Oxide nucleation on thin films of copper duringin Situ oxidation in an electron microscope

Abstract

single-crystalline thin films of copper were oxidized at an isothermal temperature of 425°C and at an oxygen partial pressure of 5×10⁻³ Torr in situ in a high-resolution electron microscope. The specimens were prepared by epitaxial vapor deposition onto polished {100} and {110} faces of rocksalt and mounted in a hot stage inside an ultra-high-vacuum specimen chamber of the microscope. Large amounts of sulfur, carbon, and oxygen were detected by Auger electron spectroscopy on the surface of the as-received films and were removed in situ by ion-sputter etching immediately prior to the oxidation. The nucleation and growth characteristics of Cu₂O on Cu were studied. The predominantly observed crystallographic orientations of Cu₂O on {100} and {110} copper films were epitaxial, parallel {100} and {110} orientations, respectively. In addition, a Cu₂O {111} orientation with Cu₂O 〈770〉//Cu 〈110〉 was found frequently on {100}-oriented copper films. The distinct particle shapes observed most frequently were square and hexagonal, representing {100} and {111} orientations, respectively. An induction period of about 30 min was found, which did not depend on the film thickness but did depend strongly on the oxygen partial pressure and the oxygen exposure prior to the oxidation. Neither stacking faults nor dislocations were found to be associated with the Cu₂O nucleation sites. The growth of Cu₂O nuclei was found to be linear with time. The experimental findings, including results from oxygen dissolution experiments and from repetitive oxidation-reduction-oxidation sequences, fit well into the framework of an oxidation process involving (a) the formation of a surface-charge layer, (b) oxygen saturation in the metal and formation of a supersaturated zone near the surface, and (c) nucleation, followed by surface diffusion of oxygen and bulk diffusion of copper for lateral and vertical oxide growth, respectively.