Formation of GaN Nanocrystal on Si and Its Photoelectrochemical Application
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Formation of GaN Nanocrystal on Si and Its Photoelectrochemical Application Katsushi Fujii1, Takashi Kato1, Keiichi Sato1, In-Ho Im1, Ji-Ho Chang2 and Takafumi Yao1 1 Center for Interdisciplinary Research, Tohoku University, Aramaki Aza Aoba 6-3, Aoba-ku, Sendai, Miyagi 980-8578, JAPAN. 2 Department of Nano Semiconductor Physics, Korea Maritime University, 1 Dongsam-Dong, Yeong Do-Gu, Busan 606-791, SOUTH KOREA. ABSTRACT GaN nanodots and nanorods were successfully grown on Si (111) substrates by molecular beam epitaxy. Photocurrent densities of GaN nanodots were quite small compared with thick GaN layer grown by metal-organic vapor phase epitaxy. The current density, however, increases with GaN nanodot density. The highest photocurrent density of the GaN nanodots was higher than that of the layer structure with similar thickness (up to 10 nm) to the nanodot height. GaN nanorods have much higher photocurrent density than that of GaN nanodots. Enough nanostructure size for light absorption is important to achieve good photoelectrochemical performance. INTRODUCTION Photoelectrochemical water splitting is an important techniques for hydrogen evolution from water[1]. GaN is one of the suitable materials for the photo-illuminated working electrode considered from the band edge energies and the stability in solutions [2]. Hydrogen generation by photoelectrochemical reaction was observed using n-type GaN [3]. Complete water splitting was also demonstrated using RuO2 dispersed GaN powder [4]. For photoelectrochemical hydrogen generation using GaN photo-illuminated working electrode, we have used flat surface GaN epitaxial layers because the crystals have been made by epitaxial technique. The larger reaction area is expected to be the better efficiency from the surface chemical reaction point of view. Making nano-structures are one of the good ways to increase the reaction area. Thus, we investigated the growth of GaN nano-structure on Si substrate by molecular beam epitaxy (MBE), and evaluated the photoelectrochemical characteristics in this report. EXPERIMENT The GaN nano structures were made on n-type (111) Si substrates by plasma-assisted MBE [5,6]. The details of the growth conditions are described in the following sections. The surface morphologies after the nanostructure growth were evaluated by atomic force microscope (AFM) and scanning electron microscope (SEM). Photocurrent densities of the samples were measured directly connected to Pt counterelectrodes. The electrodes of GaN nanodots and GaN nanorods samples were the back surface, i.e., on the Si. The photocurrents were measured without bias except for the relationship between current density and applied bias. The electrolyte was 1.0 mol/L HCl to minimize the GaN corrosion [7]. Thus, the photoelectrochemical reaction was not water splitting but Cl--ion
oxidation and H+-ion reduction. The illumination used was 350 mW/cm2 Xe lamp when it was needed. The GaN epitaxial layer on a (0001) sapphire substrate grown by metal-organic vapor phase epitaxy (MOVP
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