Aluminium layers deposited onto GaN/(0 0 0 1) sapphire and AlN/(0 0 0 1) sapphire substrates by molecular beam epitaxy have been characterised and compared with aluminium grown directly onto (0 0 0 1) sapphire. Aluminium. layers with very flat surfaces having RMS roughness in the range 0.2-0.7 nm as measured by atomic force microscopy were grown, Complementary scanning electron microscopy, reflection high energy electron diffraction and conventional transmission electron microscopy observations confirmed that the Al layers adopted a twinned island micro structure, exhibiting a {1 1 1}-oriented growth surface and epitaxial relationships of the form < 1 1 2 > (A1)parallel to < 1 1 0 0 > (nitride) parallel to < 1 1 2 0 > (sapphire) and < 1 1 2 > (A1) parallel to < 1 1 2 0 > (sapphire). (C) 2002 Published by Elsevier Science B.V.