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Jens Mohrmann, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Romain Danneau

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[Persistent hysteresis in graphene-mica van der Waals heterostructures](https://mdr.nims.go.jp/datasets/f9a0b5e9-cea1-4921-a242-223ed5755c16)

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Persistent hysteresis in graphene-mica van der Waals heterostructuresThis content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: 144.213.253.16This content was downloaded on 13/01/2015 at 00:11Please note that terms and conditions apply.Persistent hysteresis in graphene-mica van der Waals heterostructuresView the table of contents for this issue, or go to the journal homepage for more2015 Nanotechnology 26 015202(http://iopscience.iop.org/0957-4484/26/1/015202)Home Search Collections Journals About Contact us My IOPscienceiopscience.iop.org/page/termshttp://iopscience.iop.org/0957-4484/26/1http://iopscience.iop.org/0957-4484http://iopscience.iop.org/http://iopscience.iop.org/searchhttp://iopscience.iop.org/collectionshttp://iopscience.iop.org/journalshttp://iopscience.iop.org/page/aboutioppublishinghttp://iopscience.iop.org/contacthttp://iopscience.iop.org/myiopsciencePersistent hysteresis in graphene-mica vander Waals heterostructuresJens Mohrmann1, Kenji Watanabe2, Takashi Taniguchi2 andRomain Danneau11 Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz Platz 1, D-76021 Karlsruhe, Germany2Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba, JapanE-mail: romain.danneau@kit.eduReceived 7 July 2014, revised 11 October 2014Accepted for publication 31 October 2014Published 8 December 2014AbstractWe report the study of electronic transport in graphene-mica van der Waals heterostructures. Wehave designed various graphene field-effect devices in which mica is utilized as a substrate and/or gate dielectric. When mica is used as a gate dielectric we observe a very strong positive gatevoltage hysteresis of the resistance, which persists in samples that were prepared in a controlledatmosphere down to even millikelvin temperatures. In a double-gated mica-graphene-hBN vander Waals heterostructure, we found that while a strong hysteresis occurred when mica was usedas a substrate/gate dielectric, the same graphene sheet on mica substrate no longer showedhysteresis when the charge carrier density was tuned through a second gate with the hBNdielectric. While this hysteretic behavior could be useful for memory devices, our findingsconfirm that the environment during sample preparation has to be controlled strictly.Keywords: graphene, mica, boron nitride, heterostructure, hysteresis(Some figures may appear in colour only in the online journal)1. IntroductionAs a membrane of only one atomic layer of sp2-hybridizedcarbon, graphene is extremely sensitive to its environment.Therefore, it is important to understand the interaction ofgraphene with the supporting substrate or gate dielectricmaterials. This is particularly relevant when graphene isintegrated into a stack of other two-dimensional (2D) mate-rials, forming a new material with properties that will bedefined by interfacial effects and by the interaction of itscomponents. The building blocks of these so-called ‘van derWaals heterostructures’ [1] are materials with layered crystalstructures in which single or few layers can be isolated andmanipulated using the same techniques that are being used toisolate and manipulate graphene sheets.Out of the variety of dielectric materials, so far, hex-agonal boron nitride has been the center of attention [2],allowing the discovery of the effect of a Moiré superlattice onthe electronic transport in graphene [3–5].Other promising candidates that have not yet beeninvestigated as comprehensively are the members of the micagroup of naturally occurring sheet silicates. The most com-monly used one is muscovite mica, with the formulaKAl2[Si3, Al]O10(OH)2 [6]. The structure is a stack of triplelayers, each consisting of two identical tetrahedral layers of(Si, Al)2O5 sandwiching a layer of octahedrally coordinatedaluminum atoms. The triple layers are negatively charged dueto the substitution of about one fourth of the fourvalent Si bytrivalent Al. They are separated and only weakly bound byinterlayer potassium cations, compensating for the negativecharge. The layered structure allows cleaving the crystalsalong the (001) planes using either a scalpel or Scotch tape,leading to macroscopic, atomically flat surfaces. In this pro-cess, the potassium ions get distributed between the cleavedsurfaces, causing a varying, high electrostatic surface poten-tial [7] and making the surface strongly hydrophilic.NanotechnologyNanotechnology 26 (2015) 015202 (6pp) doi:10.1088/0957-4484/26/1/015202Content from this work may be used under the terms of theCreative Commons Attribution 3.0 licence. Any furtherdistribution of this work must maintain attribution to the author(s) and thetitle of the work, journal citation and DOI.0957-4484/15/015202+06$33.00 © 2015 IOP Publishing Ltd Printed in the UK1mailto:romain.danneau@kit.eduhttp://dx.doi.org/10.1088/0957-4484/26/1/015202http://crossmark.crossref.org/dialog/?doi=10.1088/0957-4484/26/1/015202&domain=pdf&date_stamp=2014-12-08http://crossmark.crossref.org/dialog/?doi=10.1088/0957-4484/26/1/015202&domain=pdf&date_stamp=2014-12-08http://creativecommons.org/licenses/by/3.0Depending on the humidity, this leads to the formation ofwater adlayers, which have been studied using various tech-niques [8–12].Mica is a common material for electronic componentssuch as capacitors. Due to its high dielectric strength andpermittivity of εr = 6.4–9.3 [13, 14] it is used for insulation. Inparticular, in high frequency applications its high resistivityand low loss tangent are valuable. Furthermore, mica is flat,transparent, flexible and cheap. These properties also makemica an appealing substrate and dielectric for graphenedevices [15, 16]. So far, thin mica layers have been utilized asa dielectric for organic electronics [17–19] and carbonnanotube field-effect transistors [20].It was shown that graphene on mica conforms to theatomically flat mica substrate and becomes atomically flatitself [21]. On the other hand, Xu et al showed that wateradsorbed on the surface due to the strong hydrophilicity canbe encapsulated between graphene and the mica surface[22, 23]. As for the electronic properties, Shim et al [24] usedRaman spectroscopy to probe substrate-induced charge dop-ing. Ponomarenko et al [25] and Kretinin et al [26] measuredthe conductivity of graphene on various substrates, includingmica, to investigate the role of charged impurities on chargecarrier mobility in graphene. However, no detailed study ofelectronic transport in graphene-mica systems has beenreported yet. Here, we show electronic transport experimentsin graphene-mica heterostructures in which mica is used as agate dielectric and/or substrate. We find a very strong positivehysteretic behavior of the resistance with respect to tuning thegate voltage. It persists down to millikelvin temperatures andcan be strongly tuned by varying the sweeping speed, thesweeping range and the charging time at a fixed gate voltage.In addition, we show that when mica is only used as a sub-strate, no hysteresis is observed. Finally, we discuss thepossible mechanisms responsible for the behavior.2. Experiments, results and discussionsIn our study, we have designed different graphene-mica het-erostructures to investigate the role of mica as a dielectric andas a substrate. In total, twelve different samples were fabri-cated and characterized. Electronic transport experimentshave been performed in a two-terminal configuration usingstandard low-frequency lock-in detection and/or using aLR700 resistance bridge. We have used a cryo-free dilutionrefrigerator BlueFors LD250 for the low temperaturemeasurements.Our first sample design is depicted in figure 1(a). Thinmica crystals (muscovite mica V1 grade, supplied by PlanoGmbH) are directly deposited onto a heavily doped Si waferwith 320 nm SiO2 using mechanical exfoliation [27]. Usingan optical microscope, mica crystals below 10 nm thicknesscan easily be located, as the apparent color of the SiO2/micastack heavily depends on the crystal's thickness [20]. Then, agraphene sheet is transferred on top using a PMMA-based drytransfer at ambient conditions [2]. Figure 1(b) shows theresistance versus the gate voltage for a device that consists ofa stack of 320 nm SiO2 and 31 nm mica. The gate sweep(figure 1(b)) was performed with a ramping speed of0.25 V s−1 and a charging time (pause) of two minutes at thehighest and lowest gate voltage. Starting at a back gate vol-tage of VBG = 0 V, the sample seems to be undoped at first.However, after decreasing the gate voltage to −75 V one canclearly see a very large hysteresis on the following gatesweeps in which the charge neutrality point is shifted from−55.5 V on the up sweep to +49 V on the down sweep. Withthe charge neutrality point shifting upward (downward) forpositive (negative) gate voltages, the sample shows positivehysteresis. In this situation, the charge carrier density cannotbe directly related to the gate voltage through the simplecapacitor model = ⋅ε εn Ved Gr0 anymore. The potential at thegraphene sheet will instead be the sum of the potential