Dielectric properties of SrTiO3 thin films with Ca and Zr partial substitutions for active microwave applications

The structure and dielectric properties of SrTiO3 thin films with either 5% Ca or Zr substitutions have been investigated. Thin films of these materials were deposited onto (100) LaAlO3 substrates by pulsed laser deposition (PLD) at temperatures of 750 degrees C in 350 mTorr of oxygen. As-deposited films, characterized by x-ray diffraction, were single phase and well oriented with omega - scan widths of < 0.2 degrees for the (002) reflection. The capacitance and dissipation factor were measured at 1 MHz to 1 GHz for the deposited films as a function of temperature and electric field (80 kV/cm) using Ag interdigital capacitors deposited on top of the SrTiO3 films. For unsubstituted SrTiO3, a peak in the capacitance vs. temperature curve was observed at similar to 60K. The temperature dependence of the dissipation factor was similar and also exhibited a maximum at SOK with a value of 0.025. A 50% change in the capacitance can be achieved for the SrTiO3 film at similar to 80 kV/cm. Substitution of 5% Ca on the Sr site does not change the temperature dependence of the capacitance but leads to an increase in dissipation factor at 60 K to similar to 0.05. In addition, the capacitance decreases more rapidly with lower electric fields for partial Ca substitution compared to pure SrTiO3. Substitution of 5% Zr for Ti leads to a dramatic reduction in the capacitance and correspondingly, the field dependence of the capacitance. However, the Zr substituted films have extremely low losses, similar to 0.004. The ability to control both the magnitude of the dielectric constant and the losses will make these materials useful for frequency-tunable active microwave applications.