Ferroelectric Resistive Switching in High-Density NanocapacitorArrays Based on BiFeO3 Ultrathin Films and Ordered PtNanoelectrodesZengxing Lu,†,§ Zhen Fan,† Peilian Li,† Hua Fan,† Guo Tian,† Xiao Song,† Zhongwen Li,† Lina Zhao,†

Kangrong Huang,† Fengyuan Zhang,† Zhang Zhang,† Min Zeng,† Xingsen Gao,*,† Jiajun Feng,§

Jianguo Wan,§ and Junming Liu*,†,§

†Institute for Advanced Materials and Laboratory of Quantum Engineering and Quantum Materials, South China Normal University,Guangzhou 510006, China§Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

*S Supporting Information

ABSTRACT: Ferroelectric resistive switching (RS), mani-fested as a switchable ferroelectric diode effect, was observed inwell-ordered and high-density nanocapacitor arrays based oncontinuous BiFeO3 (BFO) ultrathin films and isolated Ptnanonelectrodes. The thickness of BFO films and the lateraldimension of Pt electrodes were aggressively scaled down to<10 nm and ∼60 nm, respectively, representing an ultrahighferroelectric memory density of ∼100 Gbit/inch2. Moreover, the RS behavior in those nanocapacitors showed a large ON/OFFratio (above 103) and a long retention time of over 6,000 s. Our results not only demonstrate for the first time that the switchableferroelectric diode effect could be realized in BFO films down to <10 nm in thickness, but also suggest the great potentials ofthose nanocapacitors for applications in high-density data storage.

KEYWORDS: BiFeO3, ultrathin film, nanocapacitor array, high-density memory, resistive switching, Schottky emission

■ INTRODUCTION

Ferroelectrics, possessing stable and electrically switchablepolarization,1,2 have been extensively studied as candidates fornonvolatile memory elements.3−5 Traditional ferroelectricrandom access memory (FeRAM),1,4 relies on the capacitivereadout of information (i.e., polarization), which is, however,reading-destructive. Thus, an additional writing after reading(rewriting) is required, significantly limiting the widespreadapplications of FeRAM.6−8 One solution to this drawback maybe a nondestructive readout of polarization based on theferroelectric resistive switching (RS) effect. This RS effect hasbeen widely studied in ferroelectric diodes and ferroelectrictunnelling junctions (FTJs), in both of which polarizationreversal can effectively tune the charge transport properties.BiFeO3 (BFO), as being simultaneously a ferroelectric and a

narrow-gap semiconductor, offers a promising platform onwhich a reliable ferroelectric RS effect may be realized. Previousstudies mainly focused on the RS behavior in relatively thickBFO films (∼100 nm and above).9−12 In BFO thick films, bothpolarization tuned interface barriers and defect-mediatedconduction mechanisms in the bulk, can lead to RSphenomena.9−13 It is therefore of great difficulty tounambiguously identify the real mechanisms. In addition, thelarge thickness generally results in large bulk resistance andconsequently low readout current, limiting the miniaturizationof memory cells based on BFO thick films. These issues can be

well circumvented in BFO ultrathin films (∼10 nm and below),which have attracted increasing attention over the past fewyears. For example, Yamada et al.14 fabricated submicrometercapacitors based on the tetragonal BFO film with a thickness of3.5 nm and achieved a giant ON/OFF ratio of 104 togetherwith good retention and fatigue-resistance properties. Hu etal.15 reported that Sm-doped BFO ultrathin films (∼3 nm)grown on semiconducting Nb:SrTiO3 (Nb:STO) substratescould realize an even larger ON/OFF ratio (105) and a novelreading manner utilizing the photovoltaic effect. In particular,to fulfill the demand of large scale integration, we have recentlydemonstrated that in the high density arrays of BFO ultrathinfilms-based nanocapacitors (with lateral sizes of several tens ofnanometers), the apparent RS behavior modulated by polar-ization can be well-retained.16,17

In this work, we have further optimized the nanocapacitorcell structures, which consist of well-ordered high-density Ptnanoelectrodes and continuous BFO ultrathin films epitaxiallygrown on SrRuO3 (SRO) bottom electrodes. Compared withBFO nanodots17 or nanoislands18 used in previous nano-capacitors, continuous BFO films show advantages in terms ofcrystal quality and phase purity. The optimized nanocapacitors

Received: June 27, 2016Accepted: August 15, 2016Published: August 15, 2016

Research Article

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© 2016 American Chemical Society 23963 DOI: 10.1021/acsami.6b07792ACS Appl. Mater. Interfaces 2016, 8, 23963−23968

show well-defined switchable ferroelectric diode effect, whichhas not been achieved hitherto in BFO films at such a smallthickness (<10 nm).18 Our study therefore not only contributesto the further understanding of RS phenomena in ferroelectricultrathin films but also promotes the device miniaturization offerroelectric resistive memories.

