The effects of V/III ratio during the growth of the GaN nucleation layer (NL) on the phase of GaN epilayers grown on GaP (1 0 0) by plasma-assisted molecular beam epitaxy were investigated. When a higher V/III ratio (N-rich condition) was used, a polycrystalline NL was initiated. On the contrary, a lower V/III ratio (Ga-rich condition) showed typical reflection high-energy electron diffraction patterns of zincblende GaN (beta-GaN) with a 4-fold symmetry. In a wide range in the V/III ratio of the subsequent GaN epilayers, the beta-GaN NL resulted in single-crystalline beta-GaN epilayers without phase mixture, while the polycrystalline NL led to preferential growth of the wurtzite phase. These results show that adjustment of the V/III ratio at the NL stage can control the phase of the GaN epilayers. The resultant phase selection of the NL could be related to the change in atomic mobilities of Ga affected by the amount of excess N atoms on the GaP (1 0 0) surface, which determines the epitaxial relationship of the GaN NL with the substrate surface. (C) 2002 Elsevier Science B.V. All rights reserved.