Phase and Orientation Control of NiTiO Thin Films.

Subtle changes in the atomic arrangement of NiTiO in the ilmenite structure affects its symmetry and properties. At high temperatures, the cations are randomly distributed throughout the structure, resulting in the corundum structure with -3 symmetry. Upon cooling, the cations order in alternating layers along the crystallographic axis, resulting in the ilmenite structure with -3 symmetry. Related to this is the 3 symmetry, where the cations alternate both perpendicularly and along the axis. NiTiO with the latter structure is highly interesting as it exhibits ferroelectric properties. The close relationship between structure and properties for ilmenite-related structures emphasizes the importance of being able to control the symmetry during synthesis. We show that the orientation and symmetry of thin films of NiTiO formed by atomic layer deposition (ALD) can be controlled by choice of substrate. The disordered phase (-3), previously only observed at elevated temperatures, have been deposited at 250 °C on α-AlO substrates, while post-deposition annealing at moderate temperatures (650 °C) induces ordering (-3). We have in addition explored the symmetry and epitaxial orientation obtained when deposited on substrates of LaAlO(100), SrTiO(100) and MgO(100). The presented work demonstrates the possibilities of ALD to form metastable phases through choice of substrates.