Anomalous growth and properties of SrTiO3-NaNbO3 superlattices

Epitaxial multilayers and superlattices of quantum paraelectric perovskite SrTiO3 and NaNbO3 constituents with excellent lattice match are grown by pulsed laser deposition on MgO and LaAlO3 substrates using an La0.5Sr0.5CoO3 bottom electrode. Pseudomorphic growth of superlattices is demonstrated for the period as large as 40 nm. With decreasing period below similar to 20 nm, increase in unit-cell volume exceeding those of bulk constituents and their solid solutions is found. Superlattices exhibit both out-of-plane (normal to substrate surface) and in-plane (parallel to substrate surface) insulating behavior, and decrease in the out-of-plane dielectric permittivity with decreasing period. The results are discussed in terms of possible lattice distortion at the SrTiO3:NaNbO3 interface due to ionic charge mismatch between constituents. Presence of high-permittivity interfacial layers is suggested to be responsible for the evolution of permittivity with decreasing superlattice period.