Microwave dielectric properties of strained (Ba0.4Sr0.6)TiO3 thin films

Ferroelectric Ba0.4Sr0.6TiO3 (BST) thin films grown on (001) MgO by pulsed laser deposition show a strong correlation between their structure and thei r microwave dielectric properties. Epitaxially grown BST films are observed by x-ray diffraction to be tetragonally distorted. The oxygen deposition p ressure affects the magnitude of the tetragonal distortion (the ratio of in -plane and surface normal lattice parameters, D=a/c) of the deposited BST f ilms. D varied from 0.996 to 1.004 at oxygen deposition pressure of 10-800 mTorr. The dielectric properties of BST films measured at microwave frequen cies (1-20 GHz) exhibit an oxygen deposition pressure dependent dielectric constant (epsilon=100-600), and quality factor Q (1/tan delta=10-60). The B ST film grown at the oxygen deposition pressure of 200 mTorr exhibits the h ighest figure of merit [% tuning in epsilon xQ(0V), where % tuning is 100x( epsilon(0)-epsilon(b))/epsilon(0), and epsilon(0) and epsilon(b) are dielec tric constant at 0 and 80 kV/cm]. This corresponds to the film with the low est distortion (D=1.001). The observed microwave properties of the films ar e explained by a phenomenological thermodynamic theory based on the strain along in-plane direction of the films. (C) 2000 American Institute of Physi cs. [S0021-8979(00)01022-7].