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Thin Solid Films 516 (2

Crystallization behavior of Nd-doped SrBi2Ta2O9 thin filmsprepared by magnetron sputtering

Yibin Li a, Sam Zhang a,⁎, Weidong Fei b, Huili Wang a

a School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singaporeb School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, PR China

Available online 13 July 2007

Abstract

Nd-doped SrBi2Ta2O9 thin films are magnetron-sputtered on Pt/Ta/SiO2/Si substrates. The effect of heating rate on crystallization behavior isinvestigated with conventional furnace annealing (CFA) and rapid thermal annealing (RTA). Grazing incidence X-ray diffraction and fieldemission scanning electron microscopy reveal that the crystallization goes through three stages (phases): amorphous, fluorite and finallyAurivillius. Under RTA, the fluorite-to-Aurivillius transformation starts around 100 °C lower than that under CFA. The reasons behind thetransformation temperature drop are also discussed.© 2007 Elsevier B.V. All rights reserved.

Keywords: Phase transformation; Crystallization; RTA; Structural relaxation; SBT thin film

1. Introduction

SrBi2Ta2O9 (SBT) thin films have excellent fatigue-resistanceproperties even on conventional Pt electrodes [1,2]. That isparticularly useful in ferroelectric random access memories(FeRAM) applications. SBT consists of two SrTaO3 perovskite-like units between (Bi2O2)

2+ layers along the c axis [3]. As a result,the ferroelectric properties are very anisotropicwith the ferroelectricpolarization direction in the ab plane [4] and the ferroelectricityalong the c axis being absent or very low [5,6]. Therefore, remnantpolarization (2Pr) is relatively low thus cannotmeet the requirementfor high-density memories. Recently, we found that partialsubstitution of trivalent Nd ions for bivalent Sr ions couldmarkedlyimprove the remnant polarization (from 11 up to 18 μC/cm2) withreduced coercive field (from 98 down to 64 kV/cm) [7].

Annealing is inevitable for films obtained through a variety ofdeposition processes: metal-organic chemical-vapor deposition,magnetron sputtering, so–gel, etc. This is because the as-deposited films are amorphous and nonferroelectric. Conven-tionally, the annealing is carried out in “normal” furnaces whereheating is effected through electrical resistance. This is called“conventional furnace annealing” (CFA). In CFA, typical heating

⁎ Corresponding author. Tel.: +65 6790 4400; fax: +65 6791 1859.E-mail address: msyzhang@ntu.edu.sg (S. Zhang).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.07.054

rate is in the range of 1–30 °C/min. In the past two decades, rapidthermal annealing (RTA) has been frequently used in annealing offerroelectric films [8–10]. In RTA, the heating is realized throughincandescent lamps with very high heat rate (e.g., 100 °C/s). Inthis paper, we explore the crystallization behavior of the Nd-doped SBT (SNBT) films during annealing in a rapid thermalprocessor or conventional furnace.

2. Experimental details

Nd-doped SBT films were sputtered (the substrate temper-ature is room temperature) from sintered target of Sr0.8Nd0.3-Bi2.5Ta2O9+x with a purity of 99.9% (Super ConductorMaterials Inc. Tallman, NY). Details of the film depositioncan be found in Ref. [7]. The as-deposited films with thethickness of 230 nm were annealed at 400–850 °C underoxygen atmosphere at a ramping rate of 10 °C/min in CFA, and25 °C/s in RTA (JIPLEC, JETFIRST100). In order to verify theeffect of ramping rate on fluorite–Aurivillius transformationexcluding the structural relaxation effect, some other SNBT thinfilm samples were pre-annealed at 550 °C for 60 min, followedby re-annealing at 690 °C, 700 °C, 710 °C, and 720 °C for30 min under RTA at a ramping rate of 25 °C/s, respectively.The phase and crystal orientation of the films were analyzed by X-ray diffraction (XRD) with CuKα radiation (λ=1.5406 Å) (Philips

Fig. 1. XRD pattern of as-deposited Nd-doped SBT thin films.

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PW1830) at 40 kV and 30 mA and grazing incidence X-raydiffraction (GIXRD) with CuKα radiation (λ=1.5406 Å) (RigakuUltima 2000) at 40 kVand 40mA. In GIXRD, the incidence anglewas fixed at 2°, and the 2θ scanningwas conducted from10° to 70°with a step size of 0.02 °C and step duration of 5 s. The surfacemorphologies were observed using a field emission scanningelectron microscope (FESEM, JEOL JSM-6340F, Japan). Fordifferential thermal analysis (DTA) of the structural relaxationbehavior, SNBT thin films were also deposited on KBr substrateand then removed by dissolving in distilled water. The DTA testwas conducted (DTA7, Perkin Elmer) at a ramping rate of 10 °C/min (the same ramping rate as CFA).

3. Results

3.1. As-deposited SNBT thin films

Fig. 1 illustrates XRD patterns of the as-sputtered SNBT thinfilm. There are two peaks: a broad peak at around 28° and a

Fig. 2. FESEM surface morphology of as-deposited Nd-doped SBT thin films.

sharp peak at around 2θ=40°. The broad peak indicates theamorphous nature of the as-sputtered film; the sharp peak isfrom the underneath Pt electrode oriented in (111) direction.

