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 application requirements. 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 crystallising 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 relation is stronger for plasma-based processes. In conclusion, these results demonstrate an original approach to application-oriented crystallographic engineering in thin films.