■ EXPERIMENTAL METHODS2.1. Fabrication of Nanocapacitor Array. BFO/SRO bilayers

were epitaxially grown on SrTiO3 (STO) single-crystal substrates bypulsed laser deposition (PLD) using a KrF (λ = 248 nm) excimer laserwith a laser energy density of 0.5 J/cm2 and a repetition rate of 2 Hz.The SRO films (∼30 nm) were deposited on STO at 630 °C andannealed for 5 min under an oxygen pressure of 20 Pa. Subsequently,the BFO films (∼8 nm and ∼3 nm in thickness) were grown at 630 °Cunder an oxygen pressure of 2.5 Pa. Then, the BFO/SRO bilayerswere freely cooled to room temperature. The anodic alumina oxide(AAO) template masks with the pore sizes of 60−80 nm were adheredonto the samples, and then they were calcined at 450 °C for 30 min.The Pt nanoelectrodes were deposited on the BFO films through theAAO masks by PLD with a laser energy density of 1 J/cm2 and arepetition rate of 5 Hz at 120 °C under an oxygen pressure below 5 ×10−4 Pa. After the Pt deposition, nanocapacitors with ordered arrayswere obtained by mechanically lifting off the AAO templates. Thefabrication process was illustrated in Figure 1a.

2.2. Structural Characterizations. The information on the phasepurity, crystal structure and epitaxial quality were examined by X-raydiffraction (XRD) scan and reciprocal space mapping (RSM) analysis(X’Pert PRO, Pan-Analyzer). The surface morphologies, local domainstructures and piezoelectric and conduction properties werecharacterized using an Asylum Research Cypher scanning probemicroscopy (SPM).

■ RESULTS AND DISCUSSIONThe as-prepared BFO films show atomically flat surfaces(Figure S1a), ensuring good contacts between BFO and Pt.The films also show good crystallinity and high-quality epitaxy(see Figure S1b−d). As seen from Figure 1b, c, thenanocapcitor arrays are well-ordered, and the Pt nanoelectrodesare ∼60 nm (Figure 1b) in diameter and ∼8 nm in height(Figure 1c).

The local conduction behavior of the Pt/BFO/SROnanocapacitors was investigated using conductive atomic forcemicroscopy (CAFM). The topography and current images witha scanned area of 0.5 × 0.5 μm2 are presented in Figure 2a, b,

respectively. It can be seen that the bright regions in the currentmapping well-match the positions of the Pt electrodes,indicating that the Pt electrodes significantly enhance thereadout current. This is because the Pt electrodes form the face-to-face contacts with the film, which are flat and large in areacompared with the dot-to-face contact between the CAFM tipand the bare film. Next, some of the Pt electrodes in the 0.5 ×0.5 μm2 region were written with pulse voltages of −6 V/0.2 s,and subsequently the Piezoresponse force microscopy (PFM)amplitude and phase images were taken (Figure 2c, d). All theunpoled regions including both the bare film and the Ptelectrodes show the dark colors in the phase image (Figure 2d),indicating that the domains are initially aligned along oneparticular direction. On the contrary, the poled nanocapacitorsshow sharp phase contrast with the unpoled regions, as can beseen from Figure 2d. Although the negative poling can switch adomain upward, the initial orientation of domains is confirmedto be downward. To test the stability of those switcheddomains, the PFM images were taken at different times afterpoling. Here, even thinner films of BFO with ∼3 nm inthickness were investigated because it has been widely knownthat the polarization stability in ferroelectric films will decreaseas the film thickness decreases.19 Nevertheless, as shown inFigure S2, the bright colors of the poled nanocapacitors onlyslightly decay even at 12 h after poling. The above resultsdemonstrate that the domains in the BFO ultrathin films areswitchable, and they are considerably stable after switching.Next, the local switching characteristics were further probed

by PFM hysteresis loop measurements. Figure 3a shows thatthe amplitude loop exhibits a typical butterfly shape and thephase difference between the two polarization states is ∼180 o.The voltages at the minima in the amplitude loop coincide withthe switching voltages in the phase loop; thus, the coercivevoltage (Vc) is found to be ±4 V, and the corresponding

Figure 1. (a) Schematic flowchart illustrating the procedures offabricating the Pt/BFO/SRO nanocapcitor arrays on STO substrate.(b) 2D and (c) 3D topography images of the Pt nanodots.

