Thickness Dependence of Electrical and Structural Properties of Tensile Strained Calcium Manganese Oxide Thin Films

We have investigated the properties of CaMnO$_{3-\delta}$ thin films epitaxially grown by pulsed laser deposition on lattice mismatched substrates, (100)LaAlO$_{3}$ and (100)SrTiO$_{3}$ , leading to a tensile strain of $\sim$4$\%$ and 1.5$\%$ respectively. For our films these substrates, thickness dependence of the properties is characteristically different from what has been previously observed in thin films of hole-doped manganites. We observe that the resistivity decreases significantly as the film thickness decreases. The decrease in resistivity is more pronounced in the films on (100)SrTiO$_{3}$ with the larger lattice mismatch, the resistivity of the thinnest films being about 3 orders of magnitude lower than the of bulk CaMnO$_{3}$. Thickness dependence of the lattice constants also show deviations from the behavior expected from strain relaxation. These results suggest a coupling between tensile strain and oxygen deficiency consistent with predictions from models based on density functional theory calculations. Our results are relevant for potential catalytic applications of CaMnO$_{3-\delta}$ thin films.