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Polarization-mediated modulation of electronic and transport properties of hybrid MoS2/BaTiO3/SrRuO3 tunnel junctions Tao Li, Pankaj Sharma, Alexey Lipatov, Hyungwoo Lee, Jung-Woo Lee, Mikhail Y. Zhuravlev, Tula R Paudel, Yuri A. Genenko, Chang-Beom Eom, Evgeny Y Tsymbal, Alexander Sinitskii, and Alexei Gruverman Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04247 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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Polarization-mediated modulation of electronic and transport properties of hybrid MoS2/BaTiO3/SrRuO3 tunnel junctions Tao Li1*, Pankaj Sharma1†*, Alexey Lipatov2, Hyungwoo Lee3, Jung-Woo Lee3, Mikhail Y. Zhuravlev4,5, Tula R. Paudel1, Yuri A. Genenko6, Chang-Beom Eom3, Evgeny Y. Tsymbal1,7, Alexander Sinitskii2,7, and Alexei Gruverman1,7 1

Department of Physics and Astronomy, University of Nebraska, Lincoln, NE 68588 2 3

4

Department of Chemistry, University of Nebraska, Lincoln, NE 68588

Materials Science and Engineering, University of Wisconsin, Madison, WI 53706

Kurnakov Institute for General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russia 5 6

St. Petersburg State University, 190000 St. Petersburg, Russia

Institute of Materials Science, Technische Universität Darmstadt, D-64287 Darmstadt, Germany

7

Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588

ABSTRACT: Hybrid structures composed of ferroelectric thin films and functional twodimensional (2D) materials may exhibit unique characteristics and reveal new phenomena due to the cross-interface coupling between their intrinsic properties. In this report, we demonstrate a symbiotic interplay between spontaneous polarization of the ultrathin BaTiO3 ferroelectric film and conductivity of the adjacent molybdenum disulfide (MoS2) layer, a 2D narrow-bandgap semiconductor. Polarization-induced modulation of the electronic properties of MoS2 results in a giant tunneling electroresistance effect in the *

These authors contributed equally to this work. Current address: School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia †

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hybrid MoS2/BaTiO3/SrRuO3 ferroelectric tunnel junctions (FTJs) with an OFF/ON resistance ratio as high as 104 - a 50-fold increase in comparison with the same type of FTJs with metal electrodes. The effect stems from the reversible accumulation-depletion of the majority carriers in the MoS2 electrode in response to ferroelectric switching, which alters the barrier at the MoS2/BaTiO3 interface. Continuous tunability of resistive states realized via stable sequential domain structures in BaTiO3 adds memristive functionality to the hybrid FTJs. The use of narrow band 2D semiconductors in conjunction with ferroelectric films provides a novel pathway for development of the electronic devices with enhanced performance.

KEYWORDS: resistive switching, MoS2, ferroelectric tunnel junctions, 2D materials

Recent years have witnessed an unprecedented surge of research in various twodimensional (2D) materials that often possess unique physical and chemical properties that cannot be found in their three-dimensional counterparts.1 An important advantage of 2D materials is their planar morphology, which allows easy integration with other 2D materials and functional films, resulting in multilayered structures with new properties. 2 In particular, there is a considerable interest in a novel type of electronic devices, in which 2D materials are coupled with ferroelectric (FE) materials. Ferroelectric materials possess an electrically switchable spontaneous electric polarization, which allows a possibility of electrical modulation of the functional properties of the hybrid 2D-FE electronic structures, which are suitable for memory and logic device applications. In one type of these devices, ferroelectric field-effect transistors (FE-FETs), polarization reversal in a ferroelectric layer, used as a gate dielectric, alters the in-plane conductivity of the adjacent 2D channel. Most of these devices have been fabricated using an archetype 2D material, graphene, in 2 ACS Paragon Plus Environment

