Invited Paper SS1-ThM7
Complexities and Surprises in the Epitaxial Growth of Pure and Doped TiO₂

Thursday, October 18, 2007, 10:00 am, Room 608

TiO₂ is a transition metal oxide of considerable interest in several areas of surface, interface and thin-film science. Recent cation doping film growth studies show that certain transition metal dopants with unpaired d electrons, in concert with structural defects, impart a new form of high-temperature ferromagnetism in which dopant spins are aligned by electrons associated with the defects.^1-3 Anion (principally N) doping film growth studies show that substitutional N gives rise to a substantial red shift in the bandgap, paving the way for enhanced visible solar light absorption and the associated photophysical and photochemical energy conversion processes. However, N, which should be an acceptor in TiO₂, is actually fully compensated by conduction band electrons from interstitial Ti(III), which in turn results from Ti indiffusion during growth.^4,5 These insights were gained by our ability to grow and painstakingly characterize very well-defined, high-quality epitaxial films of pure and doped TiO₂. In the process of growing these materials with an unprecedented level of control, we have learned a great deal about the nucleation and growth of the different polymorphs of TiO₂.⁶ The roles of atomic fluxes, growth temperature, dopants, overall growth rate and substrate structural properties have been elucidated one by one, and these results constitute a rich source of insight into the way transition metal oxide films nucleate and grow. In this talk, I will give an overview of our work on pure and doped rutile and anatase homoepitaxy and heteroepitaxy. This work has been supported by the US DOE, Office of Science, Division of Materials Science and Engineering, and Division of Chemical Sciences.

¹T. C. Kaspar et al., Phys. Rev. Lett. 95, 217203 (2005).
²T. C. Kaspar et al., J. Vac. Sci. & Technol. B 24, 2012 (2006).
³T. C. Kaspar et al. Phys. Rev. B 73, 155327 (2006).
⁴S. H. Cheung et al., Surf. Sci. 601, 1754 (2007).
⁵S. A. Chambers et al., Chem. Phys., to appear (2007).
⁶R. Shao et al., Surf. Sci. 601, 1582 (2007).