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-pr essure metal organic chemical vapour deposition on sapphire (0001) substrat es. 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 mappin g around the (0002), (20 - 24), and (20 - 20) Bragg reflections in conventi onal and grazing incidence geometry, respectively. The in-plane lattice par ameter a of layers grown in this composition regime coincides with that of( 2/3,)a((sapphire)). Their rotational and tilting disorder shows a minimum a s compared to layers grown outside this regime. (GaN/AlxGa1-xN) multi-quant um well structures on top of such buffer layers are fully pseudomorphic hav ing lowest interface disorder and best surface morphology as evaluated by s pecular and diffuse X-ray reflectivity measurements. The findings are expla ined by the assumption of a 2D coincidence site lattice for the epitaxial g rowth of AlGaN on sapphire. The coincidence site lattice has hexagonal symm etry 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.