Modulation of Conductivity and Metallic Properties in Perovskite Metal Oxide Thin Films by Fluorination Treatment

We have studied the effects of ‘fluorination’ treatment on the structural, magnetic and electrical properties of La_0.67 Ca_0.33MnO _3-y thin films. Fluorination treatment involves coating the films with a fluorine containing polymer Poly-Vinyl Difluoride (PVDF) followed by an ex-situ heat treatment. The films grown in 400 mTorr show properties very close to that of the fully oxygenated bulk material. Fluorination treatment of these films results in a marginal increase in resistivity and no change in in insulator-metal transition, or lattice parameters. Films grown in lower oxygen pressures show orders of magnitude higher resistivity and no insulator-metal transition in the as-grown state. Fluorination treatment of these films decrease the resistivity by several orders of magnitude and induces the insulator-metal transition. XRD shows an additional phase which corresponds to a shortened c-lattice parameter. Identical control samples subject to the same thermal treatment without the PVDF coating do not show any significant changes. The drastic decrease in resistivity and the occurrence of the insulator-to-metal transition indicate that the fluorination promotes an increase in the charge carrier concentration. Incorporation of fluorine at oxygen vacancy sites can lead to increase in the hole doping thus promoting a higher Mn⁴⁺ to Mn³⁺ ratio. We are pursuing X-ray photoelectron spectroscopy studies to verify changes in valence states and investigating fluorination effects in SrTiO₃.