High-quality GaAs film grown on Si substrate is a promising structure for high-efficiency, low-cost, and lightweight multi-junction solar cells. However, the formation of rotational twins degrades the cell efficiency. This limitation could be circumvented by evaporating In atoms onto Si (111) substrate before the molecular beam epitaxial (MBE) growth of GaAs. This pre-evaporation step is believed to be effective in reducing the formation of rotational twin. Hence in this study, we have investigated the atomic arrangement and surface coverage of In atoms by using in-situ reflection high-energy electron diffraction and ex-situ scanning electron microscopy, to understand the effects of In pre-evaporation during the growth of GaAs on Si substrate. For comparison, Ga pre-evaporation was also studied. We found that 3D islands of GaAs and InAs are formed during Ga and In pre-evaporation, respectively, using the residual As in MBE chamber or physically absorbed As atoms. The X-ray diffraction analysis indicated that the rotational twin was reduced by In pre-evaporation, while it did not occur in Ga pre-evaporated sample. This suggests that the 3D islands of InAs formed during the In pre-evaporation stage facilitated the epitaxial growth of GaAs without rotational twin. (C) 2013 Elsevier B.V. All rights reserved.