In this paper, we investigate the influence of postgrowth thermal treatment on the structural quality and surface morphology of crystalline Y2O3 films prepared by molecular beam epitaxy (MBE) on (001) Si substrates. As-grown films are characterized by a relatively thick interfacial layer, which develops during growth. In situ annealing under ultrahigh vacuum results in the desorption of this layer and the formation of very sharp Y2O3/Si interfaces. However, in situ annealing degrades the insulating properties of the films. Ex situ annealing in O-2 or N-2 atmosphere causes the detachment of the film from the Si substrate, due to the development of large compressive strain as a result of the large difference between the thermal expansion coefficients Of Y2O3 and Si. This effect is further studied by atomic force microscopy (AFM) measurements and subsequent surface analysis, which showed the formation of an isotopically organized structure. The effect was absent in amorphous films grown at low temperatures. (C) 2004 Elsevier B.V. All rights reserved.