Relaxation processes of misfit-induced stress in GaN/AlyGa1-yN and AlxGa1-xN/AlyGa1-yN structures (x > y) were Studied. In case of GaN on relaxed AlyGa1-yN, relaxation of compressive-stress occurs in a very early stage oft lie GaN growth, while in case of AlyGa1-yN on AlxGa1-xN (x > y), tensile-stress relaxation does not Occur until the catastrophic relaxation by fracture formation. Surface roughness of the GaN grown on AlyGa1-yN can be suppressed by doping with Mg. (C) 2002 Elsevier Science B.V. All rights reserved.