Ferroelectric domain structures and thickness scaling of epitaxialBiFeO3 thin films

Yoonho Ahn a, Jeongdae Seo a, Jong Yeog Son a,n, Joonkyung Jang b,n

a Department of Applied Physics, Kyung Hee University, Yongin 446-701, South Koreab Department of Nanoenergy Engineering, Pusan National University, Busan 609-735, South Korea

a r t i c l e i n f o

Article history:Received 26 February 2015Accepted 14 April 2015Available online 21 April 2015

Keywords:FerroelectricBiFeO3

PolarizationDomainDomain wall energyThickness scaling

a b s t r a c t

We report the ferroelectric properties of the epitaxial BiFeO3 (BFO) thin films grown on Nb-doped SrTiO3

substrates. The (001) and (111) BFO thin films were grown epitaxially on a single-crystalline (100) and(111) Nb-doped SrTiO3 substrate by pulse laser deposition, respectively. The (111) BFO film gave higherremnant polarization in the ferroelectric hysteresis loop than that of the (001) film. The (001) and (111)BFO films revealed striped and mosaic ferroelectric domain structures, respectively. The ferroelectricdomain size vs. film thickness was in accordance with the scaling law of Landau, Lifshitz and Kittel. Thedomains of the (001) BFO film were larger than those of the (111) BFO film. This is because the (111) BFOfilm has a relatively large remnant polarization, even though the (111) film has a lower domain wallenergy.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Ferroelectric thin films, such as Pb(Zr,Ti)O3 (PZT) films, havebeen studied intensively because of their applications to nonvola-tile random access memory (NVRAM) [1–4]. However, lead-freeferroelectric materials, such as BiFeO3 (BFO) and BaTiO3, areattracting considerable attention because of the detrimentaleffects of lead on the human body and the environment [5–8].BFO thin films are particularly promising because of their multi-functional properties, i.e., coexistence of ferroelectric and mag-netic behavior at room temperature. Nevertheless, it is essential toenhance the relatively low ferroelectricity and piezoelectricity ofBFO films compared to those of the PZT film. Thus far, severalattempts have been made to improve the physical properties ofthe BFO films, such as substituting different ions into the A-site orB-site [9–11] and controlling the ferroelectric domain structureusing miscut and vicinal substrates [12–13].

The dielectric and switching behaviors of a ferroelectric materialare greatly influenced by the domain structure [14]. The domainstructure of a ferroelectric film can be studied on the nanometer scaleby piezoresponse force microscopy (PFM), which presents ferroelectricdomain patterns by recording piezoresponse signals based on polar-ization and mechanical deformation of materials under appliedelectric field. In principle, the domain structure is determined by theinterplay between the ferroelectric polarization energy and

ferroelectric domain wall energy [15]. Although there have beenextensive studies on the ferroelectric domains of BFO thin films, littleis known regarding the correlation between the ferroelectric proper-ties and the preferred orientation in the growth of film. In addition, itis important to understand how the film thickness affects the ferro-electric domain size, considering the ever-decreasing device size andfilm thickness. In this study, we investigate the ferroelectric domainstructure of a BFO thin film, which has a preferred orientation on asingle crystal substrate. Using the pulsed laser deposition (PLD), the(001) and (111) BFO thin films were grown epitaxially on a singlecrystalline (100) and (111) Nb-doped SrTiO3 (Nb:STO) substrate,respectively. This study examined the ferroelectric domain structuresof epitaxial (001) and (111) BFO thin films by PFM as well as how theferroelectric domain size is affected by the film thickness.

2. Experimental procedure

The BiFeO3 target was prepared using a conventional solid-state reaction method. High purity, commercially available pow-ders of Bi2O3 (99.99% pure) and Fe2O3 (99.99%) were mixed at themolar ratio. The mixture was well ground, pelletized and fired inair. The process was repeated three times: at 400 1C for 10 h, at700 1C for 12 h and at 880 1C for 5 h with intermediate grinding.A KrF excimer laser (with a wavelength of 248 nm and an energydensity of 0.5 J/cm2) was focused onto the target. The substrate–target distance was maintained at �4 cm. Once the base pressurereached �5�10�7 Torr, the substrate temperature was set to 800 1C

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2015.04.0720167-577X/& 2015 Elsevier B.V. All rights reserved.

n Corresponding authors. Tel.: þ82 31 201 3770, þ82 51 510 7348.E-mail addresses: jyson@khu.ac.kr (J. Yeog Son), jkjang@pusan.ac.kr (J. Jang).

