An ice bath photo-electrochemical (PEC) undercut etching technique to separate devices from substrates is described. Smoothly etched Si-doped (2021) GaN is produced by etching a 40 nm relaxed sacrificial layer single quantum well. This has potential for improving the active region quality of semipolar green-emitter. Removal of unetched misfit dislocations revealed an RMS surface roughness decreasing from = 5.136 to 0.25 nm. In view of the development of green-light emitters, the interplay between the effects of reactant diffusion-limited etch process and defect-selective etching is demonstrated by enhancing PEC etching performance toward a smooth n-type semipolar GaN surface.