Elsevier

Applied Surface Science

Volume 256, Issue 10, 1 March 2010, Pages 3316-3320
Applied Surface Science

Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes

https://doi.org/10.1016/j.apsusc.2009.12.026Get rights and content

Abstract

(Na0.85K0.15)0.5Bi0.5TiO3 thin films were deposited on LaNiO3(LNO)/SiO2/Si(1 0 0) and Pt/Ti/SiO2/Si(1 0 0) substrates by metal–organic decomposition, and the effects of bottom electrodes LNO and Pt on the ferroelectric, dielectric and piezoelectric properties were investigated by ferroelectric tester, impedance analyzer and scanning probe microscopy, respectively. 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. The improved piezoelectric properties could make (Na1−xKx)0.5Bi0.5TiO3 thin film as a promising candidate for piezoelectric thin film devices.

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 PE 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).

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