Logo: to the web site of Uppsala University

The properties of a sol-gel prepared Mn-modified TiO₂ film have been studied with X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The chemical composition and oxidation state of the elements have been determined. The manganese content estimated by PES of the Mn-modified film is about 10% and both Mn²⁺ and Mn³⁺ are observed. Addition of Mn is found to modify the valence band edge. The Mn 3d states are found to extend about 1 eV into the TiO ₂ band gap region. It is demonstrated that lithium insertion into the sol-gel film can be performed in a stepwise fashion in situ under ultrahigh vacuum (UHV) conditions. Lithium is distributed evenly throughout the entire film and leads to reduction of Mn³⁺ to Mn²⁺ followed by reduction of Ti⁴⁺ to Ti³⁺. The XAS and PES measurements give fully consistent results regarding the amount of inserted lithium.

The growth of ultrathin ZrO₂ films on Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) has been studied with core level photoelectron spectroscopy and X-ray absorption spectroscopy. The films were deposited sequentially by chemical vapor deposition in ultra-high vacuum using zirconium tetra-tert-butoxide as precursor. Deposition of a > 50 Å thick film leads in both cases to tetragonal ZrO₂ (t-ZrO₂), whereas significant differences are found for thinner films. On Si(1 1 1)-(7 × 7) the local structure of t-ZrO₂ is not observed until a film thickness of 51 Å is reached. On Si(1 0 0)-(2 × 1) the local geometric structure of t-ZrO₂ is formed already at a film thickness of 11 Å. The higher tendency for the formation of t-ZrO₂ on Si(1 0 0) is discussed in terms of Zr–O valence electron matching to the number of dangling bonds per surface Si atom. The Zr–O hybridization within the ZrO₂ unit depends furthermore on the chemical composition of the surrounding. The precursor t-butoxy ligands undergo efficient C–O scission on Si(1 0 0), leaving carbonaceous fragments embedded in the interfacial layer. In contrast, after small deposits on Si(1 1 1) stable t-butoxy groups are found. These are consumed upon further deposition. Stable methyl and, possibly, also hydroxyl groups are found on both surfaces within a wide film thickness range.

We present measurements of the Zr and Si core level photoelectron binding energies relative to the Fermi level and the vacuum level under a ZrO₂ growth series on Si(100). It is shown that the Zr core level binding energy is most properly referenced to the local vacuum level already from the monolayer regime. This confirms the insulating properties of ZrO₂. The Si core levels are referenced to the Fermi level and undergo shifts consistent with the disappearance of the mid-band-gap states originating from the (2×1) reconstruction on the clean Si(100) surface. The use of O 1s x-ray absorption spectroscopy (XAS) to determine the location of the conduction band edge of ZrO₂ is discussed with the aid of ab initio calculations. It is demonstrated that the conduction band edge is located at the XAS peak position and that the position relative to the valence band can be determined by aligning the O 1s XAS spectrum to the O 1s photoelectron spectrum. The study thus establishes that photoelectron spectroscopy in conjunction with x-ray absorption spectroscopy forms a most powerful tool for studies of the band alignment at metal oxide-silicon interfaces.