Characteristics of GaN–core/Au-shell hetere-nanowires: Effects of thermal annealing on the structural and photoluminescence properties Hyoun Woo Kim * , Mesfin Abayneh Kebede, Hyo Sung Kim Division of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Keywords: GaN Core-shell nanowires Au Sputtering Thermal annealing Photoluminescence abstract GaN-core/Au-shell nanowires were fabricated and the effects of thermal annealing on the structural and photoluminescence (PL) properties were investigated. The surfaces of hetero-nanowires became rough due to the thermal annealing which could be attributed to the agglomeration of Au-shell layers into the cluster-like structures. X-ray diffraction indicated that the thermal annealing enhanced the crystallinity of the Au shell. From Gaussian deconvolution studies, we observed that the Au coating added a green band to the PL spectrum, whereas the thermal annealing enhanced the ultraviolet band. We have dis- cussed the possible emission mechanisms. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Since one-dimensional (1D) heterostructures with modulated composition and interfaces are supposed to have diverse function- alities, they are believed to have potential applications in nano- device fabrication [1,2]. Accordingly, many attempts have been made to achieve 1D heterostructures with different material systems, including semiconductor/semiconductor superlattices and metal/insulator nanocables. In spite of their interesting properties and their use in various nanodevice applications, metal/semi- conductor nanocables have not been sufficiently studied. This may arise from the difficulty in forming the metal shell layers on semi- conductor core nanowires. In this study, we have coated GaN nanowires with Au-shell layers by using the plasma sputtering technique. Gallium nitride (GaN) nanowires have been the subject of an intense research owing to a variety of potential applications, including blue and ultraviolet light-emitting diodes and laser diodes, high temperature and high power optoelectronic devices, modulators and detectors [3–9]. On the other hand, gold (Au) nanostructures have exhibited novel size-dependent properties that cannot be explained classically [10]. Also, they have many applications such as biological probes [11], submicrometer metallic barcodes [12], surface-enhanced Raman spectroscopy [13,14], plasmon waveguides for optical devices [15], photonic materials [16], and chemical sensors [17]. The Au-shell layer will not only prevent the degradation of core GaN nanowires but also extend the functionalities. The fabrication of nanodevices inevitably comprises a thermal annealing process which will alter/improve the properties of core/shell nanowires. Accordingly, we have investigated the effects of thermal annealing on the structural and optical properties of as-fabricated GaN-core/ Au-shell nanowires. 2. Experimental First, we prepared core GaN nanowires on silver (Ag: about 10 nm)-coated Si substrates and by heating pure GaN powders in a tube furnace. A mixture of Ar (flow rate: 100 sccm) and NH 3 (flow rate: 20 sccm) gases flowed at 1000  C for 1 h. Second, the substrates were transferred to a turbo sputter coater (Emitech K575X, Emitech Ltd., Ashford, Kent, UK) [18]. By using a circular Au target at room temperature, the sputter time was set to 1 min in high-purity (99.999%) argon (Ar) ambient. During the sputtering process, the DC current was maintained at 65 mA. Subsequently, the GaN-core/Au-shell nanowires were annealed at 600  C for 10 min in N 2 ambient. The samples were characterized by powder X-ray diffraction (XRD, Philips X’pert MRD diffractometer), scanning electron microscopy (SEM, Hitachi, S-4200), transmission electron micros- copy (TEM, Philips CM-200), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX). The photoluminescence (PL) measurement was carried out using a 325 nm He-Cd laser. A 55 mW Kimmon laser beam was focused on the sample. * Corresponding author. Tel.: þ82 32 860 7544; fax: þ82 32 862 5546. E-mail address: hwkim@inha.ac.kr (H.W. Kim). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.04.012 Vacuum 84 (2010) 254–257