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conjunction with an organic or inorganic FE material. In another type of the 2D-FE devices, ferroelectric tunnel junctions (FTJs)15,16 graphene was used as a top electrode for application of polarization switching bias to control perpendicular-to-plane tunneling conductance across the FE layer.17 This approach provides a simple and straightforward method for interface engineering by encapsulating molecular species at the graphene-FE interface, which dramatically affects the resistive switching effect in FTJs.18 The functional performance of the FE-FET devices can be significantly improved if graphene, a zero-bandgap semiconductor, which remains highly conductive at any doping level, is replaced with a different 2D material that has a sizeable electronic bandgap. 19 Similarly, an enhanced functional behavior can be expected in the FTJs incorporating a 2D semiconducting material as one of the electrodes. This expectation stems from a theoretically predicted and experimentally demonstrated boost of the tunneling electroresistance (TER) effect in an FTJ with a semiconducting Nb-doped SrTiO3 electrode due to polarization-induced modulation of the Schottky barrier at the BaTiO3/SrTiO3 interface.20, 21 In this work, we employ molybdenum disulfide (MoS2), a transition metal dichalcogenide, as a top electrode in the BaTiO3 tunnel junctions to investigate the effect of ferroelectric polarization on their electronic and transport properties. Bulk MoS2 is a semiconductor with an indirect bandgap of 1.2 eV, while monolayer MoS2 is a semiconductor with a direct bandgap of 1.8 eV.22,23 We find that the conductance of the MoS2/BaTiO3 interface is strongly influenced by polarization direction resulting in a very asymmetric switching behavior of polarization in BaTiO3: complete switching of polarization from the downward to the upward direction and only partial switching in the opposite direction. This phenomenon is driven by the reversible accumulation or depletion of the majority carriers in MoS2 effectively changing the MoS2 3 ACS Paragon Plus Environment

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properties from semiconducting to insulating. This effect may be further assisted by transfer of ionic species, made available by an interfacial water layer, in and out of a semiconducting MoS2 electrode. A direct consequence of the polarization-dependent MoS2 conductance is a giant TER effect with the OFF/ON resistance ratio as large as 104 measured in the MoS2/BaTiO3/SrRuO3 tunnel junctions. Time-voltage-controlled partial switching of polarization provides an additional degree of freedom to modulate the resistive switching behavior adding memristive functionality to the MoS2/BaTiO3/SrTiO3 tunnel junctions.24 For this study, high-quality epitaxial ferroelectric BaTiO3 (BTO) films with thicknesses ranging from 6 to 12 unit cells (u.c.) (i.e. from 2.4 nm to 4.8 nm) were grown via pulsed laser deposition (PLD) on atomically smooth (001) SrTiO3 crystal substrates with a conductive buffer layer of SrRuO3 (SRO) (Supporting Information, Section I). In-situ monitoring of high-pressure reflection high-energy electron diffraction (RHEED) indicated a layer-by-layer growth with all the layers fully coherent with the single crystal substrate. The grown BaTiO3 films were under epitaxial compressive strain and possessed only out-of-plane polarization.25 MoS2 flakes with the thickness in the range from 3 to 9 monolayers were transferred from the MoS2 single crystal to the BTO film surface via mechanical exfoliation method using an adhesive tape (Supporting Information, Section II). No significant effect of the MoS2 thickness of this range on the resistive switching behavior was observed. Details of the scanning probe microscopy (SPM) measurements are given in the Supporting Information (Section III). First, we tested the polarization switching behavior of the fabricated MoS2/BTO/SRO junctions. Preliminary studies showed that as-grown BTO films were uniformly polarized downward, i.e. with polarization pointing towards the bottom electrode (Figure S1). Figure 1(a) shows the topographic image of a typical exfoliated MoS2 flake with an area of about several 4 ACS Paragon Plus Environment

