Crystalline properties of (1-2)-mu m-thick AlN buffer layers grown by plasma-assisted molecular-beam epitaxy (PA MBE) on c-Al2O3 substrates with different AlN nucleation layers have been studied. The best quality layers are obtained on 50-nm-thick nucleation AlN layers grown by a migration enhanced epitaxy (MEE) at substrate temperature of 780 degrees C. In this case the buffer layers possess the lowest FWHM values of the symmetric AlN(0002) and skew symmetric AlN(10-15) x-ray rocking curve peaks of 469 and 1025 arcsec, respectively, which correspond to the screw and edge threading dislocation densities of 4.7 x 10(8) cm-(2) and 5.9 x 10(9) cm-2. This improvement seems to be related with the larger diameter of the flat-top grains in the AlN nucleation layers grown in the MEE mode at high substrate temperatures. (c) 2013 Elsevier B.V. All rights reserved.