Journal of Alloys and Compounds 477 (2009) 85–89 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Structure and ferro/piezoelectric properties of SrBi 4 Ti 4 O 15 films deposited on TiO 2 buffer layer A.Z. Simões a,∗ , E.C. Aguiar b,1 , C.S. Riccardi b,1 , F. Moura b,1 , E. Longo b,1 , J.A. Varela b,1 a Universidade Federal de Itajubá (Unifei) - Campus Itabira, Rua São Paulo 377, Bairro Amazonas, CEP 35900-37 Itabira, MG, Brazil b Laboratório Interdisciplinar em Cerâmica, Departamento de Físico-Química, Instituto de Química, Universidade Estadual Paulista, Bairro Quitandinha, CEP 14800-900 Araraquara, SP, Brazil article info Article history: Received 22 August 2008 Received in revised form 3 October 2008 Accepted 14 October 2008 Available online 6 December 2008 Keywords: Thin films Buffer layer Bismuth layered compounds abstract TiO 2 buffer layer was introduced between SrBi 4 Ti 4 O 15 (SBTi) thin films and Pt bottom electrodes through the soft chemical solution. The obtained films were characterized by X-ray diffraction, atomic force microscopy and electrical properties. Unlike thin film crystallized directly onto a highly (1 1 1)-oriented Pt bottom electrode, the thin film on TiO 2 buffer layer was a single phase perovskite with random ori- entation. The dielectric and ferroelectric properties of the SBTi/TiO 2 thin films deposited on Pt coated Si substrates are evaluated, leading to the potential of the TiO 2 buffer layer for the integrated devices. Meanwhile, SBTi thin films deposited directly on (1 1 1) Pt bottom electrode reveal a weak ferroelectricity along c-axis direction. © 2008 Published by Elsevier B.V. 1. Introduction Bismuth-layer-structured ferroelectrics are considered to be candidate materials for lead-free piezoelectrics [1]. Bi 4 Ti 3 O 12 (BIT) is a promising candidate as an alternative material because of its large spontaneous polarization. Also, higher order structures such as SrBi 4 Ti 4 O 15 (SBTi) films may have larger remanent polariza- tion, due to the increased number of perovskite units generating ferroelectric properties. SBTi, which has a crystalline structure sim- ilar to BIT, is another typical layer-structured ferroelectric (m = 4) [2]. Its high Curie temperature (T c = 520 ◦ C) makes it useful over a wide temperature range [3,4]. However, the 2P r of SBTi thin films is lower (6.2 C/cm 2 , compared to 13 C/cm 2 ) [5–7], and the fatigue endurance property deteriorates with the increase of switching pulse width. Thin layers of crystalline oxides such as LaNiO 3 , SrRuO 3 , IrO 2 , and TiO 2 have been introduced between thin films of the Aurivillius family with n values of two and three, such as SrBi 2 Ta 2 O 9 and Bi 4 Ti 3 O 12 related compounds, in order to control the crystallographic orientation [8–11]. How- ever, the effects of the buffer layer on the crystal structure and piezoelectric characteristics of SBTi thin films have not yet been reported. In our previous work [12], SBTi thin films have been synthesized by the polymeric precursor method on Pt coated Si ∗ Corresponding author. Tel.: +55 31 3834 6472/6136; fax: +55 31 3834 6472/6136. E-mail address: alezipo@yahoo.com (A.Z. Simões). 1 Tel.: +55 16 3301 6643; fax: +55 16 3301 6692. substrates. The SBTi ferroelectric thin films were polycrystalline in nature with a strong (1 1 9) orientation. Kato et al. [13] pre- pared CaBi 4 Ti 4 O 15 (CBTi144) thin films by a chemical solution deposition (CSD) technique using a complex metal alkoxide on Pt coated Si substrates and amorphous TiO 2 layers. The authors had observed that the CBTi144 thin film crystallized on the amorphous TiO 2 layer was almost single phase perovskite and showed ran- dom orientation. The (1 1 1)-oriented Pt layer is most commonly stacked an electrode between the ferroelectric and the insulator layer because of its chemical and thermal stabilities at high tem- peratures. Therefore, the TiO 2 rutile could be a promising buffer layer, which can avoid the interdiffusion between the ferroelectric phase and the Pt substrate. In this paper, the effects of crystalline TiO 2 buffer layers introduced between the SBTi thin films and the Pt bottom electrodes are reported. Changes in the crystallo- graphic and microstructural appearances of SBTi thin films are elucidated. The electrical properties such as dielectric, ferroelec- tric, piezoelectric and leak current densities are characterized and compared. 2. Experimental procedure The coating solution was prepared by the polymeric precursor method, which is based on the chelation of cations with citric acid in an aqueous solution. Ethy- lene glycol was added to the metallic citrate formed and the heating of this mixture led to polymerization and resulted in a homogeneous resin. The viscosity of the coating solution was adjusted to 20cP by the controlled evaporation of water. The TiO2 buffer layer was deposited on (1 1 1) Pt/Ti/SiO2/Si substrates by the soft chemical method and microwave annealed at 500 ◦ C for 10 min. The previously prepared Ti precursor solution (with viscosity of 20 cP) was used as the Ti source 0925-8388/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.jallcom.2008.10.138