Transition from Microscale-Faceted Structures to Ultra-Dense GaN Nanowires
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Transition from Microscale-Faceted Structures to Ultra-Dense GaN Nanowires Kasif Teker1 and Joseph A. Oxenham1 1 Department of Physics and Engineering, Frostburg State University, 101 Braddock Road, Frostburg, MD 21532, U.S.A. ABSTRACT Creation of nanoscale building blocks with various sizes and shapes are critical for the progress of nanotechnology. The synthesis of GaN nanowires by chemical vapor deposition (CVD) using Ga and NH3 as source materials on SiO2/Si substrate was systematically studied. Various types of catalyst materials, including gold (film and nanoparticle), nickel (film and nanoparticle), silver, cobalt and iron, have been used. The growth runs have been carried out at temperatures between 800 and 1100oC under two different carrier gases; H2 and Ar. Initial growth runs using Ar as carrier gas resulted in microscale-faceted nanostructures and short nanorods regardless of the growth temperature or reactor pressure. We have successfully achieved ultra-dense interwoven long nanowires using hydrogen as carrier gas at 1100oC. In fact, the yield has been very high for both gold and nickel catalysts. It should be emphasized that combination of high-temperature and hydrogen has resulted in ultra-dense interwoven long GaN nanowires. These results suggest a radical change in growth kinetics at high temperatures in the presence of H2. The GaN nanowire diameters are in the range of 15 nm to 50 nm and lengths up to hundred microns. The grown nanowires have been characterized by scanning electron microscopy (SEM with EDS), atomic force microscopy (AFM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). INTRODUCTION Gallium nitride nanostructures attract interest due to their applications in optoelectronic devices and high-power/high temperature electronics. GaN nanostructures have been used to fabricate field effect transistors [1] and high brightness light emitting diodes [2]. Synthesis of GaN nanowires with various fabrication methods has been reported. These fabrication methods include metal organic vapor deposition (MOCVD) [3, 4], reaction between Ga2O3 films and ammonia [5], gas source molecular beam epitaxy (MBE) [6], and CVD [7- 11]. CVD has been the most widely used method to fabricate nanostructured materials due to its low cost, high yield, and simplicity of operation. Moreover, the CVD process enables direct device fabrication on prepatterned substrates. In addition, GaN nanostructures attract interest due to their significant piezoelectric effect, which provides great potential for the integration of nanoelectronics and piezoelectricity [12]. It is very attractive to develop self-powered devices that enable continuous operations of implantable biological sensors and personal electronic devices. In that regard, GaN nanowires are great candidates for converting mechanical energy into electricity. This paper presents CVD growth of GaN nanowires using Ga and NH3 as source materials with various catalyst materials. We have studied the effects of various catalyst materials, growth tempera
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