ferroelectric and piezoelectric properties of bismuth titanate

8/3/2019 Ferroelectric and Piezoelectric Properties of Bismuth Titanate

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Ferroelectric and piezoelectric properties of bismuth titanatethin lms grown on different bottom electrodes by soft

chemical solution and microwave annealingA.Z. Simo es a ,*, M.P. Cruz b ,c ,d , A. Ries a , E. Longo a , J.A. Varela a , R. Ramesh c ,d

a Chemistry Institute, Universidade Estadual Paulista (UNESP), Rua Prof. Francisco Degni s/n, 14801-970 Araraquara, SP, Brazilb Centro de Ciencias de la Materia Condensada (CCMC), UNAM, Km 107, Carretera Tijuana-Ensenada, Ensenada, BC, C.P. 22800, Mexico

c Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, United Statesd Department of Physics, University of California, Berkeley, CA 94720, United States

Received 1 June 2006; accepted 9 August 2006Available online 11 September 2006

Abstract

Bismuth titanate (Bi 4Ti3O12 , BIT) lms were evaluated for use as lead-free piezoelectric thin lms in micro-electromechanicalsystems. The lms were grown by the polymeric precursor method on LaNiO 3 /SiO2 /Si (1 0 0) (LNO), RuO 2 /SiO2 /Si (1 0 0) (RuO 2 )and Pt/Ti/SiO 2 /Si (1 0 0) (Pt) bottom electrodes in a microwave furnace at 700

8 C for10 min. Thedomain structure was investigatedby piezoresponse force microscopy (PFM). Although the converse piezoelectric coefcient, d 33 , regardless of bottom electrode isaround ( $ 40 pm/V), those over RuO 2 and LNO exhibit better ferroelectric properties, higher remanent polarization (15 and 10 m C/ cm2 ), lower drive voltages (2.6 and 1.3 V) and are fatigue-free. The experimental results demonstrated that the combination of thepolymeric precursor method assisted with a microwave furnace is a promising technique to obtain lms with good qualities for

applications in ferroelectric and piezoelectric devices.# 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Thin lms; B. Chemical synthesis; D. Ferroelectricity

1. Introduction

The superior ferroelectric properties of Bi 4 Ti3 O12 (BIT) thin lms among bismuth-layer-structured ferroelectriccompounds have attracted intense interest, making these lms some of the most competitive candidates forpiezoelectric devices against lead containing materials [1]. Bismuth titanate is composed of a triple perovskite unit

sandwiched between (Bi 2 O2 )2+

layers. The pseudo-orthorhombic BIT unit possesses the lattice parameters of a = 0.5450, b = 0.54059, and c = 3.2832 nm, and exhibits the spontaneous polarizations P s = 50 and 4 m C/cm 2 alonga - and c-axes, respectively [2]. Therefore, lms having a larger fraction of a -axis-oriented grains should exhibit betterferro- and piezoelectric properties. However, BIT lms prefer to grow with c-axis perpendicular to the lm surfacewhen common Pt (1 1 1) electrodes are used. A suitable electrode with better lattice matching with the long c-axis isrequired in order to fabricate a - or a / b-axis orientation [3,4] . Because of their complex chemical composition and

www.elsevier.com/locate/matresbuMaterials Research Bulletin 42 (2007) 975981

* Corresponding author. Tel.: +55 16 3301 6600; fax: +55 16 3322 7932.E-mail address: [email protected] (A.Z. Simoes).

0025-5408/$ see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2006.08.006
mailto:[email protected]://dx.doi.org/10.1016/j.materresbull.2006.08.006http://dx.doi.org/10.1016/j.materresbull.2006.08.006mailto:[email protected]


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crystallographical structure, single crystals of these materials are difcult to be grown. Thus, most of the currentstudies are focused on the applications of thin lms [5]. Ferroelectric thin lms are constrained by substrates andtherefore their properties can be affected by many factors, such as orientation, properties of the substrate (latticeparameters and thermal expansion coefcient), and lm thickness. For some applications, as for example inferroelectric memories, large remanent polarization and good fatigue-free characteristics are required [6].Unfortunately, lms of BIT grown over common platinum coated silicon substrates do not satisfy these requirements.This might be due to an interfacial reaction between platinum and bismuth which can lead to undesired electricalproperties [7]. Therefore, the substitution of metallic electrodes based on noble metals, like platinum, with conductiveoxides is an alternative to reach better electrical properties caused by the high oxygen afnity of these electrodes [8].For this purpose, it is essential to understand the ferroelectric properties of the BIT lms deposited on electrodes basedon metallic oxides such as LNO and RuO 2 .

