Dry transfer of CVD graphene using MoS_2-based stamps
Luca Banszerus, Kenji Watanabe, Takashi Taniguchi, Bernd Beschoten, Christoph Stampfer
DDry transfer of CVD graphene using MoS -based stamps Luca Banszerus,
1, 2
Kenji Watanabe, Takashi Taniguchi, Bernd Beschoten, and Christoph Stampfer
1, 2, ∗ JARA-FIT and 2nd Institute of Physics, RWTH Aachen University, 52074 Aachen, Germany Peter Gr¨unberg Institute (PGI-9), Forschungszentrum J¨ulich, 52425 J¨ulich, Germany National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan (Dated: June 5, 2017)Recently, a contamination-free dry transfer method for graphene grown by chemical vapor depo-sition (CVD) has been reported that allows to directly pick-up graphene from the copper growthsubstrate using a flake of hexagonal boron nitride (hBN), resulting in ultrahigh charge carrier mo-bility and low overall doping. Here, we report that not only hBN, but also flakes of molybdenumdisulfide (MoS ) can be used to dry transfer graphene. This, on one hand, allows for the fab-rication of complex van-der-Waals heterostructures using CVD graphene combined with differenttwo-dimensional materials and, on the other hand, can be a route towards a scalable dry transfer ofCVD graphene. The resulting heterostructures are studied using low temperature transport mea-surements revealing a strong charge carrier density dependence of the carrier mobilities (up to valuesof 12,000 cm /(Vs)) and the residual charge carrier density fluctuations near the charge neutralitypoint when changing the carrier density in the MoS by applying a top gate voltage. The high room temperature mobility and the tun-able charge carrier density make graphene an interest-ing material for many applications such as high fre-quency electronics , ultra-sensitive Hall sensors andspintronics . In order to realize such applications, itis necessary to make high quality graphene available ona large scale. Graphene grown by chemical vapor depo-sition (CVD) has recently made numerous advances con-cerning its growth and transfer . We previouslyreported that the electronic properties of CVD grapheneare equivalent to devices built from high quality exfoli-ated graphene if transfer-related degradations and con-taminations are avoided . The highest electronic qual-ity in CVD graphene has so far been achieved by us-ing exfoliated hexagonal boron nitride (hBN) crystals by(1) picking-up CVD-graphene directly from the catalyticcopper foil (substrate material) and by (2) subsequentlyencapsulating it with another hBN crystal . Here, wereport on CVD-graphene that has been dry-transferredfrom the copper foil using a similar scheme. Instead ofhBN, we use molybdenum disulfide (MoS ) to transfergraphene. Expanding this transfer process from usingflakes of exfoliated hexagonal boron nitride to a largerclass of two-dimensional (2d) materials has numerousadvantages: Firstly, van-der-Waals heterostructures con-sisting of different 2d materials have attracted large at-tention in recent years, as they allow for new device prop-erties, e.g. proximity induced spin-orbit interaction or applications in the field of optoelectronics . Secondly,high quality large area hBN with a low adhesion to itssubstrate has not been successfully grown so far, whichlimits the size of the heterostructures that can be ob-tained using the dry transfer to the size of the exfoliatedhBN flake. Thus, finding alternative, scalable 2d materi-als to transfer graphene and to serve as a substrate thatpreserves the intrinsic electronic properties of graphenecould speed up the scaling, opening up the way towardstrue high quality graphene applications. Transition metaldichalcogenides (TMDCs) such as MoS can by now be grown on different substrate materials such as sap-phire with high structural and electronic quality. Besidesopening up a larger set of possible material combinationsto enable new device functionalities, using a broader setof synthetic and thus potentially scalable 2d materials forthe transfer could be a future route towards scaling highquality CVD graphene to arbitrary sizes. Our findingssuggests that, similar to the established stacking tech-niques for exfoliated van-der-Waals materials a muchwider range of 2d materials can be used for the transferprocess.Graphene is grown using a low pressure CVD pro-cess on the inside of enclosures folded from copperfoil , resulting in individual graphene crystals of a fewhundred micrometer in size on the copper. In orderto weaken the adhesion between the graphene and thecopper substrate and thus facilitate the transfer process,the graphene is stored under ambient conditions for afew days, during which a thin cuprous oxide (Cu O)layer forms at the graphene-to-Cu interface . Anoptical image of a typical graphene crystal with anoxidized interface is shown in Fig. 1a. Following ourprevious reports on dry graphene transfer , a polymerstack consisting of a thick layer of poly(vinyl alcohol)(PVA) and a thin layer of poly(methyl methacrylate)(PMMA) is prepared. After exfoliating MoS flakes ofvarious thicknesses between 10 nm and 70 nm on thepolymer, the stack is placed on a polydimethylsiloxane(PDMS) stamp. Using a mask aligner, the TMDCis brought into contact with the graphene at 125 ◦ C.After picking-up the graphene, the MoS /graphenestack is placed on an exfoliated hBN flake. Thereafter,the polymers are dissolved in water, acetone and iso-propanol. Fig. 1b shows an optical microscope image ofa heterostructure consisting of hBN, graphene and MoS .We use scanning confocal Raman microscopy whichis a fast and non-invasive optical method to probethe structural and electronic properties of graphene a r X i v : . [ phy s i c s . a pp - ph ] J un (a)
