Advanced Materials Research Vols. 399-401 (2012) pp 926-929 Online available since 2011/Nov/22 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.399-401.926
Highly c-axis oriented Barium Titanate ferroelectric films deposited on SrTiO3 substrate using an off-axis sputtered conductive oxide layer as bottom electrode Wei Zhang, Meiling Yuan, Xianyang Wang, Jun Ouyanga Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Engineering Ceramics Key Laboratory of Shandong Province, School of Materials Science and Engineering, Shandong University, Jinan, Shandong 250061, China a
Corresponding author:
[email protected]
Keywords: BaTiO3, ferroelectric films, preferred orientation, RF-magnetron sputtering, off-axis sputtering
Abstract. BaTiO3 (BTO) thin films were grown on (100) SrTiO3 (STO) single crystal substrates using the RF-magnetron sputtering technique (RFMS) in both pure Ar and mixed Ar/O2 (20% O2) atmosphere. A La0.5Sr0.5CoO3 (LSCO) layer was deposited as the bottom electrode by a 90° off-axis single-target RFMS. θ-2θ X-ray diffraction measurements showed that BTO thin films grown in both cases had a highly preferred c-axis orientation (001). From hysteresis measurements, it was confirmed that both films are ferroelectric. The ferroelectric polarizations 2Pr were 6.6 µC/cm2 and 27.1 µC/cm2, for the BTO films grown in pure argon and in mixed Ar/O2 atmosphere, respectively. Introduction Perovskite-type oxide barium titanate (BaTiO3) is one of the most intensively studied lead-free ferroelectrics due to its many useful electrical properties. Particularly, BaTiO3-based thin films have received much attention in recent years for their important applications in ultrahigh density memory devices [1]. The properties of BaTiO3 thin films reported in most papers are dielectric and piezoelectric properties [2-5]. There were very few reports investigating ferroelectricity of BTO thin films deposited by RFMS. Moo-Chin Wang et al. deposited BTO thin films on Si substrate using RFMS, with a reported Pr of about 5 µC/cm2 [6]. The BTO thin films grown on (La,Sr)2CuO4 film by Yukio Watanabe’s group had a Pr of about 1.5 µC/cm2 [7]. One of the few investigations that reported desirable ferroelectric properties of BTO films was a “strain engineering” study of the BTO films grown on special substrates, which are expensive and not commonly available [8]. Therefore, it is desirable to improve the ferroelectric properties of BTO thin films grown on common substrates. Under this background, the BTO/LSCO/STO structure is used in this work. First, STO, LSCO and BTO have good lattice match with each other which may lead to highly preferred orientation of the BTO film and hence good ferroelectric properties. Second, STO is a very common single crystal substrate. Third, the LSCO layer used as bottom electrode provide ferroelectric films with good fatigue properties. Due to similar crystalline structure and lattice constants to many perovskite ferroelectric materials, thin films of conductive perovskite oxides, La0.5Sr0.5CoO3 (LSCO), SrRuO3 (SRO), LaNiO3 (LNO) and La0.5Sr0.5MnO3 (LSMO) are advantageous as compared with metal films when
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used as the bottom electrode layer in ferroelectric capacitor structures. LSCO has a pseudo-cubic perovskite structure with a lattice constant of 3.835 Å and an electrical resistivity of 90 µΩ·cm at 300 K. It was reported that LSCO was used as bottom electrode for Bi4Ti3O12 thin films grown on MgO, LAO and Al2O3 substrates [9][10], and for BaTiO3 thin film on SiO2/Si substrates [11]. However, there have been very few reports on the fabrication of BTO films on STO substrate with conductive oxide bottom electrode. The purpose of this study is to create a deposition process for high quality BTO ferroelectric thin films with preferred crystalline orientation and excellent ferroelectric properties. Experimental BTO thin films were deposited by RF-magnetron sputtering from a BaTiO3 ceramic target (99.99% purity) with a diameter of 50 mm. The LSCO bottom electrode layer was deposited on (100) STO single crystal substrates using a 90° off-axis single-target RFMS [12] (see Fig. 1). Before these substrates were loaded into the deposition chamber, they were cleaned ultrasonically in acetone and ethanol for 10 min each, then pre-sputtered (ion etched) for 5 min. Ar/O2 mixed gas has been the most widely used sputtering atmosphere for BTO thin films [2][13][14]. In this study, to investigate the influence of O2 on the properties of BTO films, both pure argon and Ar/O2 mixed gases (20% O2) were used and the pressure during deposition was kept at 1.9 Pa. The detailed deposition conditions are listed in Table 1. Fig. 1 (a) on-axis geometry (b) 90° off-axis geometry.
Table 1. Deposition conditions Sputtering methods BTO layer: on-axis RFMS LSCO layer: 90° off-axis RFMS Gas atmosphere Pure Ar O2+Ar (20% O2) Substrate Temperature (°C) 700 Base pressure (Pa) 1.5×10-4 Sputtering pressure (Pa) 1.9 Distance between target and substrate (mm) 60 Sputtering power (W) 105 Deposition time (min) 60 Post-deposition cooling rate (°C /min) 5 The microstructure of BTO thin films were measured by θ-2θ X-ray diffraction (XRD) with Cu Kα radiation at room temperature. The ferroelectric hysteresis loops were measured using an aixACCT TF Analyzer. Before ferroelectric measurements, a photolithography process followed by a room temperature direct current sputtering yielded circular Pt top electrodes with a diameter of 100 µm.
