Influence of oxygen pressure on the structural and dielectric properties of laser-ablated Ba0.5Sr0.5TiO3 thin films epitaxially grown on (001) MgO for microwave phase shifters

Abstract

Highly oriented Ba0.5Sr0.5TiO3 (BST) thin films were grown on MgO (001) single-crystal substrates by pulsed-laser deposition at 800 °C in oxygen pressure ranging from 1.2 × 10−3 to 40 Pa. A strong correlation was observed between the growth process, structure and dielectric properties for the BST films. The dielectric properties in the low frequency range 10 k–1 MHz were measured in the interdigital capacitor configuration. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D = a/c), surface morphology and dielectric response of the films are strongly dependent on the oxygen deposition pressure. With increase in oxygen pressure, the in-plane strain for the BST films changes from compressive to tensile. The BST film grown at 25 Pa exhibits the best overall dielectric properties. This corresponds to the film with a slight in-plane tensile strain (D = 1.0012). It is believed that a reasonable tensile strain, which increases the ionic displacement and thus promotes the in-plane polarization in the field direction, could enhance the tunability and dielectric constant. Based on the BST film grown at 25 Pa, distributed phase shifters were successfully fabricated with promising performance. The phase shifter shows a relatively low insertion loss of about 3.5 dB at zero bias and 10 GHz, a good return loss better than −15 dB for all phase states from dc to 16 GHz and a differential phase shift of about 43° with 120 V dc bias at 10 GHz.