Atomic layer deposition of thermoelectric layered cobalt oxides

Layered cobalt oxides based on the hexagonal CoO₂ layer, e.g., Na_xCoO₂ and [CoCa₃O₃]_0.62CoO₂ (or “Ca₃Co₄O₉”), are promising thermoelectric materials. Here, the authors investigate the atomic layer deposition (ALD) of these materials in a thin-film form; this is not trivial, in particular, for the former compound, as both Na and Co are little challenged as components of ALD thin films. The authors employ diketonate precursors for all the metal constituents and ozone as the source of oxygen. In both cases, a postdeposition heat-treatment in O₂ is applied to get crystalline coatings; the processes are found amazingly robust in terms of metal precursor pulsing ratios. A striking difference between the two processes is the resultant morphology: while the Ca₃Co₄O₉ films grow highly homogeneous and smooth, the Na_xCoO₂ coatings exhibit a rather unique reproducible 10–20 μm scale channel-like island structure for all x values investigated. Finally, the authors characterized their ALD Ca₃Co₄O₉ films for their chemical, structural, and physical property details not previously reported.