Materials Letters 154 (2015) 25–28

and the oxygen partial pressure was fixed to 10 mTorr. After deposi-tion, the thin filmwas cooled slowly to room temperature at 100 1C/hin oxygen ambient with a pressure of 300 Torr.

The thickness of the present BFO films was 100 nm (unlessotherwise noted). The thickness was measured by taking a cross-sectional image of the sample by scanning electron microscopy(SEM). The crystal structures of the BFO films were examined byXRD. Atomic force microscopy (AFM) and PFM measurements(VEECO Dimension 3100 AFM system) were used to obtain thesurface morphology, root-mean-square (RMS) roughness, and ferro-electric domain structure of each film. Circular top electrodes,100 μm in diameter and 100 nm in thickness, were fabricated. Todo so, a Pt layer was deposited on the BFO film by RF magnetronsputtering using a dot-patterned shadow mask. Subsequently, allsamples were annealed at 400 1C for 5 min before obtaining theferroelectric hysteresis loops. The ferroelectric hysteresis loops weremeasured using an RT66A test system (Radiant Technologies, Inc.).

3. Results and discussion

Fig. 1 shows the XRD θ–2θ and Φ-scan patterns for the BFOfilms. The (001) and (002) XRD peaks found for the (100) Nb:STOsubstrate indicate that the BFO thin film grew preferentially alongthe (00l) orientation (Fig. 1(a)). The full width at half maxima(FWHMs) of the (001) and (002) peaks were approximately equal to0.81. The lattice misfit between the (001) BFO film and (100) Nb:STOsubstrate was approximately 1.54%. The c-lattice constant obtainedfrom the (00l) peaks of the (001) BFO film was 4.00 Å. On the otherhand, a (111) peak was observed for the (111) Nb:STO substrate. TheFWHM in this case was measured to be 1.11 (Fig. 1(b)). A c-latticeconstant of 3.97 Å was obtained from the (111) peak. The latticemisfit between the (111) BFO thin film and (111) Nb:STO substratewas approximately 1.52%. Based on the measured FWHMs of the

(001) and (111) BFO films, both BFO films showed good crystallinityalong the out-of-plane orientation.

To confirm the in-plane orientation, XRD Φ-scans were per-formed for the (111) peak of the (001) BFO film and the (110) peakof the (111) BFO film. Four-fold symmetry was observed for the(111) peak of the (001) BFO thin film (Fig. 1(c)). On the other hand,three-fold symmetry was observed for the (110) peak of the (111)BFO thin film (Fig. 1(d)). As shown in Fig. 1(c) and (d), each unitcell of the BFO thin film and Nb:STO substrate was positionedparallel to the in-plane direction. The a-lattice constants of the(001) and (111) BFO thin films were 3.94 and 3.97 Å, respectively.The rocking curve FWHMs of the (111) and (110) peaks were0.9 and 1.01, respectively, suggesting that both the BFO films arewell crystallized along the in-plane orientation.

The ferroelectric properties of the BFO thin films were char-acterized by measuring the ferroelectric polarization under anapplied electric field with a frequency of 10 KHz. Fig. 2 shows thetypical ferroelectric hysteresis behavior of Pt/(001) BFO/Nb:STO

Fig. 1. XRD patterns of (a) (001) and (b) (111) BiFeO3 thin films. (c) Φ scans of the (111) XRD peak of the (001) BiFeO3 thin film and of the (111) XRD peak of the Nb-dopedSrTiO3 substrate. (d) Φ scans of the (110) XRD peak of the (111) BiFeO3 thin film and the (110) XRD peak of the Nb-doped SrTiO3 substrate.

Fig. 2. Ferroelectric hysteresis loops of the (001) and (111) BiFeO3 thin films.

