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 $0.8eV$ 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 $1eV$ 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.