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Results and discussion As can be seen from Fig. 2, which shows the XRD θ-2θ patterns of BTO thin films, all the BTO films show a pure perovskite structure with highly (001) orientation. The good lattice matchings between STO and LSCO (a = 3.835 Å), and between LSCO and BTO leads to the “cube-on-cube” growth of the electrode layer and the film. In addition, the 90° off-axis sputtering of LSCO helps to reduce the sputtering damage and maintain the stoichiometry of the oxide layer, which in return improves its crystalline quality and that of the film. In Fig. 2, the full-width at half-maximum (FWHM) of the (001) BTO peaks are 0.283 and 0.349, for the BTO thin films grown in Ar/O2 mixed gases and pure argon, respectively. A decrease in FWHM can be explained by a better crystallinity of the BTO film deposited in Ar/O2 mixed gas than that in pure argon.
Fig. 2 X-ray diffraction patterns of the BTO thin film on STO substrate with LSCO bottom electrode (a) in mixed Ar/O2 (b) pure argon.
Fig. 3 Ferroelectric hysteresis loops of the BTO film growth in two kinds of gas atmosphere.
Generally, the oriented growth of thin films is related to the lattice match of films with the substrates and the minimization of interface energy during the film growth process. The highly preferred c-axis orientation of the BTO thin films can be attributed to the small differences in lattice parameters between film and substrate, which are summarized in Table 2. Table 2. Lattice parameters of film and substrate materials Bulk BaTiO3 c-axis BTO film on STO substrate (growth in pure Ar) BTO film on STO substrate (growth in Ar+O2) SrTiO3 substrate
4.034 Å 4.069 Å 4.058 Å 3.905 Å
The polarization vs. voltage (P-V) hysteresis loops of the BTO films were measured under an applied voltage of 40 V and a frequency of 100 Hz. Fig. 3 shows the hysteresis loops of BTO thin films sputtered in pure argon and Ar/O2 (20% O2) mixed gases, respectively. The BTO thin films deposited in Ar/O2 (20% O2) mixed gas exhibit larger remnant polarization (2Pr = 27.1 µC/cm2) and coercive voltage (Vc = 7.4 V) than those of the BTO films deposited in pure Ar (2Pr = 6.6 µC/cm2 and Vc = 2.2 V). This is because BTO thin films deposited in pure argon were oxygen deficient,
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which will lead to increased leakage current and degraded ferroelectric properties. The addition of oxygen during film deposition reduced the number of oxygen vacancies, thereby improving the ferroelectric properties of BTO thin films. Conclusions BTO thin films with highly preferred c-axis orientation have been successfully grown on (100) STO substrates with LSCO bottom electrode by a combined on-axis and 90° off-axis RF-magnetron sputtering process. The XRD θ-2θ results showed only peaks of perovskite structure with c-axis orientation for the BTO thin films deposited in both pure argon and Ar/O2 mixed gases (20% O2). The twice remnant polarization of BTO thin films grown in pure argon and Ar/O2 (20% O2) mixed gases are 6.6 µC/cm2 and 27.1 µC/cm2, respectively. References [1] L.W. Martin, Y.H. Chu, R. Ramesh: Materials science and Engineering R. Vol. 68 (2010), P. 89 [2] Chi-Shiung Hsi, Fu-Yuan Hsiao, Nan-Chung Wu and Moo-Chin Wang: Jpn. J. Appl. Phys. Vol. 42(2003), P. 544-548 [3] Yiping Guo, Kazuyuki Suzuki, Kaori Nishizawa, Takeshi Miki, Kazumi kato: Journal of Crystal Growth. Vol. 284 (2005), P. 190-196 [4] Yang Xiang, Rui Zhang, Wenwu Cao: Journal of Applied physics. Vol. 106 (2009), P. 1-5 [5] Hideyuki Imai, Isaku Kanno, Ryuji Yokokawa, Kiyotaka Wasa, and Hidetoshi Kotera: Jpn. J. Appl. Phys. Vol. 49 (2010), P. 09MA09-01 [6] Moo-Chin Wang, Fu-Yuan Hsiao, Chi-Shiung Hsi, Nan-Chung Wu: Journal of Crystal Growth. Vol. 246 (2002), P. 78-84 [7] Y. Watanabe, Y. Matsumoto, H. Kunitomo, M. Tanamura, E. Nishimoto: Jpn. J. Appl. Phys. Vol. 33 (1994), P. 5182 [8] K. J. choi, M. Biegalski and C. B. Eom et al.: Science. Vol. 306 (2004), P. 1005-1009 [9] Park. B.H, Noh. T.W, Lee. J, Kim. C.Y, Jo. W: Applied Physics Letters. Vol. 70 (1998), P. 1101-1103 [10] Jo. W, Kim. K. H, Noh. T.W: Applied Physics Letters. Vol. 66 (1995), P. 3120-3122 [11] C. Pollak, A. Busic, K. Reichmann, H. Hutter: Thin Solid Films. Vol. 405 (2002), P. 218-223 [12] C. B. Eom, J. Z. Sun, B. M. Lairson, et al.: Physica C. Vol. 171 (1990), P. 354-382 [13] T. Yoshimura, N. Fujimura, T. Ito: Journal of Crystal Growth. Vol. 174 (1997), P. 790-795 [14] A. Ianculescu, B. Despax, V. Bley, T. Lebey, R. Gavril, N. Dragan: Journal of the European Ceramic Society. Vol. 27 (2007), P. 1129
New Materials, Applications and Processes 10.4028/www.scientific.net/AMR.399-401
Highly C-Axis Oriented Barium Titanate Ferroelectric Films Deposited on SrTiO3 Substrate Using an Off-Axis Sputtered Conductive Oxide Layer as Bottom Electrode 10.4028/www.scientific.net/AMR.399-401.926
