TOF atom-probe investigation of metal oxides

Abstract

An energy-focused time-of-flight atom-probe field-ion microscope has been successfully employed to investigate the composition depth profile and the oxides species of metal oxide layers. Depth profiles of p-type semiconducting thin films formed by in situ oxidation of materials of three different crystal structures (bcc iron, hcp cobalt, and fcc nickel) showed, without exception, a metal-rich near surface. After iron has been oxidized in pure research-grade oxygen, iron oxide layers field evaporate mostly as FeO+, Fe2+, Fe+, O+, and O+2. A small number of FeO2+, FeO+2, and FeO+3 have also been observed. However, iron oxide layers formed by oxidation in air field evaporate exclusively as FeH2+2 and Fe(OH2)2+2. No Fe3O4 or Fe2O3 have been detected in both cases. No hydroxides and very few hydrides are found for nickel and cobalt when they are oxidized in air. Cobalt oxide layers field evaporate as CoO+, Co2+, O+2, and O+ as well as a small number of Co+, CoO2+, and CoH+. Although we also expected Co3O4 ions, none was found. Nickel oxide layers field evaporate mainly as NiO+, Ni2+, O+2, O+, and Ni+. In addition to these, some NiO+2 and masses which can be assigned as Ni2O3+3 and Ni2O4+3 are detected. We have also discovered that the oxygen content in the thick oxide layer does not, in general, vary uniformly, possibly due to oxygen diffusion along grain boundaries. The oxide films are polycrystalline in nature, and the adhesive strength of the oxide to the metal is found to be weakest at the oxide/metal interface.