We have observed both metallic and semiconducting behavior in epitaxial thin films of the metallic oxide CaRuO_3 deposited under identical conditions on (100) SrTiO_3 substrates of varying crystalline quality. X-ray diffraction studies showed that while semiconducting films with enlarged unit cells were obtained on single crystal SrTiO_3 substrates, metallic films with lattice parameters close to the bulk material grew on poor crystalline quality SrTiO_3 substrats. The films deposited on (100) LaAlO_3 substrates consistently showed metallic behavior. Atomic force microscope images suggest that the semiconducting films had a coherent two dimensional nucleation. In contrast, three dimensional island-like incoherent growth was seen in the metallic films . It is believed that in the coherent films a strain induced substitution of the small Ru~4+ cations by the larger Ca~2+ cations occurs, breaking the conduction pathway within the three dimensional network of the RuO_6 octahedra and leading to a metal-insulator transition. This unique phenomenon-which is not observed in bulk mateiral-can be significant in technologiclly important epitaxial perovskite oxide heterostructures.
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