Synthesis and characterization of β-Ga 2 O 3 nanowires on amorphous substrates using radio-frequency powder sputtering S.Y. Lee a , H.C. Kang b,n a Pohang Accelerator Laboratory, POSTECH, Pohang 790-834, Republic of Korea b Department of Materials Science and Engineering, Chosun University, Gwangju 501-759, Republic of Korea article info Article history: Received 17 September 2014 Received in revised form 5 November 2014 Accepted 16 November 2014 Communicated by K. Deppert Available online 27 November 2014 Keywords: A1. Nanostructures A1. Characterization A1. Growth models A3. Physical vapor deposition processes B1. Gallium compounds abstract We investigated the growth mechanism and microstructure of β-Ga 2 O 3 nanowires (NWs) deposited on amorphous SiN/Si(0 0 1), SiO x /Si(0 0 1), and glass substrates using radio-frequency powder sputtering. During growth, the β-Ga 2 O 3 changed from an initial amorphous thin film into NWs. Since the deposition was performed in an Ar gas environment, the initial amorphous thin films were non-stoichiometric Ga oxide (Ga 2 O 3 x ). Oxygen-deficient deposition led to the formation of intermediate thin films by the phase separation of Ga 2 O 3x into metallic Ga clusters and stoichiometric β-Ga 2 O 3 . The Ga clusters acted as catalyst seeds in the growth of the β-Ga 2 O 3 NWs through the self-catalytic vapor–liquid–solid mechanism. We found that the growth of β-Ga 2 O 3 NWs is possible at temperatures greater than 450 1C, above which phase separation and Ga cluster formation occur. We also observed that NWs inherit the planar defects, such as twin boundaries, of the host Ga seed. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Monoclinic gallium-oxide (β-Ga 2 O 3 ) nanowires (NWs) have potential applications in ultraviolet optoelectronic devices and high-temperature gas sensors by virtue of their unique and out- standing properties, such as a wide bandgap of 4.9 eV and excellent thermal stability [1–4]. Several techniques, such as chemical vapor deposition and thermal evaporation, are employed to synthesize β-Ga 2 O 3 NWs, and the unprecedented properties of the NWs, distinct from those of bulk materials, have been characterized [5–8]. Typi- cally, a metal catalyst such as Au is used as a seed material to grow NWs by a vapor–liquid–solid (VLS) mechanism [5,9]. It is also well- known that high-temperature annealing of β-Ga 2 O 3 powders is a very efficient method for fabricating long NWs without catalysts via a vapor–solid mechanism [10,11]. Most of these techniques are performed at temperatures greater than 800 1C. Recently, much effort has been devoted to the synthesis of β-Ga 2 O 3 NWs by physical vapor deposition [12–15], in which Ga 2 O 3 thin films are grown under oxygen deficient conditions. These conditions lead to the spontaneous separation of Ga atoms at the growing surfaces and the formation of metallic Ga clusters from non-stoichiometric Ga-oxide (Ga 2 O 3x ) thin films. The metallic Ga clusters act as liquid seeds, providing nucleation sites for the growth of NWs via a self-catalyzing VLS growth mechanism. The growth of NWs following phase separa- tion depends sensitively on the experimental parameters, such as the growth temperature and atmosphere. It is hypothesized that the required growth temperature of NWs could be reduced below 500 1C, a level at which appropriate phase separation and NW growth could still occur. In addition, physical deposition techniques for synthesizing NWs are adaptable and would be advantageous for use in the Si-based semiconductor industry. We recently reported the synthesis of β-Ga 2 O 3 NWs on sap- phire(0 0 0 1) substrates using a radio-frequency (RF) powder- sputtering method [14,15]. We found that the epitaxial thin films initially present transform into NWs and that the formation of metallic Ga clusters is essential for the self-catalytic VLS growth of NWs. In this work, we investigated the dependence of both the growth temperature and the nature of the amorphous substrate on the formation of NWs. The microstructures of the samples grown on different amorphous substrates, such as SiN/Si(0 0 1), SiO x /Si (0 0 1), and glass, were characterized. We found that there was a lower limit on the growth temperature required to initiate the self-catalytic VLS growth of NWs. No meaningful dependence on the substrate type was observed. 2. Experimental The synthesis of β-Ga 2 O 3 NWs was carried out using the RF powder-sputtering method reported previously [16]. β-Ga 2 O 3 Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth http://dx.doi.org/10.1016/j.jcrysgro.2014.11.030 0022-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ82 62 230 6938; fax: þ82 62 230 7311. E-mail address: kanghc@chosun.ac.kr (H.C. Kang). Journal of Crystal Growth 412 (2015) 25–30