The termination of the α-Al2O3 (0001) surface:: a LEED crystallography determination

We have performed a tensor low-energy electron diffraction (TLEED) determination of 10 diffracted beams at off-normal incidence from the (0001) surface of alpha-Al2O3 produced by oxygen plasma cleaning/annealing cycles. Structures with single domain O- or Al-terminated surface layers give a poor agreement (Pendry R-factor=0.42). Such domains are related by steps in the (11(2) over bar 0) direction that produce an extra plane of symmetry in the LEED pattern. Allowing a mixture of domains due to steps produces a good agreement. An optimized mixture of 2:1 O/Al-terminated domains lowers the R-factor to 0.26 with an inner potential of 19.2 eV. At the R-factor minimum, we find that in both domains, the outermost atomic layer relaxes outward, with some twisting and relief of buckling in deeper layers. An outward relaxation of the top layer is in disagreement with theoretical predictions. However, such predictions are for vacuum-cleaved surfaces that are not realistic descriptions of the surface used in our experiment. (C) 1998 Elsevier Science B.V. All rights reserved.