The ZnO non-polar (1010) surface: an X-ray structural investigation

We performed a structural analysis of the non-polar ZnO (10 (1) over bar 0) surface by means of grazing incidence X-ray diffraction. The analysis was conducted on ten rods, smooth surface domains, though of small coherent wid th, having been obtained after several Ar+ sputtering-annealing cycles. The surface derived from the bulk structure exposes one ZnO dimer per unit cel l, parallel to the [001] axis. All the existing models, derived from ab ini tio calculations or low-energy electron diffraction (LEED) analyses, consis t in a surface dimer whose O and Zn atoms are shifted inwards, with the O p ointing outwards with respect to Zn. Whereas the LEED studies conclude on a dimer distance greater than in the bulk, the theoretical studies agree on a dimer contracted by amounts ranging from 5.5 to 7.5%. This contraction is interpreted as a result of the strong ionicity of ZnO, and is associated w ith a moderate dimer rotation. The latter, however, is found between 2.3 an d 11.4 degrees Despite the discrepancies between the models, the Zn atom is always found shifted downwards by more than 0.25 Angstrom. This is unambig uously rejected by our data, which show that the Zn atom keeps very close t o its bulk position. It is displaced downwards by Delta Z(zn) = -0.06+/-0.0 2 Angstrom, and it moves along [001] towards O by Delta X-zn =0.05 +/- 0.02 Angstrom. We denote a trend for the O atom to be displaced downwards too, with a concomitant displacement towards Zn. The faint X-ray scattering of O prevents us from assessing its position with accuracy. Depending on the ch oice of position for Zn in the error bar range, the buckling is evaluated a s between -6.5 and 3 degrees, or between -4 and 0.5 degrees. The dimer dist ance is evaluated equal to 1.90 Angstrom, with a deviation equal either to 0.06 or 0.11 Angstrom. (C) 2000 Elsevier Science B.V. All rights reserved.