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

High Resolution Image Download MS PowerPoint Slide The interfacial energetics between p-type InP and a series of metal oxides, including TiO 2, Nb 2 O 5, Ta 2 O 5, and HfO 2, were evaluated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and optical absorption spectroscopy. The energy of the conduction band minimum ( E cb ) of TiO 2 and Nb 2 O 5 was more negative (i.e., further from the vacuum level) than the conduction band minimum at the surface of InP ( E cb,s,InP ), whereas E cb for Ta 2 O 5 and HfO 2 was more positive than E cb,s,InP . The data are consistent with the electrochemical behavior of p-InP coated with various metal oxide candidate protection layers, with TiO 2 and Nb 2 O 5 facilitating interfacial transfer of photogenerated minority-carrier electrons in p-InP photocathodes, and Ta 2 O 5 and HfO 2 blocking photogenerated electrons in p-InP from readily transferring across the oxide-coated photocathodes. The energy of the valence band maximum ( E vb ) for all of the oxides was much more negative than E 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.