a Schematic illustration of the SPD with the AlGaN/AlN/GaN heterostructures. b Scanning electron microscopy (SEM) image of the SPD. The inset shows a magnified section containing the AlGaN/AlN/GaN-based HEMT. c Cross-sectional high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the AlGaN/AlN/GaN hetero-stacks in the SPD, and the corresponding energy-dispersive X-ray spectroscopy (EDS) element mapping including Ga, Al, and N. d High-resolution TEM image acquired from the AlGaN/AlN/GaN hetero-stacks. The inset is the corresponding selected area electron diffraction (SAED) of GaN.