Electronic structure of a laterally graded ZrO_2-TiO_2 film on Si(100) prepared by metal-organic chemical vapor deposition in ultrahigh vacuum

摘要

A TiO_2-ZrO_2 film with laterally graded stoichiometry has been prepared by metal-organic chemical vapor deposition in ultrahigh vacuum. The film was characterized in situ using synchrotron radiation photoelectron spectroscopy (PES) and x-ray absorption spectroscopy. PES depth profiling clearly shows that Ti ions segregate toward the surface region when mixed with ZrO_2. The binding energy of the ZrO_2 electronic levels is constant with respect to the local vacuum level. The binding energy of the TiO_2 electronic levels is aligned to the Fermi level down to a Ti/Zr ratio of about 0.5. At a Ti/Zr ratio between 0.1 and 0.5, the TiO_2 related electronic levels become aligned to the local vacuum level. The addition of small amounts of TiO_2 to ZrO_2 results in a ZrO_2 band alignment relative to the Fermi level that is less asymmetric than for pure ZrO_2. The band edge positions shift by -0.6 eV for a Ti/Zr ratio of 0.03. This is explained in terms of an increase in the work function when adding TiO_2, an effect that becomes emphasized by Ti surface segregation.