AVS 49th International Symposium
    Surface Science Monday Sessions
       Session SS2-MoM

Paper SS2-MoM10
Polar ZnO(0001)-Zn and ZnO(000-1)-O Surfaces: Geometric and Electronic Structure, Stabilization Mechanisms

Monday, November 4, 2002, 11:20 am, Room C-112C

Session: Oxide Structure and Surface Chemistry
Presenter: O. Dulub, Tulane University
Authors: O. Dulub, Tulane University
U. Diebold, Tulane University
Correspondent: Click to Email
The geometric and electronic structure of the (0001)-Zn and (000-1)-O polar surfaces of ZnO were studied with Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), as well as Low Energy Electron Diffraction (LEED). Sharp (1x1) LEED patterns were recorded for both surfaces. The STM images of the ZnO(0001)-Zn surface reveal flat triangular terraces of single step height (~2.6 Å), exhibiting two domains rotated by 180 degrees with respect to each other. STM shows a high density of triangular pits of various sizes and slightly rounded small holes. Triangular islands of different sizes were also observed on the terraces, and the smaller ones exhibit size-dependent special shapes. STM images from the O-terminated (000-1) surface reveal a quite different morphology. The surface is composed of flat, well ordered terraces without pits or added small islands. The terraces are separated mainly by double-layer high step edges (~5.2 Å) that include an angle of 120 degrees. STS indicates a slightly (but reproducibly) different electronic structure of the two polar surfaces. The filled states on the O-terminated face are shifted toward the Fermi level. Based upon the STM and STS results, two stabilization mechanisms are proposed for (0001)-Zn and (000-1)-O surfaces. A charge transfer from the O- to the Zn-terminated surface makes the latter more metallic. Since metallic Zn is known to have an extremely high vapor pressure even at low temperatures, it possibly evaporates off the surface. Depleting the Zn-terminated surface of 1/4 of zinc atoms removes the infinite dipole moment and therefore stabilizes the crystal. The special shapes of the smallest ('magic') islands observed in STM images of the (0001)-Zn surface are consistent with the proposed model.