Band offsets in transition-metal oxide heterostructures

We measured valence band offsets in Ta2O5-WO3, Ta2O5-Nb2O5 and WO3-Nb2O5 heterostructure couples by in situ x-ray photoelectron spectroscopy, immediately following the bi-layer growth in ultra-high vacuum. Conduction band offsets were estimated using the measured valence band offsets in conjunction with the literature values for the respective band gaps. The offsets between Ta2O5 and WO3 and between Ta2O5 and Nb2O5 layers were strongly asymmetric, with 0.8-1.1 eV (0.1-0.2 eV) barriers for the conduction (valence) bands, depending on the particular couple and the stacking sequence. Such asymmetry can be very useful in switching devices.