Thickness-induced insufficient oxygen reduction in La_2−xCe_xCuO_{4 ± δ}thin films

A series of electron-doped cuprate La2-xCexCuO4 +/-delta thin films with different thicknesses have been fabricated and their annealing times are adjusted carefully to ensure the highest superconducting transition temperature. The transport measurements indicate that, with the increase of the film thickness (< 100 nm), the residual resistivity increases and the Hall coefficient shifts in the negative direction. Furthermore, the x-ray diffraction data reveal that the c-axis lattice constant c(0) increases with the decrease of film thickness. These abnormal phenomena can be attributed to the insufficient oxygen reduction in the thin films. Considering the lattice mismatching in the ab plane between the SrTiO3 substrates and the films, the compressive stress from the substrates may be responsible for the more difficult reduction of oxygen in the thin films.