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 (Ecb) of TiO2 and Nb2O5 was more negative (i.e., further from the vacuum level) than the conduction band minimum at the surface of InP (Ecb,s,InP), whereas Ecb for Ta2O5 and HfO2 was more positive than Ecb,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 (Evb) for all of the oxides was much more negative than Evb,s,InP, consistent with observations that these protection layers effectively block hole transport and consequently suppress oxidative degradation of the underlying p-InP photocathodes.
Covelli et al. (Mon,) studied this question.