XRD and Optical Characterisation of GaN and Associated Substrate Materials

GaN is a promising material for opto-electronic devices. The biggest problem in growing epitaxial films arises from the lack of bulk GaN as a substrate material. This paper describes HRXRD measurements on the two possible substrate materials SiC and sapphire and on a GaN layer grown on sapphire. HRXRD measurements show this structure to be columnar. The most common crystal defects are tilt and twist of the columns, which correlate with screw and edge dislocations. HRXRD measurements of symmetrical reflections gave information about the tilt of the columns, while twist of the columns could only be measured by asymmetric Bragg reflections. PL measurements showed a pronounced correlation between twist of the columns and PL intensity. From reciprocal space maps of asymmetrical reflections the amount of tilt and the width of the columns could be established.