400 800 1200 1600 2700 C oun t s ( a . u . ) Raman shift (cm -1 )MoS Counts (a.u.) (c) (b) hBN G 2D
GGraphene (d) hBNMoS Figure 1. (a)
Optical image of a CVD grown graphene flakeon copper foil. (b)
Microscope image of a dry transferredhetero-stack consisting of hBN/graphene/MoS . (c) TypicalRaman spectrum of graphene encapsulated between hBN andMoS . (d) left: Raman map of the intensity of the MoS A peak, measured for the flake depicted in (b); right: Ramanmap of the intensity of the graphene G-peak. including defects, doping and strain, as well as nm-scalestrain variations . A typical Raman spectrum ofa MoS /graphene/hBN heterostructure is shown inFig. 1c. The E and the A mode of MoS are centeredat 386 cm − and 411 cm − , respectively . The hBNpeak is centred at 1365 cm − . The graphene G-peak islocated around 1582 cm − and the 2D-peak is centredat 2686 cm − indicating low doping and little strain inthe transferred graphene layer . Compared to grapheneencapsulated between two flakes of hBN, the full-width-at-half-maximum (FWHM) of the 2D peak, Γ , isslightly elevated to around 20 cm − indicating still lowamounts of nanometre-scale strain variations within thelaser spot , which is a good indication for high chargecarrier mobility in the graphene . A more detailedstudy on strain and doping inhomogeneities of grapheneon MoS and other substrate materials has recently beenpublished . These findings are very similar to thoseobtained for high quality graphene transferred usinghBN . Fig. 1d shows the Raman maps of the A mode of MoS and the intensity of the graphene G-peak,corresponding to the heterostructure depicted in Fig. 1b.The data shows that the entire MoS flake is coveredwith graphene, demonstrating a reliable transfer process.In order to investigate the charge transport propertiesof the resulting van-der-Waals stack, we fabricateddual-gated Hall bar devices with one-dimensional Cr/Auedge contacts (see left inset of Fig. 2b). The Hall baris patterned from the heterostructure by electron beamlithography and reactive ion etching using argon andSF as etching gases. Contacts are fabricated by electronbeam lithography followed by electron beam evaporationof Cr and Au. Fig. 2a shows the four-terminal sheetresistivity of the device as function of the applied topgate voltage, V TG , and the applied back gate voltage, V BG , measured at a temperature of 1.6 K. Fig. 2bdepicts line cuts through the data at V TG = 3 V and V TG = − TG (red area in the upper partof Fig. 2a). This indicates that the Fermi energy in theMoS is tuned into its conduction band, allowing chargecarriers in the MoS to screen the applied top gatepotential (see right inset of Fig. 2b). In contrast, for V TG < has moved intothe band gap, allowing to continuously tune the chargecarrier density in graphene by changing the top gatevoltage. At the same time, the absence of charge carriersin the MoS leads to a decreased dielectric screeningof charge traps and defects located in MoS and theMoS /graphene interface, which strongly increase theresidual charge carrier density fluctuations in graphene.This results in a reduced maximum resistance and abroadening of the resistance peak as seen for the redtrace in Fig. 2b and the lower part of Fig. 2a at thecharge neutrality point.For quantifying the influence of the disorder poten-tial in the MoS on the charge transport in the graphenelayer, in both the screened V TG > . V TG < . µ and the residual charge carrier density fluctuations n ∗ at the charge neutrality point. Fig. 3a depicts the field-effect mobility determined by the Drude formula σ = neµ as function of the charge carrier density in the graphenelayer at constant top gate voltages of V TG = 3 V (blue)and at V TG = − µ = 10,000 cm /(Vs).At high charge carrier densities, the mobility is inde-pendent of whether or not the Fermi level of MoS istuned into its conduction band. We note that the MoS is not significantly contributing to transport, as the mo-bility in MoS is typically orders of magnitudes lowerthan in graphene. More importantly, the formation ofa Schottky barrier further suppresses transport throughthe MoS . Evidence for the absence of a significant par-allel conducting channel through the MoS can be seenin Fig. 2a. If the MoS was contributing significantlyto transport, an increase of the conductivity is expected,once there are free carriers in the MoS layer, i.e. at V TG = 2 . V , which is not observed in the experiment.Similar observations have been made in previous reportsof graphene on other TMDC materials . The inset ofFig. 3a presents the top gate dependence of the chargecarrier mobility at a constant charge carrier density of n = − . × cm − in the graphene layer. The mobil-ity of the graphene increases from µ = 8,000 cm /(Vs) toaround µ = 11,000 cm /(Vs) when populating the con-duction band of the MoS . We attribute this behaviourto the self-screening of charge carriers in the graphene -30 30-20 2010 0-10-4-2024 V (V) BG V (V) BG V T G ( V )
