We report the role of a thin AlN buffer layer introduced between AlxGa1-xN and GaN grown on sapphire (0 0 0 1) by MOVPE. The heterostructures have high x values ranging from 0.35 to 0.50. After growing the 20 nm thick AlN buffer layer on GaN/sapphire (0 0 0 1) epitaxy, the 1.0 mum thick AlxGa1-xN epitaxial layers were grown by increasing the flow rate of TMA. The measured Al mole fractions of AlxGa1-xN/GaN heteroepitaxy grown using the thin AlN buffer from each TCD rocking curve are 0.35, 0.37, 0.45 and 0.50, respectively. As the incorporation rate of Al in the AlxGa1-xN increases, the crystallinity is improving and the RMS values scanned by AFM of their surfaces become lower. The optoelectronic characteristics of these heteroepitaxial layers were evaluated by cathodoluminescence and found to improve with the increase of x. To the contrary these trends of improvement, it is usually known that the properties such as crystallinity, surface morphology and optical properties of the AlxGa1-xN/GaN heteroepitaxial layers grown without AlN buffer layer become worse with the increase of x above 0.2. The electrical resistivities of Al0.35Ga0.65N/GaN, Al0.37Ga0.63N/GaN, Al0.45Ga0.55N/GaN and Al0.5Ga0.5N/GaN heterostructure, which were grown with the thin AlN buffer, measured by four point probing method are 13.5, 18.1, 31.7 and 36.2 MQ, respectively. The resisitivity increases with the raising of x. It may be caused by the increment of the intrinsic own-resistance. (C) 2002 Elsevier Science B.V. All rights reserved.