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square microns transferred on the surface of the epitaxial BTO film. The MoS2 flakes used in this study were 2-7 nm thick and were characterized by atomic force microscopy and Raman spectroscopy (Figures S2 and S3). Figure 1(b) shows typical hysteresis loops obtained in the MoS2/BaTiO3/SrTiO3 junctions by Piezoresponse Force Microscopy (PFM) spectroscopic testing. A phase loop reveals a phase change of about 180° indicating the switchable polarization in the BTO film underneath MoS2.26 Noteworthy, a PFM amplitude loop is strongly asymmetric with respect to the sign of the applied DC bias. Figures 1(c-f) show the PFM images of the polarization states of the MoS2/BaTiO3/SrTiO3 junctions resulting from application of an external bias to the MoS2 flake (to switch polarization in BTO, the MoS2 surface was scanned with an electrically biased PFM tip). Contrast inversion in the PFM phase images (Figures 1(d, f)) is an indication of polarization switching in the BTO film underneath the MoS2 flake. This process is accompanied by appearance or disappearance of a zero amplitude dark line in the PFM amplitude images (Figures 1(c,e)) associated with a 180º domain wall, which coincides with the edge of the MoS2 flake as the BTO film outside of the flake remains unaffected during the switching measurements. It is seen that the MoS2/BTO/SRO junction with the upward polarization appears with a darker contrast in Figure 1(e) than the same junction with the downward polarization in Figure 1(c). The darker contrast in the PFM amplitude image suggests a lower PFM amplitude signal for the upward polarization, which is consistent with the weak PFM amplitude signal for the negative half of the DC bias cycle in the PFM hysteresis loop (Figure 1(b)). This effect may, in principle, be due to a low polarization value for the upward state. Note, however, that the hysteresis loops acquired on a free BTO film surface are quite symmetric (Figure S1). Similarly, the BTO films sandwiched between oxide electrodes are also characterized by rather symmetric


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PFM loops.27 On the other hand, it is known that that the PFM signal can be strongly affected by the conductivity of the electrodes. Hence, given a semiconducting nature of the MoS2 electrode, we attribute the low PFM amplitude signal to the interplay between the screening charges in the MoS2 and polarization charges of BTO films and the resulting change in the electrical conductivity of the MoS2/BTO interface.20 This interplay is also manifested in remarkable asymmetry of the polarization switching from the downward to the upward direction and back. We found that application of a single negative voltage pulse (with an amplitude typically above 5 V and duration of 100 ms) led to complete switching of polarization from the downward to the upward direction in the whole volume of BTO underneath the MoS2 flake (Figures 2(a,b)). In contrast, application of a symmetric positive pulse resulted only in local switching of the upward polarization to the downward state right underneath the tip. This behavior is illustrated by Figures 2(c,d)) showing an array of circular domains with downward polarization written by positive voltage pulses of various duration and amplitude. The tip-generated field drops as a non-linear function of distance from the tip-sample contact in the plane of the 2D MoS2 sheet,28,29 which results in a strong domain size dependence on the pulse magnitude, but only in a weak dependence on the pulse duration (Figures 2(e,f)). This finding is corroborated by the theoretical modeling showing that the domain area grows exponentially with the increasing voltage V applied to the SPM tip. Using a simple 2D model for MoS2 and the Thomas-Fermi approximation for the local electrochemical potential, we solved the Poisson equation and obtained the electrostatic potential as a function of distance from the tip (for details, see Supporting Information, Section V). For the tip voltage depleting the carriers in MoS2, we found that the solution distinguishes two regions: one where the carriers are 6 ACS Paragon Plus Environment

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fully depleted and the other where only partial depletion occurs. Within the former, we obtained an analytic expression for area As, where the electrostatic potential exceeds the coercive voltage Vc of the ferroelectric and thus leads to polarization reversal: = ( )

(1)

where A0 is the area of the tip and α is a fitting parameter. This functional form of the domain area versus voltage fits our experimental data very well (Figure 2(f)), reproducing the observed steep increase in the switched domain size when the applied positive voltage exceeds 5 V. The asymmetric switching behavior is indicative of the dependence of the electronic properties of MoS2 on polarization direction. The uniform down-to-up switching, which implies that an equipotential is established over the entire MoS2 layer, suggests that MoS2 behaves as an effective conductor when polarization is oriented downward. On the contrary, the localized upto-down switching of polarization shows that MoS2 is more insulating when polarization is upward. This difference in the switching behavior is schematically illustrated in Figures 3(a,b).The observed asymmetry in the switching process along with the asymmetric PFM responses of oppositely poled junctions is indicative of the strong effect of BTO polarization on the carrier transport across the MoS2/BTO interface as is discussed below. In a uniformly polarized ferroelectric capacitor with metal electrodes, polarization of both directions is completely screened by accumulation of the charges on the electrodes. The situation is different if one of the metal electrodes is replaced by a semiconductor, such as MoS2. If polarization of BTO is switched upward (toward MoS2) by application of a negative bias, then the positive bound polarization charge is expected to attract additional negative charge from the tip. For MoS2 of n-type, this would lead to further filling of its conduction band with electrons, thereby completely screening the polarization and ensuring its stability. This should also enhance 7 ACS Paragon Plus Environment

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the conductivity of n-type MoS2. In contrast, for the downward polarization, the negative bound charge would expel electrons from MoS2 making it less conductive and presumably making polarization less stable. However, in terms of the polarization direction effect on MoS2 conductivity, our experimental data shows a completely opposite behavior: a strong PFM signal from the MoS2/BTO with the downward polarization indicates a more metallic behavior of MoS2, contrary to the case of MoS2/BTO polarized upward, which exhibits a weak PFM signal (Figures 1(c,e)). In other words, MoS2 behaves as a p-type semiconductor. (This observation is consistent with the field effect measurements in the MoS2/PZT structures,19 where MoS2 also exhibited transport properties compatible with p-type conductivity.) In addition, our data show that both polarization states in the MoS2/BTO/SRO junctions exhibit equally strong retention (see Supporting Information, Section VI). The p-type semiconducting properties of MoS2 imply that holes (rather than electrons) are the majority carriers in MoS2. With polarization pointing downward, accumulation of holes at the MoS2/BTO interface results in enhanced MoS2 conductivity, so that the MoS2 flake behaves as a relatively good conductor. This allows complete switching from the downward to the upward polarization in the whole volume of BTO by application of a single negative pulse (Figures 2(a,b)). When this happens, the positive polarization charges deplete holes at the MoS2/BTO interface causing MoS2 to behave more as an insulator, i.e. the overall conductivity of MoS2 is reduced. Under application of a positive voltage, an area under the tip forms a fully hole-depleted area, where the electrostatic potential drops logarithmically away from the tip (see Eq.(12) in Supporting Information). As a result, the upward polarization can only be switched locally in the area around the tip-sample contact where the potential exceeds the coercive voltage of ferroelectric BTO.


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Note that, since the MoS2 flakes were transferred on the BTO surfaces in ambient conditions, it is likely that an adsorbate water layer is present at the MoS2/BTO interface. Given this, the polarization-dependent switching mechanism can be further facilitated by transfer of some ionic species, in and out of MoS2, that could form in the interfacial water layer.30 Figure 3(c) illustrates the changes in the band structure of MoS2/BTO associated with polarizationinduced charge redistribution. A stable nature of both upward and downward polarization states supports the proposed model, particularly, in the case of the upward polarization, which requires negative charges for screening. It can be assumed that application of the negative bias to MoS2 pushes the Fermi level down to allow a sufficient amount of the electrons injected from the tip to be accumulated and trapped at the interface, stabilizing the polarization. This seems to be quite possible because the band gap of 1.2 eV in MoS2 is much less than the maximum possible potential shift due to the applied bias of several Volts. To further elucidate polarization-induced changes in the band structure of MoS2/BTO, we have performed Kelvin Probe Force Microcopy (KPFM) measurements (Supporting Information, Section IV). 31,32 Figures 4(a,b) show the surface potential maps of the MoS2/BaTiO3/SrTiO3 junction as a function of polarization direction with the corresponding PFM phase maps shown in Figures 4(c,d)). For reference, the topographic image of this particular junction is shown in Figure 4(e). The surface potential maps for the MoS2/BTO in Figures 4(a,b) show a decrease in the contact potential difference (CPD) for the upward polarization (in agreement with accumulation of negative charges at the MoS2/BTO interface) in comparison with the CPD for the downward polarization. A cross-section analysis in Figure 4(f) quantifies this change: it can be seen that the CPD of the MoS2 flake changes by more than 0.2 V upon reversal of the polarization in BTO (Figure S4). More importantly, the CPD is positive (about 0.25 V) even for


