In this paper, we investigate the influence of focused ion beam irradiation and subsequent rapid thermal annealing on the surface topology and the optical parameters of GaAs substrates. We describe the dose dependency and the influence of the acceleration voltage on the ellipsometric spectra and report the presence of distinct threshold doses for the build-up of surface tension and for the subsequent formation of gallium droplets on the surface. (C) 2004 Elsevier B.V. All rights reserved.