Electrical characteristics and microstructures of

Pr₆O₁₁-doped Bi₄Ti₃O₁₂ thin films

CHEN Min(陈 敏)^{1, 2}, HUANG Ke-long(黄可龙)², MEI Xiao-an(梅孝安)¹,

HUANG Chong-qing(黄重庆)¹, LIU Jing(刘 靖)¹, CAI An-hui(蔡安辉)³

1. Department of Physics, Hunan Institute of Science and Technology, Yueyang 414000, China;

2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

3. Powder Metallurgy Research Institute, Central South University, Changsha 410083, China

Received 9 January 2008; accepted 5 May 2008

Abstract: Pr₆O₁₁-doped bismuth titanate (Bi_xPr_yTi₃O₁₂, BPT) thin films with random orientation were fabricated on Pt/Ti/SiO₂/Si substrates by rf magnetron sputtering technique, and the structures and ferroelectric properties of the films were investigated. XRD studies indicate that all of BPT films consist of single phase of a bismuth-layered structure with well-developed rod-like grains. For samples with y=0.06 , 0.3, 1.2 and 1.5, I—E characteristics exhibit negative differential resistance behaviors and their ferroelectric hysteresis loops are characterized by large leakage current. Whereas for samples with y=0.6 and 0.9, I—E characteristics are of  simple ohmic behaviors and their ferroelectric hysteresis loops are saturated and undistorted. The remanent polarization (P_r) and coercive field (E_c) of the BPT Film with y=0.9 are above 35 μC/cm² and 80 kV/cm, respectively.

Key words: ferroelectric; film; bismuth titanate; doping

1 Introduction

Bismuth titanate layered perovskite films have received much attention because of their potential for technological applications[1-3]. Bismuth titanate, Bi₄Ti₃O₁₂ (BIT), is a typical ferroelectric material with the spontaneous polarization lying on the a-c plane at about 4.5? to the a-axis and exhibiting two independently reversible components of approximate magnitude of    4 μC/cm² along the c-axis and 50 μC/cm² along the a-axis [2]. Generally, the formula of doped bismuth titanate is (Bi₂O₂)²⁺(A_m–1B_mO_3m+1)^2–, where A means mono-, di-, or trivalent ions, or a mixture of them; B means quadri- or quinquevalence ions, such as Ti⁴⁺, Nb⁵⁺, Ta⁵⁺; and m means integer number (＞1). The electrical conductivity, dielectric and optical properties of many doped (Sr, Nb, W, etc) bismuth titanate perovskites based on Bi₄Ti₃O₁₂ compound have been studied[4-6]. In 1999, PARK et al[2] first reported that ferroelectric lanthanum- substituted bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂) thin films by pulsed laser deposition exhibited a fatigue-free characteristic after 3×10¹⁰ read/write cycles. Therefore, this material is considered as a promising candidate for non-volatile memory devices. They suggested that the fatigue-free characteristics of Bi_3.25La_0.75Ti₃O₁₂ thin films resulted from some Bi ions being substituted near the Ti-O octahedron layers by La ions, and that there was some room for improvement by varying the amount of La substitutes, or by substituting other rare-earth ions. Recently, these kinds of materials doped with rare earth elements attract much attention for a large remnant polarization and fatigue-free characteristic[1-3, 6-8]. In order to obtain better properties, a lot of works about Bi₄Ti₃O₁₂ doped with the rare earth elements such as La, Pr, and Sm have been reported[9-12]. However, the effects of Bi ions substituted by rare earth ions have not been clarified. The authors of this article first reported the work about Bi₄Ti₃O₁₂ doped with Pr₆O₁₁ and the results indicated that the substitution of Pr ion for Bi ion can much improve the ferroelectric properties of Bi₄Ti₃O₁₂ ceramics[13]. Praseodymium as another rare earth element may be a good additive of Bi₄Ti₃O₁₂. In the present work, Pr₆O₁₁-doped Bi₄Ti₃O₁₂ thin films were fabricated with rf magnetron sputtering technique, and their structures and electrical properties were studied in details.

