To understand the substrate orientation dependence of the surface morphology of GaN buffer layers grown on GaAs(111) surfaces. the activation energies for As desorption from GaAs(111)A and (111)B surfaces were calculated by the ab initio pseudopotential method. It was found that the activation energy for As desorption from GaAs(111)A was three times smaller than that from GaAs(111)B. These values of the As desorption energies are used to explain a previous finding that a high-quality GaN layer could be grown only on GaAs(111)A. (C) 2002 Elsevier Science B.V All rights reserved.