Polarization Switching-Induced Charge Injection Follow

An Unusual Mechanism for Negative Di ff erential Resistance in

Ferroelectric Nanocapacitors: Polarization Switching-Induced Charge Injection Followed by Charge Trapping

Peilian Li,

^†

Zhifeng Huang,

^†

Zhen Fan,*

^,†

Hua Fan,

^†

Qiuyuan Luo,

^†

Chao Chen,

^†

Deyang Chen,

^†

Min Zeng,

^†

Minghui Qin,

^†

Zhang Zhang,

^†

Xubing Lu,

^†

Xingsen Gao,*

^,†

and Jun-Ming Liu

^†,‡

†Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

‡Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

*^S Supporting Information

ABSTRACT: Negative differential resistance (NDR) has been extensively investigated for its wide device applications. However, a major barrier ahead is the low reliability. To address the reliability issues, we consider ferroelectrics and propose an alternative mechanism for realizing the NDR with deterministic current peak positions, in which the NDR results from the polarization switching-induced charge injection and subsequent charge trapping at the metal/ferroelectric interface. In this work, ferroelectric Au/BiFe_0.6Ga_0.4O₃ (BFGO)/Ca_0.96Ce_0.04MnO₃

(CCMO) nanocapacitors are prepared, and their ferroelectricity and NDR behaviors are studied concurrently. It is observed that the NDR current peaks are located at the vicinity of coercive voltages (V_c) of the ferroelectric nanocapacitors, thus evidencing the proposed mechanism. In addition, the NDR effect is reproducible and robust with good endurance and long retention time. This study therefore demonstrates a ferroelectric-based NDR device, which may facilitate the development of highly reliable NDR devices.

KEYWORDS: negative differential resistance, ferroelectric nanocapacitors, polarization switching, charge injection, charge trapping

1. INTRODUCTION

Negative differential resistance (NDR), a nonlinear transport phenomenon where the current decreases with increasing applied voltage, has attracted considerable attention owing to its numerous device applications, such as diodes, oscillators, amplifiers, and analog-to-digital converters.¹⁻⁴ It has been found that a wide variety of materials could exhibit the NDR, including semiconductor quantum wells,⁵ metal-oxide hetero- structures,⁶ polymer−nanoparticle composites,⁷ and biological molecules.⁸For most of them, however, the greatest challenge is to achieve a reproducible and stable NDR effect. For example, the voltage corresponding to the current peak of NDR (V_peak) may vary from sample to sample, and it may also shift successively and even disappear during the cyclic voltage sweeps. These reliability issues may be caused by the uncertainty and complexity associated with the conventional mechanisms of NDR, such as charge trapping and detrap- ping,^6,9,10 filament formation and rupture,^11,12 and redox reaction.^8,13 An innovative and alternative mechanism for generating the NDR effect is therefore of great need for the design of highly reliable NDR devices.

Ferroelectrics, a class of materials whose spontaneous polarization can be electrically switched,^14,15may offer a unique mechanism for realizing the NDR with high reliability. For a

semiconducting ferroelectric material, the switching of polar- ization direction can easily modulate the carrier injection through the metal/ferroelectric interface.¹⁶ Therefore, the coercive voltage (V_c) of the polarization switching that triggers the carrier injection may result in a deterministicV_peakfor the NDR (Figure 1a). Note that this work deals with the current− voltage (I−V) characteristics measured in the low frequency regime whose time scale is much longer than the polarization switching time. Therefore, the current of the NDR shown in Figure 1a does not represent the displacement current of polarization switching. Instead, it is formed by the charge injection which is triggered by the polarization switching and subsequently suppressed by the charge trapping (see the detailed descriptions below).

A ferroelectric capacitor can be considered as a series connection of two back-to-back Schottky barriers formed at the metal/ferroelectric interfaces and a bulk resistance (taking the n-type junction as an example). When a positive voltage is applied to the top electrode, the bottom Schottky barrier which is reverse-biased mainly controls the current flow. (Note: the

Received: April 22, 2017 Accepted: July 25, 2017 Published: July 25, 2017

Research Article www.acsami.org

voltage sweep rate is sufficiently low so that the displacement current is insignificant.) Let us take a further look at the bottom barrier, in which a dead layer (DL) (a thin dielectric layer) is assumed to exist between the ferroelectric layer (FE) and the metal electrode (M), as shown in Figure 1b−d.¹⁷ If the polarization is initially oriented upward, the negative polar- ization charge at the bottom barrier can induce an upward band bending, suppressing the electron injection (Figure 1b).

