Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes
Introduction
Lead-free piezoelectric materials are desired inevitably as alternate materials of lead-based materials for the sake of environmental protection and biocompatibility, because the lead toxicities of producing, using and recycling are problematic for devices associated with PbZrO3–PbTiO3 (PZT) and other lead-based piezoelectrics [1]. As bismuth sodium titanate Na0.5Bi0.5TiO3 (NBT) exhibits strong ferroelectricity (Pr = 38 μC/cm2) at room temperature and high Curie temperature Tc of 320 °C [2], it has been considered as one of the key lead-free piezoelectric materials. In comparison with pure NBT, NBT-based solid solutions modified with BaTiO3 or Bi0.5K0.5TiO3 show the improved piezoelectric and dielectric properties because of the existent rhombohedral–tetragonal morphotropic phase boundary [3], [4]. Recently, (Na0.82K0.18)0.5Bi0.5TiO3 thick film on Pt coated alumina substrate shows high remnant polarization (2Pr) 56.6 μC/cm2, dielectric constant 810 and excellent piezoelectric coefficient 109 pm/V [5], whereas (Na0.85K0.15)0.5Bi0.5TiO3 (NBT-KBT15) thin film on Pt/Ti/SiO2/Si substrate appears remnant polarization (2Pr) 27.6 μC/cm2 and dielectric constant 360 [6]. (Na1−xKx)0.5Bi0.5TiO3 (NBT-KBT100x) thin films deposited on Pt electrodes are only focused on the ferroelectric and dielectric properties [6], and never be involved in the piezoelectric performance. For device applications, it is very important to select a suitable bottom electrode for the ferroelectric thin films since the structural and electrical properties such as surface roughness, leakage current and dielectric properties are strongly related with the bottom electrode [7], [8]. Therefore, several kinds of metal oxide and compound metallic electrodes have been suggested instead of the conventional pure metal electrodes such as Pt and Au. Among metal compounds, LaNO3 (LNO) has been considered as a promising candidate for the thermally stable bottom electrodes because of the low room temperature resistivity, chemical stability and pseudocubic perovskite crystal structure with a lattice parameter a = 3.84 Å matching with most of the perovskite ferroelectric materials [8], [9], [10]. The enhancement of dielectric and piezoelectric properties observed with PZT thin films grown on electrode LNO compared with those on electrode Pt may be ascribed to the difference of grain size and of crystallographic orientation of the thin films [9]. We naturally associate whether the change of grain size and crystallographic orientation can improve dielectric and piezoelectric properties for NBT-KBT100x thin film deposited on LNO bottom electrode?
In this paper, NBT-KBT15 thin films were deposited on LNO/SiO2/Si(1 0 0) and Pt/Ti/SiO2/Si(1 0 0) substrates by using metal–organic decomposition (MOD), and the bottom electrode effects on ferroelectric, dielectric and piezoelectric properties of NBT-KBT15 thin films were investigated by ferroelectric tester, impedance analyzer and scanning probe microscopy. It is expected that the present research may offer useful guidelines to the design and application of piezoelectric NBT-based thin films and devices.
Section snippets
Experimental
LNO thin film was firstly fabricated on SiO2/Si(1 0 0) substrate by sol–gel process. Stoichiometric amounts of lanthanum acetate (La(CH3COO)3·1.5H2O) and nickel acetate (Ni(CH3COO)2·xH2O) were dissolved in acetate acid (CH3COOH) and water (with CH3COOH/H2O ratio of 5:1). The mixture was constantly stirred until a transparent and stable green precursor solution was obtained, and the final concentration of the solution was about 0.2 M. The precursor solution was spun at 400 rpm for 10 s and 4000 rpm
Results and discussion
Fig. 1 presents XRD patterns of LNO thin film and NBT-KBT15 thin films deposited on LNO and Pt bottom electrodes. It can be seen that LNO thin film shows a perovskite structure, and NBT-KBT15 thin films are polycrystalline films exhibiting dominant perovskite phase without preferred orientation. In addition, the second phase identified as Bi2Ti2O7 structure of NBT-KBT15 thin films can be observed, and it is consistent with the existences of pyrochlore phase reported for NBT thin films [11]. It
Conclusions
NBT-KBT15 lead-free thin films were deposited on LNO and Pt electrodes by MOD. They are of polycrystalline microstructure and unsaturated P–E loops. For the thin films deposited on LNO and Pt electrodes, the remnant polarization 2Pr are about 22.6 and 8.8 μC/cm2 under 375 kV/cm, the dielectric constants 238 and 579 at 10 kHz, the dielectric losses 0.06 and 0.30 at 10 kHz, the statistic d33eff values 95 and 81 pm/V. In comparison with the thin film deposited on Pt electrode, the thin film deposited
Acknowledgements
This work was supported by NNSF of China (10672139, 10825209 and 50872117), Nature Science Foundation of Hunan Province (07JJ5002), Changjiang Scholar Incentive Program ([2009]17), and Project of Hunan's Prestigious Fu-rong Scholar Award ([2007]362).
References (21)
- et al.
Mater. Lett.
(2007) - et al.
Solid State Commun.
(2007) - et al.
J. Cryst. Growth
(2008) - et al.
Thin Solid Films
(2007) - et al.
Scripta Mater.
(2007) Nature
(2004)- et al.
Sov. Phys. Solid State
(1961) - et al.
Jpn. J. Appl. Phys.
(1991) - et al.
Jpn. J. Appl. Phys.
(1999) - et al.
Appl. Phys. Lett.
(2008)
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2023, Ceramics InternationalEffect of annealing temperature on piezoelectric and mechanical properties of (Bi<inf>0.5</inf>Na<inf>0.5</inf>)TiO<inf>3</inf>-(Bi<inf>0.5</inf>K<inf>0.5</inf>)TiO<inf>3</inf>-BaTiO<inf>3</inf> thin films
2016, Ceramics InternationalCitation Excerpt :One is that the nucleation and crystal growth of 85BNT–10BKT–5BT start from the amorphous phase, thus pyrochlore structure, as an intermediate phase, is incompletely changed into pervoskite structure. The other one is during annealing, the evaporation of Na, K, and Bi results in nonstoichiometric defects, leading to the formation of pyrochlore structure [19]. The typical surface morphology of 85BNT–10BKT–5BT thin film annealed at 700 °C is shown in Fig. 2(b).
Effects of LaNiO<inf>3</inf> seeding layers on the crystal structure and electrical properties in 0.94(Bi<inf>0.5</inf>Na<inf>0.5</inf>)TiO<inf>3</inf>-0.06BaTiO<inf>3</inf> thin films
2015, Ceramics InternationalCitation Excerpt :It is worth noting that the P–E loops of LNO-buffered BNT–BT films are nearly fully saturated, which indicate that LNO seeding layers effectively block the space charges at the films/electrode interface. While, the P–E loops of BNT–BT films directly deposited on the Pt substrates (without LNO seeding layer) are not completely saturated because of the large leakage current density induced by charge carriers (as shown in the inset of Fig. 4) [18]. The observed remanent polarization 2Pr values are about 12 and 18 μC/cm2 for the BNT–BT films deposited on Pt and LNO substrates, respectively, while the measured loops of both samples show almost the same coercive field (Ec ~150 kV/cm).