The hexagonal polar ZnO(0001)-(1 x 1) surfaces: structural features as stemming from X-ray diffraction

The ZnO surfaces, apart from their role in catalytic processes, raise funda mental questions regarding the key parameters, which stabilise ionic materi al surfaces. The ZnO structure consists of hexagonal planes stacked with th e sequence aBbAaB... The polar (0001) surfaces should be unstable because o f the normal dipole moment. However, a long-range (1 x 1) order is obtained after several Ar+-800 degrees C cycles. We investigated, by grazing incide nce X-ray diffraction (XRD), both the O- and Zn-terminated faces, issued fr om a single substrate. In both cases, the coherent domain width was found b etween 180 and 100 Angstrom. This is the average size of the terraces, limi ted by bilayer steps, and thus either a or b terminated. Several intensity rods (along the surface normal) were analysed, attesting that the surface i s close to bulk truncation (no stacking fault). The effects of relaxation o r non-stoichiometry are strongly damped by the contribution of the two type s of terraces, In the Zn case, the atomic displacements are not significant (outward relaxation of +0.05 Angstrom), whereas in the O case, four layers have to be displaced (inward relaxation by -0.3 Angstrom). The fits are gr eatly improved allowing for partial occupancy in the topmost planes: 0.75 f or the external plane in the Zn case, a result which fits well with the ele ctrostatic arguments for the cancellation of the dipole moment, and 1.3, 0. 7, for the two topmost planes on the O surface. Impurity or Zn atoms substi tuting the O atoms could explain these latter values. (C) 2000 Elsevier Sci ence B.V. All rights reserved.