The surface diffusion process substantially influences the epitaxial growth of semiconductor nanowires. The spontaneous incorporation of Ga and the uniform axial-distribution over long ZnO nanowires are experimentally observed, which is attributed to the surface diffusion on ZnO sidewalls. The existence of abundant oxygen vacancies on the nanowire sidewalls greatly enhances the surface diffusion. Simulation on the diffusion dynamics is performed, and the strong bond energy between Ga and O atoms is verified. With the assistance of an oxygen vacancy, the diffusion barrier for Ga atoms decreased from 1.26 to 0.85 eV, resulting in a two orders of magnitude increase in the surface diffusion coefficient. The present work demonstrates the key role of surface diffusion process in the enhanced impurity incorporation during the growth of nanostructured semiconductors. (C) 2015 Published by Elsevier B.V.