The polar (so called Tasker type III) surfaces of ionic crystals, especially oxides, are intrinsically unstable in their bulk configuration. The mechanisms that stabilize single crystal surfaces by canceling the macroscopic dipole moment are widely studied and very peculiar for individual materials. In the case of the ZnO (0001) and (000-1) orientations ordered defects and terracing lead to rough surfaces [1], unfavorable for application in model catalysts and spintronic devices.
Theoretical studies (e.g., see [2]) suggest that the stabilizing mechanism of the polar ZnO surfaces is considerably different in the limit of few monolayer thick films. The structural rearrangement of Zn- and O-atoms from the polar Wurtzite (bulk) structure to the non-polar Boron-Nitride (BN) structure may remove the dipole moment without surface defects and roughening. While predictions are based on free standing films, we investigated the detailed film structure and morphology of ZnO films grown of Ag(111) by surface X-ray diffraction (SXRD) and scanning tunneling microscopy (STM).
ZnO films, 0.3 to 5 monolayers (ML) in thickness, were grown by pulsed laser deposition in 10-7 mbar oxygen atmosphere on Ag(111) at 300K, followed by annealing at 680K. SXRD indicates that ZnO grows with its basal (0001) plane, and parallel alignment of the hexagonal lattice vectors, on the Ag(111) surface. Related to a low interaction with the Ag(111) surface the ZnO films can relax in-plane, resulting in a 7/8 coincidence with the substrate (Ag: 2.89Å). Compared to bulk ZnO, SXRD shows an in-plane expansion of the ZnO lattice by 1.6% (3.30Å vs. 3.25Å).
In contrast to single crystal surfaces, the 2ML thick ZnO film forms a flat double-layer over the whole surface. Stabilization is achieved by the transition of the Wurtzite- towards the BN structure with reduced polarity, i.e., O ions move to the plane of Zn ions: The Zn-O bonds (bulk: 1.97Å) within the hexagonal plane shorten to 1.92Å and the intra-plane Zn-O bonds expand to 2.20Å, i.e., the ions change their four-fold coordination (Wurtzite structure) to a three-fold one.
[1] O. Dulub, U. Diebold, G. Kresse: PRL 90 (2003) 016102
[2] C. L. Freeman, F. Claeyssens, N. L. Allan, J. H. Harding: PRL 96 (2006) 066102 |