Separation of Structural Defects in MOCVD Grown GaN and AlN Films on c-Plane Sapphire by HRXRD

A novel triple-axis diffractometer equipped with a parabolically curved graded multilayer, which replaces the conventional analyzer crystal, is used to characterize structural properties of epitaxial GaN and AlN films grown on c-plane sapphire. From the angular broadening of the symmetric rocking curves, the lateral correlation length and the tilt angle is obtained by two independent methods. Additionally, the measurement of asymmetric reflections yields further information concerning the anisotropic dislocation structure in hexagonal GaN. For AlN, the out-of-plane disorder (tilt) is such small that coherent X-ray scattering from individual crystallites is observed. Despite the poor epitaxial quality with respect to the in-plane orientation (twist), the angular broadening of X-ray rocking curves remains small. The AFM images and the XRD measurements of asymmetric reflections confirm the high degree of the in-plane rotational disorder (twist).