Microstructure and high temperature transport properties of high quality epitaxial SrFeO 3 - δ films C. Solís a , M.D. Rossell b , G. Garcia c , A. Figueras a , G. Van Tendeloo b , J. Santiso a, ⁎ a Centro de Investigación en Nanociencia y Nanotecnologia, CIN2/CSIC, Campus UAB, 08193 Bellaterra, Spain b Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium c Grup de Nanomaterials i Microsistemes, UFMI, Departament de Física, UAB, 08193 Bellaterra, Spain ABSTRACT ARTICLE INFO Article history: Received 4 February 2008 Received in revised form 3 April 2008 Accepted 2 June 2008 Keywords: Thin films Microstructure Conductivity Oxygen sensors We report the high temperature electronic transport properties of SrFeO 3 - δ epitaxial thin films obtained by pulsed laser deposition on NdGaO 3 (110) substrates. The films show total conductivity higher than the bulk material and apparent activation energy of about 0.12 eV in O 2 , lower than reported values for SrFeO 3 - δ films. The conductivity dependence with oxygen partial pressure shows a power dependence with an exponent close to +1/4, in agreement with expected point defect equilibrium. For a given oxygen partial pressure, the temperature coefficient of resistance (TCR) shows a low positive value of about 1.5–2.5 10 - 3 K - 1 , which is still suitable for resistive oxygen sensing applications. The transport properties of the films are discussed in view of their particular microstructure. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Perovskite-type oxides exhibiting high electronic conductivity at elevated temperatures are interesting for their potential applications, such as high-temperature solid oxide fuel cells, or gas sensing materials. In the case of gas sensors, they represent an alternative for resistive-type sensors based on SnO 2, TiO 2 and CeO 2 –ZrO 2 semiconducting oxides [1–3]. SrFeO 3 - δ perovskite oxide has received special attention as potential candidate for oxygen sensors in direct fuel injection engines due to its strong sensitivity to oxygen partial pressure variations and negligible cross-sensitivity to temperature fluctuations [4–7]. Besides, the study of the SrFeO x (2.5 b × b 3) system is also important from a fundamental point of view since a clear correlation between the oxygen vacancies ordering and its sensing properties has been already pointed out [8]. Although thin films are expected to have a faster response, most of the sensing studies in SrFeO 3 - δ have been performed in bulk samples [9–11]. However, non- stoichiometric SrFeO 3 - δ films have been prepared by different techniques: polycrystalline thin films by sol gel [12,13] and citrate method [14]; while highly crystalline films have been prepared by pulsed laser deposition (PLD)[3–5,15–19]. The films showed a large oxygen sensitivity at high temperature, particularly in the low oxygen pressure range (pO 2 ~ 10 - 4 atm) where there was observed a phase transformation from cubic perovskite to brownmillerite structure [5]. High temperature gas sensors based on SrFeO 2.5 + x material have been already fabricated onto microhotplates and successful chemical sensor functionality has been demonstrated [20]. More recently severely reduced SrFeO 2 ceramics have shown infinite-layer structure with very promising ionic conductivity at lower temperatures [21]. In the present work, we report a complete study of film microstructure and high temperature conductivity properties, under different atmospheres, of high quality epitaxial SrFeO 3 - δ thin films obtained by PLD. 2. Experimental A dense ceramic pellet of pure SrFeO 3 - δ (SFO) compound was prepared by a solid-state reaction from a stoichiometric (Sr:Fe=1:1) mixture of SrCO 3 and Fe 2 O 3 sintered in air at 1000 °C in order to be used as target for the PLD experiment. X-ray diffraction (XRD) of the sintered targets confirmed the complete formation of the cubic phase, in good accordance with the reported results [22]. SrFeO 3 - δ films were deposited by using a Nd:YAG pulsed laser with tripled frequency (355 nm wavelength), 9 ns pulse length,10 Hz repetition rate, and 2– 3 J/cm 2 energy density per pulse. Different number of pulses was used in order to grow films from 37 nm to 240 nm. The films were deposited on NdGaO 3 (110) (NGO) single crystal substrates at oxygen pressures of 1 ×10 - 2 mbar, and substrate temperatures of 750 °C. The NdGaO 3 substrate has a perovskite structure with orthorhombic lattice parameters a = 0.54333 nm, b = 0.55036 nm, and c = 0.77157 nm [23]. Therefore its (110) plane cut exhibits a pseudocubic lattice with Solid State Ionics 179 (2008) 1996–1999 ⁎ Corresponding author. Research Centre for Nanoscience and Nanotechnology, CIN2 (CSIC-ICN), Campus UAB, 08193 Bellaterra, Barcelona, Spain. Tel.: +34 935814700; fax: +34 935813717. E-mail address: jose.santiso@cin2.es (J. Santiso). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.06.004 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi