Effect of annealing temperature on ferroelectric properties of (Bi, Nd)₄(Ti, V)₃O₁₂ thin films

YE Zhi(叶 志)¹, TANG Ming-hua(唐明华)^{1, 2}, CHENG Chuan-pin(成传品)¹, ZHOU Yi-chun(周益春)^{1, 2},
ZHENG Xue-jun(郑学军)^{1, 2}, HU Zeng-shun(胡增顺)¹

1. Faculty of Materials and Optoelectronic Physics, Xiangtan University, Xiangtan 411105, China;

2. Key Laboratory of Advanced Materials and Rheological Properties of Ministry of Education, Xiangtan 411105, China

Received 10 April 2006; accepted 25 April 2006

Abstract: Thin films of Nd³⁺/V⁵⁺-cosubstituted bismuth titanate, (Bi_3.5Nd_0.5)( Ti_2.96V_0.04)O₁₂ (BNTV), were fabricated on the Pt(111)/Ti/SiO₂/Si(100) substrates by a chemical solution deposition technique and annealed at different temperatures of 650, 700, 750 and 800 ℃. The surface morphology and ferroelectric properties of the samples were studied in detail. The result shows that the film annealed at 800 ℃ indicates excellent ferroelectricity with a remanent polarization of 2P_r=40.9 μC/cm², a coercive field (E_C) of 114 kV/cm at an applied electrical field of 375 kV/cm. The substitution of Ti-site ion by V⁵⁺ ions could improve the upper limit of the optimal annealing temperature by decreasing the space charge density in BNT thin film. Additionally, the mechanism concerning the dependence of ferroelectric properties of BNTV thin films on the annealing temperature was discussed.

Key words: ferroelectric properties; annealing; scanning electron microscopy; crystallization

1 Introduction

More and more attentions have been attracted to the ferroelectric thin films recently, because of its far-ranging applications such as nonvolatile random access memories (FeRAMs), microelectromechanical systems, pyroelectric detectors, integrated optical modulators, actuatiors and infrared sensors [1, 2]. Although lead zirconate titanate (PZT) has some of the better ferroelectric characteristics of lower processing temperature, PZT thin films suffer from serious fatigue and imprint problems [3]. As the instead of the conventional lead-based materials, films of some kinds of bismuth layer-structured ferroelectrics (BLSFs) have the characteristics of large remanent polarization, high fatigue resistance, good retention, lead-free chemical composition and so on [4, 5]. Among BLSFs, Bi₄Ti₃O₁₂ (BIT) is the most widely studied thin film. Many researchers have attempted to enhance polarization of BIT thin films using various techniques. ‘Site engineering’ may be an effective technique for improving ferroelectric properties [6]. La-substituted Bi₄Ti₃O₁₂(BLT) films have been very attractive for their larger ferroelectricity than BIT and good fatigue resistance. However, the reports of BLT thin films with large ferroelectricity are limited to the films prepared at relatively high temperature above 650 ℃. In order to obtain lower temperature deposition of BLT, WATANABE et al [7] reported that La and V-cosubstituted BIT (BLTV) thin films prepared at a relatively low temperature of 600 ℃ by MOCVD showed large ferroelectricity.

In our group, the BNT films of excellent ferroelectric properties have been fabricated by chemical solution deposition (CSD) [8, 9]. While UCHIDA et al [1] reported that substitution of Ti-site ion by other ions with higher charge valences (e.g., V⁵⁺ and W⁶⁺) improved the ferroelectric properties by the decrease of space charge density. So UCHIDA et al [1] fabricated Nd³⁺/V⁵⁺-cosubstituted BIT thin films (BNTV) by CSD, which showed larger remanent polarization than BNT thin films [(Bi_3.5Nd_0.5)Ti₃O₁₂: P_r=32 μC/cm², E_C=126 kV/cm; (Bi_3.5Nd_0.5)(Ti_2.98V_0.02)O₁₂: P_r=37 μC/cm², E_C=119 kV/ cm, 450 kV/cm].

