Fabrication and characterization of iron and fluorine co-doped BST thin films for microwave applications F. Stemme • M. Bruns • H. Geßwein • M. Schroeder • M. Sazegar • M. D. Drahus • R.-A. Eichel • F. Paul • C. Azucena • J. R. Binder Received: 10 October 2012 / Accepted: 11 January 2013 / Published online: 25 January 2013 Ó Springer Science+Business Media New York 2013 Abstract The effects of fluorine co-doping by means of a post-thermal annealing process of iron-doped BST thin films in a fluorine-containing atmosphere have been investigated. XPS and ToF-SIMS sputter depth profiling verified a homogeneous fluorine distribution in the thin films. By employing EPR, it was shown that singly charged (Fe 0 Ti –V  O )  defect complexes, as well as ‘isolated’ Fe 0 Ti centres with a distribution of F  O sites in remote coordina- tion spheres exist in the fluorinated films. Tunability enhancement due to fluorine co-doping as well as a Q-factor enhancement due to iron doping is demonstrated. Introduction The tunable dielectric barium strontium titanate (Ba 1-x Sr x TiO 3 , BST) is one of the most promising materials for microwave applications in the GHz range. It may be used in phase shifters or tunable filters [1–5]. The incorporation of acceptor and donor dopants in the materials structure is an effective way to tailor the materials properties. Paul et al. [6] and Zhou et al. [7] presented a transition metal- fluorine co-doping in screen-printed BST thick films as a suitable way to achieve low material losses and an acceptable tunability in the microwave range up to 30–40 GHz. Regarding the influence of the microstructure on the dielectric properties Friederich et al. [8], Zhou et al. [9] and Paul et al. [10] have shown that the mean grain diameter strongly influences the tunability of the thick films. Thus, the influence of grain size or crystal quality superimposes the influence originating from the dopants. However, acceptor–donor co-doping using fluorine as donor dopant has not yet been demonstrated for thin films. We modified a radio frequency (RF) co-sputter process to introduce iron as an acceptor dopant into BST thin films [11] and demonstrated the influence of the acceptor dopant F. Stemme  H. Geßwein  M. Schroeder  J. R. Binder (&) Institute for Applied Materials (IAM-WPT), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany e-mail: joachim.binder@kit.edu F. Stemme  F. Paul Department of Microsystems Engineering, IMTEK, University of Freiburg, Georges-Ko ¨hler-Allee 102, 79110 Freiburg, Germany M. Bruns Institute for Applied Materials (IAM-ESS), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany M. Bruns Karlsruhe Nano Micro Facility (KNMF), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany M. Sazegar Institute for Microwave Engineering and Photonics, Technische Universita ¨t Darmstadt, Merckstraße 25, 64283 Darmstadt, Germany M. D. Drahus Department of Physical Chemistry, University of Freiburg, Albertstraße 21, 79104 Freiburg, Germany R.-A. Eichel Research Centre Juelich, Institute of Energy and Climate Research (IEK 9), 52435 Ju ¨lich, Germany C. Azucena Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 123 J Mater Sci (2013) 48:3586–3596 DOI 10.1007/s10853-013-7156-5