Single crystalline epitaxial Si(111)/Y2O3(111)/Si(111) heterostructures were grown by molecular beam epitaxy and the morphology, structure, and defects were characterized in detail. The growth of a closed and smooth layer-system is demonstrated by means of reflection high energy electron diffraction measurements. X-ray reflectometry and high resolution Rutherford backscattering (RBS) experiments show low surface and interface roughnesses. Channeling RBS as well as x-ray diffraction pole figure studies demonstrate the type A/B/A epitaxy relationship of the Si(111)/Y2O3(111)/Si(111) heterostructure and reveal the existence of defects in the epitaxial Si(111) layer. These defects are studied in detail with high resolution transmission electron microscopy, disclosing microtwin formation and type B Si grains as the major defects. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3043540]