Solid State Communications 140 (2006) 430–434www.elsevier.com/locate/ssc

Improved fatigue endurance in Mn-doped Bi3.25La0.75Ti3O12 thin films

S.K. Singh∗, H. Ishiwara

Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan

Received 2 May 2006; received in revised form 8 September 2006; accepted 10 September 2006 by X.C. ShenAvailable online 26 September 2006

Abstract

Mn-doped Bi3.25La0.75Ti3O12 (BLT) thin films are fabricated by depositing sol–gel solutions on Pt/Ti/SiO2/Si〈100〉 substrate. Mn-doping inBLT films influences the ferroelectric properties, structural orientation, as well as the surface morphology of the films, even if the Mn concentrationis low. Mn-doping at 1% of Ti-sites in BLT films enhances the remanent polarization and reduces the coercive field by approximately 25%.However, polarization decreases gradually by Mn-doping more than 1%, which may be due to the trapping of charged particles (holes or electrons)at defects located at the interface or grain boundaries generated by excess Mn-doping. The reduced polarization due to the Mn-doping recoversduring switching cycles higher than 107–104 for the x value of 0.05–0.2 in Bi3.25La0.75Ti3−xMnxO12. Under the high switching field, theprobability of field-assisted unpinning of domains is expected to be high and this may be the main cause for increase in polarization. Mn-dopingsignificantly improves the fatigue endurance in BLT films because of de-trapping of charges from defects under high switching field.c© 2006 Published by Elsevier Ltd

PACS: 77.55.+f; 77.80.-e; 85.50.Gk

Keywords: A. Ferroelectric Bi3.25La0.75Ti3O12 thin film; B. Chemical solution deposition; C. Electrical properties; C. Fatigue

1. Introduction

Non-volatile ferroelectric random access memory (FeRAM)satisfies most conditions required for new-generation idealmemories, such as low power operation, fast write/readtime, infinite write/read cycles, compatibility with Si pro-cess, and minimum added process cost [1,2]. Many mate-rials have been studied as candidates for FeRAM, whichinclude Pb(Zr1−xTix)O3 (PZT), SrBi2Ta2O9 (SBT), andBi4Ti3O12 (BTO). Among them, PZT has the advantage oflarge remanent polarization (Pr ) but shows fatigue behavioron metal Pt electrodes [3]. In addition it contains lead atoms,which are not environmentally friendly. SBT shows superiorfatigue properties but these films require high growth temper-ature and they show smaller Pr than BTO [4]. La-substitutedBTO, (Bi, La)4Ti3O12 (BLT), is a more promising candidatefor FeRAM than SBT and PZT because of high fatigue en-durance limit, lead-free chemical composition, moderate depo-sition temperature and relatively large Pr lying between PZTand SBT [5].

∗ Corresponding author. Tel.: +81 459245874; fax: +81 459245147.E-mail address: singh@neuro.pi.titech.ac.jp (S.K. Singh).

0038-1098/$ - see front matter c© 2006 Published by Elsevier Ltddoi:10.1016/j.ssc.2006.09.022

The substitution effects at Bi3+ and Ti4+ sites in BTO filmsto the ferroelectric properties have been well studied [6–11].It has been shown that the Bi-site and Ti-site substitutionshave different effects on the ferroelectric properties. Bi-sitesubstitution enhances the chemical stability of oxygen vacancyby improving the height of the potential barrier for hoppingand enhances the mobility of domains by changing the domainstructure. On the other hand, Ti-site substitution induces thedistortion of oxygen octahedra and reduces the oxygen vacancyconcentration by the compensation effect, which result inenhancement of the Pr . Co-substitution in BTO films at Bi- andTi-sites has also been reported to be more effective in enhancingpolarization and improving fatigue endurance.

BTO films exhibit fatigue-free behavior up to 107

polarization switching cycles on Pt electrodes and showreduction in polarization after 107 switching cycles [12].La-doped BTO films show high fatigue endurance [5], andthis fatigue endurance in BLT films is specified by variousmechanisms such as the existence of the self-regulating(Bi2O2)

2+ layers compensating space charges near the Ptelectrodes [4], the oxygen stability in perovskite-like slabsbeing favored by Bi-free environment [13], faster domain wall

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unpinning by the cycling field than domain wall pinning due tocharge trapping [14,15], and surface configurations of crystalsinducing specific Pt-ferroelectric interface properties [16].Doping can stabilize the oxygen defects and control theferroelectric properties in the BTO films [10,11]. It hasbeen reported that Mn-doped Bi3.6La0.4Ti3O12 films show theimproved ferroelectric properties, but no detailed discussionsare presented on the fatigue endurance and leakage currentproperties [17]. In this article, we discuss the improvedfatigue endurance in Bi3.25La0.75Ti3O12 thin films controlledby Mn-doping.

2. Experimental procedure

Mn-doped BLT films were fabricated by a sol–gel methodon Pt/Ti/SiO2/Si〈100〉 substrate. For spin-coating, solutionswith compositions Bi:La:Ti:Mn = 3.45:0.75:3 − x:x (x =

0–0.2) were used. Excess bismuth (0.20) with respectto Bi3.25La0.75Ti3−xMnxO12 was added to compensate forbismuth evaporation during thermal processing. The gel filmswere formed by spin-coating the solution at 2500 rpm for 20 s,drying at 240 ◦C for 3 min, and prefiring at 350 ◦C for 10 minin air. This process was repeated several times to obtain thefilms of desired thickness and then the films were annealedat 750 ◦C for 30 min in oxygen atmosphere. Electron beamevaporation was used for deposition of Pt top electrodes of3.14 × 10−4 cm2 area through a shadow mask. The crystallinestructure of the films was investigated with a multipurposeX-ray diffractometer (X, Pert-Pro MPD, Philips). The electricalproperties of the film capacitors were measured using astandardized ferroelectric test system (Radiant Technologies,RT66a) and an HP4156A precision semiconductor parameteranalyzer (Hewlett-Packard). The surface morphologies of thefilms were observed using atomic force microscopy (AFM).

3. Results and discussion

Fig. 1 shows X-ray diffraction (XRD) patterns ofBi3.25La0.75Ti3−xMnxO12 (x = 0, 0.03, 0.05, 0.1, 0.15 and 0.2)thin films. It is observed that all the films are well crystallizedand possess a single phase bismuth-layered structure. There isno evidence of any secondary phase such as the fluorite phase orthe pyrochlore phase, but the small change in crystallographicorientation is observed in the Mn-doped BLT thin films. Thatis, the intensity of 00l peaks decreases while the intensity of(117) peak increases as the Mn concentration increases. Theintensity ratio of I(117)/I(006) peaks varies from 0.15 to 0.50 asthe Mn concentration increases from 0% to 6.6% (x = 0.2) inthe BLT films. This change seems to influence the ferroelectricproperties, because BLT possesses two polar axes (along the aand c axes).

The surface morphology of the films was studiedusing AFM. Fig. 2 shows the surface morphologies ofBi3.25La0.75Ti3O12 and Bi3.25La0.75Ti2.9Mn0.1O12 thin films onPt/Ti/SiO2/Si structures annealed at 750 ◦C for 30 min in O2atmosphere using a rapid thermal annealing (RTA) furnace. TheBLT film is composed of large plate-like grains of 0.5–1 µm

Fig. 1. X-ray diffraction patterns of Bi3.25La0.75Ti3−xMnxO12 thin films withx = 0, 0.03, 0.05, 0.1, 0.15, and 0.2 deposited on Pt/Ti/SiO2/Si structures andannealed at 750 ◦C for 30 min in O2 atmosphere.

Fig. 2. Surface morphologies of (a) Bi3.25La0.75Ti3O12 and (b) Bi3.25La0.75Ti2.9Mn0.1O12 films deposited on Pt/Ti/SiO2/Si structures and annealed at750 ◦C for 30 min in O2 atmosphere.

in size with small rod-like grains of about 0.2 µm in widthand 0.3–0.8 µm in length around them. The small percentageof Mn-doping affects the surface morphology of the film,which decreases plate-like grains and increases rod-like grainsin the Mn-doped films. The root mean square values of thesurface roughness (Rrms) were measured for various filmsusing AFM images. As expected, the Rrms value in Mn-dopedBLT films increased from 0.4 to 0.7 µm with increasing Mndopant concentration from 0% to 3.3% (x = 0 and 0.2). Weconclude from these results that the surface roughness increaseswith decreasing fraction of the c-axis-oriented grains, which isconsistent with our previous result [10].

