AVS 60th International Symposium and Exhibition
    Spectroscopic Ellipsometry Focus Topic Thursday Sessions
       Session EL-ThP

Paper EL-ThP3
Properties of Sm Doped CeO2 Thin Films Prepared by Liquid Solution Deposition

Thursday, October 31, 2013, 6:00 pm, Room Hall B

Session: Spectroscopic Ellipsometry Poster Session
Presenter: S. Zollner, New Mexico State University
Authors: K.N. Mitchell, New Mexico State University
C.A. Rodriguez, New Mexico State University
T. Willett-Gies, New Mexico State University
Y. Li, New Mexico State University
S. Zollner, New Mexico State University
Correspondent: Click to Email
Cerium(IV) oxide, also known as CeO2 or ceria, is a transparent (insulating) oxide of the rare earth metal cerium. It is an ionic conductor with applications in fuel cells, as a catalyst, or for photovoltaic water splitting (hydrogen production). Thin films of ceria produced by RF magnetron sputtering on sapphire at 770C have been studied extensively by Arwin's group (S. Guo et al., J. Appl. Phys. 77, 5369, 1995). They found changes in grain size, surface morphology (visible in AFM images), and optical constants varying with the film thickness. By contrast, we report analysis results for relatively thick (300-500 nm) ceria films prepared by liquid solution deposition (dip-coating) followed by annealing. We also investigate the effect of samarium doping (up to 20at.%) of ceria. The rare earth metal samarium usually forms a sesquioxide Sm2O3. Therefore, doping ceria with Sm is expected to lead to the formation of oxygen vacancies, which enhances the ionic conductivity of ceria. Our ellipsometry spectra (ellipsometric angles and depolarization) can be described very well in the transparent region (below 3 eV) using a Tauc-Lorentz dispersion model for ceria, if small amounts of surface roughness and thickness non-uniformity across the wafer are taken into account. Once these thickness parameters have been determined for our films, we obtain the optical constants of CeO2:Sm using a basis spline expansion. We find the typical dispersion expected for an insulator with a direct band gap near 3.7 eV. Samarium doping causes a significant decrease of the refractive index in the transparent region. Most likely, the films with high Sm content are less dense (have more voids, perhaps due a smaller crystallite size) than pure ceria films. An increase in disorder due to Sm doping was also found in x-ray diffraction studies of electrodeposited ceria films (Phok and Bhattacharya, phys. status solidi (a) 203, 3734, 2006). As expected from Kramers-Kronig consistency, we find a significant reduction of the height of the main absorption peak at 4 eV. The direct band gap, however, remains at 3.7 eV, independent of Sm content. There is, however, a significant decrease in the slope of the onset of absorption with increasing Sm content. In addition to ellipsometry results, we will also report AFM, XRD, Raman, and (perhaps) FTIR ellipsometry results for our Sm-doped ceria films.