Photo-Electrochemical Etching in the Process of Direct H2 Generation by Illumination of GaN-Based Material Structures Immersed in Water

摘要

Hydrogen is considered a promising candidate for energy storage. We investigated the cleanest way for hydrogen production by direct photo electrolysis of water with GaN/AlGaN based p-n structures used as working electrodes. Besides the H_2 production rate, an important consideration is the material etching (corrosion) that accompanies the photo-electrochemical process. The GaN-based structures were grown on sapphire substrates by the chloride hydride vapor phase epitaxy and used as a photo anode immersed into an aqueous electrolyte. For a p-n GaN/AlGaN structure we observed a H_2 production rate of 0.6 mL/cm~2×h. Corrosion of the electrode proceeds in two steps. First, there is a near vertical etching process, which is associated with defects in the material and penetrates deep into the structure. Subsequently, the process involves etching of n-type layers in lateral direction resulting in the formation of voids and cavities. The lateral etching is due to net positive charges arising from the spontaneous and piezoelectric polarization in the structure and positively charged ionized donors in the space charge region of the p-n junction.