Thickness dependence of microwave dielectric tunability in Ba0·5Sr0·5TiO3 thin films deposited by pulsed laser deposition

In this study, the Ba0·5Sr0·5TiO3 (BST) thin films with different thicknesses are fabricated on fused silica quartz substrate by using the pulsed laser deposition (PLD) technique. The deposition was done using a laser fluence of ∼2 J/cm2 and the number of pulses was varied from 2500 to 20,000 to change the thickness of the thin films. X-ray diffraction (XRD) patterns and Raman spectrum were used for the phase confirmation. The FESEM technique was used to observe the microstructure of BST thin films and the average grain size was found to be 208, 255, 267, 300, and 393 nm for the BST thin films with thicknesses of 150, 300, 450, 600 and 1000 nm. The optical bandgap (Eg) of all films was ∼3.54 eV. Dielectric properties of the BST thin films were measured not only in the low-frequency region (1 kHz-1 MHz) but also in the microwave frequency region (0.5 GHz-4 MHz). The microwave dielectric properties of the BST thin films were also measured as a function of voltage using the circular patch capacitor (CPC). The highest microwave tunability was found to be ∼55.5% for the BST thin film with a thickness of 300 nm. It was found that too small a film thickness where the polarizable dipoles are constrained by proximity to an amorphous substrate and thick films where the grains approach the size of that in bulk are not suitable for yielding high dielectric tunability.