Therefore, the current flowing through the barrier is low when the applied positive voltage is much lower thanV_c. When the applied voltage approaches V_c, the polarization switching occurs. The positive polarization charge can induce a large electricfield (E_d) within the dead layer:¹⁷

≈ ε ε

E P

d

0 d (1)

wherePis the polarization,ε0is the vacuum permittivity, andεd

is the static dielectric constant of the dead layer. GivenP= 60 μC/cm²andεd= 100, typical values for normal ferroelectrics, the resultantE_dcan be as large as∼6.8 MV/cm. This largeE_d leads to a significant barrier lowering and in turn triggers an intense electron injection (Figure 1c). As the applied voltage further increases, the electron injection continues but will be suppressed gradually. This is because the barrier height increases with the applied voltage due to the electron trapping.

The injected electrons have a certain probability to be trapped inside the dead layer or the depletion region of ferroelectric layer. These trapped electrons can compensate for the positive polarization charge, resuming the Schottky barrier gradually and making the subsequent electron injection more difficult (Figure 1d). Therefore, the current decreases with increasing applied voltage.

As described above, the proposed mechanism for the NDR in ferroelectrics consists of two aspects: (i) current rise caused by the polarization switching-induced charge injection and (ii) current drop due to the charge trapping (hereafter this

mechanism is termed as “PSCICT”). It is worth noting that the NDR effects in ferroelectrics were reported previously, but those studies did not show evident correlations between the V_peakandV_c.¹⁸⁻²⁰In addition, the reported mechanisms, such as polarization relaxation,¹⁸charge trapping and detrapping,¹⁹ diffusion-limited conduction,²⁰and interband tunneling due to the band overlap,²¹ were completely different from the mechanism proposed here. The merit of the PSCICT mechanism is that the coercive voltage V_c determines the position of current peak, i.e.,V_peak, which is a core ingredient of physics for ensuring the high reliability of the NDR effect.

To realize the PSCICT mechanism, the carrier transport through the extrinsic conduction channels, e.g., domain walls, grain boundaries, and dislocations, should be avoided.

Otherwise, the NDR will be overwhelmed by the extrinsic leakage currents whose magnitudes increase monotonically with increasing applied voltage. As a result, high-quality epitaxial ferroelectric thin films capped by the nanosized electrodes are demanded. Additionally, according to eq 1, a larger polarization is favorable for a more significant NDR. In this regard, ferroelectric BiFe_0.6Ga_0.4O₃(BFGO), which exhibits a stable supertetragonal phase, a giant polarization of∼150μC/

cm²,^22,23 and a narrow bandgap (∼2.95 eV, see Figure S1), could be a preferred candidate material.

In this work, we fabricate the ferroelectric Au/BFGO (∼20 nm)/Ca_0.96Ce_0.04MnO₃(CCMO) nanocapacitors (∼0.1μm²in area) to check the validity of the NDR mechanism proposed above. We choose the CCMO as the bottom electrode for (i) the lattice matching (the in-plane lattice constants of BFGO and CCMO stable phases are 3.77 and 3.74 Å, respectively)^22,24 and (ii) constructing an n-type Schottky contact with the BFGO (the work function of CCMO is∼5.2 eV,^25,26and the BFGO may be an n-type semiconductor with an electron affinity of ∼3.3 eV^27,28). With these nanocapacitors, we find that the NDR current peaks are located at the vicinity of V_c, thus validating the PSCICT mechanism. It is further shown that Figure 1.Schematic illustrations of the mechanism for NDR in ferroelectrics. (a) Typical NDR characteristics in a ferroelectric capacitor. Energy band diagrams of a metal electrode (M)/dead layer (DL)/ferroelectrics (FE) structure at different voltage regimes: (b) regime i, (c) regime ii, and (d) regime iii. The three voltage regimes are indicated in (a) with different colors, and their boundaries are drawn schematically. Although the exact mechanism for the electron injection is unknown, it may be realized via thermionic emission (green arrows),field emission (brown arrows), and trap-assisted tunneling (blue arrows), all of which are strongly dependent on the interface barrier.

ACS Applied Materials & Interfaces Research Article

the NDR effect of the Au/BFGO/CCMO nanocapacitors has good reproducibility, high endurance, and long retention.

2. EXPERIMENTAL PROCEDURE

The BFGO thinfilms of∼20 nm in thickness together with the∼5 nm CCMO buffer layers were grown on the (001)-oriented LaAlO₃ (LAO) substrates by pulsed laser deposition with a KrF excimer laser (λ= 248 nm). During the deposition, the substrate temperature was kept at 650 °C and the oxygen pressure was 15 Pa. After the deposition, the samples were cooled to room temperature at a rate of 5

°C/min in oxygen ambient of 1.0 atm. Then, the Au top electrodes with a lateral size of∼300 nm and a thickness of∼12 nm wereex situ grown on the film surfaces using polystyrene spheres as the templates.²⁹The crystal structures were examined by X-ray diffraction (XRD; PANalytical X′Pert PRO). Atomic force microscopy (AFM), piezoresponse force microscopy (PFM), conductive atomic force microscopy (C-AFM), and scanning Kelvin probe microscopy (SKPM) were performed using a commercial atomic force microscope (Cypher, Asylum Research) to study the topography, ferroelectricity, conductance, and surface potential. For the current−voltage (I−V) measurements with various voltage sweep rates, the Keithley 6430 SourceMeter was used.

3. RESULTS AND DISCUSSION

Figure 2a shows the XRDθ−2θscan of the BFGO/CCMO/

LAO epitaxial thinfilm. Only the (00l) diffraction peaks from the BFGO layer and LAO substrate are observed with no detectable impurity phases. The diffraction peaks of CCMO are not observed, probably due to (i) the very small thickness of the CCMO layer (∼5 nm) and (ii) the lattice constants of CCMO being close to those of LAO (seeFigure S2for details).

It is noteworthy that the BFGOfilm exhibits sharp diffraction peaks and thickness fringes, indicating a good crystallinity.

According to the 2θvalue of the BFGO (001) peak (∼19.1°), the out-of-plane lattice constant of BFGO is determined to be as large as 4.61 Å, which is a typical feature of the supertetragonal phase.

The AFM was performed after the deposition of Au top electrodes onto the BFGO film surface. As clearly shown in Figure 2b, the BFGO film surface is quite flat, and the Au electrode arrays are well-ordered and of uniform size (∼300 nm in later size and∼12 nm in thickness). The combined XRD and AFM results demonstrate that our Au/BFGO/CCMO nano- capacitors are of high structural quality.

To probe the ferroelectricity, the PFM hysteresis loops were measured for the BFGOfilm.Figure 2c presents the butterfly like amplitude loop and the rectangular phase loop with 180° switching, demonstrating the ferroelectricity of the BFGOfilm.

The asymmetry of±V_cis due to the internal biasfield in the BFGO film, which may be caused by the asymmetric built-in fields at the ferroelectric/electrode interfaces^30,31 and/or asymmetric distributions of trapped charges.^32,33 To gain more evidence for ferroelectricity, the PFM imaging was performed after poling the outer and inner square regions with

−5 and +5 V, respectively. As seen inFigure 2d, the±5 V poled regions show sharp phase contrast of∼180°, indicating that the domains in the two regions are aligned in the opposite directions. In addition, it is deducible that the as-grown region has a downward polarization because this region has the same color as the +5 V poled region. The self-polarization of the as- grown BFGO film may be associated with the electrostatic boundary conditions^34,35 and/or the strain effects³⁶⁻³⁸ (see detailed discussion in Figure S3). Note that the PFM phase contrast after the poling may be caused by the electrostatic interactions between the tip and sample surface.³⁹ To clarify this, the surface potential was monitored by the SKPM technique.Figure S3a−fshows that the SKPM contrast decays gradually as the waiting time increases from 0 to 1 h, whereas the PFM phase contrast persists. Therefore, the PFM signals are mainly contributed from the electromechanical responses of the ferroelectric domains, further confirming the ferroelectric nature of the BFGOfilm.

Figure 2.Topography, crystal structure, and ferroelectricity. (a) XRDθ−2θscan of the BFGOfilm (20 nm) grown on the CCMO-buffered LAO substrate. (b) Topographic image of well-ordered Au electrodes grown on the BFGOfilm. Inset shows the device structure of the Au/BFGO/

CCMO nanocapacitors. (c) Local PFM hysteresis loops of phase (pink) and amplitude (blue) signals. (d) PFM out-of-plane phase image obtained after poling the outer and inner regions with−5 and +5 V, respectively.

ACS Applied Materials & Interfaces Research Article

Figure 3a presents the typical I−V curves of the nano- capacitors measured with the sequence of 0 V→4 V→−4 V

→0 V using different voltage sweep rates of 0.15, 0.25, 0.3, and 0.47 V/s. Prior to the measurement, a preset pulse (−4 V, 1 s) was applied. The NDR characteristics are clearly observed with two current peaks located at +2.3 and −1.8 V, respectively.

These current peaks are not from the displacement current of polarization switching, whose magnitude is estimated to be on the order of only ∼10 pA at the applied voltage sweep rates.

Instead, the current peaks are probably due to the charge injection and subsequent trapping.^9,10 One can further distinguish the nature of the current peaks by calculating the integral of current over time (to be shown later).

It is also observed fromFigure 3a that theV_peakvalues agree well with the coercive voltages (V_c) of the BFGO films, indicating that the current jump (i.e., charge injection) is triggered by the polarization switching. This is consistent with the PSCICT mechanism proposed above in theIntroduction.

In addition, the observations of relatively symmetricI−Vcurves and NDR behaviors in both voltage polarities suggest that both the Au/BFGO and BFGO/CCMO interfaces form Schottky barriers and the charge injection and trapping occur at both interfaces (seeFigures S4−S7for more evidence).

Figure 3b also reveals that the current peak and valley values (i.e., I_peakand I_valley) in the NDR region become larger as the sweep rate increases. This is because the charge trapping is a relatively slow process compared with the charge injection, and thus the overshoot of the current beyond its steady-state value is larger when the voltage is swept faster.⁹

Note that without the ferroelectric BFGO layer the Au/

CCMO-only heterostructure does not show any NDR behaviors (Figure S8). Also note the measured current actually corresponds to the injected charge (Q_I) rather than switched polarization charge. If one integrates the measured current over time in the peak region, the injected charge density (Q_I/A, where A is the electrode area) much larger than the

ferroelectric polarization of BFGO can be yielded (seeFigure S9for the detailed calculation methods). TheQ_I/Avalues are 98, 61, 51, and 48 mC/cm²for the positive voltage sweeps with the rates of 0.15, 0.25, 0.3, and 0.47 V/s, respectively (Figure 3b). For the negative voltage sweeps with the rates of 0.15, 0.25, 0.3, and 0.47 V/s, the respective Q_I/A values are −77,

−72,−66, and −60 mC/cm²(Figure 3b). TheseQ_I/Avalues are 2 orders of magnitude larger than the ferroelectric polarization of BFGO (∼150 μC/cm²). Hence, one can rule out the displacement current of the polarization switching as a major contributor to the measured current. As the PSCICT mechanism states, the injected charge carriers have a certain probability (k) to be trapped, and therefore the trapped charge (Q_T) is proportional to theQ_I, asQ_T=kQ_I. Our SKPM results have suggested that the trapped charge density (Q_T/A) can be comparable to or even larger than the ferroelectric polarization.

The k value may therefore be estimated as ∼0.01, consistent with those reported previously.⁹ Another observation from Figure 3b is that theQ_I(and consequentlyQ_T) increases as the sweep rate decreases, indicating that more charge carriers can be injected and subsequently trapped as the sweep rate becomes lower.

To further support that the NDR in the Au/BFGO/CCMO nanocapacitors is caused by the PSCICT mechanism, theI−V characteristics were measured using the unipolar voltage sweeps.Figure 3c shows the I−V curves measured with 0 V

→4 V→0 V for three sequential cycles. Prior to thefirst cycle, a preset pulse (−4 V, 1 s) was applied. The NDR behavior is observed only in the first cycle while it disappears in the following two cycles. (Note: the NDR effects refer to the regions near the current peaks at∼V_crather than the satellite peaks. Those satellite peaks may be caused by the trap emission or tip vibration.) This observation can be well explained by the PSCICT mechanism as follows. The polarization is switched from upward to downward at ∼V_c in thefirst cycle, and the induced electron injection and subsequent electron trapping at Figure 3.Characterization of the NDR behaviors. (a) LogarithmicI−Vcurves measured with different voltage sweep rates and PFM phase loop. (b) Current peak and valley values (I_peakandI_valley) and injected charge density (Q_I/A) as a function of voltage sweep rate. The solid and hollow symbols indicate the values obtained with 0 V→4 and 0 V→−4 V, respectively.I−Vcurves measured with (c) 0 V→+ 4 V→0 V and (d) 0 V→−4 V→ 0 V for three sequential cycles. Insets in panels a, c, and d show the sequences of applied voltages.

ACS Applied Materials & Interfaces Research Article

the BFGO/CCMO interface (an n-type Schottky contact) give rise to the NDR current peak (Figure 1). In the following two cycles, however, neither polarization switching nor electron detrapping occurs because the voltages with the same polarity are applied. The trapped electrons therefore make the Schottky barrier relatively high, suppressing the conduction in the following cycles. Similar NDR behaviors are observed in the case of unipolar negative voltage sweeps (Figure 3d), where the polarization is switched upward and the electron injection and trapping occur at the Au/BFGO interface. Note that the slight inconsistence between theV_peakvalues inFigures 3a and3c,d may be associated with the device-to-device variation.

Finally, the reproducibility, fatigue, and retention perform- ances of the Au/BFGO/CCMO NDR structures were tested.

The NDR behaviors were observed in more than 35 devices with various electrode areas, and theQ_Ivalues derived from the NDR current peaks are shown statistically inFigure 4a. TheQ_I scales in approximate linearity with the electrode area, which not only demonstrates a good reproducibility but also rules out thefilament-type resistive switching as the origin of the NDR.⁴³ Figure 4b shows the I−V curves measured with the cyclic voltage sweeps, where the NDR current peaks can still be observed after 1000 cycles. Such fatigue performance is better than that of the TiO₂-based NDR devices reported recently.⁶In terms of the retention measurement, the delay time between the preset pulse and the measurement pulse was varied. As shown inFigure 4c,d, the NDR behaviors can be well retained for a delay time up to 30 min, which may promise nonvolatile memory applications. We believe that better fatigue and retention data can be obtained if the issue of tip drift during the measurement can be addressed.

4. CONCLUSIONS

In summary, we have proposed a mechanism for realizing the NDR based on ferroelectrics, in which the NDR is caused by the polarization switching-induced charge injection and subsequent charge trapping at the metal/ferroelectric interface.

This mechanism is then verified by the experimental observation of NDR current peaks at the vicinity ofV_c in the ferroelectric Au/BFGO (∼20 nm)/CCMO nanocapacitors.

Moreover, the NDR in these nanocapacitors is shown to be reproducible and stable with an endurance of∼1000 cycles and a retention of∼30 min. Further optimizing the ferroelectricity and the interface properties (e.g., Schottky barrier height, dead layer thickness, and trap density) may advance the NDR characteristics. This study therefore demonstrates an alternative type of NDR devices based on ferroelectrics, which may benefit the development of highly reliable NDR devices.

■

ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acsami.7b05634.

UV−vis absorption spectrum of the BFGOfilm (Figure S1), XRDθ−2θscan of the∼50 nm CCMO grown on the LAO substrate (Figure S2), PFM and SKPM images taken after different waiting times in the air (Figure S3), schematics showing the theoretical band alignment (Figure S4), I−V characteristics of the Ti/BFGO/

CCMO heterostructures (Figure S5), SKPM images taken after different waiting times in the dry Ar gas (Figure S6), C−f characteristics of the Ti/BFGO/

CCMO heterostructures (Figure S7), I−V curves of the Au/CCMO nanocapacitors (Figure S8), and methods to calculate theQ_I(Figure S9) (PDF) Figure 4.Reproducibility, fatigue, and retention performances of the Au/BFGO/CCMO NDR device. (a) Injected charge (QI) as a function of electrode area. Solid lines are linearfits to the data. (b)I−Vcurves measured in the cyclic test.I−Vcurves measured with (c) 0 V→4 V→0 V and (d) 0 V→−4 V→0 V for different delay times. The delay time is defined as the interval between the preset pulse and the measurement pulse, as shown in the insets.

ACS Applied Materials & Interfaces Research Article

■

AUTHOR INFORMATION Corresponding Authors

*E-mail: fanzhen@m.scnu.edu.cn(Z.F.).

*E-mail: xingsengao@scnu.edu.cn(X.G.).

ORCID

Peilian Li:0000-0003-0902-9689

Zhen Fan:0000-0002-1756-641X

Xingsen Gao:0000-0002-2725-0785

Jun-Ming Liu:0000-0001-8988-8429 Notes

The authors declare no competingfinancial interest.

■

ACKNOWLEDGMENTS

The authors thank the National Key Research Program of China (Nos. 2016YFA0201002 and 2016YFA0300101), the State Key Program for Basic Researches of China (No.

2015CB921202), National Natural Science Foundation of China (Nos. 51602110, 11674108, 51272078, and 51431006), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014), the Science and Technology Planning Project of Guangdong Province (No. 2015B090927006), and the Natural Science Foundation of Guangdong Province (No. 2016A030308019).

■

(1) Sollner, T. C. L. G.; Goodhue, W. D.; Tannenwald, P. E.; Parker,^REFERENCES C. D.; Peck, D. D. Resonant Tunneling through Quantum Wells at Frequencies up to 2.5 THz.Appl. Phys. Lett.1983,43, 588−590.

(2) Brown, E. R.; Söderström, J. R.; Parker, C. D.; Mahoney, L. J.;

Molvar, K. M.; McGill, T. C. Oscillations up to 712 GHz in InAs/AlSb Resonant-Tunneling Diodes.Appl. Phys. Lett.1991,58, 2291−2293.

(3) Mathews, R. H.; Sage, J. P.; Sollner, T. C. L. G.; Calawa, S. D.;

Chen, C. L.; Mahoney, L. J.; Maki, P. A.; Molvar, K. M. A New RTD- FET Logic Family.Proc. IEEE1999,87, 596−605.

(4) Broekaert, T. P. E.; Brar, B.; van der Wagt, J. P. A.; Seabaugh, A.

C.; Morris, F. J.; Moise, T. S.; Beam, E. A.; Frazier, G. A. A Monolithic 4-bit 2-Gsps Resonant Tunneling Analog-to-Digital Converter.IEEE J.

Solid-State Circuits1998,33, 1342−1349.

(5) Chang, L. L.; Esaki, L.; Tsu, R. Resonant Tunneling in Semiconductor Double Barriers.Appl. Phys. Lett.1974,24, 593−595.

(6) Du, Y.; Pan, H.; Wang, S.; Wu, T.; Feng, Y. P.; Pan, J.; Wee, A. T.

S. Symmetrical Negative Differential Resistance Behavior of a Resistive Switching Device.ACS Nano2012,6, 2517−2523.

(7) Tseng, R. J.; Ouyang, J.; Chu, C.-W.; Huang, J.; Yang, Y.

Nanoparticle-induced Negative Differential Resistance and Memory Effect in Polymer Bistable Light-emitting Device. Appl. Phys. Lett.

2006,88, 123506.

(8) Jangjian, P.-C.; Liu, T.-F.; Li, M.-Y.; Tsai, M.-S.; Chang, C.-C.

Room Temperature Negative Differential Resistance in DNA-based Molecular Devices.Appl. Phys. Lett.2009,94, 043105.

(9) Kiehl, R. A.; Le, J. D.; Candra, P.; Hoye, R. C.; Hoye, T. R.

Charge Storage Model for Hysteretic Negative-differential Resistance in Metal-Molecule-Metal Junctions.Appl. Phys. Lett.2006,88, 172102.

(10) Xie, X. N.; Gao, X.; Wang, Y.; Wang, J.; Loh, K. P.; Wee, A. T. S.

Negative Differential Resistance Based on Electron Injection/

Extraction in Conducting Organic Films.Appl. Phys. Lett. 2009,95, 063301.

(11) Bernede, J. C.; Abachi, T. Differential Negative Resistance in Metal/Insulator/Metal Structures with an Upper Bilayer Electrode.

Thin Solid Films1985,131, L61−L64.

(12) Lin, J.; Ma, D. Origin of Negative Differential Resistance and Memory Characteristics in Organic Devices Based on Tris(8- hydroxyquinoline) Aluminum.J. Appl. Phys.2008,103, 124505.

(13) Tang, Q.; Moon, H. K.; Lee, Y.; Yoon, S. M.; Song, H. J.; Lim, H.; Choi, H. C. Redox-Mediated Negative Differential Resistance

Behavior from Metalloproteins Connected through Carbon Nanotube Nanogap Electrodes.J. Am. Chem. Soc.2007,129, 11018−11019.

(14) Ratcliff, W.; Lynn, J. W.; Kiryukhin, V.; Jain, P.; Fitzsimmons, M. R. Magnetic Structures and Dynamics of Multiferroic Systems Obtained with Neutron Scattering. npj Quantum Mater. 2016, 1, 16003.

(15) Jain, P.; Stroppa, A.; Nabok, D.; Marino, A.; Rubano, A.; Paparo, D.; Matsubara, M.; Nakotte, H.; Fiebig, M.; Picozzi, S.; Choi, E. S.;

Cheetham, A. K.; Draxl, C.; Dalal, N. S.; Zapf, V. S. Switchable Electric Polarization and Ferroelectric Domains in a Metal-Organic-Frame- work.npj Quantum Mater.2016,1, 16012.

(16) Lee, D.; Baek, S. H.; Kim, T. H.; Yoon, J. G.; Folkman, C. M.;

Eom, C. B.; Noh, T. W. Polarity Control of Carrier Injection at Ferroelectric/Metal Interfaces for Electrically Switchable Diode and Photovoltaic Effects.Phys. Rev. B: Condens. Matter Mater. Phys.2011, 84, 125305.

(17) Tagantsev, A. K.; Gerra, G. Interface-induced Phenomena in Polarization Response of Ferroelectric Thin Films.J. Appl. Phys.2006, 100, 051607.

(18) Scott, J. F.; Melnick, B. M.; Cuchiaro, J. D.; Zuleeg, R.; Araujo, C. A.; McMillan, L. D.; Scott, M. C. Negative Differential Resistivity in Ferroelectric Thin-Film Current-Voltage Relationships.Integr. Ferroe- lectr.1994,4, 85−92.

(19) Chen, H. D.; Udayakumar, K. R.; Li, K. K.; Gaskey, C. J.; Cross, L. E. Dielectric Breakdown Strength in Sol-gel Derived PZT Thick Films.Integr. Ferroelectr.1997,15, 89−98.

(20) Dawber, M.; Scott, J. F. Negative Differential Resistivity and Positive Temperature Coefficient of Resistivity Effect in the Diffusion- limited Current of Ferroelectric Thin-film Capacitors. J. Phys.:

Condens. Matter2004,16, L515−L521.

(21) Maity, A. K.; Lee, J. Y.-m.; Sen, A.; Maiti, H. S. Negative Differential Resistance in Ferroelectric Lead Zirconate Titanate Thin Films: Influence of Interband Tunneling on Leakage Current.Jpn. J.

Appl. Phys.2004,43, 7155−7158.

(22) Fan, Z.; Xiao, J.; Liu, H.; Yang, P.; Ke, Q.; Ji, W.; Yao, K.; Ong, K. P.; Zeng, K.; Wang, J. Stable Ferroelectric Perovskite Structure with Giant Axial Ratio and Polarization in Epitaxial BiFe_0.6Ga_0.4O₃ Thin Films.ACS Appl. Mater. Interfaces2015,7, 2648−2653.

(23) Yan, J.; Gomi, M.; Yokota, T.; Song, H. Phase Transition and Huge Ferroelectric Polarization Observed in BiFe_1−xGa_xO₃ Thin Films.Appl. Phys. Lett.2013,102, 222906.

(24) Zeng, Z.; Greenblatt, M.; Croft, M. Charge Ordering and Magnetoresistance of Ca_1−xCe_xMnO₃. Phys. Rev. B: Condens. Matter Mater. Phys.2001,63, 224410.

(25) Lee, H. S.; Park, H. H.; Rozenberg, M. J. Manganite-based Memristive Heterojunction with Tunable Non-linear I-V Character- istics.Nanoscale2015,7, 6444−6450.

(26) Asanuma, S.; Akoh, H.; Yamada, H.; Sawa, A. Relationship between Resistive Switching Characteristics and Band Diagrams of Ti/

Pr_1−xCaxMnO3 Junctions. Phys. Rev. B: Condens. Matter Mater. Phys.

2009,80, 235113.

(27) Yi, H. T.; Choi, T.; Choi, S. G.; Oh, Y. S.; Cheong, S. W.

Mechanism of the Switchable Photovoltaic Effect in Ferroelectric BiFeO₃.Adv. Mater.2011,23, 3403−3407.

(28) Clark, S. J.; Robertson, J. Band Gap and Schottky Barrier Heights of Multiferroic BiFeO3.Appl. Phys. Lett.2007,90, 132903.

(29) Fan, H.; Fan, Z.; Li, P.; Zhang, F.; Tian, G.; Yao, J.; Li, Z.; Song, X.; Chen, D.; Han, B.; Zeng, M.; Wu, S.; Zhang, Z.; Qin, M.; Lu, X.;

Gao, J.; Lu, Z.; Zhang, Z.; Dai, J.; Gao, X.; Liu, J.-M. Large Electroresistance and Tunable Photovoltaic Properties of Ferroelectric Nanoscale Capacitors Based on Ultrathin Super-tetragonal BiFeO₃ Films.J. Mater. Chem. C2017,5, 3323−3329.