Figure 2. (a) Topography, (b) CAFM current mapping at a scanningbias of +2V, and PFM (c) amplitude and (d) phase images of thenanocapacitor arrays. The blue circles in c and d indicate the Ptnanoelectrodes poled with a pulse voltage of −6 V/0.2 s.

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coercive field (Ec) is 5 × 103 kV/cm. It is noteworthy that thisEc is 10 times larger than those observed in typical BiFeO3 thickfilms.9,10 This is because as the film thickness decreases,increase of the epitaxial strain, effect of interfacial layers, lesssites of domain nucleation and pinning of domain walls willinduce a significant enhancement of Ec.

20−22 Additionally, whenpiezoresponse force microscopy (PFM) is used to measure thehysteresis loops, the contact resistance between the tip and filmis considerably high and thus the tip/film contact can undertakea large portion of the applied voltage. All the above factorscontribute to the large Ec measured by PFM in BFO ultrathinfilms.15,16 At the same time, similar local switching behaviorwith lower VC of ±2 V (Figure S4a) was observed in the ∼3nm-thick BFO film. These results further prove the ferroelectric

nature of the BFO ultrathin films. One advantage of thenanocapacitor structure is that the BFO domains underneath Ptelectrodes are easier to be switched than those in the bare film.This can be evidenced by comparing voltage-dependenthysteresis loops and phase images measured for the nano-capacitors and the bare film. As shown in Figure 3b, at theapplied voltage of ±4 V, the phase loop recorded on the Ptelectrode is more saturated than that recorded on the bare film.To further confirm this observation, we have conductedmultiple measurements at different locations of the sample(both on and off the electrodes). Similar results that at theapplied voltage of 4 V, the phase loops recorded on theelectrodes are more saturated than those recorded on the barefilm, have been well reproduced (seen in Figure S3). Inaddition, the different domain switching behavior between thebare film and the nanocapacitors was further studied by DCpoling. After DC poling with −4 V, BFO domains in thenanocapacitors were completely switched while those in thebare film remained nearly unchanged, as can be seen in Figure3c, d. When the poling voltage increased to −5 V, all thedomains in both the nanocapacitors and the bare film wereswitched. Therefore, the Pt nanoelectrodes can effectivelyfacilitate the domain switching in BFO ultrathin films. Thiseffect can be accounted for by several factors associated withthe face-to-face contacts formed between Pt nanoelectrodesand the film, such as improved uniformity of the electric fielddistribution, lowered interface barrier, and favorable sites fordomain nucleation.16,23

Having confirmed the switchable polarization in BFOultrathin films, we further investigated the polarization-dependent conduction behavior using CAFM. Figure 4ashows the PFM phase image superimposed with the 3Dtopography, which was taken after +6 V/−6 V DC poling and asubsequent pulse poling (+6 V/0.2 s) on a specific Pt electrode.The 180° phase contrast reveals that the polarizations in the +6V and −6 V poled regions are antiparallel. The correspondingcurrent image measured at a scanning bias of +3 V is shown inFigure 4b. In this image, larger currents are observable on thePt electrodes which were positively poled (i.e., Pdown). To gainmore information on the polarization-dependent conductionbehavior, the local current−voltage (I−V) characteristics weremeasured on a typical Pt electrode. Figure 4c shows twodifferent diode-like I−V curves for different polarization states(poled with pulse voltages of +6 V/0.2 s and −6 V/0.2 s,respectively). The forward direction of the diode can bereversed from positive to negative once the polarizationswitches from Pdown to Pup. Therefore, in terms of the

Figure 3. Comparison of the polarization switching behavior betweenthe regions on Pt nanodot and bare film. (a) Amplitude and phasehysteresis loops measured on a typical Pt nanodot. (b) Comparison ofphase hysteresis loops between the Pt nanodot and the bare film. (c)Amplitude and (d) phase images scanned after poling with −4 V and−5 V for different regions.

Figure 4. (a) PFM Phase and (b) CAFM current images superimposed with the 3D topography measured on an area of 2 × 2 μm2. In a and b, theupper and lower areas are poled with +6 V and −6 V, respectively, and the white circle indicates the Pt nanoelectrode poled with a pulse voltage of+6 V/0.2 s. The current image is scanned with a bias of +3 V. (c) I−V curves measured on a typical Pt/BFO/SRO nanocapacitor in two oppositepolarization states.