Some attempts have been made to enhance the crystallization ability of ferroelectric thin lms and metallic oxideelectrodes [3,9] . For obtaining good crystallized lms by chemical solution deposition, heat treatments at hightemperatures for a long time, $ 2 h, are normally necessary. These long heat treatments can cause several damages tothe stack, leading to interdiffusion between the lm and the substrate, and sometimes loss of stoichiometry (due to theloss of volatile cation). So, it is important to decrease the temperature and time of thermal treatment. Recently, the useof a domestic microwave furnace has been developed as a way to process materials and has opened an opportunity to

enhance crystallization with a lower annealing processing time. This leads to a decrease the interfacial reactionsbetween ferroelectric thin lms and electrodes and also improves the control over the crystallographic orientation of the thin lms [10] .

Among various methods such as metal-organic chemical vapour deposition, pulsed laser deposition and solgel, thepolymeric precursor method has its advantages over the other production techniques include its low cost, goodcompositional homogeneity, relatively low processing temperatures and the ability to coat large substrate areas [11,12] .

In this work, we present our ndings on the preparation of piezoelectric BIT lms on LaNiO 3 , RuO 2 and Pt/Ti/SiO 2 / Si substrates by the polymeric precursor method combined with the domestic microwave oven with the advantage of reducing the time of thermal treatment.

2. Experimental

The bottom electrodes thin lms (LNO and RuO 2 ) were spin-coated on SiO 2 /Si (1 0 0) substrates by a commercialspinner at 5000 revolutions/min for 30 s (spin coater KW-4B, Chemat Technology). Bottom electrodes of commercialplatinum coated silicon substrates were also used. Each deposited layer was pre-red at 400 8 C for 2 h in aconventional oven. After the pre-ring, each layer was crystallized in a microwave oven at 700 8 C for 10 min using aSiC susceptor, which absorbs the microwave energy and rapidly transfers the heat to the lm. No post-annealingtreatment was performed after crystallization. Using the same procedure, BIT thin lms were deposited by spinningthe precursor solution on the desired substrates. Through this process, we obtained thicknesses of about 150 nm for thebottom electrodes and about 300 nm for BIT, by repeating the spin-coating and heating treatment cycles. Themicrowave oven used here was a simple domestic model similar to that described in literature [10] . Phase analysis wasperformed at room temperature by X-ray diffraction (XRD) in BraggBrentano geometry (Rigaku 2000) at Cu K aradiation. Furthermore, topography and thickness were examined using atomic force microscopy (AFM) (DigitalInstruments, Nanoscope IIIa) and scanning electron microscopy (Topcom SM-300), respectively. The top Ptelectrodes were prepared by photolithography with 8 10

4 mm 2 dot area. The ferroelectric properties of thecapacitors were measured by a Radiant Technology Tester RT6000 A in a virtual ground mode. The piezoelectricmeasurements were done using a setup based on an atomic force microscope [13] in a Multimode Scanning ProbeMicroscope with Nanoscope IV controller.

3. Results and discussion

Fig. 1 shows the XRD results for BIT lms deposited on different bottom electrodes. All peaks were assigned to aBIT-type structure. No reections were detected that would be indicative of second phases. Film orientations dependin general on surface and interface energies. The (1 1 7) and (2 0 0) orientations for the lms deposited on LNO and

RuO 2 bottom electrodes are expected to be due to good lattice matching. In contrast, the predominant c-oriented

A.Z. Simoes et al. / Materials Research Bulletin 42 (2007) 975981976


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crystallites in the lm deposited on Pt are thought to be driven by their low surface energies [14]. In the case of the(2 0 0) oriented lm, the lattice mismatch between bottom electrode and the peculiar long c-axis of BIT-type structure

is proposed to have its origin in the different OO bond lengths along the c-axis and other ( h k l) directions [15] . Fromthe application point of view, random-oriented BIT lms are more favourable than (0 0 l) oriented lms. Therefore,this suggest that the lms deposited on oxides electrodes have a suitable orientation to reach excellent ferroelectric andpiezoelectric properties.

Fig. 2 shows polarization hysteresis loops of the BIT lms deposited on different bottom electrodes. Remanentpolarizations ( P r) of 10 and 15 m C/cm 2 with drive voltages of 2.3 and 1.6 V were observed in the lms deposited onLNO and RuO 2 bottom electrodes, respectively. For the lms deposited on Pt substrates the remanent polarization isreduced to 8.5 m C/cm 2 due the stronger contribution of the crystallites grown in the c-axis direction. These resultsclearly demonstrate that controlling the orientation of grains in a lm is a key point to improve the ferroelectricproperties. Another factor can be the inhibition of domain wall movement caused by a high concentration of dipolecomplexes at the lmsubstrate interface. This can originate from the thermal shock caused by the rapid heating of the

SiC susceptor on platinum which favours the accumulation of the static charges at the interface lmsubstrate. It issupposed that the real temperature in the susceptor may be some degrees higher, which, allied to the effect of themicrowave energy may cause degradation of the lmelectrode interface and hence a loss of the ferroelectricproperties [16]. This effect is more evident in the lms deposited on platinum since it does not act as sink for oxygenvacancies compared to the oxides which accommodate relatively large concentration of oxygen defects [16]. For thislm the drive voltage is around 4 Vand is considered large for use as ferroelectric memories. From hystereses curves, itis clear that the remanent polarization of (1 1 7)- and (2 0 0)-oriented lm is larger than that of (0 0 l)- and (1 1 7)-oriented lms. Since LNO and RuO 2 electrodes give better ferroelectric properties, it is believed that if oxygenvacancies accumulate near the lmelectrode interface, the conductive oxide can consume the vacancies by changingits nonstoichiometry. Therefore, the accumulation of oxygen vacancies near the interface is reduced. This implies less

A.Z. Simoes et al. / Materials Research Bulletin 42 (2007) 975981 977

Fig. 1. X-ray diffraction patterns of Bi 4Ti3 O12 lms deposited at 7008 C for 10 min on: (a) LaNiO 3 , (b) RuO2 and (c) Pt.

Fig. 2. PV hystereses loops of Bi 4 Ti3 O12 lms deposited on LaNiO 3 , RuO2 and Pt bottom electrodes at 7008

C for 10 min.


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charge trapping and domain wall pinning in the interface region. Therefore, the effect of the LNO and RuO 2 electrodesmay be attributed to their function as an oxygen vacancy sink [17,18] . As a consequence of oxygen vacancies

accumulated at the lmelectrode interface, a shift of the hysteresis loop along the electric eld axis towards thepositive side was observed and may lead to a failure of the capacitor. These charges may originate during the heattreatment process due to the decomposition of the polymeric precursor [12]. According to the electrostatic modelproposed by Robels et al. [19] , this horizontal shift of the curve represents the internal bias, which is closely connectedto the electrode/lm interface. These ndings suggest that the slightly higher voltage shift observed in the lmsdeposited on Pt substrates resulted from the nature of the bottom electrode, since the only difference in the lmpreparation is the type of substrate used. In our case it is observed that microwave crystallization with platinum asbottom electrode induces a permanent voltage shift in the hysteresis loops of the BIT lms due to the generation of trapped charges at the defect sites near the electrodelm interface. These results indicate the unsuitability of the lmsfor use as memories owing to the signicant difference between + V c and V c , respectively.

Fig. 3 presents the fatigue endurance of the BIT thin lms as a function of switching cycles. P * is the switched

polarization between two opposite polarity pulses and P^

is the nonswitched polarization between the same twopolarity pulses. The P * P ^ or P * ( P ^) denote the switchable polarization, which is an important variable fornonvolatile memory application. Fatigue resistance was observed up to 10 10 cycles with oxide electrodes indicatingthat the conductive oxide can consume the oxygen vacancies accumulated near the lmelectrode interface. On theother hand, for the lms deposited on platinum coated silicon substrates no fatigue resistance was observed indicatingthat the sparks formed by the high electric eld in the microwave environment damaged the lmelectrode interfaceand therefore affect the switching characteristics of BIT lms.

