Preparation and ferroelectric properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 thin films deposited on Pt electrodes using LaNiO3 as buffer layer

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Abstract

(Na0.5Bi0.5)0.94Ba0.06TiO3 thin films were deposited on Pt/Ti/SiO2/Si (1 1 1) and LaNiO3/Pt/Ti/SiO2/Si (1 1 1) substrates by a sol–gel process. The phase structure and ferroelectric properties were investigated. The X-ray diffraction pattern indicated that the (Na0.5Bi0.5)0.94Ba0.06TiO3 thin film deposited on Pt/Ti/SiO2/Si (1 1 1) substrates is polycrystalline structure without any preferred orientation. But the thin film deposited on LaNiO3/Pt/Ti/SiO2/Si substrates shows highly (1 0 0) orientation (f  81%). The leakage current density for the two thin films is about 6 × 10−3 A/cm2 at 250 kV/cm, and thin film deposited on LaNiO3/Pt/Ti/SiO2/Si substrates possessed a much lower leakage current under high electric field. The hysteresis loops at an applied electric field of 300 kV/cm and 10 kHz were acquired for the thin films. The thin films deposited on LaNiO3/Pt/Ti/SiO2/Si substrates showed improved ferroelectricity.

Introduction

Lead-based ferroelectric materials represented by PbTiO3 and Pb(Zr, Ti)O3 are widely used due to their excellent ferroelectric and piezoelectric properties. However, lead toxicity is a tremendous disadvantage to the environment. Inevitably, developing lead-free materials is of great importance. Sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) is considered to be one of the promising materials since it was discovered by Smolenski et al. [1]. Bulk ceramics of NBT have shown strong ferroelectricity at room temperature (remnant polarization Pr = 38 μC/cm2 and coercive field Ec = 73 kV/cm) with a relative high Curie temperature of 320 °C. NBT-based bulk ceramics modified with BaTiO3 (BT) show improved properties due to the existence of rhombohedral–tetragonal morphotropic phase boundary (MPB) [2], [3]. The piezoelectric constant d33 of the NBT-BT ceramics with the MPB composition can reach up to 125 pC/N [4], and the (1 0 0)-oriented crystals grown by spontaneous nucleation method showed a piezoelectric constant of d33 = 450 pC/N and electric field induced strain up to 0.25% [5]. Also, for MPB composition, some researchers got bulk ceramics with pyroelectric coefficient Pi = 3.9 × 10−4 cm−2 K−1 [6], which possibly resulted from the large spontaneous and remnant polarization (Pr = 40 μC/cm2) and lower depolarization temperature (Td = 100 °C) [4]. However, NBT-BT in the form of thin films has rarely been studied because the existence of precipitation when adding BT to NBT. Recently, we use ammonia solution to stabilize the solution. Meanwhile, buffer layers are widely used to improve the properties of the thin films, such as LaNiO3 and NaNbO3 [7], [8], [9]. LaNiO3 (LNO) has a pseudo cubic perovskite structure (a = 3.84 Å), which is not only a good candidate for the electrode material due to its good metallic conductivity, but also a promising buffer layer material for oriented growth.

Section snippets

Experimental

Two solutions were fabricated with nominal compositions of Na0.5Bi0.5TiO3 (NBT) and BaTiO3 (BT), respectively.

The NBT sol was synthesized first. Sodium acetate, CH3COONa·3H2O, bismuth nitrate, Bi(NO3)3·5H2O, were mixed and stirred in acetic acid at 70 °C for 120 min, which was denoted as sol 1. On the other hand, acetylacetone (CH3COCH2COCH3) was mixed with 2-methoxyethanol, stirred for 30 min, and we added titanium tetra-n-butyl into the mixture. The mixture was stirred constantly until a

Results and discussion

Fig. 2 shows the XRD patterns of the NBT-BT thin films deposited on Pt/Ti/SiO2/Si (1 1 1) and LNO/Pt/Ti/SiO2/Si (1 1 1) substrates. The XRD patterns indicated that the thin film deposited on Pt/Ti/SiO2/Si substrates is polycrystalline structure. But, films deposited on LNO/Pt/Ti/SiO2/Si substrates showed a highly preferred orientation of (1 0 0). The degree of grain orientation can be calculated by Lotgering factor f, which is defined as [10]f=pp01p0wherep=iI(h00)iI(hkl)p0=iI(h00)iI(hkl)

Conclusions

(Na0.5Bi0.5)0.94Ba0.06TiO3 thin films were deposited on Pt/Ti/SiO2/Si (1 1 1) and LNO/Pt/Ti/SiO2/Si (1 1 1) substrates by a sol–gel process. The X-ray diffraction pattern indicated that the NBT-BT thin film deposited on Pt/Ti/SiO2/Si (1 1 1) substrates is polycrystalline structure without any preferred orientation. But the NBT-BT thin film deposited on LNO/Pt/Ti/SiO2/Si substrates shows highly (1 0 0) orientation. NBT-BT thin film deposited on LNO/Pt/Ti/SiO2/Si substrates possessed a much lower leakage

Acknowledgement

The authors would like to acknowledge the support from the National Natural Science Foundation of China under grant Nos. 50972108 and 50932007.

References (14)

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