Enhanced dielectric and ferroelectric properties of PZT thin films derived by an ethylene glycol modified sol–gel method Guoping Lu, Hanting Dong, Jianguo Chen & Jinrong Cheng
Journal of Sol-Gel Science and Technology ISSN 0928-0707 Volume 82 Number 2 J Sol-Gel Sci Technol (2017) 82:530-535 DOI 10.1007/s10971-017-4311-5
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Author's personal copy J Sol-Gel Sci Technol (2017) 82:530–535 DOI 10.1007/s10971-017-4311-5 ORIGINAL PAPER: SOL-GEL AND HYBRID MATERIALS FOR DIELECTRIC, ELECTRONIC, MAGNETIC AND FERROELECTRIC APPLICATIONS
Enhanced dielectric and ferroelectric properties of PZT thin films derived by an ethylene glycol modified sol–gel method Guoping Lu1 Hanting Dong1 Jianguo Chen1 Jinrong Cheng1 ●
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Received: 10 October 2016 / Accepted: 15 January 2017 / Published online: 4 February 2017 © Springer Science+Business Media New York 2017
Abstract PbZr0.52Ti0.48O3 thin films on stainless steel substrates were fabricated by an ethylene glycol modified sol–gel method. Perovskite structure of PbZr0.52Ti0.48O3 thin films was examined by the X-ray diffraction analysis. With the increase of ethylene glycol, PbZr0.52Ti0.48O3 thin films with enhanced dielectric and ferroelectric properties were got. Crack-free PbZr0.52Ti0.48O3 thin films with thickness of 1.6 μm were prepared for ethylene glycol content of 50%, and the dielectric permittivity and dielectric loss were 224 and 5.4% respectively at the frequency of 1 kHz showing better dielectric properties compared with PbZr0.52Ti0.48O3 thin films without ethylene glycol modified. The remnant polarization (Pr) of 50% content ethylene glycol modified PbZr0.52Ti0.48O3 thin films was nearly twice reaching 12.3 μC/cm2, and lower leakage current density was obtained with 4.6 × 10−6 A· cm−2 under the field of 50 kV· cm−1. Such enhanced performance indicated that the PbZr0.52Ti0.48O3 thin films prepared on stainless steel substrates by the ethylene glycol modified sol–gel process exhibited their potentiality in applications.
Graphical abstract
Keywords PZT thin films Modified sol–gel SS substrates Ferroelectric properties ●
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1 Introduction
* Jinrong Cheng
[email protected] 1
School of Materials Science and Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, People’s Republic of China
Lead zirconate titanate (PZT) film with composition near the morphotropic phase boundary has attracted great attention for applications in microelectromechanical systems, actuators, sensor, and high power capacitors because of its superior dielectric, ferroelectric, and piezoelectric properties [1–4]. Ferroelectric thin films deposited on metal substrates are beneficial to integrating films with engineering system, revealing high work efficiency, rapid response, and high signal-noise ratio [5]. Moreover, stainless steel substrate serves as bottom electrode avoiding the complex micro-machined processes in the fabrication process [6]. Sol–gel methods have been used extensively for the
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fabrication of high quality PZT thin films due to their easy stoichiometry control and cost-effectiveness [7, 8]. In the application of many devices, the thickness of ferroelectric films exceeding 3 μm is needed. However, the thickness of single layer PZT thin film derived from the conventional sol–gel method is generally limited to 0.1 μm [9]. It is very difficult to prepare crack-free PZT thin films with thickness above 3 μm by the multiple-spinning process. As a consequence, polyvinylpyrrolidone (PVP) has been introduced into the precursor solutions to prepare PZT and PLZT thick films, however, addition of PVP will deteriorate the ferroelectric and dielectric properties [10–12]. Chelating ligand like ethylene glycol (EG) is commonly used in chemical sol preparation as stabilizing agents. It also acts as strong nucleophile to promote the chemical reaction for the grain growth which facilitates three dimensional grain growths by establishing the linkage between molecules (like bridges or chelating rings) [13, 14]. The introduction of EG promotes the reaction to achieve the modification of PZT thin film on dielectric and ferroelectric properties. In this work, PZT thin films with Zr/Ti ratio of 52/48 were deposited on the stainless steel substrates due to their great ferroelectric, piezoelectric and electromechanical properties [15, 16]. PVP was added into the precursor solution to increase the thickness of single layer PZT thin films according to the reference papers [10, 17]. The 2methoxyethanol solvent was mixed with EG. The effects of EG concentration on the structural, dielectric and ferroelectric properties of PZT thin films were investigated. It is found that the ferroelectric and dielectric properties of PZT thin films were improved with the help of EG.