fromthe back gate and an unknown time and gate voltagedependent potential VH that is responsible for the hysteresis[28]. After the stay at maximal gate voltage, VH almostcompletely compensates for VG. The two potentials thus areof the same order of magnitude in the present case. As theadditional potential changes during the gate sweeps, thecurves get distorted, and the intrinsic properties of the gra-phene sheet cannot be properly determined [29, 30]. Thepositive gate hysteresis observed here can be caused bycharge trapping in adsorbates, in bulk or in surface chargetraps [31, 32] and/or electrochemical effects in water in thevicinity of the graphene sheet [30, 33]. It has been argued thatthe first water adlayers that form on a cleaved mica surface aredipole-ordered, forming a layer of ferroelectric ice [11, 34].Figure 1. (a) Sketch of a graphene on mica field-effect device. (b)Gate sweep at 0.25 V s−1 with a charging time of two minutes at±75 V measured on a graphene monolayer on a 320 nm SiO2/31 nmmica dielectric bilayer. The second x-axis shows the displacementfield, defined as = ⋅ε εD Vd GrSiO02. The charge neutrality point for theup and down sweeps shift by 104.5 V.2Nanotechnology 26 (2015) 015202 J Mohrmann et alHowever, like in the experiments using ferroelectric coatingsby Zheng et al [35, 36], the sole polarization of H2O mole-cules should induce a negative hysteresis [31, 37]. Thus, ifthere are effects caused by the polarization of water mole-cules, they are obscured by the much stronger positivehysteresis.In order to elucidate the effect of trapped water at thegraphene-mica interface, we have prepared samples inside ofa glove bag in a nitrogen atmosphere with relative humidity<5%. Additionally, the sample stage and mica substrate wereheated to 140 °C during the transfer to further reduce theamount of water adsorbed on the surface that could gettrapped at the mica/graphene interface. The amount of waterthat is adsorbed by the mica surface depends on the humidity,and it was reported that it completely disappears below 2%relative humidity (RH) [22]. Graphene sheets on suspendedPMMA membranes were prepared in advance outside of theglove bag. Then, all of the following procedures were donewithin the controlled nitrogen atmosphere: mica was cleavedand exfoliated onto a SiO2 substrate, a suitable crystal wasselected using an optical microscope and the graphene wastransferred on top before the sample was removed from theglove bag so that one could consider that water was nottrapped between the graphene and the mica.Figure 2 shows the resulting gate sweeps of graphene ona stack of 320 nm SiO2 and 12 nm mica, together with theeffect of varying the charging time at the minimal andmaximal gate voltage before reversing the sweeping direction.To decrease distortions, the sweep speed was increased to0.83 Vs−1, and the sweeping range was decreased to ±50 V.Still, a very large hysteresis remains, which can be stronglytuned by varying the charging time (here, from 0 to 6 min).Figure 2(b) shows the difference between the position of thecharge neutrality points for the up and down sweeps withrespect to the charging time. Starting at 12 V for the directreversion of the sweeping direction, the hysteresis increasesup to 56 V for a charging time of 10 min without showing asign of saturation. Raman spectroscopy measurements haveshown that a direct contact between the graphene and themica surface can cause strong p-doping, which gets sup-pressed by interfacial water layers [26]. As we don’t observethis doping effect, it seems likely that despite the preparationin a dry atmosphere, water might get trapped at the interfacebetween mica and graphene after exposure to ambient air,suppressing the charge transfer. As shown in figure 3, weobserve interfacial water even when graphene was directlyexfoliated onto mica inside an argon glovebox with <5 ppmwater. The image was recorded at 30% relative humidity andshows islands of water with a height of 0.4 nm at the edge ofthe graphene sheet. In the inner part, the water formed aclosed film. The mobility of the interfacial water seems todepend on details of the sample fabrication [38]. While Xuet al [22] see a clear dependence of the amount of trappedwater with respect to the humidity during graphene depositionand report that graphene permanently traps and immobilizeswater adlayers for weeks, Severin et al [39] observe reversibledewetting of the mica/graphene interface when changing thehumidity.Although a lot of charges seem to accumulate in thevicinity of the graphene sheet during the gate sweeps, whicheffectively screen the gate, the curves only get shifted, whiletheir shape does not show a