“…The amorphous layer may result in a polycrystalline and defected epilayer [15,16], and recent efforts have achieved its elimination mainly by employing Al pre-deposition and a low-temperature AlN buffer layer, resulting in interface of good structural quality [15 -21]. However transmission electron microscopy (TEM) observations on such epilayers have shown that they contain large densities of defects, mainly threading dislocations and inversion domain boundaries (IDBs), as well as stacking faults, pinholes, voids, hillocks, and grain boundaries [10,11,16,17,22,23]. IDBs in particular have been shown to emanate from substrate surface steps as well as from the AlN/GaN interface [22,23].…”