Thickness dependent electrical resistivity modulation in tensile-strained epitaxial thin films of calcium manganese oxide

We report our studies of the thickness dependence of electrical resistivity and lattice constants in strained epitaxial thin films of calcium manganese oxide. Our results indicate the potential of bi-axial lattice mismatch strain as a handle for modulating electrical resistivity. We observe thickness dependence of lattice constants consistent with what is expected for strain relaxation for films thicker than 400 angstrom. At lower thickness values, anomalies are observed suggestive of reduced oxygen stoichiometry. We observe a remarkable decrease in electrical resistivity with decreasing film thickness. The resistivity of our thinnest films (5-7 nm) is about three orders of magnitude lower than the resistivity of bulk CaMnO3. Resistivity increases as the film thickness increases, along with the progression of strain relaxation. It is noteworthy that the thickness dependence of resistivity we observe in CMO thin films is the opposite of what has been reported for their hole-doped rare earth manganite counterpart La0.67Ca0.33MnO3 (LCMO), where tensile lattice mismatch strain suppresses metallicity, leading to the increase in resistivity with film thickness. We believe that the enhanced conductivity in our thinnest films is related to the possible oxygen deficiency promoted by tensile strain. Recent x-ray absorption measurements have revealed reduced oxygen content and associated changes in Mn valence states in tensile-strained CMO thin films, as also predicted by density functional theory calculations. This report is the first observation of electrical transport behavior possibly indicative of strain-oxygen stoichiometry coupling.