Nearly strain-free AlGaN on (0001) sapphire:: X-ray measurements and a new crystallographic growth model

Two micrometer thick AlxGa1-xN layers with 0 < x < 0.4 were grown by low-pressure metal organic chemical vapour deposition on sapphire (0001) substrates. For Al concentrations 0.18 < x < 0.25 the layers are found to be nearly strain-free as determined by high resolution X-ray reciprocal space mapping around the (0002), (20 - 24), and (20 - 20) Bragg reflections in conventional and grazing incidence geometry, respectively. The in-plane lattice parameter a of layers grown in this composition regime coincides with that of(2/3,)a((sapphire)). Their rotational and tilting disorder shows a minimum as compared to layers grown outside this regime. (GaN/AlxGa1-xN) multi-quantum well structures on top of such buffer layers are fully pseudomorphic having lowest interface disorder and best surface morphology as evaluated by specular and diffuse X-ray reflectivity measurements. The findings are explained by the assumption of a 2D coincidence site lattice for the epitaxial growth of AlGaN on sapphire. The coincidence site lattice has hexagonal symmetry with the lattice parameter three times a(A(0.22)Ga(0.78)N) equals two times a(sapphire). (C) 2000 Elsevier Science B.V. All rights reserved.