An advanced Bragg diffraction imaging technique to characterize defects: The examples of GaN and AlN

Abstract

An advanced X-ray Bragg diffraction imaging technique known as Rocking Curve Imaging (RCI) was implemented and developed at the European Synchrotron Radiation Facility (ESRF), where it complements the simpler and faster, but less accurate, “white beam topography” technique. These techniques allow the observation and characterization of defects in bulk crystals, as well as in crystalline layers with a thickness in the μm range. We recall the basics of X-ray Bragg diffraction imaging (historically called X-ray topography) and describe the technical aspects of RCI, including the approximations we often use to analyse the images, and the information we can extract from the various produced maps (Integrated Intensity, Full Width Half Maximum and Peak Position). We show, as examples, results of its application to aluminium nitride (AlN) and gallium nitride (GaN) crystals that are used as substrates for microelectronic devices. More in particular we describe the images of individual dislocations in crystals of both materials, and the particular arrangement of threading dislocations in ammonothermally grown GaN that lead to subgrain boundaries and hexagonal shaped (“honeycomb”) defects. These examples show that the RCI Bragg diffraction imaging technique allows obtaining unique results, in the sub-μm spatial resolution and the μradian range angular resolution ranges, when the characterization of high-quality crystals is required.