Scanning tunneling microscopy study of clean and copper covered zinc oxide surfaces
Description
The surfaces of ZnO are of great interest due to their wide application in catalysis, gas sensing and microelectronic fabrication, yet their properties are still a subject of debate. Scanning tunneling microscopy and spectroscopy, low energy electron diffraction, ion scattering spectroscopy, and x-ray photoelectron spectroscopy techniques were used to study the geometric and electronic structure of the polar Zn-terminated (0001), O-terminated (0001¯), and the non-polar (101¯0) and (112¯0) surfaces of ZnO. After sputtering and annealing at 450--750°C in ultra-high vacuum, all surfaces exhibit a (1 x 1) structure with a distinctly different terrace and step morphology. The (0001)-Zn surface exhibits a high density of one layer-high triangular holes and islands. Islands and holes on subsequent terraces are rotated by 180° with respect to each other and have exclusively O-terminated step edges approximately 0.27 nm high, that produce excess O on the terraces. Images of the (0001¯)-O surface display smooth terraces separated by (mostly) 0.53 nm high double-layer steps that include an angle of 120°, running along either [0001¯] or [112¯0] direction The stability of polar surfaces is one of the open questions of modern surface science. The presence of an accumulating dipole moment normal to the surface leads to an energetically highly unstable situation. However, some polar surfaces are quite stable, and a prime example is the basal surfaces of ZnO, i.e. the (0001)-Zn and the (0001¯)-O surfaces. Simple theoretical arguments show that polar surfaces of wurtzite ZnO can be stabilized by reducing each surface charge density by a factor of ∼1/4. Small islands (16--34 Ain size) observed in STM images of Zn-terminated (0001) surface have special shapes that are consistent with a stabilizing Zn/O concentration ratio of ∼3/4. STS curves show a slight but reproducible difference between this and the O-terminated (0001¯) surfaces. No metallic surface states were observed on the polar surfaces, in contrast to theoretical predictions. These results give evidence for two different stabilization mechanisms of the polar surfaces of ZnO: possibly, a charge transfer from the O-side to the Zn-side, and a partial vaporization of ∼1/4 of Zn atoms on the Zn-side The growth of Cu on the non-polar ZnO(101¯0) surface was also investigated. Cu nucleation sites are found to be strongly correlated with surface defects such as steps and impurities. Images of Cu deposited on a 'freshly-annealed' ZnO(101¯0) surface show preferential nucleation of exclusively 3D islands at the step edges oriented perpendicular to the atomic row direction. A completely different mode of Cu growth was observed on the same face when the density of defects on terraces is high. Both 2D and 3D islands randomly distributed across the terraces were observed