Structure of the a-Al2O3„0001... surface from low-energy electron diffraction: Al termination and evidence for anomalously large thermal vibrations

We use dynamical low-energy electron diffraction ~LEED! to determine the surface structure of a-Al2O3(0001). Sapphire surfaces are prepared in three different ways, and the diffraction results are analyzed using an exhaustive search of possible models. For all sample processing conditions, the clearly favored structure has a single Al layer termination and a large first interlayer contraction. In addition, we find that the aluminum atoms at the surface have unusually large vibrational amplitudes at room temperature, suggestive of an anharmonic vibrational mode.