High thermoelectric power in a NaxCoO2 thin film prepared by sputtering with rapid thermal annealing

• Fabrication of NaxCoO2 thin films by means of RF-magnetron sputtering.
• The mechanism of Na ion migration was also investigated.
• Rapid thermal annealing method restrains Na diffusion and the layer crystal structure is maintained.

NaxCoO2 thin films were fabricated by means of RF-magnetron sputtering. We measured and analyzed the thermal properties and changes of the NaxCoO2 crystal structure by XRD, SEM, Raman spectra, and XPS analyses. Sodium ions diffused from the bulk of the thin film to the surface as the temperature increased. The diffused Na ions reacted with oxygen ions and Na2O was formed on the surface of the thin film, resulting in a decrease of the carrier concentration and a change of the crystal structure from a layer to a spinel structure. The Seebeck coefficient of the NaxCoO2 thin film annealed at 550 °C is larger than the value (100 μV/K) for single crystal NaCo2O4.