Synthesis and Characterization of Glomerate GaN Nanowires
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Glomerate GaN nanowires were synthesized on Si(111) substrates by annealing sputtered Ga2O3/Co films under flowing ammonia at temperature of 950 °C. X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy and Fourier transformed infrared spectra were used to characterize the morphology, crystallinity and microstructure of the as-synthesized samples. Our results show that the samples are of hexagonal wurtzite structure. For the majority of GaN nanowires, the length is up to tens of microns and the diameter is in the range of 50–200 nm. The growth process of the GaN nanowires is dominated by Co–Ga–N alloy mechanism.
KeywordsNanowires Magnetron sputtering Alloy mechanism
Gallium nitride (GaN) has gained considerable attentions due to its wide and direct band gap (3.39 eV at room temperature), high thermal stability and strong resistance to radiation [1, 2, 3, 4, 5]. GaN-based materials are expected to be a good candidate for high-power electronic devices, light-emitting diodes, and laser diodes in the blue and UV wavelength regions [6, 7, 8]. In recent years, more and more research efforts have been devoted to the one-dimensional nanoscale materials because of their fascinating electronic, optical and mechanical properties in fabrication of novel nanodevices [9, 10, 11, 12]. The GaN nanowires are of interest due to the giant electrogyration effects . Many attempts have been made to synthesize GaN nanowires using various techniques such as the carbon-nanotube-confined reaction, the anodic alumina template method, arc discharge, laser ablation, catalytic chemical vapour deposition, and the oxide-assisted growth route [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. Compared to these techniques, the radio frequency (RF) magnetron sputtering is one of newly developed methods, which has many advantages on synthesis of GaN nanowires such as simplicity for deposition of multicomponent, effective charge of sputter-time, no corrosive gas and low processing temperatures .
In this work, GaN nanowires were synthesized by ammoniating Ga2O3/Co thin films deposited on Si(111) substrates with RF magnetron sputtering method. The metal Cobalt was used as the buffer layer, which was expected to change the surface energy distribution and to enhance the formation of GaN nanowires. To our knowledge, so far no experimental study has been done on GaN nanowires in this method.
Gallium nitride nanowires were synthesized by the following steps. First, the silicon substrate was ultrasonic cleaned in absolute ethyl alcohol and de-ionized water for 30 min in sequence. Second, the Co films were deposited on Si substrates by sputtering a Co target (99.99%) for 10 s with a JCK-500A RFMS. The thickness of Co layer was about 10 nm. The background pressure of the sputtering chamber was about 5.5 × 10−4 Pa, and Ar (purity: 99.999%) under 2 Pa pressure was introduced into the chamber as the sputtering gas. The distance between the target and the substrate was 8 cm. Under these conditions, the Ga2O3(purity: 99.999%) thin films were grown on Co-coated Si(111) substrates by sputtering a sinter Ga2O3target for 90 min. The thickness of Ga2O3layer was about 500 nm. Finally, the Ga2O3/Co films were ammoniated in an ammonia atmosphere with a flow rate of 500 ml/min in a horizontal tube furnace. The ammoniating temperatures was 950 °C, and the duration of ammoniating is 10 min. After reaction, a deposit of light-yellow layer was found on the substrate surface.
We studied the structure, morphology, composition and crystallinity of the as-synthesized samples using X-ray diffraction (XRD, RigaKu D/max-rB Cu Kα), scanning electron microscope (SEM, Hitachi S-570), high resolution transmission electron microscope (HRTEM, Tecnai F30) and Fourier transform infrared spectroscopy (FTIR, TENSOR27).
Results and Discussions
In summary, the glomerate GaN nanowires were synthesized on the Si(111) substrate using Co as the catalyst. The diameters are in the range of 50–200 nm and the lengths are up to several tens of microns. Most of the nanowires are single-crystalline wurtzite structured GaN crystals grown with the  direction. The catalytic growth mechanism of GaN nanowires is described as the alloy mechanism.
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