The effect of increasing the width of the (1 I 1)B facet on the molecular beam epitaxy (MBE) growth of a GaAs mesa structure consisting of (00 1) top and (1 1 1)B side surfaces was studied. The growth occurred only on the (0 0 1) surface in the beginning of the MBE deposition. while growth was observed on both surfaces as the (1 1 1)B facet width was intentionally increased by depositing a very thick epitaxial layer of more than 3 mum thickness. It is indicated that growth on (1 1 1)B starts when the Ga adatom concentration on (1 1 1)B is raised to the critical value for 2D nucleation by increasing the facet width. which reduces the efficiency of adatom flow from (1 1 1)B to (0 0 1) by intersurface diffusion. We show that a simple rate equation gives the critical value of similar to0.37 ML for the Ga coverage. (C) 2002 Published by Elsevier Science B.V.