AlN thin films were grown on Al2O3 (0001) by MBE using NH3-clusters ionized with the energy of 4-7 eV/molecule at 960 degrees C. The a-axis is shrunken and the c-axis is extremely elongated at the initial growth stage (up to the film thickness of about 20 nm). The films thicker than 20 nm have the relaxed a and c-axis lengths close to the unstrained values, which obey the Poisson relation. (C) 2016 Elsevier B.V. All rights reserved.