We studied the morphology of GaP/(001)GaAs and GaAs/(001)GaP heterostructures grown by metal-organic vapour-phase epitaxy and found wire-like surface undulations elongated in the [110] direction. We attribute this elongation to anisotropic lateral growth rates in the [110] and [110] directions, which are due to a different roughness of monolayer surface steps. In III-V materials grown by molecular beam epitaxy. such surface corrugations are usually elongated in [110]. We explain this difference by the two growth methods having inverted ratios of lateral growth rates in [110] and [110]. Resonant tunnelling diodes fabricated from the GaP/GaAs heterostructures showed very symmetric I-V characteristics. Their peak-to-valley ratio was limited to 2. most probably due to the corrugation of the GaP barriers. (C) 2002 Elsevier Science B.V. All rights reserved.