Control of nonlinear dielectric property of oxide artificial superlattices by the lattice strain for application to devices

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This study was aiming at controlling dielectric properties of perovskite type artificial superlattice by the strain induced in the crystal lattice for applications to devises. It was revealed in the previous studies that the artificial superlattices with the stacking period of 10 perovskite cells showed an extremely high dielectric permittivity, but its temperature dependence was not known in spite of the importance for the applications for practical devices. In this study, a technique to measure temperature dependence of the dielectric permittivity of ultra thin films in microwave region was developed first and it was applied to the artificial superlattices made by a molecular beam epitaxy method. It was found the high dielectric permittivity of artificial superlattice was stable with the change of temperature, which was an important result for the application of artificial superlattices. The artificial superlattices were formed by a conventional RIP sputtering and their dielectric properties ware measured. It was found from the measurements that the superlattices of BaTiO3/SrTiO3 indicated the enhancement of permittivity even thought the super diffractions could not be observed in the XRD profiles. The electrode effects on the nonlinear dielectric properties of BaTiO_3-SrTiO_3 films were investigated. The films were deposited on SrRuO_3 bottom electrodes formed on single crystal SrTiO_3 substrate by RIP sputtering process. Top electrodes of In, Au, Ag and Pt were deposited on the films and the dielectric properties were measured. it was found that the dielectric permittivity markedly depended on the electrode metals and the results were analyzed using the theory of Schottky barriers.