We have investigated the c(4 x 4) surface reconstructions of GaAs(0 0 1) using scanning tunneling microscopy. We found that, by the As-4 flux in molecular beam epitaxy, the Ga-As dimer (c(4 x 4)alpha) and the As-As dimer (c(4 x 4)beta) structures at the top of the surfaces are distinguished depending on the growth temperature and annealing time. Annealing for more than 2 h at 350 degrees C is required to obtain a stable c(4 x 4)p surface reconstruction dominantly under the As-4 flux conditions. Furthermore, the Ga atoms in the c(4 x 4)alpha Ga-As heterodimers are segregated onto the surface and migrate to the step edges with the step barrier formation by the transition from alpha to beta phases with decreasing annealing temperature. (c) 2006 Published by Elsevier B.V.