4D-STEM at interfaces to GaN: Centre-of-mass approach & NBED-disc detection

Highlights: • Experiment and simulation of 4D-STEM at material interfaces for the determination of internal electric fields. • Two methods: The centre-of-mass (COM) approach with high beam convergence & disc detection using nano-beam electron diffraction (NBED). • Strong effects directly at GaN/AlN and GaN/vacuum interfaces are observed for COM and disc detection. • The determined polarization-induced electric field in a GaN/AlN superlattice is reduced with increasing specimen thickness. 4D-scanning transmission electron microscopy (4D-STEM) can be used to measure electric fields such as atomic fields or polarization-induced electric fields in crystal heterostructures. The paper focuses on effects occurring in 4D-STEM at interfaces, where two model systems are used: an AlN/GaN nanowire superlattice as well as a GaN/vacuum interface. Two different methods are applied: First, we employ the centre-of mass (COM) technique which uses the average momentum transfer evaluated from the intensity distribution in the diffraction pattern. Second, we measure the shift of the undiffracted disc (disc-detection method) in nano-beam electron diffraction (NBED). Both methods are applied to experimental and simulated 4D-STEM data sets. We find for both techniques distinct variations in the momentum transfer at interfaces between materials: In both model systems, peaks occur at the interfaces and we investigate possible sources and routes of interpretation. In case of the AlN/GaN superlattice, the COM and disc-detection methods are used to measure internal polarization-induced electric fields and we observed a reduction of the measured fields with increasing specimen thickness.