An investigation into the photo-electrochemical (PEC) etching of free-standing GaN wafers produced by hydride vapor phase epitaxial growth (HVPE) has found that etching is only possible with UV illumination in an acidic or basic electrolyte. Through photo-current measurement and X-ray analysis it was determined that lack of etching in a neutral electrolyte can be attributed to the formation of an oxide film on the GaN surface. Surface damage was also found to be a significant factor, with the etching rate and photo current density of surfaces treated by grinding and mechanical polishing being markedly less compared to a finely polished surface. Subsequent investigation of the luminescence and the etching characteristics of the intentionally-introduced scratches indicated that subsurface damage is difficult to remove from GaN by PEC etching due to the trapping of photo-excited carriers. A peculiar surface feature of concentric ring structures made up of alternating small and large pores was observed on the GaN surface along with small island regions, which is attributed to variations in the electronic properties of the GaN crystal that is created during HVPE growth. (C) 2015 Elsevier Ltd. All rights reserved.