Diffusion of F atoms from fluoride substrates promotes the epitaxial growth of metal fluorides

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We report the growth of YF3 epitaxial thin films by magnetron sputtering using MgF2(100) substrates as a fluorine source instead of fluorination gases. We find that the substrate selection based on the relative film/substrate Gibbs free energies of formation can drive F-ion diffusion from the fluoride substrates into the Y thin films. Due to the high diffusivity of F ions in YF3, this method can grow ∼40 nm thick epitaxial thin films with uniform F distribution. These results indicate that fluoride substrates can serve as the F-ion source, providing a novel synthetic route for growing metal-fluoride thin films.