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the upward polarization state, again suggesting the dominant role of positive charges in the MoS2/BTO/SRO junctions. Switchable and stable polarization in the MoS2/BTO/SRO junctions opens a possibility for testing their polarization-dependent electronic transport characteristics (the TER effect). The resistive switching measurements of the FTJs were performed using a standard protocol described elsewhere.33 Note, that similar to our previous work,34 we find that holes dominate the tunneling mechanism. The band diagram in Figure 3(c) indicates that the upward (toward MoS2) polarization (the right panel in Figure 3(c)) forms an additional barrier in the MoS2 electrode, which is expected to produce a much higher resistance of the FTJ (OFF state) as compared to the downward polarization (the left panel in Fig. 3(c)), where such a barrier is absent (ON state). Indeed, the I-V characteristics in Figure 5(a) show that the tunneling resistance measured in the junction after application of a positive bias (RON) is much lower than that of the junction poled by a negative pulse of -6 V (ROFF). (To switch the whole area of BTO underneath the MoS2 flake to the downward state, several positive pulses were applied in several locations on the MoS2 surface). The ROFF/RON ratio exceeds a level of 104 at the read voltage of 0.1 V when the whole area of the BTO films underneath the MoS2 flake is uniformly poled (Figure 5(b)). For comparison, in the BTO-based FTJs employing Co as a top electrode and with the same thickness of the barrier (6 u.c.) the maximum value of the TER effect is of the order of 200.35 Furthermore, a strong time-voltage dependence of the switched area when switching occurs from the OFF (upward polarization) to the ON state (downward polarization) (Figure 2) allows continuous tuning of the OFF/ON resistance ratio by several orders of magnitude by varying the write pulse amplitude and duration thereby adding memristive functionality to the MoS2/BaTiO3/SrTiO3 junctions. Figure 5(c) shows a gradual change in the OFF/ON resistance


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ratio upon an increase in the domain area with the downward polarization due to application of the positive pulses of an increasing duration (see Supporting Information, Section VII for details). The overall resistance of the junction can be modeled by treating the upward and downward domains as Rup and Rdown resistors connected in parallel.36 Using a fraction of domains with downward polarization, s, as a variable the device resistance can be expressed as:

1 s (1 − s) = + Reff Rdown R up

(2)

Figure 5(d) shows that even this simple model provides good fitting of the experimental data. Feasibility of memristive functionality is further emphasized by remarkable stability of the intermediate polydomain structures. In conclusion, we have demonstrated that polarization-induced modulation of conductivity of MoS2, a narrow band 2D semiconductor, allows drastic enhancement of the OFF/ON resistance ratio in the hybrid ferroelectric tunnel junctions. The effect occurs due to the reversible accumulation or depletion of the majority carriers in MoS2 in response to ferroelectric polarization switching effectively changing the MoS2 properties from semiconducting to insulating. The MoS2-based FTJs provide remarkable advantages in comparison to the FTJs with graphene or other electrodes, which could be summarized as follows: (1) In the MoS2-based FTJs, both polarization states are stable in contrast with the graphene/BTO/LSMO FTJs where special interfacial engineering is required to stabilize polarization;17 (2) MoS2/BTO/SRO FTJs are characterized by a much larger electroresistance effect; (3) A possibility of continuous change in the switched domain area by using variable amplitude/duration voltage pulses allows realization of tunable and stable TER effect adding memristive functionality to the MoS2/BaTiO3/SrTiO3 FTJs; (4) Precise domain control can be used to modulate the lateral 11 ACS Paragon Plus Environment

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conductivity of MoS2, which can find application in the field effect transistors. The use of hybrid 2D semiconductor-ferroelectric structures opens a novel pathway for development of the highdensity, ultrafast, non-volatile multilevel memory devices with non-destructive low-power readout. In addition, local polarization control accompanied by the spatially-confined change in MoS2 conductivity may enable fabrication of p-n junctions, conduction channels superlattices and other nanoscale-patterned multi-terminal electronic devices.

ACKNOWLEDGEMENT This work was supported by the National Science Foundation (NSF) through Materials Research Science and Engineering Center (MRSEC) under Grant DMR-1420645 (thin film fabrication and theoretical modeling) and under Grant ECCS-1509874 (tunnel junction fabrication and electrical characterization). The authors also acknowledge the support by the Center

for

Nanoferroic

Devices

(CNFD),

a

Semiconductor

Research

Corporation

Nanoelectronics Research Initiative (SRC-NRI) under Task ID 2398.002, sponsored by NIST and the Nanoelectronics Research Corporation (NERC).

