The effects of nitrogen addition on methane-based ECR plasma etching of GaN were studied. The etch rate 30 nm/min and r.m.s.. roughness 2.6 nm were obtained when the GaN sample was etched by a methane-based gas mixture without N-2. The addition of N-2 gas resulted in a decrease of etch rate and a smoother etched surface. The r.m.s. roughness became less than 0.4 nm even only 1.5 seem N-2 gas was added to the mixture. In situ XPS measurements showed that, without N-2, heavy N-depletion took place on the etched surface, resulting in appearance of Ga metal on the surface. In contrast, the loss of N was compensated when the N-2 gas was added, and the etched surface approached the stoichiometric one with the increase of N-2 gas flow. This suppression of preferential loss of N was considered to be the main reason that improved the etched surface morphology. (C) 2002 Elsevier Science B.V. All rights reserved.