).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.
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