|
Metal surfaces having a structure-free electronic properties are more preferable to widely-used semiconductor surfaces in microscopically probing the electronic properties of artificially-fabricated nanometer-scale structures on it. Formation and characterization of an atomically-flat insulating layer onto an equally flat metal substrate is relevant to isolate the structure of interest from the influence of the bulk. In the present study, a Cu was chosen as the substrate material because of its relative ease in preparing atomically-flat surfaces, and the ideal condition for its oxidation and the resultant surface properties were analyzed using non-contact atomic force microscopy (NC-AFM) microscopically as well as morphologically. The Cu(110) surface was chosen for its larger initial oxidation rate in comparison with the Cu(001) surface and expected anisotropy that draws scientific interest. In contrast to a (2 x 1) structure that have been extensively studied, a more oxidized c(6 x 2) structure, emerging typically at an enormous exposure over 103 L has not been reported in number. Especially its transition from/to the (2 x 1) phase is interesting in addition to its microscopic features. A clean Cu(110) surface was oxidized by exposure to oxygen gas with a pressure of 1 x 10-5 Torr while its temperature was changed from RT to 773 K. The duration for oxidation was chosen so that the total exposure amount was from 1 x 104 to 1.2 x 105 L. The observed dominant reconstruction on the surface was c(6 x 2) structure. The quality of oxide layer was found to be improved by increasing the oxidation temperature, and atomically-flat single-phase terraces wider than 20 nm were obtained at 573 K. With the temperature raised to 773 K, a portion of surface exhibited a (2 x 1) reconstruction, probably due to desorption or reduced adsorption of oxygen atoms. On the other hand, microscopic observation of a c(6 x 2) unit cell exhibited a contrast arising from subsurface Cu and O atoms in addition to topmost Cu atoms that were the only significant feature in scanning tunneling microscopy (STM) images. This result is to authenticate a proposed structural model. |