We report successful growth of a crystallized AlOx layer on top of AlN/GaN heterostructures by using RF-plasma molecular-beam epitaxy for exploring a new-type oxide/nitride heterostructure system. The insertion of an AlOx, buffer layer, which was formed by following three steps of (i) an Al metal deposition at 150 degrees C, (ii) an oxidation of the Al metal by oxygen plasma irradiation, and (iii) an annealing of the oxidized layer at 800 degrees C, facilitated the formation of a crystalline AlOx, layer on top of the AlN/GaN structures. Surface morphologies observed by atomic force microscope showed that the AlOx, buffer layer was directly formed on the nitride structure and fully covered the AlN layer. The AlOx, top layer grown on the buffer layer had a flat and smooth surface. A cross-sectional transmission electron microscopy micrograph revealed that the AlOx thin film grown at 800 degrees C on the nitride structure was fully crystallized. (C) 2014 Elsevier B.V. All rights reservecl.