Effects of substrate on the dielectric and tunable properties of epitaxial SrTiO₃thin films

Tunable dielectric thin films of SrTi O3 (STO) were prepared on different single-crystalline substrates, including insulating LaAl O3, conductive Nb-doped STO (NSTO), and superconducting Y Ba2 Cu3 O7-δ. Substrate effects including morphology, orientation, and lattice mismatch induced strains were investigated. We found that a change of substrate used for STO thin films can strongly affect the dielectric properties of STO thin films in terms of dielectric constant, loss tangent, and tunability. Effects of substrate properties on the temperature dependence of dielectric constant and loss tangent were investigated. At low temperatures, STO thin films under minimal strain yield high dielectric constant and low loss tangent while the thin films under either tensile or compressive strain exhibit the reduced dielectric constant and high loss. The tunability of about 77% in STONSTO system, close to the value found in STO single crystal, was observed at 10 K. Physical origin of observed phenomena was discussed.