In the present study, Nd³⁺/V⁵⁺-cosubstituted BIT thin film was fabricated on platinum-coated silicon [Pt(111)/Ti/SiO₂/Si(100)] substrate by CSD. In order to investigate the temperature dependence of ferroelectric properties of BNTV thin films, the films are crystallized at a moderate annealed temperature (650-800 ℃).

The (Bi_3.5Nd_0.5)(Ti_2.96V_0.04)O₁₂ (BNTV) thin films were fabricated on Pt(111)/Ti/SiO₂/Si(100) substrates using a CSD technique at room temperature. The precursor solution for the coating was prepared by first dissolving appropriate amounts of bismuth nitride [Bi(NO₃)₃?5H₂O], neodymium nitride [Nd(NO₃)₃] and vanadium (III) acetylacetonate [V(C₅H₈O₂)₃] in acetic acid [CH₃COOH]. The 10%(in more fraction) excess bismuth nitride was needed to compensate the Bi loss during the high thermal process. A stoichiometric amount of titanium butoxide [Ti(OC₄H₉)₄] was added to the mixed precursor solution rapidly. Then, acetylacetone as a stabilizing agent to stabilize the solution was added to the solution. Finally, glacial acetic was mixed to adjust the concentration. A clear yellow sol with a molar concentration of 0.05 mol/L was obtained. The solution was spin-coated on substrates at a rate of 3 000 r/min for 50 s, followed by a drying process at 100 ℃ for 5 min and a pyrolysis process at 400 ℃ for 5 min with a rise rate of 1 ℃/s to remove residual organic compounds. These processes were repeated eight times to achieve desired film thickness. The resultant films were annealed for crystallization for 10 min in air by a rapid thermal annealing process at various temperatures of 650, 700, 750 and 800 ℃, respectively.

The constituent phase and crystal orientation of the films were identified by X-ray diffraction (XRD) using a Rigaku D/max-rA diffractometer with Cu K_α radiation at 40 kV. The surface morphology and the thickness of the films were investigated using a JSM-5600LV scanning electron microscope (SEM). For electrical measurements, Au was sputtered on the surface of the films as top electrode with diameters of 0.2 mm using a metal mask. Then, the ferroelectric properties were measured using a radiant technology precision workstation ferroelectric tester with the virtual ground mode.

Fig.1 shows the XRD patterns of BNTV thin films annealed under various temperatures ranging from 650 to 800 ℃ at an interval of 50 ℃. The diffraction peaks were identified and indexed using the standard XRD data of Bi_3.6Nd_0.4Ti₃O₁₂ powder. It is found that all of the films consist of a single phase of a bismuth layered structure showing the preferred (00l) and (117) orientation. This indicates that the film already have the good crystallinity when the annealing temperature is above 650 ℃. With increasing annealing temperature, it can be seen from Fig.1 that the (00l) and (117) diffrac- tion peaks become stronger and sharper. Compared with those of films annealed at other temperatures, the film annealed at 800 ℃ has stronger (00l) and (117) peaks， and the latter peak is the strongest one. Moreover, the intensity of (0014) peak rapidly increases with increasing annealing temperature, and finally equals to the (006) peak which is the second stronger peak. Thus, we can see that the film annealed at 800 ℃ has better crystallinity. ZHONG et al [8] reported that the BNT thin films exhibit preferred (00l) orientation at an annealing temperature above 700 ℃. While some researchers reported that the BNT films have the absolute (117) preferred orientation compared with the (00l) peaks [1, 10]. However, the XRD results of our V-doped BNT films reveal that the mixed (00l) and (117) orientations have the comparable intensity.

Fig.1 XRD patterns of BNTV thin films annealed at different temperatures varying from 650 to 800 ℃

The surface morphologies of the sample annealed at 800 ℃ is shown in Fig.2. It is found that the film shows dense and smooth microstructure in Fig.2 (a) which is observed at a relatively low magnification, and the average grain size of the sample is approximately 0.25 μm. We also find from Fig.2 (b), with a relatively high magnification, that the grains randomly distributed on the substrate are not uniform, but consist of mixed rod-like and plated-like ones, which is in good agreement with [10]. It has been reported that, for La-doped BIT thin films, the rod-like grains are presented in (117) preferentially oriented films, while the plate-like grains may correspond to the c-axis or (00l) preferred orientation [1, 11, 12]. Thus, the mixed rod-like and plated-like grains found in the present BNTV thin films are consistent with the XRD results shown in Fig.1, where

Fig.2 SEM surface morphology of BNTV thin film annealed at 800 ℃ under two different magnifications

mixed (00l) and (117) orientations are observed. Additionally, the film thickness estimated from the cross-sectional micrograph by SEM is about 400 nm.

