AVS 54th International Symposium | |
Surface Science | Thursday Sessions |
Session SS1-ThM |
Session: | Oxide Surface Structure II |
Presenter: | E.H. Morales, Tulane University |
Authors: | E.H. Morales, Tulane University M. Batzill, University of South Florida U. Diebold, Tulane University |
Correspondent: | Click to Email |
In2O3 and Sn-doped In2O3 (Indium-Tin Oxide, ITO) exhibit optical transparency combined with low electrical resistivity, and find application in flat panel displays and solar cells. Relatively little is known about their atomic-scale surface properties, mainly because of challenges in preparing single crystal samples. We have grown epitaxial In2O3 and ITO films on Yttrium Stabilized Zirconia. The (100) surface has polar character, and the (111) orientation is non-polar. The films were prepared using oxygen-plasma assisted electron beam epitaxy in ultra high vacuum (UHV) conditions. The growth was monitored with Reflection High Energy Electron Diffraction (RHEED). Samples were characterized with X-ray Photoemission Spectroscopy (XPS) and Angle Resolved XPS (ARXPS) using Al-Ka radiation and Low Energy Electron Diffraction (LEED) in-situ, as well as synchrotron-based Ultra-Violet Photoemission Spectroscopy (UPS) . The films were stoichiometric, except for ITO(100), where ARXPS indicates Sn segregation. In2O3 (100) shows faceting in LEED, while ITO(100) stays flat with a 1x1 surface termination. Thus, it appears that Sn-segregation to the surface stabilizes the polar In2O3 (100). In2O3 (111) exhibits a (√3x√3)R30° reconstruction in LEED. Up to 9 at% Sn in ITO (111) does not seem to alter this reconstruction. Resonant photoemission measurements indicate a Sn-derived band gap state with resonance at 30 eV photon energy on the ITO (100) film; this gap state is far less pronounced on ITO (111). Interestingly, the valence band maximum is located at 2.5 and 2.7 eV below the (surface) Fermi level for ITO (100) and (111), respectively. This is ca. 1 eV higher than expected for a heavily n-type doped material with a direct optical band gap of 3.7 eV. Reasons for this apparent discrepancy will be discussed.