As environmental problems such as global warming and air pollution arise due to fossil fuel-based energy production, the need for alternative energy development is emphasized. Especially, hydrogen energy has much attention to the ideal next generation alternative energy. Recently, solar water splitting, which exploits the reaction phenomenon of photoelectrode material and water under solar irradiance, has been one of promising candidates for generating hydrogen ecofriendly. The efficiency of solar water splitting can be improved by enhancing the reaction between the light and photoelectrode material based on following strategies: 1) widening a surface area of photoelectrode material and 2) reducing the strong surface reflection at the interface between photoelectrode material and water. These two goals can be achieved by introducing surface nanostructures such as nanowire, nanopore, and tapered nanostructure.
Meanwhile, various photoelectrode materials such as TiO2, ZnO, WO3, and so forth are used for solar water splitting. However, these conventional materials exhibit weak points such as wide energy bandgap that absorbs only the ultraviolet region of the solar spectrum, low charge carrier mobility, and chemical corrosion in the electrolyte. On the other hand, GaN can overcome these limitations due to tunable band gap, an outstanding electrical feature, and chemical stability. Also, other photoelectrode materials are difficult to manipulate surface texture, whereas GaN can be easily fabricated in nanostructure by using conventional semiconductor processing.
Thus, in this study, we investigate the water splitting characteristics of photo-electrochemicals by fabricating GaN anti-reflective nanostructures and measuring current density in bulk structures and nanostructures. After the deposition of SiO2 and Ag on GaN, a thermal dewetting technique of Ag thin film and SiO2 etching are utilized to form a nanoparticle mask, and a nanostructure is formed by using an ICP RIE etching process. The measurement of reflectance is performed by using ultraviolet-visible spectrometer for the optical analysis of the nanostructures. This result demonstrates that the reflectance of the nanostructure is 20% lower than that of the bulk structure. The photocurrent density of GaN with anti-reflective nanostructures is measured under the condition of artificial solar light with an intensity of 100 mW /cm2. As a result, the planar GaN is 0.1 mA / cm2 at a voltage of 0.8 V (versus Ag / AgCl), the photocurrent density of 0.15 mA /cm2 and 0.2 mA/cm2 is measured respectively, and the photocurrent density is improved 1.5 to 2 times in the GaN nanostructure compare to the planar GaN.