An STM study of the initial oxidation of single-crystalline zirconium surfaces

The microstructural development of thin (thickness < 10 nm) oxide layers grown on Zr surfaces by thermal oxidation was investigated by in-vacuo STM and XPS. To this end, single-crystalline Zr(0001) and Zr(10 1 ¯ 0) surfaces were prepared under UHV conditions by a cyclic treatment of ion-sputtering and in-vacuo annealing steps and then exposed to dry O2(g) in the temperature range of 300–450 K (at pO2 = 1×10− 4 Pa). Oxidation proceeds by the fast formation of a dense arrangement of tiny oxide nuclei, which cover the entire Zr surface. The initial oxide cluster size is about 1.2 ± 0.1 nm. The transport processes on the oxidizing surface become promoted with increasing temperature and thereby the oxide clusters rearrange into bigger agglomerates with increasing oxidation time. At the same time, a long-range atomic order develops in the oxide overgrowths, as evidenced from the emergence of a bonding/non-bonding fine structure in the resolved oxide-film upper valence band, as measured in-situ by XPS.