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- Karlsson, P. G., et al.
(author)
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Metal organic chemical vapor deposition of ultrathin ZrO2 films on Si(1 0 0) and Si(1 1 1) studied by electron spectroscopy
- 2007
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In: Surface Science. - 0039-6028 .- 1879-2758. ; 601:4, s. 1008-1018
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Journal article (peer-reviewed)abstract
- The growth of ultrathin ZrO2 films on Si(100)-(2x1) and Si(111)-(7x7) 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 > 50Å thick film leads in both cases to tetragonal ZrO2 (t-ZrO2), whereas significant differences are found for thinner films. On Si(111)-(7x7) the local structure of t-ZrO2 is not observed until a film thickness of 51Å is reached. On Si(100)-(2x1) the local geometric structure of t-ZrO2 is formed already at a film thickness of 11Å. The higher tendency for the formation of t-ZrO2 on Si(100) 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(100), leaving carbonaceous fragments embedded in the interfacial layer. In contrast, after small deposits on Si(111) 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.
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- Karlsson, P.G., et al.
(author)
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Ultrathin ZrO2 films on Si-rich SiC(0 0 0 1)-(3 × 3) : Growth and thermal stability
- 2007
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In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 601:11, s. 2390-2400
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Journal article (peer-reviewed)abstract
- The growth and thermal stability of ultrathin ZrO2 films on the Si-rich SiC(0 0 0 1)-(3 × 3) surface have been explored using photoelectron spectroscopy (PES) and X-ray absorption spectroscopy (XAS). The films were grown in situ by chemical vapor deposition using the zirconium tetra tert-butoxide (ZTB) precursor. The O 1s XAS results show that growth at 400 °C yields tetragonal ZrO2. An interface is formed between the ZrO2 film and the SiC substrate. The interface contains Si in several chemically different states. This gives evidence for an interface that is much more complex than that formed upon oxidation with O2. Si in a 4+ oxidation state is detected in the near surface region. This shows that intermixing of SiO2 and ZrO2 occurs, possibly under the formation of silicate. The alignment of the ZrO2 and SiC band edges is discussed based on core level and valence PES spectra. Subsequent annealing of a deposited film was performed in order to study the thermal stability of the system. Annealing to 800 °C does not lead to decomposition of the tetragonal ZrO2 (t-ZrO2) but changes are observed within the interface region. After annealing to 1000 °C a laterally heterogeneous layer has formed. The decomposition of the film leads to regions with t-ZrO2 remnants, metallic Zr silicide and Si aggregates. © 2007 Elsevier B.V. All rights reserved.
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- Richter, J H, et al.
(author)
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Electronic structure of lithium-doped anatase TiO2 prepared in ultrahigh vacuum
- 2005
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 71:23, s. 1-235418
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Journal article (peer-reviewed)abstract
- Insertion of lithium in anatase TiO2, giving LixTiO2, is performed under ultrahigh vacuum (UHV) conditions and studied using synchrotron radiation based electron spectroscopy. Core level photoemission spectra are directly compared to results obtained after electrochemical insertion, illustrating the usefulness of the UHV approach. The growth of a state of mainly Ti 3d character in the band gap is monitored and the amount of charge transferred from Li to the band gap state is quantified. The result that the Ti 3d level is occupied by 0.85 +/- 0.10 electronic charge is in good agreement with theoretical predictions. Binding energy shifts of the core levels suggest that the population of the Ti 3d states does not follow a simple rigid band behavior. It is concluded that the formation of the Li-poor phase (x < 2%) is associated with pinning of the Fermi level to the bottom of the conduction band. The Li-poor phase can therefore be envisaged as related to defects. Changes in the valence photoemission spectrum and O 1s x-ray absorption spectrum are interpreted in terms of a decreased O 2p-Ti 3d interaction upon Li insertion. Shifts in the sample work function are finally found to agree reasonably well with the measured cell voltage for electrochemical Li insertion into a nanoporous anatase film.
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