Comparative study of threading dislocations in GaN epitaxial layers by nondestructive methods

An investigation of the threading dislocations (TDs) density in two high quality GaN epitaxial layers grown on sapphire (Al2O3) substrate, with the thickness of 300 and 1000 nm, respectively, is presented in this paper. In the case of GaN300 sample, taking into account that the finite size effect exerts an increased influence, further advanced methods are necessary to find the correct results besides the standard analysis of the Full Width at Half Maximum (FWHM) of the X-ray rocking curves peaks as in the case of the thicker sample, GaN1000. Thus, we explored the tails of the rocking curves using a universal power law fit and subsequently, we used modified version of the Williamson-Hall plots. Analysis of the XRD profiles allowed us to avoid the overestimation of the threading dislocations density determined by the influence of the correlation between dislocations. Moreover, we successfully separate the contribution of the strain and size broadening based on the analysis of refined Williamson Hall plots and demonstrate the nearly uniform dislocation density depth distribution of the GaN studied films, with no supplementary nucleation or annihilation centers are present inside the epitaxial film using, both grazing incidence and grazing exit configurations of the asymmetric geometry.