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Remote modulation doping in van der Waals heterostructure transistors - Nature Electronics
Abstract . Doping is required to modulate the electrical properties of semiconductors but introduces impurities that lead to Coulomb scattering, which hampers charge transport. Such scattering is a particular issue in two-dimensional semiconductors because charged impurities are in close proximity to the atomically thin channel. Here we report the remote modulation doping of a two-dimensional transistor that consists of a band-modulated tungsten diselenide/hexagonal boron nitride/molybdenum disulfide heterostructure. The underlying molybdenum disulfide channel is remotely doped via controlled charge transfer from dopants on the tungsten diselenide surface. The modulation-doped device exhibits two-dimensional-confined charge transport and the suppression of impurity scattering, shown by increasing mobility with decreasing temperature. Our molybdenum disulfide modulation-doped field-effect transistors exhibit a room-temperature mobility of 60?cm 2 ?V –1 ?s – 1 ; in comparison, transistors that have been directly doped exhibit a mobility of 35?cm 2 ?V –1 ?s – 1 . You have full access to this article via your institution. Download PDF Download PDF Main . Doping while suppressing Coulomb scattering is essential when fabricating high-performance electronic devices based on conventional semiconductors 1 , 2 . However, substitutional doping introduces excess carriers that generally leave behind ionized dopants in the channel, which disturb the transport of charge carriers. To avoid this, a modulation doping technique in which ionized dopants are spatially separated from the channel has been widely used in epitaxial heterostructures based on semiconducting nanowires 3 , complex oxides 4 , 5 and compound semiconductors 6 , 7 . Two-dimensional (2D) carrier gases created in this way have also been used as a platform to fabricate high-electron-mobility transistors (HEMTs) and to study quantum transport phenomena 8 , 9 . Charge carrier transport in 2D semiconductors, such as transition metal dichalcogenides (TMDCs), is strongly affected by both intrinsic defects and extrinsic effects, because their carriers are intrinsically confined to their atomically thin bodies 10 , 11 , 12 , 13 , 14 . Unlike 2D carrier gases, which are artificially created in bulk semiconductor heterostructures, carriers in 2D materials are substantially affected by scattering events via long-range Coulomb interactions induced by the surrounding environment 15 , 16 , 17 . This makes the investigation of the fundamental physics and physical properties of 2D materials challenging. In particular, to experimentally explore collective quantum phenomena—including the quantum Hall effect 18 , 19 , 20 , valley Hall effect 21 , 22 , 23 , magic-angle superconductivity 24 , 25 and moiré interlayer excitons 26 , 27 —high-quality materials and interfaces that can eliminate unwanted scattering are required. In addition, charged impurity scattering degrades device performance, such as mobility, limiting the practical applications for high-speed 2D electronics. Previous studies have shown that external scattering sources of 2D materials can be suppressed by encapsulation with hexagonal boron nitride (hBN) or a polymer 28 , 29 , 30 . However, reducing intrinsic scattering caused by impurity states, such as ionized dopants, remains a challenge 16 , 31 . Doping in 2D semiconductors can be achieved by either atomic substitution of elements or molecular surface doping via charge transfer 32 , 33 , 34 , 35 . Substitutional doping is prevalent in bulk semiconductors and has also been extensively explored in 2D semiconductors. Charge transfer doping is a simple and powerful approach that is uniquely applicable to atomically thin van der Waals (vdW) semiconductors 36 , 37 . Neither method can, however, avoid charged impurity scattering because ionized dopants exist