STM study of the geometric and electronic structure of ZnO(0001)-Zn, (000-1)-O, (10-10), and (11-20) surfaces

O. Dulub, L. A. Boatner, U. Diebold

Department of Physics, Tulane University, New Orleans, Louisiana 70118, U.S.A.
Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.

Surf. Sci. 519 (2002) 201-217

The geometric and electronic structure of clean (0001), (000-1), (11-20), and (10-10) faces of ZnO single crystals have been studied with scanning tunneling microscopy (STM) and spectroscopy (STS), low-energy electron diffraction (LEED), and low-energy He+ ion scattering spectroscopy (LEIS). All surfaces exhibit a (1×1) termination but distinctly different terrace and step structures. On the zinc-terminated (0001)-Zn surface, the terraces are covered with triangular islands and pits of different sizes, rotated by 180° with respect to those in the neighboring terraces. Single-layer steps with a height of ~2.7 Å are observed. Vicinal surfaces of (0001)-Zn consist of terraces separated by alternating straight and saw-tooth-shaped steps. On the oxygen-terminated (000-1)-O surface, flat hexagonal terraces are separated by predominantly ~5.3 Å high-double-layer steps. The terraces are wide (~500 Å) and smooth with no added islands and holes. They are not covered with a saturation coverage of hydrogen. Near-atomic-resolution images of the prism (10-10) surface show flat, rectangular terraces separated by single-layer steps (~3 Å) running perpendicular to the <0001> and <1-210> directions. A high density of terraces with atomic rows running preferentially along the <0001> directions was observed on the as-grown (11-20) surface. This surface is the least stable and tends to form long grooves that are ~250 Å wide and ~50 Å deep along the <1-100> directions. STS measurements show semiconductor-like behavior of all the surfaces, but a slightly different I-V characteristic of the (000-1)-O face. Based on these results, structural models for the different surfaces are proposed and related to the stability and reactivity of ZnO surfaces.

Reprints available from U. Diebold (diebold at iap_tuwien_ac_at).

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