The mechanism of nitridation of (001) GaAs surface using RF-radical source was systematically studied with changing substrate temperature, nitridation time and supplying As molecular beam. It was found from atomic forth microscopy (AFM) measurements that supplying As is very important to suppress the re-evaporation of As atoms and to keep the surface smooth. Reflection high-energy electron diffraction (RHEED) measurements shows that surface lattice constant (SLC) of GaAs of 0.565 nm decreases with increasing the substrate temperature and that it finally relaxes to the value of c-GaN of 0.452 nm, at 570 degrees C in both [110] and [(1) over bar 10] directions without concerning with the supply of As molecular beam. But, in the medium temperature range (between 350 and 520 degrees C), SLC of [110] direction was smaller than that of [(1) over bar 10] direction. This suggests a relation between the surface structure and the relaxing mechanism of the lattice. The valence band discontinuity between the nitridated layer and the GaAs layer was estimated by using X-ray photoemission spectroscopy (XPS). It was between 1.7 and 2.0 eV, which coincides well with the reported value of c-GaN of 1.84 eV. This suggests that the fabricated GaN layer was in cubic structure. (C) 2009 Elsevier B.V. All rights reserved.