Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
Gallium nitride (GaN) has band edge potentials that include water redox potentials [1]. However, photocurrent between a GaN anode and metal cathode decreases with time, because the etching reaction proceeds on the GaN anode. In a previous study, NiO island structure was shown to prevent the etching reaction [2]. Recently, we have reported that NiO layered structure on the surface is effective for preventing the etching [3]. In this study, we further suppressed the time degradation of photocurrent.
Al0.1Ga0.9N(AlGaN)/n-GaN layers were grown on a sapphire substrate by metal organic chemical vapor deposition. A 1-nm-thick Ni layer was formed on the AlGaN surface by electron beam evaporation, and annealed at 500 °C for 15 min under O2. As a reference, NiO layer was formed on the AlGaN surface by annealing of a 3-nm-thick Ni layer deposited by magnetron sputtering. The sample was used as an anode in a tandem cell with a Pt cathode [3].
A cross-section transmission electron microscopy confirmed that the annealing transformed the vapor deposited Ni layer (1 nm) into a NiO layer (1.7 nm) and the magnetron-sputtered Ni layer (3 nm) into a NiO layer (6.7 nm). The 1.7-nm-thick NiO layer was well adhered to the AlGaN surface compared with the 6.7-nm-thick NiO layer. The initial photocurrent density of the NiO(1.7 nm)/AlGaN/n-GaN was 0.19 mA/cm2 and 85% of that was maintained after 30 h. On the other hand, the initial photocurrent density of the reference sample was 0.16 mA/cm2 and it was reduced to 85% for only 6 h. The results indicate that the thickness and the deposition method of the Ni layer affect the adhesion between the NiO layer and the AlGaN surface, and that the well-adhered NiO protective layer is effective for the suppression of the time degradation of the photoccurent.
References
[1] K. Fujii, T. Karasawa, and K. Ohkawa, Japanese Journal of Applied Physics, 2005, 44 (Part 2), 16–19.
[2] K. Ohkawa, W. Ohara, D. Uchida, and M. Deura, Japanese Journal of Applied Physics, 2013, 52(Part 2), 08JH04.
[3] Y. Ono, Y. Uzumaki, K. Kumakura, and T. Komatsu, Book of Abstracts (International Conference on Artificial Photosynthesis), 2017, P4-17.
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