaN nanowires indicated stronger emission stability compared with carbon nanotubes. 1. Introduction Synthesis of gallium nitride (GaN) nanowires has attracted much attention because of their one-dimensional distinctive optical, electrical, and magnetic nanostructure properties. 1-8 Many research groups have studied the synthesis of GaN nanowires using various methods such as laser ablation, pyrolysis, and catalytic chemical vapor deposition (CCVD). 9-12 GaN is an important semiconductor material with a wide direct band gap (3.4 eV) because of its various applications in blue and ultraviolet light emission, high-temperature electronic devices, and high-power electronic devices. Recently, GaN nanowires promised bright hope for use as sensors, electronic devices, logic gates, light-emitting diodes, and diode lasers. 1-7 To apply the GaN nanowires to various areas, several research groups have studied optical properties such as Raman and photoluminescence (PL). There have been a few reports about room-temperature PL measurements of GaN nanowires. How- ever, most room-temperature PL measurements performed on the GaN nanowires revealed no quantum confinement effect, showing red shifts because most GaN-nanowire diameters were larger than the GaN excitonic Bohr radius, a B , 11 nm. 13,14 Only Chen et al. reported a blue shift in the band edge emission peak from GaN nanowires with 10-40 nm diameters at room- temperature PL measurements, indicating that some parts of GaN nanowires have smaller diameters than the excitonic Bohr radius of GaN, 11 nm. 15 However, there was no report showing blue shift in low-temperature PL from thin GaN nanowires which have smaller diameters than the Bohr radius, 11 nm. Low- temperature PL measurements present more precise spectro- scopic information than room-temperature PL measurements by reducing thermally activated nonradioactive recombination processes and thermal line broadening. The PL peak shifts of 80 and 54 meV were reported in the temperature range of 10- 340 K for bulk GaN and GaN quantum dots, respectively. 16 The main reduction of the band gap at high-temperature originates from interaction with LO phonons, resulting in the red shift of the band gap. 16,17 Therefore, we performed low- temperature PL measurements to investigate the exact relation with the quantum confinement effect on the size effect from the thin GaN nanowires with 10-50 nm diameters without reflecting on the temperature effect. We previously reported the synthesis of GaN nanowires by a thermal CCVD method using a mixture source of Ga and GaN powder. 11 However, in this work differently from the previous work, we used only Ga powder as a Ga source and separated a Ga source from a substrate about 10 mm. Even though the optical and electrical properties of GaN materials were announced by many research groups, there have been rare reports for field emission from GaN materials. 18-24 Berishev et al. 18 and Sugino et al. 19 investigated the electron field emission from GaN films. Ward et al. reported the field emission from GaN pyramid arrays for the high-power and high- temperature microelectronic devices. 20 They reported that the * To whom correspondence should be addressed. Tel: +82-2-2293-4744. Fax: +82-2-2220-0768. E-mail: cjlee@hanyang.ac.kr. ² Department of Nanotechnology, Hanyang University. ‡ Division of Advanced Materials Science Engineering, Hanyang Uni- versity. § Pohang University of Science and Technology. | Sungkyunkwan University. 11095 J. Phys. Chem. B 2005, 109, 11095-11099 10.1021/jp044334c CCC: $30.25 © 2005 American Chemical Society Published on Web 05/12/2005