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Al0.30Ga0.70N/GaN heterostructures grown on sapphire substrates by rf-plasma-assisted molecular beam epitaxy are investigated. The heterostructures consist of a 20 nm Al0.30Ga0.70N barrier layer deposited on a 2 μm semi-insulating GaN epilayer. Room-temperature mobilities averaging 1350 cm2/V s and a sheet charge density of 1.1×1013 cm-2 are consistently achieved. Central to our approach is the iron-doped semi-insulating GaN epilayer and a pulsed N/Ga atomic deposition technique which makes it possible to simultaneously reduce threading dislocations and achieve a smooth surface. The heterostructures were examined by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. Structural characterizations reveal a smooth surface morphology, coherent and sharp interfaces, and a low density of the threading dislocations. [ABSTRACT FROM AUTHOR]