Epitaxial growth and control of the sodium content in NaxCoO2 thin films

Single-phase c-axis oriented NaxCoO2 thin films were grown on (001) SrTiO3 single-crystal substrates, using pulsed laser deposition. X-ray diffraction analysis indicates the epitaxial growth of NaxCoO2 thin films in two domains, rotated in-plane by 15 and 45 degrees relative to [100] SrTiO3. The sodium stoichiometry x of the films can be controlled in a range of 0.38 < x < 0.84 by in-situ post-deposition annealing the NaxCoO2 films at 720 - 760 °C in oxygen for 10 - 30 min. γ - NaxCoO2 films are obtained with a full width at half maximum of the (002) NaxCoO2 rocking curve below 0.2 degrees. The post-deposition annealing can substitute commonly used chemical deintercalation of Na which is typically associated with a loss in crystallinity.