Synthesis and characterization of nanostructured BSTO thin-films for microwave applications

Nanophase synthesis of ferroelectric thin-films of Ba0.6Sr 0.4TiO3 (BSTO) was studied systematically for applications in tunable microwave components. Synthesis of nanostructured BSTO was performed using a pulsed-laser deposition system with real-time in-situ process control. The main research goal was to utilize the pulsed laser deposition parameters to control the grain growth for low microwave loss nanostructured BSTO thin-films on crystalline substrates such as LaAlO3. These parameters include the energy density of the laser pulses, wavelength, oxygen partial pressure, distance between the target and the substrate, and the substrate temperature. The nanostructural characterization was performed using XRD, SEM and AFM. Microwave characterization was done using coplanar waveguide lines to characterize the frequency dependent dielectric properties (&egr; r and tandelta). BSTO films were grown at the same measured temperature and energy density but in different oxygen ambient pressures from 19 mTorr through 300 mTorr. Using contact mode AFM, the grain size was found to decrease as the oxygen ambient pressure was reduced from 150 mTorr to 38 mTorr. The growth process changed when the pressure was increased above 150 mTorr. Nanocluster structures rather than nanoparticles were found at 225 mTorr. Average grain sizes less than 100 nm were obtained for oxygen pressures below 75 mTorr. The XRD spectra indicate the highly crystalline nature of the film. Microwave measurements, performed between 9--18 GHz, suggest the nano-structured BSTO thin-films on LaAlO3 (LAO) substrates are highly tunable (up to 25%). Surface potential imaging indicated that the surface potential of polarized areas to be the sum of contributions from the surface charge and from the vertical part of the oriented dipoles in the areas.