Ultrathin single crystalline Tm2O3 films have been grown on Si (0 0 1) substrate by molecular beam epitaxy using metallic Tm source at a substrate temperature of 600 degrees C and an atomic oxygen ambient pressure of 2 x 10(-7) Torr. The epitaxial relationship between the Tm2O3 films and the Si substrates is Tm2O3 (1 1 0)//Si (0 0 1), Tm2O3 [0 0 1]//Si[110] or Tm2O3[1 -1 0]//Si[110]. Higher oxygen pressure would change the preferential growth orientations from (1 1 0) to (1 1 1) with the growth mode from epitaxy to non-epitaxy. After annealing in O-2 ambience at 450 degrees C for 30 min, the single crystalline films exhibit a dielectric constant of 10.8 and a leakage current density of 2 x 10(-3) A/cm(2) at an electric field of 1 MV cm(-1) with an equivalent oxide thickness of 2.3 nm. Small angle X-ray reflectivity measurements were carried out to investigate the annealing effect in the improvement of electrical properties of the films. (C) 2011 Elsevier B.V. All rights reserved.