Dielectric properties of thin film strontium titanium oxide at microwave frequencies

The voltage tunable dielectric function of thin film ferroelectrics has applications to integrated microwave filters, phase shifters, and matching networks. To properly engineer these devices, the nonlinear dielectric constant and loss tangent of thin film ferroelectrics must be fully characterized at the frequencies of interest. We have developed and used a microstrip resonator technique to carry-out the necessary characterization of epitaxial SrTiO{dollar}sb3{dollar} (STO) films over a wide range of temperatures, frequencies and bias conditions. We find that the dielectric constant is roughly independent of frequency at least up to 20 GHz (especially at high bias) as would be expected of bulk STO. In contrast to the bulk behavior, however, it is observed that the dielectric constant of thin film STO achieves a maximum of approximately 2000 at about 50 K whereas the bulk dielectric constant continues to climb as the temperature is lowered. We attempt to explain this behavior in terms of the dielectric properties of bulk STO which have been tuned by an internal bias. This bias may be attributed to film strain, defect charging, or electrode effects. Our models attempt to distinguish these effects. The resonator technique also allows us to subtract-out background losses (originating in the resonator's conductors, substrate, and coupling) to arrive at a loss tangent for the ferroelectric capacitor. The loss tangent of the STO capacitor is in the 10{dollar}sp{lcub}-3{rcub}{dollar} to 10{dollar}sp{lcub}-2{rcub}{dollar} range, decreasing with increasing bias and is independent of frequency between 6 and 20 GHz. Data on {dollar}rm Basb{lcub}0.5{rcub}Srsb{lcub}0.5{rcub}TiOsb3{dollar} is also discussed.