Fig. 3 shows I –V characteristics of the Mn-doped BLTthin films. As shown in the figure, the leakage current density

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Fig. 3. I –V characteristics of Pt/Bi3.25La0.75Ti3−xMnxO12/Pt capacitors withx = 0, 0.03, 0.05, 0.1, 0.15, and 0.2 compositions.

Fig. 4. Variation of polarization (2Pr ) and coercive field (Ec) with various Mnconcentrations from x = 0–0.2 in Bi3.25La0.75Ti3−xMnxO12 thin films.

increases with the Mn concentration in BLT films. The valuesof the leakage current density at a 200 kV/cm applied field varyfrom 8×10−7 to 6×10−5 A/cm2 with Mn concentration of x =

0–0.2 in BLT films. Mn-doping seems to increase the electricalconductivity in BLT crystallites, since a similar tendency hasbeen observed in Mn-doped SBT thin films [18]. A reasonfor the increased leakage current density with Mn-doping maybe the orientation modification of the BLT films. The leakagecurrent density in a BLT film is known to be smaller alongthe c-axis than along other axes [11]. As mentioned earlier,the XRD data show that the preferred c-axis orientation ofgrains decreases and (117) orientation increases with increasingMn concentration. It is also considered in the Mn-dopedBLT films that oxygen vacancies are generated by the chargecompensation phenomenon in substitution of Mn3+ ions forTi4+ ions. Since it is known in a bulk crystal that oxygenvacancies increase the conductivity of the crystal throughgeneration of holes in the oxygen annealing process [19,20],increase of the leakage current in the Mn-doped BLT films maybe explained by this mechanism.

Fig. 4 shows the remanent polarization (2Pr ) and coercivefield (Ec) of Bi3.25La0.75Ti3−xMnxO12 thin films with variousMn concentrations from x = 0–0.2 at an applied electricfield of 500 kV/cm and Fig. 5 shows the polarization–electricfield (P–E) hysteresis loops of Bi3.25La0.75Ti3−xMnxO12 thinfilms with x = 0 and 0.03 at a 500 kV/cm. In the low Mnconcentration region, the remanent polarization increases with

Fig. 5. P–E hysteresis loops of Bi3.25La0.75Ti3−xMnxO12 thin films withx = 0 and 0.03 at a 500 kV/cm electric field.

the Mn-doping concentration. Then, the polarization becomesmaximum (2Pr = 30 µC/cm2) in the 1% Mn-doped (x =

0.03) BLT film. This value is larger by approximately 22%than that in the undoped BLT film (24 µC/cm2) under the sameapplied field, as shown in Fig. 5. As the dopant concentrationincreases beyond 1% (x = 0.03), the polarization startsdecreasing and when the Mn concentration is higher than 3.3%(x = 0.1) the films show smaller polarization than that of theundoped film. The polarization further decreases slowly withMn concentration and the 6.6% Mn-doped film (x = 0.2)

shows 2Pr = 15 µC/cm2.The coercive field (Ec) of the BLT films decreases with

increasing Mn-doping concentration. The undoped BLT filmshows a coercive field of 142 kV/cm for an applied field of500 kV/cm, while Ec decreases to 110 kV/cm in the 1%Mn-doped film for the same applied field, which correspondsto a decrease by approximately 23%. As the Mn concentrationexceeds 1%, the coercive field decreases rather slowly withincreasing Mn-concentration. For the film with 6.6% Mn, thecoercive field decreases to 100 kV/cm at an applied fieldof 500 kV/cm. We can assume the Mn-doping concentrationdependence of the remanent polarization as follows. In thelow concentration region the remanent polarization is enhanceddue to modification in the preferred orientation of grains inthe film and structural distortion of the oxygen octahedra.However, when the Mn concentration exceeds 1% moreoxygen vacancies are generated, domain pinning caused by thevacancies becomes stronger [19], and the remanent polarizationis decreased.

