The effect of in situ annealing was investigated on GaN dots grown on Si(1 1 1) substrates by droplet epitaxy. In particular, the effect of irradiation by nitrogen plasma during annealing was examined. The density of GaN dots on the sample annealed with nitrogen plasma was approximately 1.0 x 10(11) cm(-2), which was much higher than the density of dots on the sample without nitrogen plasma (1.2 x 10(8) cm(-2)). X-ray photoelectron spectroscopy showed that the Ga 2p peaks of the samples annealed both with and without irradiation by nitrogen plasma consisted mainly of Ga-N peaks, while that of the unannealed sample contained an extremely large Ga-O component, which is observed at the higher binding energy side of the Ga-N peak. These results indicate that the degree of nitridation of the dots was much improved by annealing. The high density of GaN dots on the sample annealed with nitrogen plasma, in which case the dot diameters were as small as 20 nm, was presumably enabled through the suppression of both the re-evaporation of Ga atoms and the coarsening of Ga droplets by irradiation by the surface by nitrogen plasma. (c) 2006 Elsevier B.V. All rights reserved.