within the atomically thin lattice or on its surface close to the channel. In this Article, we report modulation doping in 2D semiconductors using vdW band engineering and remote charge transfer doping. Our heterostructure consists of a molybdenum disulfide (MoS 2 ) embedded channel, an hBN tunnel barrier and a tungsten diselenide (WSe 2 ) layer. The WSe 2 layer is chemically doped with an n-type molecular dopant triphenylphosphine (PPh 3 ), and we show that this doping can modulate the charge carrier density in the underlying MoS 2 channel by remote charge transfer, without degrading the mobility. The resulting MoS 2 modulation-doped field-effect transistors (MODFETs) exhibit a room-temperature mobility of 60?cm 2 ?V –1 ?s –1 , compared with a mobility of 35?cm 2 ?V –1 ?s –1 for directly doped devices. Temperature-dependent mobility measurements show that almost complete suppression of charge scattering can be achieved in the doped MoS 2 channel. Band-modulated vdW heterostructure for remote doping . Figure 1a shows a schematic of the vdW heterostructure used to demonstrate modulation doping in a 2D semiconductor. To fabricate the WSe 2 /hBN/MoS 2 heterostructure, individually exfoliated 2D constituent flakes are stacked in a layer-by-layer manner in sequence using standard transfer methods. In particular, the electrical contacts using either metal or graphene were made only on MoS 2 before the stacking of hBN and WSe 2 layers (the fabrication details are available in Methods and Supplementary Fig. 1 ). The cross-sectional transmission electron microscopy image (Fig. 1b ) confirms the constitution and interfaces of a representative WSe 2 /hBN/MoS 2 heterostructure, consisting of four-layer WSe 2 , trilayer hBN and trilayer MoS 2 . The thickness of each layer was determined by atomic force microscopy before stacking (Supplementary Fig. 2 ). The trilayer MoS 2 channel was primarily used in this study because it is still thin enough to be modulated by the back gate, and small contact resistance can be achieved even at low temperatures 38 . In this heterostructure, with the backbone of a WSe 2 /MoS 2 type-II junction, the electrons introduced into the top WSe 2 layer via molecular doping spontaneously transfer to MoS 2 due to the conduction band offset. This consequently leads to the doping of the confined MoS 2 channel (Fig. 1c ), which is similar to the 2D electron gas (2DEG) in traditional bulk MODFETs or HEMTs 6 . In particular, WSe 2 was chosen as the top layer because of the easier charge transfer doping than hBN with a large bandgap in addition to the lower electron affinity than MoS 2 . Also, note that the insertion of a vdW hBN spacer layer between WSe 2 and MoS 2 not only enables the effective confinement of doped carriers but also reduces remote optical phonon scattering from WSe 2 and extrinsic scattering arising from the alloy disorder and interface roughness typically existing in bulk material systems 39 . Fig. 1: Band-modulated WSe 2 /hBN/MoS 2 heterostructure for remote doping. a , b , Schematic ( a ) and cross-sectional bright-field scanning transmission electron microscopy image ( b ) of the representative heterostructure consisting of WSe 2 , hBN and MoS 2 layers. Scale bar in b , 2?nm. c , Band diagram of the doped WSe 2 /hBN/MoS 2 FET consisting of MoS 2 as the active channel layer, hBN as the interfacial layer, WSe 2 as the chemically doped layer and PPh 3 as the n-type dopant. This structure leads to the confinement of electrons in the conduction band of MoS 2 . E C , E V and E F indicate the conduction band edge, valence band edge and Fermi level energy, respectively. Full size image Remote modulation doping in MoS 2 FETs . We first reveal the charge transfer interactions in the abovementioned heterostructure by optical characterization, including photoluminescence (PL) and Raman spectroscopy. Figure 2a shows a schematic (top) and optical microscopy image (bottom) of the representative WSe 2 /hBN/MoS 2 heterostructure with