The effect of wafer curvature on x-ray rocking curves from gallium nitride films

The full width at half maximum (FWHM) of x-ray rocking curves is often used as a measure of the crystalline quality of thin films. In this paper, the effects of wafer curvature on the x-ray rocking curves (ω-scans) obtained from a 2in. 0001-oriented sapphire wafer and an epitaxial 0001-oriented GaN film grown on such a wafer are discussed. Beam height reduction can limit curvature-related effects for symmetric and asymmetric reflections; these effects are generally significant for low dislocation density GaN films. Due to the shape of the area illuminated by the x-ray beam in the skew symmetric geometry, beam width reduction is instead appropriate, but this still gives an underestimate of ω-FWHM values obtained with an open detector and an overestimate of ω-FWHM values obtained with an analyzer. Therefore, skew symmetric ω-FWHM values from curved samples are often unreliable. Additionally, Williamson–Hall analysis using ω-scans is rendered unreliable in the presence of significant curvature. Variations in ω-FWHM values for different experimental configurations are attributed to nonspherical wafer curvature, changes in illuminated area for each reflection, and changes in sampled volume at each step in the ω-scan. Although this paper focuses on GaN films, the results are applicable to a wide range of other materials exhibiting wafer curvature.