Photoelectron Spectroscopic Determination of the Interfacial Energetics of Metal Oxide Protection Layers on p-InP Photocathodes

The interfacial energetics between p-type InP and a series of metal oxides, including TiO₂, Nb₂O₅, Ta₂O₅, and HfO₂, were evaluated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and optical absorption spectroscopy. The energy of the conduction band minimum (<b>E</b> _cb) of TiO₂ and Nb₂O₅ was more negative (i.e., further from the vacuum level) than the conduction band minimum at the surface of InP (<b>E</b> _cb,s,InP), whereas <b>E</b> _cb for Ta₂O₅ and HfO₂ was more positive than <b>E</b> _cb,s,InP. The data are consistent with the electrochemical behavior of p-InP coated with various metal oxide candidate protection layers, with TiO₂ and Nb₂O₅ facilitating interfacial transfer of photogenerated minority-carrier electrons in p-InP photocathodes, and Ta₂O₅ and HfO₂ blocking photogenerated electrons in p-InP from readily transferring across the oxide-coated photocathodes. The energy of the valence band maximum (<b>E</b> _vb) for all of the oxides was much more negative than <b>E</b> _vb,s,InP, consistent with observations that these protection layers effectively block hole transport and consequently suppress oxidative degradation of the underlying p-InP photocathodes.