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conduction behavior, the Pt/BFO/SRO nanocapacitors behavelike switchable ferroelectric diodes. It should be highlighted thatthis is the first time to realize switchable ferroelectric diodeeffect in such BFO-based ultrathin films (<10 nm in thickness).To further characterize the switchable diode effect, the

current hysteresis loops were measured on a typical Ptelectrode by sweeping the bias voltage with a sequence of 0→ 6 V→ 0 → −6 V→ 0, as shown in Figure 5a. The currentloops show distinct hysteresis and excellent repeatability,indicating significant and stable RS behavior. The switchingfrom high resistance state (HRS) to low resistance state (LRS)occurred at threshold voltages of ±4 V, which are consistentwith the coercive voltages of polarization switching. Moreover,the current loop plotted on the semilogarithmic scale shows animpressive ON/OFF ratio of above 1 × 103 at 2 V (inset ofFigure 5a). For the ∼3 nm thick film, similar RS behavior withan ON/OFF ratio of 700 at 1 V was also observed (FigureS4b). It is noteworthy that such ON/OFF ratios are generallylarger than those observed in ferroelectric thick films with athickness above 100 nm,9,10,24,25 and are also comparable withthose observed in ultrathin films.6,26−29 As further shown inFigure 5b, the ON/OFF current values, read at +2 V, onlyslightly deviate from device to device, indicating goodreproducibility of the RS behavior in the Pt/BFO/SROnanocapacitors. In addition, the retention properties are alsoquite good as both HRS and LRS could be well retained forover 6000 s (Figure 5c). Next, we have studied the fatigueproperties of the nanocapacitors. Unfortunately, the largest

number of fatigue cycles obtained so far is limited to 26 cycles(as shown in Figure 5d), probably because of the shift in the tiplocation during fatigue measurements.One remaining important question is the conduction

mechanism in the Pt/BFO/SRO nanocapacitors. It has beenpreviously suggested that the switchable diode effect inferroelectric films is caused by the polarization modulation ofSchottky barriers.9,10,18 This mechanism, specifically in ultrathinfilms, can be well described by a thermionic emission model asproposed by Pantel et al.7,30 In this model, the ferroelectric filmis thin enough to be assumed to be fully depleted. The Schottkybarrier (ΦB,i, i = 1 or 2 for top or bottom interfaces,respectively) consists of two components: one is the potentialbarrier originating from the band offset (Φi); and the other oneis the barrier variation due to the imperfect screening ofpolarization (ΔΦi). The change in polarization gives rise todifferent ΔΦi, and by this way ΦB,i is modulated. Depending onthe polarity of the applied voltage, one of the Schottky barriers(ΦB,i) will limit the conduction process. The resulting currentcan be described as12,31,32

= **−Φ +

πε ε

⎛

⎝

⎜⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟I AA T

k Texp

iq E

2B, 4

B

3

0 ifl

(1)

where A is the electrode area, A** is the effective Richardsonconstant, T is the Kelvin temperature,q is the electron charge,

Figure 5. Resistive switching behaviors for the nanocapacitors. (a) I−V hysteresis loops measured for multiple cycles. Inset shows the I−V hysteresisloop plotted on a semilogarithmic scale. (b) RS parameters measured for different nanocapacitors. (c) Retention properties of a typicalnanocapacitor. In b and c, the reading voltages is +2 V. (d) Fatigue performance of a typical nanocapacitor. The set and reset voltages are +6 V and−6 V, and the reading voltage is +1.5 V.

Figure 6. Ferroelectric resistive switching mechanism for the nanocapacitors. (a) I−V curves plotted as ln |I| vs U1/2. Schematic of the potentialenergy profiles of the Pt/BFO/SRO heterostructure in two opposite polarization states: (b) downward and (c) upward.

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ε0 is the vacuum permittivity, εifl is the image force loweringpermittivity (slightly larger than the optical permittivity εopt),and kB is the Boltzmann constant. In eq 1, E is a superpositionof the applied field Eapp, the depolarization field Edep, and thefield due to band alignment Eband. The Edep and Eband are fittingparameters because they are constants for a given polarizationstate and given ferroelectric and electrode materials.As shown in Figure 6a, the positive and negative branches of

the ln |I| − U1/2 curves almost overlap with each other,suggesting that the polarization-induced modulations of the Pt/BFO and BFO/SRO Schottky barriers are nearly identical.Moreover, in the positive (or negative similarly) branch, linearrelations between ln |I| and U1/2 are observed in certain voltageranges for both LRS and HRS. These linear relations confirmthat the thermionic emission model is applicable for describingthe conduction behavior in Pt/BFO/SRO nanocapacitors. Notethat here Uapp is used for the overall U, although U alsocomprises the contributions from Edep and Eband. This is purelyfor simplicity of data presentation, because Edep and Eband areconstant and they are small compared with the large Eapp =Uapp/d. However, to extract accurate parameters in eq 1, Edepand Eband are kept as fitting parameters in our fitting process. Itshould also be noted that in HRS, the linear relations only existin a small voltage range. This is because at low voltages (U1/2 <1.8 V1/2), other conduction mechanisms may contribute to themeasured currents. The data at low voltages in HRS aretherefore not suitable for fitting. After fitting, εifl, which iscorrelated with the slope of the linear part of the ln |I| − U1/2

curve, is calculated as ∼8.5. Considering that εifl is slightly largerthan εopt and the εopt of BFO was previously reported to be6.25,33 the calculated value of εifl in this study is thus reasonableand the validity of the thermionic model is confirmed. In thecases of Pdown, the Schottky barrier heights are calculated to be∼0.87 and ∼0.40 eV for Pt/BFO and BFO/SRO barriers,respectively (Figure 6b), leading to the forward diode I−Vbehavior. As the polarization is switched to Pup, the two barrierheights are almost symmetrically reversed and calculated to be∼0.41 and 0.87 eV, respectively (Figure 6c), resulting in thebackward diode rectifying behavior. This symmetric switchingof Schottky barriers leads to a symmetric switching of currentrectifying behavior, which in turn benefits the reading using asmall bias with either polarity. Additional interfacial effects withthe present nanostructures may be explored, such as interfacialspin and orbital coupling.34

■ CONCLUSIONIn summary, well-ordered and high-density nanocapacitorarrays, based on continuous BiFeO3 ultrathin films (<10 nmin thickness) and isolated Pt nanonelectrodes (∼60 nm inlateral size), were fabricated by PLD in combination with theAAO template method. The Pt/BFO/SRO nanocapacitorsexhibit significant and symmetric switchable diode-likerectifying behavior as controlled by the polarization, which isa unique RS behavior for the first time observed in BFOultrathin films. This RS behavior also shows a large ON/OFFratio of ∼1000 and a long retention time of over 6000 s. Theconduction mechanism governing the observed switchablediode effect can be well-described by a thermioic emissionmodel, in which the polarization can effectively modify theinterface Schottky barriers. Our results therefore suggest thatthe BFO ultrathin film-based nanocapacitors are promising ascandidates for nonvolatile ferroelectric resistive memories withultrahigh density.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b07792.

(1) Crystallographic information for the BiFeO3 (BFO)films; (2) comparison of piezoelectric hysteresis behavioron and off electrodes; (3) written out-of-plane PFMimages; and (4) piezoresponse and resistance switchinghysteresis loops measured on a single nanocapacitorfabricated on the thinner film with a thicknes of 3 nm(PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: xingsengao@scnu.edu.cn.*E-mail: liujm@nju.edu.cn.

Author ContributionsZ.L. conducted the data acquisition and helped draft themanuscript. X.S., P.L., G.T., and L.Z. participated in the samplefabrication. H.F., X.S., Z.L., and F.Z. carried out the PFM andCAFM measurement. K.H. and Z.Z. contributed to the AAOpreparation. Z.F., M.Z., J.F. and J.W. contributed to the datainterpretation. Z.F., X.G., and J.L. contributed to the datainterpretation and manuscript writing. X.G. supervised theresearch.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Key ResearchProgram of China (No. 2016YFA0201002), the State KeyProgram for Basic Researches of China (Grant2015CB921202), Natural Science Foundation of China (Grants51072061, 51272078, 51431006), the Project for GuangdongProvince Universities and Colleges Pearl River Scholar FundedScheme (2014), the International Science & TechnologyCoope r a t i on P l a t f o rm Prog r am o f Guangzhou(2014J4500016), the Natural Science Foundation of Guang-dong Province (2016A030308019), and Science and Technol-ogy Planning Project of Guangdong Province (Grant2015B090927006).

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b07792ACS Appl. Mater. Interfaces 2016, 8, 23963−23968

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