Microstructure and composition analysis of group III nitrides by X-ray scattering

The importance of Group III-nitride structures for both light-emitting devi ces and high-power field effect transistors is well known (J.W. Orton, C.T. Foxon, Rep. Prog. Phys. 61 (1998) 1). In both cases, different alloy compo sition and doping levels or type are utilised and the device performance al so depends critically on the interface quality and defect density. We have used high resolution X-ray scattering to measure the state of strain in the individual layers on an absolute scale to derive the alloy composition, i. e. we have avoided the conventional method of using the substrate as an int ernal reference since it could be strained. The composition and individual layer thickness are derived through simulation of the profile with this add itional strain information and the best-fit profile is obtained with an aut omatic procedure. These structures are laterally inhomogeneous arising from defects breaking up the structure into narrow vertical columns of nearly p erfect material and this produces significant broadening of the diffraction pattern. This broadening in the diffraction pattern has been modelled usin g an extended dynamical scattering model (P.F. Fewster, X-Ray Scattering fr om Semiconductors, Imperial College Press, World Scientific, Singapore, 200 0) to yield the size distribution of perfect crystal regions. The measureme nt of the rotation about an axis defined by the growth direction of the GaN with respect to the sapphire is determined and is found to be small. Howev er, a poor quality sample indicates that a large range of rotations is poss ible in these structures. (C) 2001 Elsevier Science B.V. All rights reserve d.