Improvement of Photoelectrochemical Reaction for Hydrogen Generation from Water using N-face GaN
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1202-I07-03
Improvement of Photoelectrochemical Reaction for Hydrogen Generation from Water using N-face GaN Katsushi Fujii1, Keiichi Sato1, Takashi Kato1, Tsutomu Minegishi1,2, Takafumi Yao1 1 Center for Interdisciplinary Research, Tohoku University, Aramaki Aza Aoba 6-3, Aoba-ku, Sendai, Miyagi 980-8578, JAPAN 2 Department of Chemical System Engineering, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, JAPAN ABSTRACT Photoelectrochemical properties of Ga- and N-face GaN grown by hydride vapor phase epitaxy (HVPE) were investigated. The properties were also compared with Ga-face GaN grown by metal-organic vapor phase epitaxy (MOVPE). The flatband potentials were in order of Gaface GaN grown by MOVPE < N-face GaN < Ga-face GaN. The highest photocurrent density at zero bias was obtained from the N-face GaN. The photocurrent density was over 3 times larger than that of Ga-face GaN. INTRODUCTION Photoelectrochemical water splitting is one of expecting renewable energy techniques to generate hydrogen from light and water [1]. The energy conversion efficiency improvement is one of the keys to realize the hydrogen production using this technique. Photoelectrochemical water splitting used a photo-absorbed semiconductor as a working electrode to generate electronhole pairs, which are utilized to split water. Therefore, increasing of light absorption by band gap shrinkage is important to improve the energy conversion. A few semiconductors have been reported to split water by the absorption of visible light [2]. The reduction of generated carrier loss also improves the efficiency. Co-catalyst on the working electrode is typically used to reduce the carrier loss. Nitride semiconductors are good candidates for the photo-illuminated working electrodes considered from the band edge energy and the chemical stability in solutions [3,4]. Band gap shrinkage for nitride semiconductor using InGaN increases the light absorption [5]. Co-catalyst for GaN also improves the photoelectrochemical water splitting [6]. In addition, semiconductor properties like carrier lifetime and surface orientation affect the light to carrier conversion efficiency [4,7]. Thus, the property tuning is also important to improve the photoelectrochemical efficiency. In this report, we evaluated the surface polarity of Ga- and N-face GaN in order to reduce the carrier loss and to compare with conventional Ga-face GaN grown by MOVPE. EXPERIMENT We used 1.5 mm thick freestanding n-type GaN grown by hydrogen vapor phase epitaxy (HVPE) for the evaluation of Ga- and N-face GaN. The carrier concentration (C.C.) and mobility were 1.8 × 1018 cm-3 and 280 cm2/Vs, respectively. The contact electrodes were the backsides of the photoelectrochemical reaction faces. The Ga-face GaN samples grown by metal-organic vapor phase epitaxy (MOVPE) (n-type, C.C. = 2.2 ~ 10.0 × 1018 cm-3, 4.5 ~ 4.2 µm in thickness)
were also used as references. The contact electrodes of the references were placed at the front surface edges because the insulating sap
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