Fig. 2 shows the as-sputtered morphology of the film. Thefilm is dense and looks defect free. Owing to the amorphousnature (thus very low contrast), the image appears not sharp.

3.2. Crystallization behavior under CFA

Fig. 3 shows the variations in GIXRD results for the filmsannealed at various temperatures (650 °C–850 °C) for 30 minunder CFA. At temperature 650 °C, the fluorite phase appears.At 750 °C, another group of peaks appears in addition to that ofthe fluorite peaks. Those belong to the Aurivillius phase.

Fig. 4 shows the corresponding surface morphology. Fig. 5plots a narrow XRD scan result of the film annealed at 740 °Cfor 30 min. It is confirmed that no Aurivillius peaks are found at740 °C. From Fig. 3, at 800 °C, the fluorite peaks almostcompletely vanish signaling completion of the transformation.At 850 °C, peaks of the Aurivillius phase become sharper,indicating better crystallinity. Below 750 °C (Fig. 4a and b), thefilm is characterized by clusters of tiny round crystals. Thesecrystals belong to the fluorite phase. At 750 °C, part of the tinyround crystal grains transforms into rod-like crystals which arecharacteristic of the Aurivillius phase. At 800 °C, the plate/

Fig. 3. GIXRD patterns of Nd-doped SBT thin films annealed at differenttemperatures for 30 min under CFA.

Fig. 5. XRD pattern of Nd-doped SBT thin films annealed at 740 °C for 30 minunder CFA.

Fig. 6. XRD patterns of SNBT thin films (fluorite phase) pre-annealed at 400and 500 °C for 30 min under RTA.

Fig. 4. FESEM morphologies of Nd-doped SBT thin films annealed at different temperatures for 30 min under CFA: (a) 650 °C; (b) 700 °C; (c) 750 °C; (d) 800 °C;(e) 850 °C.

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Fig. 7. GIXRD patterns of SNBT thin films annealed at different temperaturesfor 10 min under RTA.

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needle-like grains become dominant. At 850 °C, all are plate/needle-like, and the grain growth is obvious (from about 100–150 nm at 800 °C to 200–300 nm).

Fig. 8. FESEM morphologies of SNBT thin films annealed at different temperat

3.3. Crystallization behavior under RTA

The phase transformation from amorphous to fluorite isevidenced in XRD profiles in Fig. 6, where the “400 °C profile”still shows an amorphous nature while after 30 min at 500 °C,the film is basically crystallized into fluorite.

Fig. 7 shows the GIXRD patterns the films annealed at 650–800 °C in RTA for 10 min. At 650 °C and 680 °C, the phase isfluorite. At 690 °C, however, the Aurivillius peaks start toappear. In order to ascertain whether the Aurivillius transfor-mation can be detected at even lower temperature, the film isannealed at 680 °C for a prolonged 30 min under RTA. No traceof Aurivillius peaks was found. At 700 °C, the Aurivillius peaksbecome dominant. At 750 °C, the fluorite phase is negligible. At800 °C, the Aurivillius peaks become shaper and more distinct,signifying increase in crystallinity. The FESEM microstructureshown in Fig. 8, however, reveals that at as low as 650 °C, thetransformation to Aurivillius phase may have started, as isevident in the small rod-like grains. At increasing temperatures,the Aurivillius phase increases. At 700 °C, the rod-like grains(the Aurivillius phase) dominate, this agrees with the XRDprofile. At 750 °C, a more uniform distribution of theAurivillius phase and better crystallinity is obtained.

Fig. 9 illustrates the GIXRD patterns of the films (the filmspre-annealed at 500 °C for 30 min) re-annealed at 690 °C,700 °C and 710 °C for 30 min under RTA. There do not appearAurivillius phase diffraction peaks except that fluorite diffrac-tion peaks were observed below 710 °C. Until 710 °C,Aurivillius diffraction peaks are visible but not dominant. At720 °C, the Aurivillius peaks are dominant and fluorite peakintensity subsides.

ures for 10 min under RTA: (a) 650 °C; (b) 700 °C; (c) 750 °C; (d) 800 °C.

Fig. 10. DTA curve of SNBT thin films removed from KBr substrate.

Fig. 9. XRD patterns of SNBT thin films pre-annealed at 690–720 °C for 10 minunder RTA.

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3.4. DTA result

Fig. 10 is the DTA curve of the film removed from the KBrsubstrate. There are two peaks observed. One diffusedexothermal peak starts at 236 °C and reaches the maximum at295 °C. Another peak locates at 500 °C and ends at 600 °C.

4. Discussion

4.1. Crystallization path

The crystallization of SBT phase from amorphous toAurivillius phase is realized via an intermediate metastablefluorite-like phase (cubic structure) [11,12]. This crystallizationprocess (path) was first illustrated by Tanaka et al. [13] in metal-organic decomposition (MOD)-derived SBT films. This path isconfirmed by our results (Figs. 3 and 6, Figs. 4 and 7).

