Formation of Hexagonal Pyramids and Pits on V-/VI-Polar and III-/II-Polar GaN/ZnO Surfaces by Wet Etching

The etching characteristics and evolution of the surface morphology of ZnO and GaN thin films during wet chemical etching were investigated. While the ZnO etch rates using acidic solutions were similar, regardless of etchant or polarity, the etch rate of the Ga-polar GaN surface was dependent on the etchant. N-polar GaN could be etched in alkali-based solutions such as KOH. The surface morphologies of etched ZnO were very similar to those of the GaN surface: hexagonal pyramids and stepped hexagonal pits formed during the etching of O-polar ZnO and N-polar GaN and of Zn-polar ZnO and Ga-polar GaN, respectively. The formation of a hexagonal etch pit on the etched Zn-polar and Ga-polar surfaces was enhanced by surface defects such as pits or threading dislocations. The etching of N-polar GaN by KOH proceeded through the evolutionary stages of hexagonal pyramids: formation, growth, dissociation, and isolation. Ga-polar GaN was etched through the formation, widening, and merging of hexagonal pits.