AVS 46th International Symposium
    Thin Films Division Thursday Sessions
       Session TF-ThA

Paper TF-ThA3
Near Surface Chemical Dependence of Electronic States at Al-Doped TiO@sub 2@(110)Ultrathin Films

Thursday, October 28, 1999, 2:40 pm, Room 615

Session: Ex-situ Characterization
Presenter: S.H. Goss, Ohio State University
Authors: S.H. Goss, Ohio State University
L. Brillson, Ohio State University
S.A. Chambers, Pacific Northwest National Laboratory
Correspondent: Click to Email
Impurity doping of TiO2 has important catalytic and photocatalytic applications.@footnote 1@ Al doping is known to enhance TiO2's chemical properties and is used extensively in surface coatings. We have used electron excited nanoscale luminescence spectroscopy (EENLS) to observe the dependence of electronic states with chemical composition of Al-doped TiO2 ultrathin films. Using incident electron beams of varying energy to probe depths from <5 to >150 nm below the free surface, we observe: mid-gap state emission at 1.4 eV due to Al doping, O vacancy emission at 2.5 eV, and near band edge (NBE) transitions at 3.0 eV. The 1.4 eV emission appears specifically within a 20 nm, 6% Al - doped TiO2 layer stacked on 4% and 2% doped layers, all grown epitaxially on TiO2 substrates. No 1.4 eV emission is evident for these deeper layers. Recombination involving this 1.4 eV level increases dramatically with annealing at 600 C under 5-x 10e7 L O2 treatment, while the 2.5 eV peak decreases. As mid-gap recombination increases, NBE emissions decrease strongly, indicating a pronounced decrease in free carrier concentration near the free surface. Auger electron spectroscopy (AES) shows Ti and O in correct proportion and only C contamination at the free surface. Mid-gap emission intensities show no correlation with surface C concentration which range from <5 to < 20 %. The O vacancy peak intensity at 2.5 eV is affected only slightly by oxygen inter-diffusion. These results demonstrate the capability of EENLS to probe ultrathin wide band gap oxide films with nanometer scale depth resolution, identifying the role of Al as a mid-gap state, and the effect of O indiffusion to further activate Al recombination as an efficient recombination center. Furthermore, the dramatic changes in free carrier concentration with oxidation and deep level activation suggest a role of recombination centers in controlling surface catalytic properties. @FootnoteText@ 1.S.A.Chambers et al., Surface Sci. 365, 625 (1996).