Synthesis of gallium nitride nanostructures by nitridation of electrochemically deposited gallium oxide on silicon substrate
© Ghazali et al.; licensee Springer. 2014
Received: 24 October 2014
Accepted: 9 December 2014
Published: 18 December 2014
Gallium nitride (GaN) nanostructures were successfully synthesized by the nitridation of the electrochemically deposited gallium oxide (Ga2O3) through the utilization of a so-called ammoniating process. Ga2O3 nanostructures were firstly deposited on Si substrate by a simple two-terminal electrochemical technique at a constant current density of 0.15 A/cm2 using a mixture of Ga2O3, HCl, NH4OH and H2O for 2 h. Then, the deposited Ga2O3 sample was ammoniated in a horizontal quartz tube single zone furnace at various ammoniating times and temperatures. The complete nitridation of Ga2O3 nanostructures at temperatures of 850°C and below was not observed even the ammoniating time was kept up to 45 min. After the ammoniating process at temperature of 900°C for 15 min, several prominent diffraction peaks correspond to hexagonal GaN (h-GaN) planes were detected, while no diffraction peak of Ga2O3 structure was detected, suggesting a complete transformation of Ga2O3 to GaN. Thus, temperature seems to be a key parameter in a nitridation process where the deoxidization rate of Ga2O3 to generate gaseous Ga2O increase with temperature. The growth mechanism for the transformation of Ga2O3 to GaN was proposed and discussed. It was found that a complete transformation can not be realized without a complete deoxidization of Ga2O3. A significant change of morphological structures takes place after a complete transformation of Ga2O3 to GaN where the original nanorod structures of Ga2O3 diminish, and a new nanowire-like GaN structures appear. These results show that the presented method seems to be promising in producing high-quality h-GaN nanostructures on Si.
Gallium nitride (GaN) is a very hard, chemically and mechanically stable wide bandgap (3.4 eV) semiconductor material with high heat capacity and thermal conductivity which makes it suitable to be used for sensors [1–8], high power electronic devices such as field-effect transistor (FET)  and optoelectronic devices such as light-emitting diode (LED) . Up to this date, many techniques have been explored to synthesize GaN nanostructures including nanowires, nanorods, nanodots and so forth since such low-dimensional nanostructures are promising for increasing the performance optoelectronic devices and the sensitivity of sensors [11, 12]. For example, GaN nanorods and nanowires have been applied for chemical sensing application as reported by Wright et al.  and Huang Y et al. , respectively, due to a large surface to volume ratio. GaN nanodots have been used in photodetectors as reported by Kumar et al..
Recently, GaN on silicon carbide (SiC) or sapphire substrate have been widely used for several specific electronic applications. However, these substrates are expensive and not available in large wafer size . According to Kukushkin et al., Si substrate seems to be more preferable for the heterostructure growth of GaN due to the availability of Si in large wafer size, the low price of Si and the maturity of Si-based technology . In addition, the integration of GaN-based devices on Si platform seems to be very attractive for the hybrid integration towards ‘More than Moore’ technology . Several vapor-phase techniques have been reported for growing GaN nanostructures directly on Si with high quality which include molecular beam epitaxy (MBE) , metal-organic chemical vapor deposition (MOCVD)  and hydride vapor phase epitaxy (HVPE) . However, these vapor-phase techniques are too expensive and their growth parameters are quite complicated. In recent years, a transformation of the grown gallium oxide (Ga2O3) structures on Si to GaN by a so-called nitridation seems to be a simple method to create a GaN/Si heterostructure . Here, a nitridation is achieved by annealing the Ga2O3 structures in ammonia gas. Li et al. reported the repeatable transformation of the CVD-grown GaN structures to Ga2O3 structures by an annealing in air and back to GaN structures by an annealing in ammonia . Moreover, there are several studies reporting the formation of GaN nanostructures by annealing the sputtered Ga2O3 layer on metal-coated Si substrates in ammonia [24–27]. To our knowledge, the nitridation of the electrochemically deposited Ga2O3 structures on bare Si substrates to form GaN nanostructures without the assistance of metal catalyzers does not appear in the published literature.
Recently, we report the growth of Ga2O3 nanostructures directly on Si without any assistance of metal catalyzer by using a simple electrochemical deposition . This liquid-phase technique provides several advantages such as high controllability of thickness and morphologies of Ga2O3 nanostructures due to a less number of growth parameters. In this work, we investigate the formation of GaN nanostructures by ammoniating the electrochemically deposited β-Ga2O3 nanostructures on Si substrate. Up to this date, no such similar work is reported where a combination of liquid-phase and vapor-phase methods is utilized to form a GaN/Si heterostructure. The effects of the ammoniating times and temperatures were studied. The mechanism for the growth of GaN was proposed and discussed.
The electrochemically deposited Ga2O3 was ammoniated in a quartz tube furnace as shown in Figure 1b, at various times of 15, 30 and 45 min and temperatures of 800°C, 850°C and 900°C under a flow of ammonia (NH3) gas of 100 sccm at atmospheric pressure. The timing chart of ammoniating process is shown in Figure 1c. Before starting the ammoniating process, the sample was put inside the quartz tube furnace and then the nitrogen (N2) gas was purged for 10 min to flush out the air in the quartz tube. After that, the temperature was increased up to the setting temperatures, i.e. 800°C, 850°C and 900°C from room temperature (RT) with a ramping rate of 28°C/min. After reaching the setting temperature, N2 gas was stopped and NH3 gas was introduced into the furnace. The furnace was immediately switched off upon reaching the setting ammoniating time. At the same time, NH3 gas was immediately stopped and N2 gas was purged back into the furnace for 1 h to remove the remaining NH3 gas during the cooling down process. The ammoniated structures were characterized using field-emission scanning electron microscopy (FESEM; Hitachi SU8083, Hitachi, Ltd, Chiyoda-ku, Japan), energy dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD; Bruker D8 Advance, Bruker AXS, Inc., Yokohama-shi, Japan).
Results and discussion
The nitridation of the electrochemically grown Ga2O3 nanostructures to form GaN nanostructures on Si platform have been studied by varying the ammoniating times and temperatures. The complete transformation of Ga2O3 nanorods to h-GaN nanowires was achieved at 900°C with a short ammoniating time of 15 min. The obtained results suggest that the effect of the ammoniating temperature in realizing a complete transformation is more prominent than the ammoniating time. Form the proposed growth mechanism, it was found that a complete transformation to GaN nanostructures can not be realized without a complete deoxidization of Ga2O3. In a complete transformation process, it seems to show the occurence of morphological change. The presented method seems to be promising for the formation of h-GaN nanostructures on Si for the applications in sensing and optoelectronics.
The authors would like to thank for the supports provided by MIMOS Berhad and Universiti Sains Malaysia. N. M. Ghazali thanks Malaysia-Japan International Institute of Technology (MJIIT) for the scholarship. This work was supported by Nippon Sheet Glass Corp, Hitachi Foundation, MJIIT, universiti Teknologi Malaysia, Malaysian Ministry of Education and Malaysian Ministry of Science, Technology and Innovation through various research grants.
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