The fatigue endurance test on the BLT capacitors was carriedout at room temperature using 1 kHz bipolar square switchingpulses with an electric field of 300 kV/cm. Fig. 6 shows thevariation in the polarization of thin film capacitors with fatiguecycles. The fatigue behavior in BLT films strongly dependson the Mn-doping concentration. In the low Mn-dopingconcentration where x < 0.05, BLT films show fatigue-freebehavior until approximately 108 switching cycles and showsmall degradation in the polarization in the course to 1010

switching cycles. When Mn-doping concentration is higher(x ≥ 0.05), BLT films shows fatigue-free behavior up to1 × 107, 3 × 106, 3 × 105, and 6 × 104 switching cycles forx = 0.05, 0.1, 0.15, and 0.2, respectively. Then the polarizationincreases, followed by small degradation in polarization. As aresult, the polarization values in the vicinity of 1010 switching

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Fig. 6. Fatigue characteristics of Pt/Bi3.25La0.75Ti3−xMnxO12/Pt capacitorsfor x = 0, 0.03, 0.05, 0.1, 0.15, and 0.2 at 300 kV/cm electric field.

cycles are almost the same for x = 0.1 and larger in thesamples with the higher Mn-concentration. These results showthat Mn-doping controls the fatigue behavior in BLT films.

Fig. 7 shows P–E hysteresis loops in Mn-doped films with(a) x = 0.0, (b) x = 0.03, (c) x = 0.05, (d) x = 0.1,(e) x = 0.15, and (f) x = 0.2 for the initial state andthe state after applying 1010 switching pulses at an electricfield of 300 kV/cm. The Pt/BLT/Pt capacitors show fatiguelessresponse up to 5 × 107 switching cycles at a 300 kV/cm

electric field and the polarization decreases by 23% after1010 switching cycles. Similarly, the polarization decreasesby 30%–7% in Mn-doped films with x = 0.03 and 0.05,respectively. On the other hand, in the samples with x = 0.1,0.15 and 0.2 increase of polarization by approximately 15%is observed after 1010 switching cycles at 300 kV/cm. Weconclude from these results that Mn doping in BTO films iseffective in increasing the fatigue endurance. We think that Mndoping increases the oxygen vacancies in BLT films which inturn increases domain pinning in the higher Mn-concentrationregion. Domain unpinning in these films can be expectedby application of repeated bipolar pulses. Thus, the wake-upphenomenon (increase of the remanent polarization) in Fig. 6can be explained by this mechanism.

4. Conclusions

The electrical properties of BLT films depend on the Mnconcentration in the films. In the low concentration range upto approximately 1% the remanent polarization is improvedand in the range higher than 1.5% the fatigue resistance isimproved. The Mn-doped Bi3.25La0.75Ti3O12 films show the

Fig. 7. P–E hysteresis loops for the initial state and the state after applying 1010 switching pulses at 300 kV/cm applied field. (a) Pt/Bi3.25La0.75Ti3O12/Pt, (b)Pt/Bi3.25La0.75Ti2.97Mn0.03O12/Pt, (c) Pt/Bi3.25La0.75Ti2.95Mn0.05O12/Pt, (d) Pt/Bi3.25La0.75Ti2.9Mn0.1O12/Pt, (e) Pt/Bi3.25La0.75Ti2.85Mn0.15O12/Pt, and (f)Pt/Bi3.25La0.75Ti2.8Mn0.2O12/Pt capacitors.

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enhanced remanent polarization and the reduced coercive fieldby approximately 25% when the Mn concentration is 1%.Mn-doping influences the microstructure of the films and thefilms show rod-like surface morphology, which is probably dueto the increased volume fraction of (117)-oriented grains. XRDresults also show partial loss of the c-axis-oriented grains. Theleakage current density in BLT films increases with Mn-dopingdue to the modification of preferred orientation of grains inthe films. However, Mn-doping increases the oxygen vacanciesin BLT films, which in turn increases domain pinning and theremanent polarization decreases in the higher Mn-concentrationregion. Under high switching field, the probability of field-assisted unpinning of domains is expected to be high and thismay be the main cause for the increase in polarization after 105

cycles in Mn-doped BLT films.

Acknowledgements

This work was performed under the auspices of the SpecialCoordination Funds for Promoting Science and Technologysupported by Ministry of Education, Culture, Sports, Scienceand Technology, Japan.

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