NiO-loading on n-type GaN was reported to prevent the anodic corrosion during the photoelectrochemical water splitting, however, the role of NiO is still not clear [1]. We try to investigate the properties of NiO-loading on n-type GaN photoanode with changing the carrier concentration including low carrier concentration in this report.The thickness of GaN layer was 2.0 µm top layer on 2.0 µm undoped layer on sapphire substrate. The top layers were undoped and Si-doped GaN with changing carrier concentration. The carrier concentrations of top layers were 1.7×10¹⁶ cm^-3 for undoped GaN, 1.6×10¹⁷ cm^-3 and 1.2×10¹⁸ cm^-3 for Si-doped GaN. The NiO were deposited on the GaN surface and annealed. The structure of NiO was dot-like on GaN surface observed by TEM. The counter electrode was Pt and the light was Xe-lamp. The electrolyte was 1.0 mol/L NaOH (pH 13.6). The photoelectrochemical experiments were performed without bias condition.The NiO-loading was improved the photocurrent and stability of Si-doped GaN samples except for the undoped one. The photocurrent densities of undoped GaN with and without NiO loaded were very low due to the high resistivity of the GaN layer. The anodic corrosion (GaN surface ething) of the higher carrier concentration was faster than that of the lower carrier concentration when NiO was not loading observed by AFM. However, the photocurrent densities and time dependences of NiO-loading Si-doped GaN samples were almost the same even when the carrier concentration was changed. In addition, the surfaces of NiO loaded Si-doped GaN were not changed, whereas the surfaces of Si-doped GaN were etched under photoelectrochemical experiment condition. From these results, NiO-loading keeps the GaN surface even under the strong oxidation enviroment like water oxidation. The flatband potentials shifted negative direction for all samples independent of the carrier concentration. When the sample with NiO-layer structure on n-type GaN was used, negative shift of flatband potential from the n-type GaN without NiO-loading was also observed. From the result, the potentials at the region of NiO particles are expect to be lower than the other GaN bare surface. Thus, hole accumulation and carrier exchange between electrode and electrolyte are estimated to occur at the NiO particles. This is the reason why the stable water splitting is performed by NiO-loading on GaN. 

[1] T. Hayashi et al., Jpn. J. Appl. Phys. 51, 112601 (2012).