multi-terminal graphene contacts. In the PL spectra for each layer, characteristic peaks—the A exciton ( A Mo ) at 1.83?eV and B exciton ( B Mo ) at 2.00?eV for trilayer MoS 2 and the indirect transition ( I W ) at 1.45?eV and A exciton ( A W ) at 1.57?eV for four-layer WSe 2 —are observed (Fig. 2b ). Primarily, in the WSe 2 /hBN/MoS 2 junction area, the intensities of all the peaks decrease as a consequence of the interlayer charge transfer process of the photogenerated excitons 40 . The PL mapping at 1.83?eV ( A Mo ) shows a homogeneous reduction in intensity over the overlapping area, tentatively implying that the charge transfer interaction occurs uniformly over the area owing to the high interface quality of the stacked WSe 2 /hBN/MoS 2 heterostructures (Supplementary Fig. 3 ). Notably, the insertion of hBN preserves the charge transfer and resultant PL quenching behaviour observed in the WSe 2 /MoS 2 heterojunction because the trilayer hBN between WSe 2 and MoS 2 is sufficiently thin for the tunnelling of charge carriers (Supplementary Fig. 4 ). Fig. 2: Remote modulation doping in WSe 2 /hBN/MoS 2 heterostructures via charge transfer. a , Schematic (top) and optical microscopy image (bottom) of the representative WSe 2 /hBN/MoS 2 FET. Red, blue, green and grey solid lines represent the edges of the MoS 2 , WSe 2 , hBN and graphene flakes, respectively. Scale bar, 10?μm. b , PL spectra of the regions including WSe 2 /hBN/MoS 2 (black solid line), MoS 2 (red solid line) and WSe 2 (blue solid line). E ph , I W , A W and B Mo indicate the photon energy, indirect transition of WSe 2 , A exciton of WSe 2 and B exciton of MoS 2 , respectively. c , Electrical sheet conductivity at V D ?=?0.1?V, σ ?=?( I D / V D )( L / W ), as a function of the gate voltage ( V BG ) for the MD (red lines) and DD (black lines) trilayer MoS 2 FETs before (dashed lines) and after (solid lines) PPh 3 doping. The inset shows a schematic of the MD and DD devices. d , Field-effect mobility ( μ ) as a function of the electron concentration ( n e ) of the FETs before (open symbols) and after (filled symbols) doping. The light blue (light red) and blue (red) filled squares (circles) represent the DD (MD) FETs after doping once and twice, respectively. The error bars indicate the standard deviation obtained from measuring more than five devices. Full size image More importantly, to verify the doping capability via charge transfer, we investigated the electrical characteristics of the MoS 2 field-effect transistor (FET) when the device was doped with PPh 3 . Two different types of MoS 2 FET are presented in this study: the band-modulated WSe 2 /hBN/MoS 2 heterostructure for remote doping and bare MoS 2 for direct doping as a control. For studies on the device characteristics depending on the doping concentration, the devices were fabricated using metal (Supplementary Figs. 5 and 6 ) and graphene (Fig. 2 ) electrodes, which exhibited Ohmic-like linear current?voltage ( I D ? V D ) characteristics at room temperature due to a low Schottky barrier. All the FETs were fabricated on hBN/SiO 2 /Si substrates to minimize external scattering from the surface roughness, charged impurities and polar optical phonons of the substrate. Figure 2c compares the representative gate-dependent transport characteristics between the modulation-doped (MD) WSe 2 /hBN/MoS 2 and directly doped (DD) MoS 2 FETs, which are doped with the same concentration of 45% PPh 3 solution ( I D ? V D curves and mobility characteristics are shown in Supplementary Fig. 7 ). Both devices similarly show negative shifts in the threshold voltage ( V th ) after PPh 3 treatment, indicating an n-type doping behaviour. The 2D sheet electron density ( n e ) is calculated as n e ?=? I D L /( qWV D μ ) using the field-effect mobility ( μ ) extracted from I D ? V BG curves, μ ?=? G m L /( V D C OX W ), where L is the channel length, W is the channel width, q is the electron charge, G m is the transconductance and C OX is the gate capacitance per unit area 41 . On doping, n e at V BG ?