dependence on the charge neu-trality point’s position. Thus, either the trapped charges aretoo far away from the graphene to act as scatterers, or thedensity of fixed charge impurities is much higher than thedensity of additional trapped charges.In order to investigate the difference between using micaas a substrate and as a gate dielectric, as well as the role ofFigure 2. Sample prepared in a controlled atmosphere: (a) the sweepswere performed starting at −50 V with a sweeping speed of0.83 V s−1. The gate was kept at ±50 V before each up and downsweep for the given time. (b) Gate voltage difference of the positionof the charge neutrality points in (a) with respect to the charging timeat maximal gate voltage.Figure 3. AFM image of the edge of exfoliated graphene on mica.The graphene was exfoliated onto freshly cleaved mica in an argonglovebox. Nevertheless, encapsulated water is visible.3Nanotechnology 26 (2015) 015202 J Mohrmann et alimpurities and adsorbates on top of the graphene sheet, wedesigned a mica-graphene-hBN van der Waals hetero-structure. Instead of the dual layer SiO2/mica dielectric, themica crystals were directly exfoliated onto pre-patternedmetal back gates. After the transfer, connection and etching ofa graphene sheet, a thin crystal of hexagonal boron nitridewas added on top; finally, a metal top gate was created inanother electron beam lithography step. The sample thusfeatures a single graphene sheet sandwiched between a backgate with a 20 nm thin mica dielectric and a local top gatewith a 13 nm thin hBN dielectric (figure 4(a)). The resultinggate sweeps are shown in figures 4(b) and (c). The second x-axis shows the displacement field = ε εD Vd Gr0 for ε ≈ 8.1rmica ,ε ≈ 3.9rhBN . First, the back gate was swept from −7 V to +7 Vand back at a sweeping speed of 0.12 V s−1 with a chargingtime of one minute between the sweeps. Then, the top gatewas swept from −4 V to +4 V and back, with the back gate at0 V. While the gate sweep using the mica back gate againshowed a very strong hysteresis of 6 V, which corresponds toa difference in the displacement field strength of⋅ −1.4 10 e cm13 2, the hysteresis in the resistance curve for thesweep using the top gate with hBN dielectric almost vanishes.As both measurements were performed not only on the samesample but on the same area of graphene, this excludesadsorbates, PMMA residues or water on top of the graphenesheet as dominant sources for the gate hysteresis, as theireffect should still be visible in the top-gated measurement.We can conclude that the hysteresis must therefore be causedby charge trapping either in the bulk mica crystal or at thegraphene/mica interface.Finally, the temperature dependence of the gate hyster-esis was investigated in another sample featuring a 50 nm thinmica dielectric on top of a metal back gate. Graphene on SiO2devices typically shows some hysteresis due to charge traps inthe oxide. Lowering the temperature is known to freeze outcharge traps and therefore suppresses the gate hysteresis [40](different behavior was observed in [41] on one device). Inour sample, the back gate voltage was swept from −10 V to10 V and back with a sweeping rate of 0.06 V s−1 and acharging time of one minute before each sweep. Figure 5shows the temperature dependence of the gate voltage hys-teresis, i.e. the shift of the gate voltage of the charge neutralitypoint between the up and down gate sweep. Starting at a shiftof 6 V at room temperature, which corresponds to a dis-placement field of ⋅ −5.5 10 e cm12 2, the hysteresis is reducedby cooling the sample until it reaches about 3.1 V or⋅ −2.8 10 e cm12 2 at 150 K. Astonishingly, further coolingonly slightly lowers the hysteresis to about 2.7 V or⋅ −2.5 10 e cm12 2, which even remains down to 7 mK andunexpectedly does not vanish.How can we explain this hysteretic behavior? Electro-chemical effects, such as those proposed by Veligura et al[30], are most likely suppressed at these very low tempera-tures. Instead, we believe the hysteresis is caused by electronstunneling into charge traps at the graphene/mica interface orin the bulk mica.Interlayer water might play a crucial role [42], especiallysince potassium ions, such as those that exist on the micasurface, are known to cause dissociation of water moleculeson metal surfaces. The valence electron of potassium adsor-bed on an ice adlayer on a metal substrate can tunnel into themetal substrate, leaving behind an ion, which causes thedissociation through electrostatic perturbation and creates trapFigure 4. Double-gated sample with both a mica back gate and hBN top gate. (a) Sample structure. The graphene is encapsulated betweentwo metal gates with mica (global