To reduce light reflection and enlarge the effective reaction surface area at the n-GaN/electrolyte interface, n-GaN epitaxial layers with naturally textured surface are utilized. The layers are combined with Cr/Au ohmic contacts on n-GaN to form working electrodes that generate hydrogen by direct photoelectrolysis of water. Although the surface reflection on the naturally textured n-GaN samples is lower than that on n-GaN epitaxial layers with flat surface, our results reveal that the photocurrent ͑I ph ͒ and gas generation rates ͑R gas ͒ obtained from the naturally rough n-GaN samples are lower than those from the flat samples. The results can be attributed to the fact that the rough n-GaN surface caused by dense surface pits leads to significant recombination of photogenerated carriers with charged defects; this occurs before carriers reach the ohmic contacts, thereby resulting in lower I ph and R gas. Related analyses have been performed and presented in this paper to initially explain the possible mechanism.

An n-GaN photoelectrochemical (PEC) cell with immersed finger-type indium tin oxide (ITO) ohmic contacts was demonstrated in the present study to enhance the hydrogen generation rate. The finger-type ITO ohmic contacts were covered with SiO₂ layers to prevent the PEC cell from generating leakage current. Using a 1M NaCl electrolyte and external biases, the typical photocurrent density and gas generation rate of the n-GaN working electrodes with ITO finger contacts were found to be higher than those with Cr/Au finger contacts. The enhancement in photocurrent density or gas generation rate can be attributed to the transparent ITO contacts which allowed the introduction of relatively more photons into the GaN layer. No significant corrosion was observed in the ITO layer after the PEC process compared with the Cr/Au finger contacts which were significantly peeled from the GaN layer. These results indicate that the use of n-GaN working electrodes with finger-type ITO ohmic contacts is a promising approach for PEC cells.

To enhance the efficiency of photogenerated electron collection in the n-type working electrode, Indium Tin oxide (ITO) finger-type ohmic contacts were immersed in NaCl electrolyte because ITO is a well-known transparent and conductive optical film and the ITO/n-GaN contact exhibited ohmic property when the carrier concentration of n-GaN were close to 1×10 19 /cm 3 . We found that the performances of the n-GaN photoelectrochemical cells with finger-type ITO ohmic contacts in photocurrent densities and hydrogen gas generation rates were both better than the n-GaN without finger-type ITO ohmic contacts. Related analyses have been performed and will be presented in this paper to explain the possible mechanism from the point of view of electrochemical analysis. Besides, after the photoelectrochemical measurements we observed that the adhesion of ITO/n-GaN contacts was pretty good. Finally, we did the surface analysis by scanning electron microscope (SEM) before and after the photoelectrochemical measurements to conform the surface morphology of ITO almost did not change in the NaCl electrolyte. This indicates that ITO is a good candidate material for the immersed ohmic contact in water splitting system.