Heteroepitaxial Growth of T-Nb₂O₅ on SrTiO₃

There is a growing interest in exploiting the functional properties of niobium oxides in general and of the T-Nb₂O₅ polymorph in particular. Fundamental investigations of the properties of niobium oxides are, however, hindered by the availability of materials with sufficient structural perfection. It is expected that high-quality T-Nb₂O₅ can be made using heteroepitaxial growth. Here, we investigated the epitaxial growth of T-Nb₂O₅ on a prototype perovskite oxide, SrTiO₃. Even though there exists a reasonable lattice mismatch in one crystallographic direction, these materials have a significant difference in crystal structure: SrTiO₃ is cubic, whereas T-Nb₂O₅ is orthorhombic. It is found that this difference in symmetry results in the formation of domains that have the T-Nb₂O₅ c-axis aligned with the SrTiO₃ <001>_s in-plane directions. Hence, the number of domain orientations is four and two for the growth on (100)_s- and (110)_s-oriented substrates, respectively. Interestingly, the out-of-plane growth direction remains the same for both substrate orientations, suggesting a weak interfacial coupling between the two materials. Despite challenges associated with the heteroepitaxial growth of T-Nb₂O₅, the T-Nb₂O₅ films presented in this paper are a significant improvement in terms of structural quality compared to their polycrystalline counterparts.