Tm2O3 crystalline films have been deposited on Si (0 0 1) by molecular beam epitaxy (MBE). Band alignments of Tm2O3/Si gate stacks were studied by X-ray photoelectron spectroscopy (XPS). According to XPS measurements, it can be noted that a valence-band offset of -3.1 +/- 0.1 eV and a conduction-band offset of 2.3 +/- 0.3 eV for the Tm2O3/Si heterojunction have been obtained. Based on analysis from O 1s energy-loss spectrum, the energy gap of Tm2O3 is determined to be 6.5 +/- 0.3 eV. A relatively thicker interfacial SiOx layer was observed for the as-annealed samples. However, no apparent change in band alignment has been observed for Tm2O3/Si heterojunction with the formation of interface layer, which has been discussed in detail. (C) 2012 Elsevier B. V. All rights reserved.