4.2. Reduction of phase transformation temperature at RTA

Our results indicate that under RTA, fluorite-to-Aurivilliustransformation starts at as low as 650 °C and completes at750 °C. In contrast, under CFA, the transformation starts at about750 °C, and mostly completes at 800 °C (still with a smallamount of fluorite phase observable in both XRD pattern (Fig. 3)and in morphology (Fig. 4d)). As such, the starting temperature isabout 100 °C lower and the completion temperature is about atleast 50 °C lower. The difference between CFA and RTA lies in

the ramping rate. The ramping rate of RTA (25 °C/s) is 150 timesmore rapid than that of CFA (10 °C/min or 0.17 °C/s). Thereduction of annealing temperature as a result of higher rampingrate is also observed in PZT studies [14].

The as-deposited amorphous film contains a large number ofdefects [15] and short-range-ordered clusters. When the as-deposited film is heated, these defects get annihilated; as aresult, some energy is released. This is called structuralrelaxation. Structural relaxation occurs during annealing and ametastable state is changed to a more stable lower energy state[16]. The energy release process needs some time. Highramping rate deprives of this time. As a result, the stored energyreleases at the crystallization temperature and effectively aidsthe phase transformation. This amounts to a decrease intransformation temperature, as what happens in PZT [17]. Inthe present case, the sputtered amorphous films should contain alarge number of defects due to sputtering deposition. DTA curve(Fig. 10) of the film includes two peaks, the left one is believedto be the release of the stored energy from the amorphous films.Here, the film is still amorphous, but not yet crystallized, asevident in the XRD profile (Fig. 6 “400 °C profile”). The rightpeak corresponds to the amorphous–fluorite transformation.Here, amorphous phase crystallizes into fluorite (note the broadpeak around 500 °C). This peak tapers off at about 600 °C,signaling completion of fluorite transformation. Under RTA,where the ramping rate is extremely high, i.e., 25 °C/s, there isno time for this energy to release. Then, at higher temperature, itreleases and effectively aids the phase transformation. As aresult, crystallization takes place at 650 °C, or 100 °C lowerthan usual.

The GIXRD results of the films re-annealed at 690 °C,700 °C and 710 °C for 30 min under RTA (shown in Fig. 9)reconfirm that Aurivillius phase does not appear (only fluoritepeaks are observed below 710 °C). The Aurivillius starts toshow up at 720 °C. Without pre-annealing, however, Aurivilliusphase starts to form at 650 °C. This supports the analysis that thestored energy plays an important role in assisting the fluorite–Aurivillius transformation. However, the re-annealed films startto transform into Aurivillius at lower temperature (around710 °C) in comparison with the CFA-treated films (740 °C),

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thus the difference is only 30 °C. But the fact is that trans-formation starting temperature under RTA is lower than thatunder CFA by 100 °C. There must be a second reason.

Dang and Gooding have modeled the effect of rapid thermalannealing on crystallization of Pb(Zr0.5Ti0.5)O3 thin films [18].During annealing process, thin film experiences phase trans-formation from amorphous to perovskite, in between whichthere exists a defect-pyrochlore (an intermediate phase havingan identical stoichiometry to that of the desired cubicperovskite, the final transition can be completed without anydiffusion). It is suggested that the long-ranged interactionsshould be introduced as a result of elastic misfits of theinhomogeneous medium. Meanwhile, the ramping rate has animportant effect on nucleation and growth of cubic perovskitephase. That is to say, an increase of heating rate reduces both thetransformation temperature and time required for transforma-tion to the perovskite. The crystallization of SBT thin film isbasically similar to PZT thin films. The explanation for PZTthin film is also applicable to the transformation to layeredstructure perovskite (generally called Aurivillius) taking placeat reduced temperature under RTA [17]. As discussed above,SNBT thin film crystallizing undergoes three stages: amor-phous–fluorite–Aurivillius. From fluorite to Aurivillius (bothhave different crystal structures), there also includes the long-ranged interaction due to elastic misfits of the inhomogeneousmedium. Thus, the rapid ramping rate can also lower thetransformation from fluorite to Aurivillius, to some extension.This may be responsible for the second reason.

In summary, two sides take effect. On one hand, thestructural relaxation (released energy) may aid for the phasetransformation to Aurivillius. On the other hand, ramping rateitself can cause the reduction of transformation temperaturebecause the ramping rate could effectively affect the long-ranged interaction. These two aspects may well account for thefact that the phase transformation to Aurivillius can start tooccur at reduced temperature under RTA while it mustcommence at higher temperature under CFA.

5. Conclusions

During the annealing process, the crystallization path ofSNBT thin film begins with amorphous phase and ends with

Aurivillius. The transformation temperature is obviouslylowered under RTA in comparison with CFA. This is attributedto two sides. One hand goes to the structural relaxationaccompanying some energy released during annealing process.At rapid heating rate, the structure relaxation is postponed tohigher temperature thus the released energy aids for phasetransformation to Aurivillius. On the other hand, the rapidramping rate itself is also beneficial to the transformation toAurivillius.

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