Fig.3 shows the polarization-electric field (P-E) hysteresis loops of the prepared BNTV thin film annealed at temperatures in the range from 650 ℃ to 800 ℃. The hysteresis loops of all the films are well saturated. By increasing the annealing temperature of the films, the degree of squareness of the hysteresis loops of the cosubstituted films is clearly improved. Fig.4 shows the changes of 2P_r and E_C with the various annealing temperatures. Under a maximum applied field of 375 kV/cm, the 2P_r values of the prepared films annealed at 650, 700, 750 and 800 ℃ are 8.8, 13.8, 22.3 and 40.9 μC/cm², and corresponding E_C are 97, 102, 120 and 114 kV/cm, respectively. Obviously, the 2P_r increases with increasing the annealing temperature and the film has the largest 2P_r value of 40.9 μC/cm² at 800 ℃. But there is a sudden decrease of E_C with increasing annealing temperature, when the temperature increases above 750 ℃. Therefore, the BNTV film has a lower E_C at 800 ℃ than at 750 ℃. Some articles reported that the remnant polarization of BNT thin films often decreases with increasing the annealing temperature at an optimal crystalline temperature below 800 ℃ [8, 10, 13]. Two possible reasons are suggested for the phenomena. One is that the element of bismuth in the film is evaporative at higher annealing temperature. Another is that the relative intensity of the (00l) and (117) in the films will change with the crystalline temperature [14]. Our experimental results have confirmed that the cosubstitution of Nd and V is effective to improve the optimal crystalline temperature. The reasons may be due to that the V⁵⁺ substitution would lead to a decrease of the space charge density of BNT thin film by compensating the charge valence of oxygen vacancies [7]. Therefore, with increasing annealing temperature, the intensity of (00l) peaks increases rapidly to catch up with the (117) peak, and the ferroelectric properties of the films become well.

Fig.3 P-E hysteresis loops of BNTV thin films annealed at different temperatures

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Fig.4 Variations of 2P_r and E_C against annealing temperature of BNTV thin films

In summary, Nd³⁺/V⁵⁺ -cosubstituted BIT thin films [(Bi_3.5Nd_0.5)(Ti_2.96V_0.04)O₁₂] were fabricated on Pt(111)/Ti/SiO₂/Si(100) substrates using a CSD technique. The effect of higher charge valences V⁵⁺ ions substitution of Ti-site ion on ferroelectric properties under different crystalline temperatures was investigated. The surface morphologies confirm the good crystallinity and crystalline orientation of mixed a axis and c axis at 800 ℃. Due to the improvement of crystalline of BNTV thin films with increasing annealing temperature, the 2P_r values of the samples increase with increasing annealing temperature, while E_C values begin to decrease with further increasing annealing temperature when the annealing temperature is above 750 ℃. So a well saturated hysteresis loop with better squareness is obtained for the polycrystalline BNTV thin films annealed at 800 ℃. The 2P_r and E_C are 40.9 μC/cm² and 114 kV/cm (at the electric field of 375 kV/cm), respectively. Thus, the effect of V⁵⁺ substitution of Ti-site to improve the upper limit of optimal annealing temperature of BNT thin film is obvious.

References

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(Edited by HE Xue-feng)

Foundation item: Project (05FJ2005) supported by the Key Project of Scientific and Technological Department of Hunan Province, China; Project(05C095) supported by the Research Funds of Educational Department of Hunan Province of China

Corresponding author: TANG Ming-hua; Tel.: +86-732-8293577; Fax: +86-732-8292468. E-mail address: mhtang@xtu.edu.cn