2 Experimental

Bi_xPr_yTi₃O₁₂ (y=0.06-1.5) targets were prepared by solid-state reaction of mixture of Bi₂O₃ (99.9%), Pr₆O₁₁ (99.95%) and TiO₂ (99.9%) powders. Raw materials of targets were mixed by ball milling with agate balls and ethanol for 24 h. The powders with binder addition were pressed into discs of 50 mm in diameter and about 4 mm in thickness at a pressure of 120 MPa. The discs were sintered at 1 100 ℃ for 2 h in air, and furnace-cooled to room temperature. BPT thin films (about 400-500 nm in thickness) were grown on Pt/Ti/SiO₂/Si substrates by rf magnetron sputtering technique. The deposition temperature and ambient oxygen pressure were optimized to be 400 ℃ and 20 Pa, respectively. The as- grown thin films were annealed for 1 h in an oxygen atmosphere at 650 ℃. For electrical measurements, the Pt electrodes of 100 nm were deposited onto BPT films at 400 ℃ through a shadow mask on an area of 2×10^{-4 cm2}. The phase constitutions were characterized by Rigaku X-ray diffractometer (D/MAX-RB) with Cu K_α radiation. The surface morphology was observed by Scanning Electron Microscope (SEM) (JEOL JSM- 6300). The ferroelectric hysteresis loops and the leakage current were measured with the Radiant RT66A Unit. The current—voltage characteristics were measured by means of a dc power supply and a digital multimeter controlled by a computer.

3 Results and discussion

3.1 Microstructures of BPT films

Fig.1 shows XRD patterns of BPT films annealed at 650 ℃. XRD studies indicate that all of the BPT films consist of single phase of a bismuth-layered structure

Fig.1 XRD patterns of BPT films annealed at 650 ℃

showing a highly (117) oriented preferential growth with a minor fraction of (00l) orientation. The results show that Pr-doping does not affect the crystal orientation of the BIT film. According to these results, it is possible to think that the Bi-layered structure is always maintained in BPT and rather insensitive to the amount of Pr. The Bi-layered perovskite structure of BIT can be maintained for such high amount of Pr-doping because the radii of Bi and Pr are very close and some Bi ions are only substituted near Ti-O octahedron layers by Pr ions.

The SEM images of surface morphology of BPT films with y=0.3 and 0.9 annealed at 650 ℃ are shown in Fig.2. The images indicate that the BPT films consist of well-developed rod-like grains with random orientation. The average length and diameter of the grains of the film with y=0.9 are about 350 nm and 150 nm, respectively. This implies that the film with y=0.9 promotes bismuth titanate grain growth greater than the film with y=0.3.

Fig.2 SEM micrographs of BPT films with y=0.3 (a) and y= 0.9(b)

3.2 Ferroelectric properties of BPT films

The P—E hysteresis loops of BPT (y=0.06-1.5) films at room temperature are shown in Fig.3. Hysteresis loops of BPT films with y≤0.3 and y≥1.2 are distorted due to leakage current. With y value increasing, the remanent polarization and coercive field of films with    y≤0.9 increase, whereas for films with y＞0.9, the remanent polarization and coercive field decrease. Well- saturated loops are measured in BPT films with y= 0.6, 0.9. The P_r and the E_c values of the BPT film with y=0.9 are 35 μC/cm² and 80 kV/cm, respectively. Without controlling the crystal orientation, the P_r of the polycrystalline BPT film with y=0.9 can exceed 35 μC/cm², much larger than that of the BLT film[2] and comparable to that of the BNTV film[14]. Meanwhile, the E_c is remarkably lower than that of the BNTV film (about 150 kV/cm)[14] and comparable to that of the BLT films[2]. Thus, the BPT film with large P_r and low E_c seems to be more suitable for application as ferroelectric random access memoer(FRAM). The Bi_2.9Pr_0.9Ti₃O₁₂ ceramic prepared by LIU et al[13] showed that the remanent polarization and the coercive were   26 μC/cm² and 52 kV/cm, respectively. The differences of ferroelectric properties between Pr-doped film and ceramic might originate from the difference of microstructure.

Fig.3 P—E hysteresis loops of Bi_xPr_yTi₃O₁₂ films: (a)y=0.06; (b)y=0.3; (c) y=0.6; (d) y=0.9; (e) y=1.2; (f) y=1.5

NOGUCHI and MIYAYAMA[15], UCHIDA et al[14] discussed the possible origin of good ferroelectric properties of the BTV ceramics and the BNTV films, respectively. They pointed out that V⁵⁺ ions substitution for Ti⁴⁺ ions caused a decrease in Bi³⁺ ions content (Bi³⁺vacancies increasing), and inhibited the generation of oxygen vacancies. However, in the present work, Pr doping is designed to substitute for Bi³⁺ ions rather than for Ti⁴⁺ ions, and the equation of composition is Bi_2.9Pr_0.9Ti₃O₁₂ according to the charge neutrality condition. It is known that the valence of Pr ion is 3⁺ and 4⁺, respectively, and Pr₆O₁₁ can be divided into two parts by

Pr₆O₁₁→Pr₂O₃+4PrO₂                                   (1)

Three Pr⁴⁺ ions should be companied by four Bi³⁺ vacancies if Pr substituted only for Bi³⁺ ions. Therefore, it is indicated that Pr⁴⁺ ions substitution for Bi³⁺ ions efficiently suppresses the generation of oxygen  vacancies, similar to V⁵⁺ ions doping into BIT[14-15], Pr³⁺ ions substitution for Bi³⁺ ions, and La³⁺ and Nd³⁺ ions doping into BIT[3, 7-16]. Both Pr³⁺ ions substitution and Pr⁴⁺ ions substitution for Bi³⁺ ions are effective to derive enough ferroelectricity by efficient decrease in space charge density caused by Bi³⁺-site substitution. This speculation is consistent with experimental results. In fact, the Pr doping into BIT draws out a much larger P_r than that of non-doped BIT and leads to a marked improvement in leakage current properties for BPT films with y=0.6 and 0.9. These experimental results imply that the Pr doping into BIT leads to an efficient decrease in space charge density.

