To investigate the mechanism of the yellow luminescence (YL) in GaN, N-face GaN epitaxial film was prepared by wafer bonding and laser lift off from sapphire substrate. The exposed N-polar surface could be etched with potassium hydroxide aqueous solution. Intriguingly, we observed the yellow-to-band-edge luminescence ratio increased by 3.2 times after KOH etching. Since KOH etching is dislocation selective and changes the surface states of GaN, we associated the outstanding increase of YL to the presence of surface states instead of dislocations. It is further confirmed by X-ray photoelectron spectroscopy studies that the Ga vacancies dominate the YL in GaN. (C) 2012 Elsevier B.V. All rights reserved.