594 Current Nanoscience, 2011, 7, 594-597 1573-4137/11 $58.00+.00 © 2011 Bentham Science Publishers Ltd. Growth Mechanism and Field Emission Characteristics of GaO/GaN Nanotips Using Iodine-assisted Enhanced Focused Ion Beam Etching Zhan-Shuo Hu 1 , Fei-Yi Hung 2, *, Shoou-Jinn Chang 1,3, *, Bohr-Ran Huang 4 , Bo-Cheng Lin 5 , Kuan-Jen Chen 6 , Tse-Pu Chen 6 and Wen-I Hsu 6 1 Institute of Electro-Optical Science and Engineering, Center for Micro/Nano Science and Technology, National Cheng Kung Uni- versity, Tainan 701, Taiwan, 2 Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Tech- nology, National Cheng Kung University, Tainan 701, Taiwan, 3 Institute of Microelectronics & Department of Electrical Engineer- ing, Center for Micr/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan, 4 Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, 5 Department of Electronic Engineering National Taiwan University of Science and Technology, Taipei 106, Tai- wan, 6 Institute of Microelectronics, National Cheng Kung University, Tainan 701, Taiwan Abstract: GaN nanorods are fabricated with an AgO mask by a process of iodine-assisted focused ion beam etching (IFIBE). The trans- formation from GaN nanorod to GaN nanotip structure, the thermal treatment uses a high temperature of 800 o C in air to increase the par- tial oxygen pressure resulting in the formation of a double mask, GaO and AgO. In addition, the Ag clusters react with the iodine gas to affect the etching rate and retain a GaO zone on the GaN nanotip arrays. Oxide-capped GaN nanotips can be applied as field emitter. The turn-on electric field was 2.2V/um when the current density was 0.1mA/cm 2 . Keywords: Ag, double masks, field emission, FIB, GaN, iodine. 1. INTRODUCTION One dimensional GaN nanostructures including, nanowires [1, 2], nanorods [3, 4], nanotubes [5, 6] and nanotips [7, 8] which pos- sess a high surface-to-volume ratio and quantumconfinement effect can be applied in field emission [8], photoconductivity [9], and field effect transistors [10]. Notably, the site control and structural properties are very important for the various applications. In order to define the site of the pattern, many methodologies have been utilized, such as traditional photolithography [7, 11], electron beam lithography [12] and nanosphere lithography [13]. However, the aforementioned lithographic methodologies not only involve a complex process, but also have line defect and are time-consuming. In this paper, iodine-assisted enhanced focused ion beam etching (IFIBE) technology was used with a double mask. This can pre- cisely and rapidly produce GaN nanotips without use of any litho- graphic techniques and a site control. Additionally, we can use IFIBE to fabricate rod-like or tip structures depending on the thermal treatment employed. In the case of nanorods, the Ag film of sputtered Ag/n-GaN structure was able to form the Ag cluster mask at low temperature. Then increas- ing the temperature and the oxygen partial pressure, the double masks which formed (AgO and GaO) assisted the GaN nanorods to transfer to GaN nanotips. During IFIBE, the silver oxide cluster is used as the first mask for reacting with the iodine gas to form AgI phase which protects the gallium oxide from gas-assist etching. Because silver is more active than gold or platinum, the silver could be oxidized to be the first mask and the formation of gallium oxide as the second mask. The IFIBE-fabricated GaN nanotips arrays possess the n-type semi- conductor and low electron affinity so that are suitable for field emitter [14]. So, this paper uses IFIBE and the thermal treatment on the Ag/n-GaN structure, not only to investigate the etching mecha- nism of the nanotips, but also study the field emission properties, so *Address correspondence to these authors at the Institute of Nanotechnol- ogy and Microsystems Engineering-Department of Materials Science and Engineering, Center for Micro/Nano Science and Technology, National- Cheng Kung University, Tainan 701, Taiwan; Tel: 886-6-2757575; Ext: 31395; Fax: 886-6-2745885; E-mails: fyhung@mail.mse.ncku.edu.tw; changsj@mail.ncku.edu.tw as to further understand the potential for the fabrication of nanos- tructures. 2. EXPERIMENT A n-GaN epilayer was grown on c-plane sapphire substrate by metalorganic chemical vapor deposition and the corresponding average carrier concentration was 8.710 18 cm -3 . The thickness of the n-GaN was about 4um. The cleaning process of the n-GaN in- volved acetone, isopropanol and hydrofluoric acid in turn and then rinsing with DI water. The cleaned n-GaN was loaded into the sput- ter chamber to deposit a Ag film (50nm) and the sputtered power was kept at 40W for 5min under a sputtering pressure of 15mTorr. Then the Ag/GaN was annealed under 2 different thermal con- ditions. In the first one: Ag/GaN was heated at 500 o C for 30min in an O 2 atmosphere to form an AgO cluster to be the first mask. The formation of this AgO cluster resulted from the strong agglomera- tion effect [15]. In the second one, AgO cluster /GaN was annealed at 800 o C for 40min in air to produce the double masks, AgO and GaO. At high temperatures, the exposed GaN surface would oxidize to form gallium oxide. The GaO not only functions as the second mask, but also stabilizes the top zone of the GaN nanotips to en- hance the properties of the field emission. Finally, the Ag/GaNs with different thermal treatments were subjected to IFIBE using a Ga + source with the vapor pressure of 0.3Torr for iodine and a chamber pressure of 5x10 -5 Torr. After IFIBE, the Ag/GaN epilay- ers became tip-structures. The crystallization of GaN epilayer was investigated by X-ray diffraction (XRD) and the field emission current-voltage (I-V) of the GaN nanotips was measured at room temperature at a pressure of ~6x10 -6 Torr. The indium-tin oxide layer was used as the anode and the indium layer on the un-etched GaN surface was employed to act as a cathode. The distance between the anode and cathode was 180um and they were separated by a mica sheet. The voltage rose to a maximum of 1100V to extract the electrons from the cath- ode to obtain the field emission I-V curves. 3. RESULTS AND DISCUSSION Fig. (1) shows the morphology of the Ag cluster on the GaN surface with annealing of 500 o C for 30 min in O 2 atmosphere, as well as an image of the GaN nanorod after IFIBE using a scanning