Stress dependence of growth mode change of epitaxial layered cobaltite γ-Na0.7CoO2

We report stress dependence of growth characteristics of epitaxial γ-Na0.7CoO2 films on various substrates deposited by pulsed laser deposition method. On the sapphire substrate, the γ-Na0.7CoO2 thin film exhibits spiral surface growth with multi-terraces and highly crystallized texture. For the γ-Na0.7CoO2 thin film grown on the (1 1 1) SrTiO3 substrate, the nano-islands of ∼30 nm diameter on the hexagonal grains are observed. These islands indicate that the growth mode changes from step-flow growth mode to Stranski-Krastanow (SK) growth mode. On the (1 1 1) MgO substrate, the large grains formed by excess adatoms covering an aperture between hexagonal grains are observed. These experimental demonstrations and controllability could provide opportunities of strain effects of NaxCoO2, physical properties of thin films, and growth dynamics of heterogeneous epitaxial thin films.