3.3 I—E characteristics of BPT films

Fig.4 shows current—voltage (I—E) characteristics of Bi_xPr_yTi₃O₁₂ samples. In the same region of applied electric field, the current values of samples with y=0.6 and 0.9 are about 1-2 order of magnitude lower than those of the samples with y=0.06, 0.3, 1.2, and 1.5. The  I—E curve of the sample with y=0.06 is not entirely symmetric with the direction of applied field and exhibits current peaks near -100 and 23 kV/cm, so called the voltage-controlled negative differential resistance (VNDR) behavior. I—E curves of samples with y=0.3, 1.2, and 1.5 are asymmetric with the direction of applied field. Near -90, -70, and -65 kV/cm, the samples with y=0.3, 1.2, and 1.5 also exhibit current peaks, respectively, which are lower and broader than those of y=0.06 sample. Both y=0.6 and 0.9 samples exhibit linear current—voltage behavior, as shown in Fig.4.

The voltage-controlled negative differential resistance(VNDR) behavior has been observed in many material systems, such as electroformed metal-insulator- metal(MIM) thin film sandwich structures, amorphous semiconductors and semiconductor quantum devices, and some electrical models have been suggested[16]. The VNDR of the film with y=0.06 can be explained based on the thermal model of filamentary conductivity[16]. Clearly, The I—E curve of the sample y=0.06 is not symmetric when the direction of the applied field is reversed because of polarization response. However, it is difficult to use conducting filamentary model [16] to describe the VNDR of films with y=0.3, 1.2 and 1.5 because it is asymmetric related to the direction of applied field. It is noticed that I—E curves of samples with y=0.3, 1.2 and 1.5 show linear character provided that the current peaks vanish. Therefore, assuming that the conducting filaments exist in these Pr-doped samples and are of characteristic of rectifying like a p-n junction, the VNDR appearing asymmetrically in these I—E curves can be explained. For applied positive field, the conducting filaments manifest high resistance, and the ohmic conductivity controls the I—E characteristics. For negative field, the conducting filaments have low resistance because of their rectifying characteristics and the current increases quickly with the electric field increasing. The larger the current is, the greater the heat effect appears. The conducting filaments are ruptured progressively due to thermal processes, resulting in an eventual decrease in current, then a current peak appears. Once all conducting filaments are ruptured, the I—E characteristic shows ohmic conductivity, that is, a linear behavior. This implies that the origin of conducting filaments should be due to the local structural change caused by Pr doping.

Fig.4 I—E curves of Bi_xPr_yTi₃O₁₂ films

4 Conclusions

1) Pr-doping does not affect the crystal orientation of Bi₄Ti₃O₁₂film. Experimental results reveal that all of BPT films consist of single phase of a bismuth-layered structure and well-developed rod-like grains with random orientation.

2) Pr-doping results in dramatic improvement in ferroelectric properties of Bi₄Ti₃O₁₂ film. The remanent polarization and coercive field of the Bi_xPr_yTi₃O₁₂ Film with y=0.9 are above 35 μC/cm² and 80 kV/cm, respectively. The improvement of ferroelectric properties of Bi₄Ti₃O₁₂ film is due to both Pr³⁺ ions and Pr⁴⁺ ions simultaneous substitution for Bi³⁺ ions.

3) For Bi_xPr_yTi₃O₁₂ films with y=0.06 , 0.3, 1.2, and 1.5, I—E characteristics exhibit negative differential resistance behaviors and their ferroelectric hysteresis loops are characterized by large leakage current, whereas for samples with y=0.6 and 0.9, I—E characteristics show simple ohmic behaviors and their ferroelectric hysteresis loops are saturated and undistorted.

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Foundation item: Project (50774034) supported by the National Natural Science Foundation of China; Project (06JJ20005) supported by the Natural Science Foundation of Hunan Province, China; Project (05A055) supported by the Educational Science Foundation of Hunan Province, China

Corresponding author: CHEN Min; Tel: +86-730-8646505; E-mail: chenmin581215@yahoo.com.cn

DOI: 10.1016/S1003-6326(08)60241-1

(Edited by YANG Bing)