Lead-free piezoelectric thin films of Mn-doped NaNbO 3 –BaTiO 3 fabricated by chemical solution deposition Wataru Sakamoto a, ⁎, Yu-ichi Hamazaki a , Hiroshi Maiwa b , Makoto Moriya a , Toshinobu Yogo a a Division of Nanomaterials Science, EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Department of Materials Science and Engineering, Shonan Institute of Technology, 1-1-25 Tsujido-Nishikaigan, Fujisawa, Kanagawa 251-8511, Japan abstract article info Article history: Received 28 June 2009 Received in revised form 7 November 2009 Accepted 19 December 2009 Available online 4 January 2010 Keywords: Lead-free piezoelectric NaNbO 3 –BaTiO 3 Thin film Chemical solution deposition Ferroelectric properties Lead-free piezoelectric thin films of NaNbO 3 –BaTiO 3 were fabricated on Pt/TiO x /SiO 2 /Si substrates by chemical solution deposition. Perovskite NaNbO 3 –BaTiO 3 single-phase thin films with improved leakage- current and ferroelectric properties were prepared at 650 °C by doping with a small amount of Mn. The 1.0 and 3.0 mol% Mn-doped 0.95NaNbO 3 –0.05BaTiO 3 thin films showed slim ferroelectric P–E hysteresis and field-induced strain loops at room temperature. The 1.0 and 3.0 mol% Mn-doped 0.95NaNbO 3 –0.05BaTiO 3 films showed remanent polarization values of 6.3 and 6.2 μC/cm 2 , and coercive field of 41 and 55 kV/cm, respectively. From the slope of the field-induced strain loop, the effective piezoelectric coefficient (d 33 ) was found to be 40–60 pm/V. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Piezoelectric materials are important in the electronics industry. In particular, Pb(Zr,Ti)O 3 (PZT)-based ceramics are widely used for piezoelectric devices because of their excellent electrical properties [1]. However, Pb is well known as a harmful element; therefore, lead- free piezoelectric materials are in high demand for environmental reasons. Among several lead-free piezoelectric materials, alkali niobate-based compounds, such as (K,Na)NbO 3 , have been exten- sively studied as PZT alternatives [2–4]. Recently, NaNbO 3 –BaTiO 3 (NN-BT) ceramics have been reported [5] and are expected to be promising candidates for lead-free piezoelectric materials. This solid solution system does not contain the volatile element potassium, so its properties are more easily controlled. At a composition of 0.9NaNbO 3 –0.1BaTiO 3 , this system has Tc and d 33 values of approx- imately 230 °C and 147 pC/N, respectively, which are relatively high compared with other lead-free piezoelectric materials [5]. Furthermore, thin-film processing is becoming increasingly im- portant for microelectromechanical system (MEMS) applications. Chemical solution deposition (CSD), a promising thin-film processing method, offers high homogeneity, low-temperature fabrication, precise control of chemical composition, and reduced equipment cost. Several research groups have utilized CSD to fabricate piezo- electric MEMS devices [6–8]. In these studies, piezoelectric thin films containing heavy metal ion, such as Pb or Bi, are applied mainly. Therefore, alkali niobate-based piezoelectric thin films are very attractive. Moreover, piezoelectric thin films of NN-BT have never been reported. However, in general, it is difficult to achieve good ferroelectric properties for alkali niobate-based thin films because of their low electrical resistivity (relatively large leakage current) [9]. Thus, improvement in ferroelectric properties is strongly required for realizing piezoelectric alkali niobate-based, such as NN-BT, thin films. In this study, NN-BT thin films were fabricated on Si-based substrates by CSD and their electrical properties were evaluated. Furthermore, they were Mn-doped to improve their ferroelectric properties. 2. Experimental procedure For thin film preparation, the chemical compositions of the precursor solutions of NN-BT and Mn-doped NN-BT were set at Na 1 - x Ba x Nb 1 - x Ti x O 3 [x =0.05, 0.1, 0.15; NN-BT100(1 - x)], and Na 0.95 Ba 0.05 (Nb 0.95 Ti 0.05 ) 1 - y Mn y O 3 (y =0, 0.01, 0.03; Mn100y% NN-BT95). Appropriate amounts of NaOC 2 H 5 , Nb(OC 2 H 5 ) 5 , Ba (OC 2 H 5 ) 2 , Ti(O i C 3 H 7 ) 4 , and Mn(O i C 3 H 7 ) 2 (Kojundo Chemical, Japan) with 10 at.% excess Na were dissolved in absolute 2-methoxyethanol and then refluxed for 20 h to yield a 0.3 M homogeneous solution. Since the starting materials are extremely sensitive to moisture, 2- methoxyethanol was dried over molecular sieves and distilled before use. Moreover, the entire procedure was conducted in a dry nitrogen atmosphere. Thin films were fabricated using the precursor solution by spin coating on Pt (200 nm)/TiO x (50 nm)/SiO 2 (500 nm)/Si (500 μm) substrates. As-deposited precursor films were dried at 150 °C for 5 min and calcined at 400 °C at a rate of 10 °C/min for 30 min in an Thin Solid Films 518 (2010) 4256–4260 ⁎ Corresponding author. Tel.: +81 52 789 2751; fax: +81 52 789 2133. E-mail address: sakamoto@esi.nagoya-u.ac.jp (W. Sakamoto). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.12.102 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf