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Ferroelectric Resistive Switching in High-Density Nanocapacitor Arrays Based on BiFeO

₃

Ultrathin Films and Ordered Pt

Nanoelectrodes

Zengxing 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 in well-ordered and high-density nanocapacitor arrays based on continuous BiFeO₃ (BFO) ultrathin films and isolated Pt nanonelectrodes. The thickness of BFO films and the lateral dimension of Pt electrodes were aggressively scaled down to

<10 nm and ∼60 nm, respectively, representing an ultrahigh

ferroelectric memory density of∼100 Gbit/inch². Moreover, the RS behavior in those nanocapacitors showed a large ON/OFF ratio (above 10³) and a long retention time of over 6,000 s. Our results not only demonstrate for thefirst time that the switchable ferroelectric diode effect could be realized in BFOfilms down to <10 nm in thickness, but also suggest the great potentials of those nanocapacitors for applications in high-density data storage.

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

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INTRODUCTION

Ferroelectrics, possessing stable and electrically switchable polarization,^1,2have been extensively studied as candidates for nonvolatile memory elements.³⁻⁵ Traditional ferroelectric random access memory (FeRAM),^1,4 relies on the capacitive readout of information (i.e., polarization), which is, however, reading-destructive. Thus, an additional writing after reading (rewriting) is required, significantly limiting the widespread applications of FeRAM.⁶⁻⁸One solution to this drawback may be a nondestructive readout of polarization based on the ferroelectric resistive switching (RS) effect. This RS effect has been widely studied in ferroelectric diodes and ferroelectric tunnelling junctions (FTJs), in both of which polarization reversal can effectively tune the charge transport properties.

BiFeO₃(BFO), as being simultaneously a ferroelectric and a narrow-gap semiconductor, offers a promising platform on which a reliable ferroelectric RS effect may be realized. Previous studies mainly focused on the RS behavior in relatively thick BFOfilms (∼100 nm and above).⁹⁻¹²In BFO thickfilms, both polarization tuned interface barriers and defect-mediated conduction mechanisms in the bulk, can lead to RS phenomena.⁹⁻¹³ It is therefore of great difficulty to unambiguously identify the real mechanisms. In addition, the large thickness generally results in large bulk resistance and consequently low readout current, limiting the miniaturization of memory cells based on BFO thickfilms. These issues can be

well circumvented in BFO ultrathinfilms (∼10 nm and below), which have attracted increasing attention over the past few years. For example, Yamada et al.¹⁴fabricated submicrometer capacitors based on the tetragonal BFOfilm with a thickness of 3.5 nm and achieved a giant ON/OFF ratio of 10⁴ together with good retention and fatigue-resistance properties. Hu et al.¹⁵ reported that Sm-doped BFO ultrathin films (∼3 nm) grown on semiconducting Nb:SrTiO₃ (Nb:STO) substrates could realize an even larger ON/OFF ratio (10⁵) and a novel reading manner utilizing the photovoltaic effect. In particular, to fulfill the demand of large scale integration, we have recently demonstrated that in the high density arrays of BFO ultrathin films-based nanocapacitors (with lateral sizes of several tens of nanometers), the apparent RS behavior modulated by polarization can be well-retained.^16,17

In this work, we have further optimized the nanocapacitor cell structures, which consist of well-ordered high-density Pt nanoelectrodes and continuous BFO ultrathinﬁlms epitaxially grown on SrRuO₃(SRO) bottom electrodes. Compared with BFO nanodots¹⁷ or nanoislands¹⁸ used in previous nanocapacitors, continuous BFOﬁlms show advantages in terms of crystal quality and phase purity. The optimized nanocapacitors

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

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show well-defined switchable ferroelectric diode effect, which has not been achieved hitherto in BFOfilms at such a small thickness (<10 nm).¹⁸Our study therefore not only contributes to the further understanding of RS phenomena in ferroelectric ultrathinfilms but also promotes the device miniaturization of ferroelectric resistive memories.

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EXPERIMENTAL METHODS

2.1. Fabrication of Nanocapacitor Array. BFO/SRO bilayers were epitaxially grown on SrTiO₃(STO) single-crystal substrates by pulsed laser deposition (PLD) using a KrF (λ= 248 nm) excimer laser with a laser energy density of 0.5 J/cm²and a repetition rate of 2 Hz.

The SROﬁlms (∼30 nm) were deposited on STO at 630°C and annealed for 5 min under an oxygen pressure of 20 Pa. Subsequently, the BFOﬁlms (∼8 nm and∼3 nm in thickness) were grown at 630°C under an oxygen pressure of 2.5 Pa. Then, the BFO/SRO bilayers were freely cooled to room temperature. The anodic alumina oxide (AAO) template masks with the pore sizes of 60−80 nm were adhered onto the samples, and then they were calcined at 450°C for 30 min.

The Pt nanoelectrodes were deposited on the BFOﬁlms through the AAO masks by PLD with a laser energy density of 1 J/cm² and a repetition rate of 5 Hz at 120°C under an oxygen pressure below 5× 10⁻⁴Pa. After the Pt deposition, nanocapacitors with ordered arrays were obtained by mechanically lifting oﬀ the AAO templates. The fabrication process was illustrated inFigure 1a.

2.2. Structural Characterizations.The information on the phase purity, crystal structure and epitaxial quality were examined by X-ray diﬀraction (XRD) scan and reciprocal space mapping (RSM) analysis (X’Pert PRO, Pan-Analyzer). The surface morphologies, local domain structures and piezoelectric and conduction properties were characterized using an Asylum Research Cypher scanning probe microscopy (SPM).

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RESULTS AND DISCUSSION

The as-prepared BFO ﬁlms show atomically ﬂat surfaces (Figure S1a), ensuring good contacts between BFO and Pt.

Theﬁlms also show good crystallinity and high-quality epitaxy (see Figure S1b−d). As seen from Figure 1b, c, the nanocapcitor arrays are well-ordered, and the Pt nanoelectrodes are ∼60 nm (Figure 1b) in diameter and ∼8 nm in height (Figure 1c).

The local conduction behavior of the Pt/BFO/SRO nanocapacitors was investigated using conductive atomic force microscopy (CAFM). The topography and current images with a scanned area of 0.5×0.5μm²are presented inFigure 2a, b,

respectively. It can be seen that the bright regions in the current mapping well-match the positions of the Pt electrodes, indicating that the Pt electrodes significantly enhance the readout current. This is because the Pt electrodes form the face- to-face contacts with thefilm, which areflat and large in area compared with the dot-to-face contact between the CAFM tip and the barefilm. Next, some of the Pt electrodes in the 0.5× 0.5μm²region 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 the unpoled regions including both the bare film and the Pt electrodes show the dark colors in the phase image (Figure 2d), indicating that the domains are initially aligned along one particular direction. On the contrary, the poled nanocapacitors show sharp phase contrast with the unpoled regions, as can be seen fromFigure 2d. Although the negative poling can switch a domain upward, the initial orientation of domains is confirmed to be downward. To test the stability of those switched domains, the PFM images were taken at different times after poling. Here, even thinner films of BFO with ∼3 nm in thickness were investigated because it has been widely known that the polarization stability in ferroelectricfilms will decrease as the film thickness decreases.¹⁹ Nevertheless, as shown in Figure S2, the bright colors of the poled nanocapacitors only slightly decay even at 12 h after poling. The above results demonstrate that the domains in the BFO ultrathin films are switchable, and they are considerably stable after switching.

Next, the local switching characteristics were further probed by PFM hysteresis loop measurements. Figure 3a shows that the amplitude loop exhibits a typical butterfly shape and the phase difference between the two polarization states is∼180ô. The voltages at the minima in the amplitude loop coincide with the switching voltages in the phase loop; thus, the coercive voltage (V_c) is found to be ±4 V, and the corresponding Figure 1. (a) Schematic flowchart illustrating the procedures of

fabricating 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 scanning bias of +2V, and PFM (c) amplitude and (d) phase images of the nanocapacitor arrays. The blue circles in c and d indicate the Pt nanoelectrodes poled with a pulse voltage of−6 V/0.2 s.

ACS Applied Materials & Interfaces

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coercivefield (E_c) is 5×10³kV/cm. It is noteworthy that this E_cis 10 times larger than those observed in typical BiFeO₃thick films.^9,10 This is because as the film thickness decreases, increase of the epitaxial strain, effect of interfacial layers, less sites of domain nucleation and pinning of domain walls will induce a significant enhancement ofE_c.²⁰⁻²²Additionally, when piezoresponse force microscopy (PFM) is used to measure the hysteresis loops, the contact resistance between the tip andfilm is considerably high and thus the tip/film contact can undertake a large portion of the applied voltage. All the above factors contribute to the largeE_cmeasured by PFM in BFO ultrathin films.^15,16 At the same time, similar local switching behavior with lower V_Cof±2 V (Figure S4a) was observed in the ∼3 nm-thick BFOfilm. These results further prove the ferroelectric

nature of the BFO ultrathin ﬁlms. One advantage of the nanocapacitor structure is that the BFO domains underneath Pt electrodes are easier to be switched than those in the bareﬁlm.

This can be evidenced by comparing voltage-dependent hysteresis loops and phase images measured for the nanocapacitors and the bare ﬁlm. As shown in Figure 3b, at the applied voltage of ±4 V, the phase loop recorded on the Pt electrode is more saturated than that recorded on the bareﬁlm.

To further confirm this observation, we have conducted multiple measurements at different locations of the sample (both on and off the electrodes). Similar results that at the applied voltage of 4 V, the phase loops recorded on the electrodes are more saturated than those recorded on the bare film, have been well reproduced (seen in Figure S3). In addition, the different domain switching behavior between the bare film and the nanocapacitors was further studied by DC poling. After DC poling with −4 V, BFO domains in the nanocapacitors were completely switched while those in the barefilm remained nearly unchanged, as can be seen inFigure 3c, d. When the poling voltage increased to −5 V, all the domains in both the nanocapacitors and the bare film were switched. Therefore, the Pt nanoelectrodes can effectively facilitate the domain switching in BFO ultrathin films. This effect can be accounted for by several factors associated with the face-to-face contacts formed between Pt nanoelectrodes and thefilm, such as improved uniformity of the electric field distribution, lowered interface barrier, and favorable sites for domain nucleation.^16,23

Having confirmed the switchable polarization in BFO ultrathin films, we further investigated the polarization- dependent conduction behavior using CAFM. Figure 4a shows the PFM phase image superimposed with the 3D topography, which was taken after +6 V/−6 V DC poling and a subsequent pulse poling (+6 V/0.2 s) on a specific Pt electrode.

The 180°phase contrast reveals that the polarizations in the +6 V and−6 V poled regions are antiparallel. The corresponding current image measured at a scanning bias of +3 V is shown in Figure 4b. In this image, larger currents are observable on the Pt electrodes which were positively poled (i.e.,P_down). To gain more information on the polarization-dependent conduction behavior, the local current−voltage (I−V) characteristics were measured on a typical Pt electrode. Figure 4c shows two diﬀerent diode-likeI−Vcurves 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 be reversed from positive to negative once the polarization switches from P_down to P_up. Therefore, in terms of the Figure 3.Comparison of the polarization switching behavior between

the regions on Pt nanodot and bareﬁlm. (a) Amplitude and phase hysteresis loops measured on a typical Pt nanodot. (b) Comparison of phase hysteresis loops between the Pt nanodot and the bareﬁlm. (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 2×2μm². In a and b, the upper 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−Vcurves measured on a typical Pt/BFO/SRO nanocapacitor in two opposite polarization states.

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conduction behavior, the Pt/BFO/SRO nanocapacitors behave like switchable ferroelectric diodes. It should be highlighted that this is the first time to realize switchable ferroelectric diode effect in such BFO-based ultrathinfilms (<10 nm in thickness).

To further characterize the switchable diode eﬀect, the current hysteresis loops were measured on a typical Pt electrode by sweeping the bias voltage with a sequence of 0

→6 V→0→−6 V→0, as shown inFigure 5a. The current loops show distinct hysteresis and excellent repeatability, indicating significant and stable RS behavior. The switching from high resistance state (HRS) to low resistance state (LRS) occurred at threshold voltages of ±4 V, which are consistent with the coercive voltages of polarization switching. Moreover, the current loop plotted on the semilogarithmic scale shows an impressive ON/OFF ratio of above 1× 10³at 2 V (inset of Figure 5a). For the∼3 nm thickfilm, similar RS behavior with an ON/OFF ratio of 700 at 1 V was also observed (Figure S4b). It is noteworthy that such ON/OFF ratios are generally larger than those observed in ferroelectric thick films with a thickness above 100 nm,^9,10,24,25and are also comparable with those observed in ultrathin films.^6,26−29 As further shown in Figure 5b, the ON/OFF current values, read at +2 V, only slightly deviate from device to device, indicating good reproducibility of the RS behavior in the Pt/BFO/SRO nanocapacitors. In addition, the retention properties are also quite good as both HRS and LRS could be well retained for over 6000 s (Figure 5c). Next, we have studied the fatigue properties of the nanocapacitors. Unfortunately, the largest

number of fatigue cycles obtained so far is limited to 26 cycles (as shown inFigure 5d), probably because of the shift in the tip location during fatigue measurements.

One remaining important question is the conduction mechanism in the Pt/BFO/SRO nanocapacitors. It has been previously suggested that the switchable diode effect in ferroelectricfilms is caused by the polarization modulation of Schottky barriers.^9,10,18This mechanism, specifically in ultrathin films, can be well described by a thermionic emission model as proposed by Pantel et al.^7,30In this model, the ferroelectricfilm is thin enough to be assumed to be fully depleted. The Schottky barrier (ΦB,i, i = 1 or 2 for top or bottom interfaces, respectively) consists of two components: one is the potential barrier originating from the band offset (Φi); and the other one is the barrier variation due to the imperfect screening of polarization (ΔΦi). The change in polarization gives rise to differentΔΦi, and by this wayΦB,iis modulated. Depending on the polarity of the applied voltage, one of the Schottky barriers (ΦB,i) will limit the conduction process. The resulting current can be described as^12,31,32

= **

−Φ +

πε ε

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟ I AA T ⎟⎟

exp k T

i

q E

2 B, 4

B

3 0 ifl

(1) whereAis the electrode area,A**is the eﬀective Richardson constant,Tis the Kelvin temperature,qis the electron charge, Figure 5.Resistive switching behaviors for the nanocapacitors. (a)I−Vhysteresis loops measured for multiple cycles. Inset shows theI−Vhysteresis loop plotted on a semilogarithmic scale. (b) RS parameters measured for diﬀerent nanocapacitors. (c) Retention properties of a typical nanocapacitor. 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−Vcurves plotted as ln|I|vs U^1/2. Schematic of the potential energy 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 lowering permittivity (slightly larger than the optical permittivityεopt), andk_Bis the Boltzmann constant. Ineq 1,Eis a superposition of the appliedfield E_app, the depolarizationfieldE_dep, and the field due to band alignmentE_band. TheE_depandE_bandarefitting parameters because they are constants for a given polarization state and given ferroelectric and electrode materials.

