Recently,the epitaxial growth of rare earth (RE) metal oxide films,such as Y2O3,Gd2O3 and Pr2O3 on Si,has gained attention from Si based industry.It has been reported that Er2O3 may be one of the most promising altemative dielectrics.However,the origin and the evolvement of interface between Er2O3 and Si substrates is not clear.Synchrotron Radiation Photoelectron Spectroscopy (SRPES) is a most powerful tool in the study of the structural and chemical information of the surface and interface of a material because of its highly sensitive for surfaces.In this work,in situ Synchrotron Radiation Photoelectron Spectroscopy (SRPES) is used to study the initial growth of Er2O3 films on Si(001) substrates at an O2 pressure of 2×10-1 Torr.