Several orientations of GaAs substrates, including (10 0), (4 1 1), (1 1 1) and (5 1 1) have been annealed in a metalorganic vapour phase epitaxy (MOVPE) horizontal reactor at different annealing temperatures and under different triniethyl-bismuth (TMBi) flux. Surface morphology of the annealed GaAs substrates was investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show islands formation on all the studied samples. The density and size of Bi islands vary greatly with annealing temperature and TMBi flow. For different substrate orientations, the activation energies were deduced from Arrhenius plot of island density. Except for (5 1 1) oriented GaAs, all the studied orientations show the same activation energy of 1.8 eV. For low annealing temperature 420 degrees C, and under different Bi flux, each oriented substrate shows a specific behaviour. For higher temperatures 700 degrees C and above Bi islands are totally removed and the substrates Lire smooth. Surface change of (100) oriented GaAs substrate was in situ monitored by laser reflectometry. (c) 2007 Elsevier B.V. All rights reserved.