As shown inFigure 6a, the positive and negative branches of the ln |I| − U^1/2 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, linear relations between ln|I|andU^1/2are observed in certain voltage ranges for both LRS and HRS. These linear relations confirm that the thermionic emission model is applicable for describing the conduction behavior in Pt/BFO/SRO nanocapacitors. Note that here U_app is used for the overall U, although U also comprises the contributions fromE_depandE_band. This is purely for simplicity of data presentation, because E_dep andE_band are constant and they are small compared with the large E_app = U_app/d. However, to extract accurate parameters in eq 1, E_dep andE_bandare kept asfitting parameters in ourfitting process. It should also be noted that in HRS, the linear relations only exist in a small voltage range. This is because at low voltages (U^1/2<

1.8 V^1/2), other conduction mechanisms may contribute to the measured currents. The data at low voltages in HRS are therefore not suitable for fitting. After fitting, εifl, which is correlated with the slope of the linear part of the ln|I|−U^1/2 curve, is calculated as∼8.5. Considering thatεiflis slightly larger than εopt and the εoptof BFO was previously reported to be 6.25,³³the calculated value ofεiflin this study is thus reasonable and the validity of the thermionic model is confirmed. In the cases ofP_down, 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−V behavior. As the polarization is switched toP_up, the two barrier heights are almost symmetrically reversed and calculated to be

∼0.41 and 0.87 eV, respectively (Figure 6c), resulting in the backward diode rectifying behavior. This symmetric switching of Schottky barriers leads to a symmetric switching of current rectifying behavior, which in turn beneﬁts the reading using a small bias with either polarity. Additional interfacial eﬀects with the present nanostructures may be explored, such as interfacial spin and orbital coupling.³⁴

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^CONCLUSION

In summary, well-ordered and high-density nanocapacitor arrays, based on continuous BiFeO₃ ultrathin films (<10 nm in thickness) and isolated Pt nanonelectrodes (∼60 nm in lateral size), were fabricated by PLD in combination with the AAO template method. The Pt/BFO/SRO nanocapacitors exhibit significant and symmetric switchable diode-like rectifying behavior as controlled by the polarization, which is a unique RS behavior for the first time observed in BFO ultrathinfilms. This RS behavior also shows a large ON/OFF ratio of∼1000 and a long retention time of over 6000 s. The conduction mechanism governing the observed switchable diode effect can be well-described by a thermioic emission model, in which the polarization can effectively modify the interface Schottky barriers. Our results therefore suggest that the BFO ultrathinfilm-based nanocapacitors are promising as candidates for nonvolatile ferroelectric resistive memories with ultrahigh density.

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ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acsami.6b07792.

(1) Crystallographic information for the BiFeO₃(BFO) films; (2) comparison of piezoelectric hysteresis behavior on and off electrodes; (3) written out-of-plane PFM images; and (4) piezoresponse and resistance switching hysteresis loops measured on a single nanocapacitor fabricated on the thinnerfilm with a thicknes of 3 nm (PDF)

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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected].

Author Contributions

Z.L. conducted the data acquisition and helped draft the manuscript. X.S., P.L., G.T., and L.Z. participated in the sample fabrication. H.F., X.S., Z.L., and F.Z. carried out the PFM and CAFM measurement. K.H. and Z.Z. contributed to the AAO preparation. Z.F., M.Z., J.F. and J.W. contributed to the data interpretation. Z.F., X.G., and J.L. contributed to the data interpretation and manuscript writing. X.G. supervised the research.

Notes

The authors declare no competingﬁnancial interest.

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ACKNOWLEDGMENTS

This work was supported by the National Key Research Program of China (No. 2016YFA0201002), the State Key Program for Basic Researches of China (Grant 2015CB921202), Natural Science Foundation of China (Grants 51072061, 51272078, 51431006), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014), the International Science & Technology C o o p e r a t i o n P l a t f o r m P r o g r a m o f G u a n g z h o u (2014J4500016), the Natural Science Foundation of Guang- dong Province (2016A030308019), and Science and Technol- ogy Planning Project of Guangdong Province (Grant 2015B090927006).

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