Intensity oscillation of reflection high-energy electron diffraction (RHEED) is observed during C-60 layer epitaxial growth on GaAs (1 1 1)B and (1 1 1)A substrates. The frequency of the oscillation coincides well with growth rate of C-60 layers, suggesting that C-60 layers grow with repeating nucleation and a step flow growth as with GaAs and other semiconductor materials. Unusual oscillation is observed in the initial C-60 layer growth on GaAs (1 1 1)B substrates with (2 x 2) reconstruction. The initial layer growth is completed at approximately half monolayer coverage by C-60 molecules. This phenomenon is explained by the model that C-60 absorption sites are limited due to As-trimers absorbed on (1 1 1)B surfaces. This model is strongly supported by the fact that no such effect is observed on GaAs (1 1 1)A substrates where no As-trimer is absorbed. (c) 2008 Elsevier B. V. All rights reserved.