Structural and electrical characterization of sputter-deposited SrTiO3 thin films

Thin films of ferroelectrics in combination with superconducting YBa2Cu3O7-delta (YBCO) find application in tuneable microwave components. The crystalline quality of the ferroelectric films directly affects the tunability and microwave losses of these components. In the present work, SrTiO3 thin films less than or equal to 300 nm in thickness were deposited by rf sputtering in an Ar + O-2 + N2O gas mixture at temperatures ranging from room temperature to 845 degrees C on LaAlO3 rhombohedral (110) substrates. The influence of the discharge plasma anisotropy on the film deposition rate and elemental composition was studied by depositing films on substrates placed both in on- and off- axes. X-ray diffraction (XRD) showed all films deposited at greater than or equal to 685 degrees C to be epitaxial (001) SrTiO3 with an enlarged unit cell in the c-direction. The unit cell lattice parameter decreased with increasing deposition temperature and often again after annealing in an oxygen ambient at 800 or 850 degrees C. Using a planar capacitor with a top electrode composed of Au on Ni-Cr, the dielectric properties of the SrTiO3 thin films evaluated as a function of de voltage bias and temperature. At 1 MHz the best films were found to have dielectric constant of similar to 500 and a loss Increases in the tunability of the dielectric constant were positively correlated to changes in the lattice parameter toward the bulk value of 0.3905 nm. The epitaxial orientation relationships of (001) SrTiO3 // (110) LaAlO3 and (101) SrTiO3 // (121) LaAlO3 were determined by TEM and the former relationship was verified by XRD. Misfit between the substrate and film gave rise to dislocations positioned 35 unit cells of SrTiO3 apart.