On the epitaxial growth in ALD Co₃O₄- and NiO-based bilayers

NiO and Co₃O₄ are versatile materials studied for a plethora of applications, yet their performance for a specific application relies on the control of their crystallographic texture and corresponding surface facets. Achieving such control can be challenging, often requiring hetero-epitaxial growth on single-crystalline substrates, which are frequently incompatible with the requirements of the application. The combination of NiO and Co₃O₄ in heterostructures provides potential to control texture due to their similar crystal structures, whilst retaining the possibility to work with more versatile substrates. In this study, atomic layer deposited (ALD) thin films based on cyclopentadienyl precursors and an oxygen plasma are adopted to tailor the crystallographic texture of NiO from 〈100〉 to 〈111〉 using an ALD Co₃O₄ template layer, and similarly, to modify the Co₃O₄ texture from 〈111〉 to 〈100〉 on a NiO template. The films are shown to conform to the crystal orientation of the template material, whilst crystallizing directly in their own stable crystal structure with corresponding metal atom coordination. Further investigation includes ALD process parameters for NiO growth: the film texture is found to depend on the choice of co-reactant and the above-highlighted hetero-epitaxial relationship is stronger for plasma-based processes. In conclusion, these results demonstrate an original approach for application-oriented crystallographic engineering in thin films.