Microstructure evolution of Ca_0.33 CoO₂ thin films investigated by high-angle annular dark-field scanning transmission electron microscopy

Microstructures of epitaxial Ca_0.33 CoO₂ thin films, which were grown on m(11̄00) plane and c(0001) plane of α-Al₂O₃ by the reactive solid-phase epitaxy (R-SPE) method and the subsequent ion-exchange treatment, were investigated in detail by using selected-area electron diffraction, high-resolution transmission electron microcopy, spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (Cs-corrected HAADF-STEM), and electron energy-loss spectroscopy (EELS). Detailed electron diffraction analyses reveal that the orientation relationships between Ca_0.33 CoO₂ thin film and substrate are [112̄0] _Ca0.33 CoO₂//[0001] _Al2O3 and (0001)Ca_0.33 CoO₂, having an angle of about 43° with (1100)_Al2O3 for the film deposited on m(11̄00) plane, and [11̄00] _Ca0.33 CoO₂//[112̄0] _Al2O3 and (0001)_Ca0.33 CoO₂//(0001] _Al2O3 for the film deposited on c(0001) plane though a Ca-Al-O amorphous layer formed between them. CoO seed layer near the interface and residual Co₃O4 phase inside the films were observed and identified by HAADF-STEM and EELS in both samples. Such microstructural configuration indicates that the processes of film growth during R-SPE are (i) oxidation of CoO into Co₃O₄ with residual CoO layer near the interface and (ii) intercalation of Na⁺ layer into Co₃O₄ to achieve the layered Na_xCoO₂ film while forming Na-Al-O amorphous layer at the interface.