STM study of the geometric and electronic structure of ZnO(0001)-Zn, (000(1)over-bar)-O, (10(1)over-bar0), and (11(2)over-bar0) surfaces

The geometric and electronic structure of clean (000 1), (000(1) over bar), (11(2) over bar 0), and (10(1) over bar 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 (I x 1) termination but distinctly different terrace and step structures. On the zinc-terminated (0 0 0 I)-Zn surface, the terraces are covered with triangular islands and pits of different sizes, rotated by 180degrees with respect to those in the neighboring terraces. Single-layer steps with a height of similar to2.7 Angstrom are observed. Vicinal surfaces of (0 0 0 I)-Zn consist of terraces separated by alternating straight and saw-tooth-shaped steps. On the oxygen-terminated (000(1) over bar)-O surface, flat hexagonal terraces are separated by predominantly similar to5.3 Angstrom high-double-layer steps. The terraces are wide (similar to500 Angstrom) 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(1) over bar 0) surface show flat, rectangular terraces separated by single-layer steps (similar to3 Angstrom) running perpendicular to the <0 0 0 1> and <1(2) over bar 10 > directions. A high density of terraces with atomic rows running preferentially along the <0 0 0 1> directions was observed on the as-grown (11(2) over bar 0) surface. This surface is the least stable and tends to form long grooves that are similar to250 Angstrom wide and similar to50 Angstrom deep along the <1(2) over bar 10 > directions. STS measurements show semiconductor-like behavior of all the surfaces, but a slightly different I-V characteristic of the (000(1) over bar)-O face. Based on these results, structural models for the different surfaces are proposed and related to the stability and reactivity of ZnO surfaces. (C) 2002 Elsevier Science B.V. All rights reserved.