Fundamentals of X-ray Diffraction Characterisation of Strain in GaN Based Compounds

Abstract

For the characterisation of group III nitrides by X-ray diffraction there are several challenges. In particular: a) reliable reference data on cell parameters of the end members and b) a simple and reliable method to obtain the alloy content of a thin film or multi-quantum layer structure that is fully strained to a template, the latter being fully relaxed or still showing some residual strain. A broad range of reference values for both cell parameters and elastic constants have been reported. In this work, we investigate the cell parameters and strain of (0001) GaN templates grown on sapphire and a bulk ammono GaN sample to derive a coherent set of cell parameters and elastic constants. The values are found to be applicable to a variety of samples, with different crystalline quality (threading dislocation density) or doping level. Subsequently different methods for determining the composition of group III nitride alloys on such strained GaN layers are compared. For simpler analysis, the template is often assumed fully relaxed. We investigate here this approximation and find that standard “relative” method fortuitously provides very good results.