STM study of the geometric and electronic structure of ZnO(0 0 0 1)-Zn, (0 0 0 1̄)-O, (1 0 1̄ 0), and (1 1 2̄ 0) surfaces

The geometric and electronic structure of clean (0 0 0 1), (0 0 0 1̄), (1 1 2̄ 0), and (1 0 1̄ 0) 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 (0 0 0 1)-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 (0 0 0 1)-Zn consist of terraces separated by alternating straight and saw-tooth-shaped steps. On the oxygen-terminated (0 0 0 1̄)-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 (1 0 1̄ 0) surface show flat, rectangular terraces separated by single-layer steps (∼3 Å) running perpendicular to the 〈0 0 0 1〉 and 〈1 2̄ 1 0〉 directions. A high density of terraces with atomic rows running preferentially along the 〈0 0 0 1〉 directions was observed on the as-grown (1 1 2̄ 0) 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 1̄ 0 0〉 directions. STS measurements show semiconductor-like behavior of all the surfaces, but a slightly different I–V characteristic of the (0 0 0 1̄)-O face. Based on these results, structural models for the different surfaces are proposed and related to the stability and reactivity of ZnO surfaces.