Defect selective photoetching of GaN: Progress, applications and prospects

Defect-selective etching methods are commonly used for a quick assessment of crystallographic and chemical inhomogeneities in various semiconductors, including nitrides. Because of the stability of GaN, "extreme" etchants such as molten bases and hot phosphoric/sulfuric acids are required for chemical etching. Photoetching provided an alternative and attractive path for room temperature etching of GaN. In this comprehensive review the introduction and subsequent modification of the photoetching method used for revealing defects and inhomogeneities in GaN are described in detail. The initial etchant, a KOH-based aqueous solution, was subsequently modified by addition of potassium peroxydisulphate (K₂S₂O₈), and later trisodium phosphate (Na₃PO₄) was added. The mechanism of photoetching in these solutions is presented and the advantages of using two- and three-component solutions are considered. This mechanism is based on generation of charge carriers (electrons and holes) by illumination of GaN with supra-bandgap light and was named photo-electrochemical (PEC) method. A correlation has been established between the carrier concentration in n-type GaN and the photoetch rate. A model is outlined that allows interpretation of large differences in the photoetch rate of inhomogeneous samples. Numerous examples of defects revealed by photoetching of GaN bulk crystals and homo- or hetero-epitaxial layers are described. The corresponding models for the formation of etch features are discussed and the results are compared with those obtained from other structural methods used for analysis of novel defects found in ammonothermally grown GaN crystals. The range of defects revealed by photoetching in GaN includes dislocations, inversion domains, nano-pipes, nano-scale and extended inhomogeneities. The importance of using photoetching for analysis of potentially new types of defect in recently grown ammonothermally GaN bulk crystals is emphasized. Future prospects of the PEC method for analysis of defects are considered.