The selective growth mechanism of GaN on a (0 0 0 1) GaN template using radio-frequency plasmaassisted molecular beam epitaxy is investigated. The effect of the mask material on selective growth is investigated using SiO(2) and Ti masks. Selective growth of GaN with excellent selectivity and a smooth surface is achieved by optimizing the growth temperature. The threshold temperatures for selective growth are 930 and 940 degrees C for SiO(2) and Ti masks, respectively. A high growth temperature is necessary to suppress the formation of polycrystals on the mask and to obtain selective growth. The different threshold temperatures are thought to be due to the different affinities between Ga adatoms and the two mask materials. Selective growth is also performed using wide masks. Re-evaporation of adatoms is found to be the principal process in the selective growth obtained using the SiO(2) mask, whereas adatom migration is the principal process with the Ti mask. Surface diffusion of adatoms on the Ti mask results in the formation of a denuded zone near the mask edge and protuberances on both sides of the selective growth. (C) 2011 Elsevier B.V. All rights reserved.