Nearly strain-free AlGaN on (0 0 0 1) sapphire: X-ray measurements and a new crystallographic growth model

Abstract

Two micrometer thick Al x Ga 1− x N layers with 0< x<0.4 were grown by low-pressure metal organic chemical vapour deposition on sapphire (0 0 0 1) 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 (0 0 0 2), (2 0−2 4), and (2 0−2 0) 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/Al x Ga 1− x N) 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 0.22Ga 0.78N) equals two times a (sapphire).