Cubic phase Y2O3 films 1.6-10 nm thick of excellent quality have been epitaxially grown on Si (111) with Y2O3(111)parallel to Si(111) using electron beam evaporation of Y2O3 in ultrahigh vacuum. Structural and morphological studies were carried out by x-ray scattering and reflectivity and high-resolution transmission electron microscopy, with the growth being in situ monitored by reflection high energy electron diffraction. There are two Y2O3 domains in the initial stage of the oxide growth with equal population, and the B-type domain of Y2O3[2 (1) over bar(1) over bar]parallel to Si[11 (2) over bar] becomes predominating over the A-type domain of Y2O3[2 (1) over bar(1) over bar]parallel to Si[2 (1) over bar(1) over bar] with increasing film thickness. Besides the excellent crystallinity of the films as derived from the small omega-rocking curve width of 0.014 degrees, our results also show atomically sharp smooth surface and interfaces. (c) 2008 American Institute of Physics.