We have studied the surface atomic structure of GaAs(6 3 1), and the GaAs growth by molecular beam epitaxy (MBE) on this plane. After the oxide desorption process at 585 degrees C reflection high-energy electron diffraction (RHEED) showed along the [-120] direction a 2x surface reconstruction for GaAs(631)A, and a 1 x pattern was observed for GaAs(631)B. By annealing the substrates for 60 min, we observed that on the A surface appeared small hilly-like features, while on GaAs(631)B surface pits were formed. For GaAs(631)A, 500 nm-thick GaAs layers were grown at 585 degrees C. The atomic force microscopy (AFM) images at the end of growth showed the self-formation of nanoscale structures with a pyramidal shape enlarged along the [5-9-3] direction. Transversal views of the bulk-truncated GaAs(631) surface model showed arrays of atomic grooves along this direction, which could influence the formation of the pyramidal structures. (c) 2006 Elsevier B.V. All rights reserved.