back gate) and hBN (local top gate) dielectrics. The same area of graphene can thus be tuned using mica asa gate dielectric or only as a substrate. (b) When tuning the mica back gate, a very large hysteresis is observed. (c) The same graphene sheetshows a vanishing hysteresis when the top gate with the hBN dielectric is used.Figure 5. Gate voltage hysteresis (and its equivalent displacementfield) versus temperature (see text) down to 7 mK. The gate wasswept between −10 V and +10 V, with a charging time of oneminute.4Nanotechnology 26 (2015) 015202 J Mohrmann et alsites [43, 44]. This effect is further enhanced by the appli-cation of an external electric field. A similar mechanism couldbe responsible for charge traps in ice adsorbed on the micasurface. Here, the potassium ions already exist at the surface,randomly distributed after cleaving the crystal. The dis-sociation of water molecules causes trapping sites in the iceadlayers at the mica surface. Charge carriers in graphenemight then tunnel into these traps when a gate voltage isapplied. This layer of trapped charges underneath the gra-phene will then screen the gate. It remains to be investigatedwhere the trapped water originates from and eventually how itcan be avoided. This could be achieved by further reducingthe humidity during sample preparation and ideally measuringthe sample without ever exposing it to moisture.3. ConclusionsTo conclude, we have investigated the use of mica as asubstrate and/or gate dielectric for graphene field-effectdevices. When using mica as a gate dielectric, we find apronounced hysteresis of the sample resistance with respect tothe gate voltage. The hysteresis persists even at millikelvintemperatures, but it vanishes in a double gated device whenthe mica is used only as a substrate and the charge carrierdensity is modulated through a top gate with hBN dielectric.This behavior can be explained by charge trapping at thegraphene-mica interface in which the interplay betweenpotassium ions at the surface of mica crystals and waterenclosed in the interface may enhance the charge trap density.Avoiding the formation of interfacial trapped water layersturned out to be difficult; solely reducing the relative humidityand heating the substrate during sample preparation did notlead to the desired effect. While the hysteretic behavior couldbe exploited in memory applications, the use of mica ingraphene devices requires strict control over the manu-facturing conditions to adjust the amount of trapped water.Furthermore, it could be necessary to encapsulate graphene-mica devices to prevent the intrusion of water into the inter-face even after sample preparation.AcknowledgmentsWe thank C Benz, J Bordaz, R Du and F Wu for theirtechnical assistance and fruitful discussions. R.D.’s SharedResearch Group SRG 1-33 received financial support fromthe Karlsruhe Institute of Technology within the frameworkof the German Excellence Initiative. This work was supportedby the EU project MMM@HPC FP7-261594.References[1] Geim A K and Grigorieva I V 2013 van der Waalsheterostructures Nature 499 419[2] Dean C R et al 2010 Boron nitride substrate for high-qualitygraphene electronics Nat. 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Rep. 46 1[44] Meyer M 2012 Ultrafast electron dynamics at Alkali, Icestructures adsorbed on a metal surface PhD Thesis FreieUniversität Berlin (www.diss.fu-berlin.de/diss/servlets/MCRFileNodeServlet/FUDISS_derivate_000000010681/PhD_thesis_MMeyer.pdf)6Nanotechnology 26 (2015) 015202 J Mohrmann et alhttp://dx.doi.org/10.1073/pnas.0502848102http://dx.doi.org/10.1109/TNANO.2004.842053http://dx.doi.org/10.1063/1.3588033http://dx.doi.org/10.1063/1.3665196http://dx.doi.org/10.1021/nn101950nhttp://dx.doi.org/10.1063/1.3630227http://dx.doi.org/10.1063/1.3630227http://dx.doi.org/10.1021/nl0259232http://dx.doi.org/10.1134/S1063783408060048http://dx.doi.org/10.1063/1.3119215http://dx.doi.org/10.1103/PhysRevLett.105.166602http://dx.doi.org/10.1063/1.3273396http://dx.doi.org/10.1063/1.3273396http://dx.doi.org/10.1016/j.susc.2013.01.005http://dx.doi.org/10.1021/nl2037358http://dx.doi.org/10.1063/1.3460798http://dx.doi.org/10.1088/1367-2630/13/4/043020http://dx.doi.org/10.1021/nl903162ahttp://dx.doi.org/10.1016/S0167-5729(01)00020-6http://www.diss.fu-berlin.de/diss/servlets/MCRFileNodeServlet/FUDISS_derivate_000000010681/PhD_thesis_MMeyer.pdfhttp://www.diss.fu-berlin.de/diss/servlets/MCRFileNodeServlet/FUDISS_derivate_000000010681/PhD_thesis_MMeyer.pdfhttp://www.diss.fu-berlin.de/diss/servlets/MCRFileNodeServlet/FUDISS_derivate_000000010681/PhD_thesis_MMeyer.pdf 1. Introduction 2. Experiments, results and discussions 3. Conclusions Acknowledgments References