Fig. 4 shows the out-of-plane (OP) and in-plane (IP) piezoresponse images of the as-grown lms after applying abias of 12 V, on an area of 2 m m 2 m m, and then an opposite bias of +12 V in the central 1 m m 1 m m area. Toobtain the domain images of the BIT lms, a high voltage that exceeds the coercive eld was applied duringscanning. The contrast in these images is associated with the direction of the polarization [13] . The white regions inthe out-of-plane PFM images correspond to domains with the polarization vector oriented toward the bottomelectrode hereafter referred to as down polarization ( Fig. 4a, d and g) while the dark regions correspond to domainsoriented upward referred to as up polarization. Grains which exhibit no contrast change is associated with zero out-of-plane polarization. Although the lms have a certain degree of preferred orientation as indicated by the XRDspectra in Fig. 1 , they still preserve their polycrystalline nature. This can be concluded from the piezo-contrast givenby the as-grown state. After a negative bias was applied, grains labeled A did not change their contrast. However inmost cases, such as in grains labeled B and C ( Fig. 4a), a polarization pointing predominantly out-of-plane wasobserved. A similar situation was observed when a positive bias was applied to the lm. We noticed that some of thegrains exhibit a white contrast associated to a component of the polarization pointing toward the bottom electrode.On the other hand, in the in-plane PFM images ( Fig. 4b, e and h) the contrast changes were associated with changesof the in-plane polarization components. In this case, the white contrast indicates polarization, e.g. in the positivedirection of the y-axis while dark contrast are given by in-plane polarization components pointing to the negative

part of the y-axis. The d 33 (V) hysteresis loops are shown in Fig. 4c, f and i. The maximum d 33 value, $ 40 pm/V, is


Fig. 3. (P * P ^) as a function of polarization cycles for Bi 4 Ti3 O12 lms deposited on different bottom electrodes at 7008 C for 10 min: (* )

LaNiO 3 , (& ) RuO2 and (~ ) Pt.


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Fig. 4. Out-of-plane (OP) and in-plane (IP) PFM images, as well as piezoresponse loops of Bi 4 Ti3O12 lmsdeposited on different bottom electrodesat 700 8 C for 10 min: (a) LaNiO 3 (OP), (b) LaNiO 3 (IP) and (c) LaNiO 3 hysteresis loop; (d) RuO 2 (OP), (e) RuO 2 (IP) and (f) RuO 2 hysteresis loop;

(g) Pt (OP), (h) Pt (IP) and (i) Pt hysteresis loop.


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similar for all samples and approaches the reported value for a BIT single crystal [19] . The enhancement of polarization could be caused by the microwave annealing whose role is to avoid interfacial reactions and increasethe lms crystallization. As can be seen, the hysteresis loop for the lm deposited on the Pt substrate ( Fig. 4i) showsan offset in the vertical direction which can be probably caused by clamping effect due to the substrate and thegeneration of trapped charges at the defect sites near the electrodelm interface caused by strong interactionbetween microwave energy with platinum electrode [20] . Although the PZT lms still have higher d 33 values,ranging from 40 to 110 pm/V [21] , the presented values reported for our BIT lms suggest that this material can beconsidered as a viable alternative for lead-free piezo-ferroelectric devices. Also, the microwave annealing for theprocessing of materials provides the advantages of low investment, rapid and uniform heating, low sinteringtemperatures and times and improved product quality. In comparison with other lead-free ferroelectrics, 40 pm/V ismuch higher than the d 33 value of SrBi 2 Ta 2 O9 lms (17 pm/V) and close to the reported value of Nd-dopedBi4 Ti3 O12 (38 pm/V) [22] .

4. Conclusions

In conclusion, BIT thin lms were successfully crystallized using a low power microwave oven, with no post-annealing treatment. The electrical measurements indicate that the use of platinum substrate as bottom electrode isinappropriate to obtain good BIT lms. This effect was not observed for lms deposited on oxide electrodes. Regularlyshaped hysteresis is observed for the lms deposited on the LaNiO 3 and RuO 2 electrodes. Furthermore, high fatigue

resistance was observed for lms deposited on LaNiO 3 and RuO 2 electrodes which shows that our lms are promising


Fig. 4. (Continued ).


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candidates for nonvolatile random access memories. Our results indicate that the BIT lms exhibit a goodpiezoelectric response of (40 pm/V). BIT lms crystallized in the microwave oven using conductor oxides as bottomelectrodes present good ferroelectric properties and piezoelectric coefcients and can be used for ferroelectric randomaccess memories and piezoelectric devices.

Acknowledgments

The authors gratefully acknowledge the nancial support of the Brazilian agencies FAPESP, CNPq, CAPES.

References

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ferroelectric and piezoelectric properties of bismuth titanate

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