Electrochemical Formation of GaN Porous Structures Under UV-Light Irradiation for Photoelectrochemical Application
n-type GaN epitaxial layers (ND = 5 × 1016 ~ 5 × 1018 cm-3) grown on free-standing GaN substrates (ND ≧ 1 × 1018 cm-3) and on insulting sapphire substrates were used for the porous formation. The Au-ohmic contact was first made on the backside or topside of the substrate to supply the electrochemical current. The electrochemical process was performed using a standard cell with three electrodes in the electrolyte which is a mixture of 1M H2SO4 and 1M H3PO4. Photo-assisted anodization was carried out under UV light by comparing the back-sided irradiation (BSI) mode with the top-sided irradiation (TSI) mode.
After the anodization for a few minutes, the high-density arrays of pores were formed in GaN layers on both free-standing substrate and sapphire substrate. From SEM observation, we found that the pore depth, wall thickness, and surface morphology of porous structures were strongly influenced by the way holes generated by the light irradiation were supplied. In the case of the BSI mode on the free-standing GaN substrates, the pore diameter and wall thickness were estimated to be about 20 nm and 50 nm, respectively. They were almost same sizes throughout all part of the porous layer from the top to the bottom. Furthermore, the pore depth was well controllable by the total electric charge whereas it has never been achieved by the TSI mode. This is because the electric holes were moderately supplied from the backside by using the BSI mode, where the top region of porous layer did not dissolve as seen in the TSI mode. As for the GaN layers grown on the insulating sapphire substrates, the anodic currents should be supplied from the top electrodes formed beside the reaction area. We found that the BSI mode was very effective even for such substrates to form the porous structures under the condition that holes were sufficiently generated and supplied from the back surface.
We optimized the structural properties of GaN porous structures in view of the application as photoelectrodes and photosensors. As for such applications, the sufficiently-thick walls showing the good electrical conductivity and photoabsorption property are required as well as the deeper pores having a large surface area. From the photo-electrochemical measurements in a NaCl electrolyte, the photo-conversion efficiency degraded in the over-etched porous structures having thinner walls obtained by the TSI mode. On the other hand, the porous structures having thicker walls over 20 nm showed higher and depth-dependent efficiency. In addition to this, photorefelctance properties greatly influenced to the photo-conversion efficiency. As compared with the planar GaN surface, the porous surface showed a low photoreflectance with a high photoabsorption. The smallest reflectance was obtained from the sample with a largest porosity on the surface for both the BSI and TSI mode. These results indicate that the control of porous structural features such as surface morphology, thickness of pore walls, and pore depth is considered crucial to the improvement of the photoelectrochemical characteristics of GaN porous structures.