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author Alexei Gruverman, E-mail: [email protected] 12 ACS Paragon Plus Environment

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FIGURE CAPTIONS

Figure 1. Switching of ferroelectric polarization in the MoS2/BTO/SRO tunnel junction. (a) Topography image of the direct-transferred exfoliated MoS2 flake on the surface of the 6-u.c.thick BTO film. A dashed rectangle indicates an area shown in images (c-f). (b) PFM amplitude and phase hysteresis loops illustrating the switching behavior of the junction. (c,d) PFM amplitude (c), and the corresponding PFM phase image (d), obtained after application of +5V bias to the MoS2 flake. (e,f) PFM amplitude (e), and the corresponding PFM phase image (f), obtained after application of -5 V bias to the MoS2 flake.

Figure 2. Asymmetric switching behavior of the MoS2/BTO/SRO tunnel junction. (a,b) PFM images of the junction induced after application of a single negative pulse of -5 V and 100 ms. (c,d) PFM images of the domain states induced by application of several positive pulses of various magnitude and duration. (e, f) Plots illustrating domain size dependence on the pulse duration for a fixed pulse amplitude of +5 V (e) and pulse amplitude for a fixed pulse duration of 0.1 s (f). Experimental data points in (f) are fitted by Eq (1).

Figure 3. Schematic illustration of the asymmetric switching behavior of the MoS2/BTO/SRO tunnel junction. (a) Complete switching from the downward to upward state under application of a negative voltage pulse. (b) Partial switching from the upward to the downward state under application of a positive voltage pulse. (c) Polarization-induced changes in the band structure of the MoS2/BTO/SRO junction.


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Figure 4. Combined surface potential (KPFM) and electromechanical (PFM) testing of the MoS2/BTO/SRO tunnel junction. (a.b) Surface potential maps and (c,d) corresponding PFM phase images obtained after application of a positive (a,c) and negative (b,d) electrical bias to the MoS2 flake. (e) Topographic image of the MoS2 flake on the surface of the 12-u.c.-thick BTO film. (f) A cross-section analysis of the surface potential maps (a,c) along the dashed line in (e) for opposite polarization directions.

Figure 5. Resistive switching behavior of the MoS2/BTO(6u.c.)/SRO tunnel junction. (a) Representative I-V curves for opposite polarization states. (b) The reading bias dependence of the ROFF/RON ratio, where RON and ROFF resistances correspond to the polarization states completely switched to the downward and upward direction, respectively. (c) Dependence of the ROFF/RON ratio on the area of domains with the downward polarization (ON state) normalized to the total area of the MoS2 flake. (d) Effective resistance of the junction as a function of the domain area with the downward polarization fitted by Eq (2).


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FIGURES

Figure 1


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Figure 2

Figure 3


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Figure 4


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Figure 5


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SUPPORTING INFORMATION AVAILABLE Supporting Information contains sections and figures that provide additional information for the main text. Section I and Figure S1 describe growth and electrical characterization of BaTiO3 films used in this study. Section II and Figures S2 and S3 describe fabrication of MoS2 flakes and morphological characterization of MoS2/BaTiO3(6u.c.)/SrRuO3 tunnel junctions. Sections III and IV along with Figure S4 describe scanning probe microscopy measurements of the MoS2/BaTiO3 structures. Section V and Figure S5 present results of modeling of the switched domain area as a function of the tip bias. Section VI and Figure S6 present the results of polarization retention testing in MoS2/BTO/SRO tunnel junctions. Section VII and Figure S7 describe measurements of the tunnel resistance as a function of the downward polarization fraction.

NOTE The authors declare no competing financial interest.


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A hybrid MoS2/BaTiO3/SrRuO3 ferroelectric tunnel junction (a) showing a possibility of localized polarization switching (b) due to depletion of the majority carriers in the MoS2 electrode for the upward (toward MoS2) polarization direction (c). 90x27mm (300 x 300 DPI)

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