Anisotropic in-plane strain in W-doped (Ba,Sr)TiO3 thin films deposited by pulsed-laser deposition on (001)MgO

The structural and the microwave dielectric properties of Ba_xSr_(1-x)TiO₃ films (BST) with x=0.5, 0.6 and 0.7, containing 1 mol % W have been investigated. The films were grown by pulsed-laser deposition on MgO (001) substrates at a temperature of 720 °C in an oxygen pressure from 3 to 500 mTorr. The film structures were determined by X-ray diffraction. The lattice parameters were fitted to a tetragonal distortion of a cubic lattice. The out-of-plane lattice parameter (c) was calculated from the position of the (004) reflection. Using c, the in-plane lattice parameter, a, was calculated from the position of the (024) and (224) reflections. A deviation in the calculated values for a, beyond the systematic error, was found in the in-plane lattice parameter, suggesting an in-plane orthorhombic distortion (a, a’). Films with x=0.7 showed a minimum in-plane distortion due to a better lattice match with the substrate. The ratio of the in-plane and out-of-plane lattice parameters was calculated as a measure of the lattice distortion (a/c and a’/c). The dielectric properties of the films deposited were measured at room temperature at 2 GHz using gap capacitors fabricated on top of the dielectric film. For all Ba/Sr ratios investigated in the W-doped material, the dielectric Q (1/cosδ) was observed to be insensitive to the oxygen deposition pressure. A peak in the change in the dielectric constant, as a function of an applied electric field (0–80 kV/cm), was observed for films deposited in 50 mTorr of oxygen. The largest K-factor, K=(ε(0)-ε(V )/ε(0)×Q(0)), for films deposited from a BST x=0.6 (1 mol % W-doped) target was observed in the film that had a minimum in-plane strain, where a∼a’ and c was greater than a and a’. Received: 4 July 2002 / Accepted: 5 July 2002 / Published online: 17 December 2002 RID="*" ID="*"Permanent address: Nuclear Research Center–Negev, P.O. Box 9001 Beer-Sheva, Israel RID="**" ID="**"Corresponding author. Fax: +1-202/767-5301, E-mail: horwitz@ccsalpha3.nrl.navy.mil