(30) Lee, J.; Choi, C. H.; Park, B. H.; Noh, T. W.; Lee, J. K. Built-in Voltages and Asymmetric Polarization Switching in Pb(Zr,Ti)O3 Thin Film Capacitors.Appl. Phys. Lett.1998,72, 3380−3382.

(31) Gruverman, A.; Kholkin, A.; Kingon, A.; Tokumoto, H.

Asymmetric Nanoscale Switching in Ferroelectric Thin Films by Scanning Force Microscopy.Appl. Phys. Lett.2001,78, 2751−2753.

ACS Applied Materials & Interfaces Research Article

(32) Warren, W. L.; Dimos, D.; Pike, G. E.; Tuttle, B. A.; Raymond, M. V.; Ramesh, R.; Evans, J. T. Voltage Shifts and Imprint in Ferroelectric Capacitors.Appl. Phys. Lett.1995,67, 866−868.

(33) Tagantsev, A. K.; Stolichnov, I.; Setter, N.; Cross, J. S. Nature of Nonlinear Imprint in Ferroelectric Films and Long-Term Prediction of Polarization Loss in Ferroelectric Memories.J. Appl. Phys.2004,96, 6616−6623.

(34) Chu, Y. H.; He, Q.; Yang, C. H.; Yu, P.; Martin, L. W.; Shafer, P.; Ramesh, R. Nanoscale Control of Domain Architectures in BiFeO3 Thin Films.Nano Lett.2009,9, 1726−1730.

(35) Chu, Y. H.; Cruz, M. P.; Yang, C. H.; Martin, L. W.; Yang, P. L.;

Zhang, J. X.; Lee, K.; Yu, P.; Chen, L. Q.; Ramesh, R. Domain Control in Multiferroic BiFeO3 through Substrate Vicinality.Adv. Mater.2007, 19, 2662−2666.

(36) Miao, P.; Zhao, Y.; Luo, N.; Zhao, D.; Chen, A.; Sun, Z.; Guo, M.; Zhu, M.; Zhang, H.; Li, Q. Ferroelectricity and Self-Polarization in Ultrathin Relaxor Ferroelectric Films.Sci. Rep.2016,6, 19965.

(37) Chen, J.; Luo, Y.; Ou, X.; Yuan, G.; Wang, Y.; Yang, Y.; Yin, J.;

Liu, Z. Upward Ferroelectric Self-polarization Induced by Compressive Epitaxial Strain in (001) BaTiO3 Films. J. Appl. Phys. 2013, 113, 204105.

(38) Luo, Y.; Li, X.; Chang, L.; Gao, W.; Yuan, G.; Yin, J.; Liu, Z.

Upward Ferroelectric Self-poling in (001) Oriented PbZr0.2Ti0.8O3 Epitaxial Films with Compressive Strain.AIP Adv.2013,3, 122101.

(39) Balke, N.; Maksymovych, P.; Jesse, S.; Herklotz, A.; Tselev, A.;

Eom, C. B.; Kravchenko, II; Yu, P.; Kalinin, S. V. Differentiating Ferroelectric and Nonferroelectric Electromechanical Effects with Scanning Probe Microscopy.ACS Nano2015,9, 6484−6492.

(40) Khan, A. I.; Chatterjee, K.; Wang, B.; Drapcho, S.; You, L.;

Serrao, C.; Bakaul, S. R.; Ramesh, R.; Salahuddin, S. Negative Capacitance in a Ferroelectric Capacitor.Nat. Mater.2014,14, 182−

186.

(41) Appleby, D. J.; Ponon, N. K.; Kwa, K. S.; Zou, B.; Petrov, P. K.;

Wang, T.; Alford, N. M.; O’Neill, A. Experimental Observation of Negative Capacitance in Ferroelectrics at Room Temperature.Nano Lett.2014,14, 3864−3868.

(42) Islam Khan, A.; Bhowmik, D.; Yu, P.; Joo Kim, S.; Pan, X.;

Ramesh, R.; Salahuddin, S. Experimental Evidence of Ferroelectric Negative Capacitance in Nanoscale Heterostructures.Appl. Phys. Lett.

2011,99, 113501.

(43) Yan, Z. B.; Liu, J. M. Resistance Switching Memory in Perovskite Oxides.Ann. Phys. (Amsterdam, Neth.)2015,358, 206−224.

ACS Applied Materials & Interfaces Research Article