2 Experimental details Lead acetate [Pb(CH3COO)2], Zirconium n-propoxide [Zr (OC3H7)4], and tetrabutyl titanate [Ti(OC4H9)4] were used as raw materials for preparing PZT thin films. Pb (CH3COO)2 was 20 mol% excess to compensate Pb loss during heat treatment. Ethylene glycol monomethylether (CH3OCH2CH2OH, 2-MOE) and ethylene glycol ((CH2OH)2, EG) were used as solvent. PVP with average molecular weight of 38,000 (k-30) was introduced into the precursor with the molar ratio of PZT:PVP = 1:1. PbZr0.52Ti0.48O3 precursor solutions of 0.5 M concentration were prepared for EG volume percent content with 0, 25, 33, and 50% respectively. The LSCO sol were prepared by using lanthanum nitrate, strontium nitrate, and cobaltous acetate as starting materials, which were mixed in a molar ratio of 1:1:2 dissolved in the mixed solvents of acetic acid and deionized water to obtain LSCO sol of 0.1 mol/L. Six layers of LSCO were firstly deposited on the stainless steel substrates and crystallized at 700 °C for 10 min to serve as a
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buffer layer, which improved properties of the ferroelectric film as other reports [18, 19]. Then, the same layers of different EG contained PZT thin films were deposited on the top of LSCO followed by the crystallization at 650 °C for 30 min. The Au electrode with diameter of 0.4 mm was sputtered on the top of PZT thin films through a shadow mask. The phase structure of identification PZT thin films was characterized by X-ray diffraction system (XRD, Rigaku D/ Max-2200V, Japan) using Cu Kα radiation. The film surfaces and cross-section morphologies were observed by a field-emission scanning electronic microscope (FESEM, JEOL, JSM-7000F for cross-section morphologies and FESEM, Hitachi, S-4800 for surface morphologies, Japan). Micro-Raman spectra (LABHR-UV) of the samples were recorded with the 514.5 nm line of an Ar+ laser. Dielectric properties were measured by an Agilent 4294A impedance analyzer at room temperature. Radiant Technology Ferroelectric tester (Precision Premier II) was used to measure the ferroelectric hysteresis loops and the J–V curve using the step delay and soak time of 100 ms.
3 Results and discussion Figure 1 exhibits XRD patterns of the PZT thin films for various EG concentrations. It can be seen that PZT thin films have the rhombohedral perovskite structure without detectable second phase. With increasing the EG content, the (110) diffraction peaks of PZT thin films shift toward higher 2θ angle indicating a decrease of lattice parameter. This phenomenon is probably due to the released internal stress, which also can be reflected by the following Raman spectra. Figure 2 shows the Raman spectra of PZT thin films for different EG contents. Raman peaks at 576.9, 570.2, 565.4, and 559.1 are identified for PZT thin films with different EG content in solvent of 0, 25, 33, and 50% respectively, which are extracted from Fig. 2. The peak of A1(TO3) mode of PZT solid solutions usually appears at
Fig. 1 XRD patterns of PZT thin films with different EG content
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Fig. 2 Raman spectra of PZT thin films with different EG content
~560 cm−1. As a result, A1(TO3) [transverse optical3] mode is used to calculate the applied stress due to its obvious peak intensity in Raman spectra. According to Lyddane-SachsTeller relation, the correlation between the phonon frequency ω and the applied stress σ can be expressed as σ 2 2 ω ¼ ω0 1 ð1Þ σ1 where σ1 is the stress, under which the phonon frequency becomes zero and ω0 is the phonon frequency under zero stress. Meanwhile, ω0 is obtained at 528.8 cm−1 and σ1 for −640.7 MPa assigned as A1(TO3) modes as reported [20]. According to Eq. (1), the corresponding stresses of different EG contained PZT thin films are calculated as 121.9, 104.2, 91.8, and 75.5 MPa respectively. It is obviously that stress in PZT thin films has a decrease with the increased volume ratio of EG in the solvent. The cause of this phenomenon can be explained that EG will further degrade Young’s modulus of PVP-contained films and contribute to compressive stress release. PVP is generally regarded as a stress-relaxing agent [21]. Figure 3 shows SEM images of PZT thin films with different EG content of 0, 25, and, 50% respectively. It is observed from cross-section morphologies that the thickness of PZT thin films is increased, when EG is added in the solvent. With the same layers spin-coated, the thickness of PZT thin film without EG addition reached ~1.2 μm. When the 25% EG content introduced, the thickness has an obvious increase reaching ~1.5 μm. However, as the content of EG increases from 25 to 50%, the thickness of PZT thin films is just increased from ~1.5 to ~1.6 μm. It illustrates that EG modified precursor solution will dramatically improve the thickness of PZT thin film and the content of EG has a little effect on the thickness. From what shown in surface morphologies, grain sizes of different EG content contained PZT thin films have obvious changes. Compared to the PZT thin film prepared without EG, film containing greater ratio of EG shows larger grain size of average around 110 nm as
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presented in Fig. 3. The addition of EG may help the formation of nucleation and promote grain growth by establishing the linkage between molecules like bridges. And then, a trend toward larger grain size distribution plays a significant role in promoting coalescence of small grains to form large grains in three dimensional directions. Figure 4 demonstrates dielectric constant (εr) and dielectric dissipation factor (tanδ) as a function of frequency for films with various EG concentrations of 0, 25, 33, and 50%. Dielectric constant of PZT thin films at the frequency from 1 k to 1 M Hz exhibits a small increase regularly with the concentration rise of EG. This indicates that there is a significant effect of EG addition on improving the dielectric constant due to the increased grain size within a certain range limit (
