Sapphire Surface Polymorphs and The Growth of Pb Overlayers

The surface structure of sapphire ($\alpha $-alumina) is an issue of long-standing interest, both scientifically and technologically. Molecular dynamics simulations showed the $\gamma $-alumina structure to have a lower energy than that of $\alpha $-alumina [1], thus suggesting a possibly modified surface structure when sapphire is treated at high temperatures. We have performed x-ray reflectivity measurements at the Advanced Photon Source to address this issue. Standard sapphire substrates were prepared by furnace annealing at 1600$^{\circ}$ C in air. The resulting surfaces showed large terraces with straight step edges. The substrates were then annealed in a UHV chamber at increasingly higher temperatures. Many new features emerged in the reflectivity curves, which could be attributed to various transition alumina structures, including the $\theta $'-, $\delta $-, and $\theta $- polymorphs [2]. Pb films were grown on these surfaces. The resulting structure and morphology was characterized. This talk will summarize our findings. [1] S. Blonski and S. H. Garofalini, Surf. Sci. \textbf{295}, 263 (1993). [2] I. Levin and D. Brandon, J. Am. Ceram. Soc. \textbf{81}, 1995 (1998).