A Thin Film Source in a Solid-State Diffusion Experiment: CoO on SrTiO3

To realize a chemical diffusion experiment for simple quantitative analysis of one-dimensional diffusion profiles requires the fabrication of a planar and chemically sharp interface between two phases, one serving as the diffusion source and the other as the material to be studied. We demonstrate a thin film source on top of single crystals or epitaxial films for the example of cobalt (II) oxide (CoO) grown on top of SrTiO3 (STO) by ion beam sputtering. After deposition at room temperature, a nearly single-crystalline epitaxial film with a flat and chemically sharp interface is present. Diffusion annealing leads to a partial formation of the Co3O4 phase. We report the conditions where compact and stable CoO layers with flat interface are maintained, serving as a constant source for Co diffusion. Exemplarily, the formation of a Co-diffusion profile is studied by using three different methods: energy dispersive X-ray spectroscopy in a transmission electron microscope, atom probe tomography, and time-of-flight secondary ion mass spectroscopy. The origin of differences in the diffusion constant probed on different sample scales is discussed.