110 R (k Ω ) R ( k Ω ) -40 40-20 20 00628410 MoS CB MoS VB (a) (b) hBN MoS grapheneCr/Au hBN Figure 2. (a)
Resistivity of the MoS /graphene/hBN Hallbar as function of the applied top gate and back gate voltage,measured at 1.6 K. (b) Line cuts of the data shown in (a),taken at V TG = 3 V (blue) and V TG = − µ m. Below, a schematic cross-section of the sample isdepicted. Inset right: Schematic band structure of the MoS .The top gate allows to either tune the Fermi energy into theband gap or into the conduction band. from the disorder potential in the MoS . However, atlow charge carrier densities, the extracted mobility de-creases for V TG = − in-terface. These effects are less present, when the Fermienergy is located in the conduction band of the MoS as charge carriers can screen the Coulomb potential andcharge traps are already occupied by carriers in the MoS (blue curve in Fig. 3a).We now focus on the charge carrier density fluctua-tions near the charge neutrality point by plotting theconductance vs. charge carrier density on a double loga-rithmic scale (Fig. 3b) for both traces shown in Fig. 3a.Following the scheme of Couto et al. , we performline fits to this double logarithmic representation of thedata in order to extract n ∗ . At V TG = 3 V, where σ ( e / h ) n* n*
100 n (10 cm -2 )n (cm -2 ) μ ( c m / ( V s )) (b) (a) −4 −2 0 2 410 V TG (V) n ( c m - ) μ ( c m / ( V s )) −4 −2 0 2 4V TG (V) Figure 3. (a)
Charge carrier mobility µ vs. charge carrier den-sity n for two back gate traces in Fig. 2a at V TG = 3 V (blue)and V TG = − T = 1 . n = − . × cm − (indicated by the black arrow) as function of the top gate volt-age. (b) Conductance σ vs. charge carrier density n for themeasurements in (a) plotted on a double logarithmic scalewhich allows to extract the residual charge carrier densityfluctuations n ∗ near at the charge neutrality point. The insetpresents the top gate dependence of n ∗ . the Fermi energy of MoS is tuned into its conductionband, we extract n ∗ = 4 . × cm − , while inthe case where the Fermi energy lies in the band gap,we measure n ∗ = 2 . × cm − . This drasticincrease of the charge carrier density fluctuations atcharge neutrality by almost one order of magnitude (Seealso inset of Fig. 3b) demonstrates the importance ofa homogeneously charged substrate and the absenceof charge traps for high quality graphene devices.Furthermore, we emphasize that the concentration ofCoulomb scatterers and defects in the MoS is subjectto growth methods and fabrication techniques andmight be heavily improved by processing in a glove boxand direct encapsulation with hBN. Furthermore, thepresence of a band gap in MoS allows to precisely tunethe number of charge carriers, available for screeningin the substrate material, which might be of use, forexample, when studying the interaction between twovan-der-Waals materials.In this work, we demonstrated that MoS crystalscan be used to delaminate CVD graphene from theunderlying copper showing that the dry transfer methodcan potentially be applied to a large number of other2d materials resulting in more complex van-der-Waalsheterostructures. This allows for tailoring the electronicproperties of the resulting heterostructure by combiningappropriate combinations of 2d materials, as has beendemonstrated previously in heterostructures assemblesfrom exfoliated flakes. Confocal Raman microscopy ver-ifies the high structural quality, reflected in a low valuesof Γ . Low temperature transport measurements showcarrier mobilities on the order of µ = 10,000 cm /(Vs),which are lower than what has been reported for drytransferred CVD graphene encapsulated in hBN. Weattribute this observation to scattering with a stronglyvarying disorder potential and charge transfer into trapstates present in the MoS . We demonstrate that both, the charge carrier mobility, as well as the charge carrierdensity fluctuations at the charge neutrality point ofthe graphene are affected by the disorder potentialand charge traps. By increasing the charge carrierdensity in the MoS by a top gate voltage, its scatteringpotential can be screened, allowing to tune the electronicproperties of the graphene. ACKNOWLEDGMENTS
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