Band offsets between Si and epitaxial rare earth sesquioxides (RE2O3, RE=La,Nd,Gd,Lu): Effect of 4f-shell occupancy

Internal photoemission of electrons and holes into cubic Nd2O3 epitaxially grown on (100)Si reveals a significant contribution of Nd 4f states to the spectrum of the oxide gap states. In contrast to oxides of other rare earth (RE) elements (Gd, Lu) epitaxially grown in the same cubic polymorph, to hexagonal LaLuO3, and to polycrystalline HfO2, the occupied Nd 4f states produce an additional filled band 0.8 eV above the O 2p derived valence band. The unoccupied portion of the Nd 4f shell leads to empty electron states in the energy range of 1 eV below the RE 5d derived oxide conduction band. The exposed Nd 4f states suggest the possibility to use this metal and, possibly, other REs with low f-shell occupancy to control the interface band offsets by selective interface doping.