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0902-T10-25.1

Microwave Phase Shifters: Epitaxial Growth by Pulsed Laser Deposition of Dielectric Ba0.5Sr0.5TiO3 Thin Films on MgO Y. Y. Tse1, P.M. Suherman2, T.J. Jackson2 and I. P. Jones1 1) Department of Metallurgy and Materials 2) Department of Electronic, Electrical and Computer Engineering, School of Engineering, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom. ABSTRACT Ba0.5Sr0.5TiO3 (BSTO) thin films were grown on (001) MgO using pulsed-laser deposition (PLD). The microstructures of in-situ and ex-situ annealed BSTO films were studied by X-ray diffraction and transmission electron microscopy (TEM). The films showed a cube on cube epitaxial relationship with BSTO // MgO. They were essentially single crystals with a columnar structure and possessed smooth surfaces. The interfaces of the BSTO films and substrates were atomically sharp, with misfit dislocations. Better crystallinity and full strain relaxation was obtained in films grown in 10-1 mbar oxygen and annealed ex-situ. A 30% increase in dielectric tuneability was achieved compared with in-situ annealing and deposition at 10-4 mbar. Threading dislocations are the dominant defects in the films grown in 10-1 mbar oxygen and annealed ex-situ, while the films with in-situ annealing show columnar structures with low angle boundaries. INTRODUCTION The electrical properties of BaxSr(1-x)TiO3 (BSTO) films have drawn increasing attention recently because of their potential application in agile high frequency devices [1]. For microwave applications such as tuneable filters, microstrip line phase shifters, antennae and varactors, high quality BSTO films, with high tuneability and low dielectric losses are desirable [2, 3]. BSTO thin films can be grown epitaxially on a single crystal substrate such as MgO by PLD which allows extremely good stoichiometric transfer of the cationic species from the target to the substrate [4]. MgO can be chosen as the substrate because of its small dielectric constant (∼ 9) and low dielectric loss (tan δ < 10-6 at 10GHz, 77K). However, in the BSTO/MgO system, there is a fairly large lattice mismatch (∼ 6.1%) and the difference in thermal expansion leads to the formation of defects such as dislocations and stacking faults in the films [5]. These defects increase the dielectric loss of the film. The volume fraction and formation mechanism of the defects depend on the processing conditions such as deposition temperature, oxygen pressure, deviation from stoichiometry and substrate surface finish. In this work, the microstructures of BSTO films which were grown in different oxygen pressures and with different post-annealing conditions are discussed in relation to their microwave properties. EXPERIMENTAL PROCEDURE Ba0.5Sr0.5TiO3 thin films with a nominal thickness of 350 nm were grown epitaxially on polished MgO (001) single crystals by PLD. The laser fluence of 1.5 Jcm-2 at 5 Hz pulse rate was used in all depositions. Oxygen pressure for the deposition ranged from 10-4 to 10-1 mbar. In-situ annealing was carrie