The ZnO non-polar (100) surface: an X-ray structural investigation

We performed a structural analysis of the non-polar ZnO (100) surface by means of grazing incidence X-ray diffraction. The analysis was conducted on ten rods, smooth surface domains, though of small coherent width, having been obtained after several Ar⁺ sputtering–annealing cycles. The surface derived from the bulk structure exposes one ZnO dimer per unit cell, parallel to the [001] axis. All the existing models, derived from ab initio calculations or low-energy electron diffraction (LEED) analyses, consist in a surface dimer whose O and Zn atoms are shifted inwards, with the O pointing 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 with a moderate dimer rotation. The latter, however, is found between 2.3 and 11.4°. Despite the discrepancies between the models, the Zn atom is always found shifted downwards by more than 0.25 Å. This is unambiguously rejected by our data, which show that the Zn atom keeps very close to its bulk position. It is displaced downwards by ΔZ_Zn=−0.06±0.02 Å, and it moves along [001] towards O by ΔX_Zn=0.05±0.02 Å. 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 choice of position for Zn in the error bar range, the buckling is evaluated as between −6.5 and 3°, or between −4 and 0.5°. The dimer distance is evaluated equal to 1.90 Å, with a deviation equal either to 0.06 or 0.11 Å.