Y. Ahn et al. / Materials Letters 154 (2015) 25–2826

and Pt/(111) BFO/Nb:STO capacitors. The remnant polarization (Pr)and coercive field (Ek) of the (111) BFO thin film were 60.2 μC/cm2

and 277 kV/cm, respectively, which are larger than the corre-sponding values, Pr�49.2 μC/cm2 and Ek�243 kV/cm, of the (001)BFO film. The saturation polarizations (Ps) of the (111) and (001)films were 85.3 and 69.9 μC/cm2, respectively.

We examined the surface morphology for the (001) and (111)BFO thin films by using AFM. The (001) BFO thin film showed aterrace pattern with smooth and clear steps arising from layer-by-layer growth (Fig. 3(a)) [16]. On the other hand, round rectangulargrains were observed for the (111) BFO thin film, which isindicative of an island growth mode (Fig. 3(c)) [16]. The root-mean-square (RMS) roughnesses of the (001) and (111) BFO thinfilms were 1.4 and 1.7 nm, respectively. The ferroelectric domainstructures were also observed by PFM, which was established byan Rh-coated tip (f0 �25 kHz, k �3 N/m) applied 10 kHz ACvoltage. The PFM image of the (001) BFO film showed stripepatterns with opposite contrast to that observed in the AFM image(Fig. 3(b)). In contrast, a mosaic domain structure was observed forthe PFM image of the (111) BFO film (Fig. 3(d)). The ferroelectricdomain sizes of the (001) and (111) BFO thin films were 98 and80 nm, respectively.

The domain size was related to the film thickness using theLandau, Lifshitz, and Kittel (LLK) scaling law [17]. This law, whichwas developed originally for ferromagnetic domains, was extendedto the ferroelectric domains by Mitsui and Furuichi [18]. Accordingto the LLK scaling law, the domain width grows in proportion to thesquare root of the film thickness. The mean domain width (w) as afunction of the film thickness (d) follows a power law, w¼ Adγ withγ¼1/2. In Fig. 4, the average domainwidth was plotted as a functionof the film thickness (log–log scale). The least squares fit was usedto derive the scaling exponent γ for the (001) and (111) BFO films of50, 80, 100, and 150 nm in thickness. The values fitted to the LLKscaling law were γ¼0.56 and 0.52 for the (001) and (111) BFO thinfilms, respectively. The scaling exponent of the (001) film wasslightly larger than that of the (111) film. This indicates that the

domain wall energy of the (001) film was greater than that of the(111) BFO thin film because the domainwall energy is compatible tothe experimental scaling exponent, γ [17].

4. Conclusions

In summary, epitaxial (001) and (111) BiFeO3 thin films weredeposited on single crystalline (100) and (111) Nb-doped SrTiO3

substrates, respectively. The (111) BFO thin film exhibited aremnant polarization higher than that of the (001) BFO thin film.The epitaxial (001) and (111) BiFeO3 (BFO) thin films showedstriped and mosaic ferroelectric domains, respectively. From thedomain size as a function of the film thickness, the (111) BFO filmwas confirmed to have a domain wall energy lower than that ofthe (001) BFO thin film. The (001) BFO thin film had a domain sizelarger than that of the (111) BFO thin film. This is because the (111)film has relatively large remnant polarization compared to the

Fig. 3. AFM ((a) and (c)) and PFM ((b) and (d)) images of the (001) (top) and (111) (bottom) BiFeO3 thin films.

Fig. 4. Domain size vs. the film thickness of the (001) and (111) BiFeO3 thin films.The dotted lines are the fitted curves from which the scaling exponents (γ) werederived.

Y. Ahn et al. / Materials Letters 154 (2015) 25–28 27

(001) BFO thin film despite the (111) film having a lower domainwall energy.

Acknowledgments

This study was supported by the National Research Foundationof Korea (NRF) grant funded by the Korea government (No.2012R1A2A2A01046451). JJ gratefully acknowledges the supportby National Research Foundation Grants funded by the KoreanGovernment (MEST) (Nos. MEST-2013–027519 and 2014-001690)

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