=?0?V increases from 1.1?×?10 12 to 4.8?×?10 12 ?cm –2 and from 8.1?×?10 11 to 5.6?×?10 12 ?cm –2 for the MD and DD devices, respectively. Variations in n e are similar for the two cases despite slightly less doping in the MD device. This implies that PPh 3 molecules can modulate the carrier density of the underlying MoS 2 channel via remote charge transfer across the WSe 2 /hBN layers without a substantial loss in the doping efficiency. The electron density of both MoS 2 devices can be controlled according to the PPh 3 concentration and the number of dopings, as confirmed by the control experiments, and its upper limit is 1.5?×?10 13 ?cm –2 and 1.1?×?10 13 ?cm –2 for the DD and MD samples, respectively (Fig. 2d and Supplementary Fig. 6 ). Note that the electron transport occurs dominantly through MoS 2 and the contribution of WSe 2 in the stacked heterostructures is negligible because pristine WSe 2 intrinsically exhibits p-type characteristics with a high series resistance (Supplementary Fig. 8 ). Even after doping, WSe 2 shows much lower electron conductivity than MoS 2 (Supplementary Fig. 9 ). In addition to the ability to modulate the carrier density, the notable aspect of remote modulation doping is presented in Fig. 2d , which plots μ as a function of n e at V BG ?=?0 for all the devices studied in this work. Even after doping, μ remains virtually unchanged at approximately 60?cm 2 ?V –1 ?s –1 for the MD device, as depicted in Fig. 2d . However, the counterpart DD device shows a degradation in μ from 60 to 35?cm 2 ?V –1 ?s –1 , which is consistently observed for all the DD devices. This result partly supports the fact that charged impurity scattering is suppressed even at room temperature in the case of heterostructure MD FETs, and the detailed transport characteristics and scattering mechanisms are further discussed below. Temperature-dependent electron transport in MoS 2 MODFETs . To evaluate the charge transport properties, we performed the temperature-dependent electrical characterization of the graphene-contacted devices using four-probe measurements to eliminate the effect of contact resistance. Figure 3a shows four-probe conductivity ( σ 4p ) as a function of V BG for the MD device at various temperatures ranging from 300 to 10?K. As the temperature decreases, σ 4p increases for V BG ?>??50?V and vice versa for V BG ?contact resistance than Au-contacted counterparts, although slightly nonlinear I D ? V D characteristics are observed at low temperatures (Fig. 3a , inset). To further reveal information on the contact quality, we analysed the Schottky barrier height ( ? B ) and contact resistance ( R c ). From the Arrhenius plots of ln( I D / T 3/2 ) versus 1/ k B T (Fig. 3b ), ? B is extracted as a function of V BG using the 2D thermionic emission equation \(I_{{{\mathrm{D}}}} = A_{2{{{\mathrm{D}}}}}T^{3/2}{{{\mathrm{exp}}}}\left( {\frac{{ - E_{{{\mathrm{A}}}}}}{{k_{{{\mathrm{B}}}}T}}} \right)\left[ {1 - {{{\mathrm{exp}}}}\left( { - \frac{{qV_{{{\mathrm{D}}}}}}{{k_{{{\mathrm{B}}}}T}}} \right)} \right]\) , where A 2D , T , E A and k B are the 2D Richardson constant, temperature, activation energy and Boltzmann constant, respectively (Fig. 3c ) 42 , 43 . Considering that V D leads to barrier lowering, the activation energy is given by \(E_{{{\mathrm{A}}}} = q\left( {\phi _{{{\mathrm{B}}}} - \frac{{V_{{{\mathrm{D}}}}}}{\eta }} \right)\) , where η is the ideality factor. For \(V_{{{\mathrm{D}}}} > \frac{{3k_{{{\mathrm{B}}}}T}}{q}\) , the equation can be reduced to \(I_{{{\mathrm{D}}}} = A_{2{{{\mathrm{D}}}}}T^{3/2}{{{\mathrm{exp}}}}\left[ { - \frac{q}{{k_{{{\mathrm{B}}}}T}}\left( {\phi _{{{\mathrm{B}}}} - \frac{{V_{{{\mathrm{D}}}}}}{\eta }} \right)} \right]\) (see the detailed derivation in Supplementary Information ). As shown in Fig. 3c , using this equation, the ? B value of MD devices extracted from the slope of the Arrhenius plot approaches 0?eV as the unpinned Fermi level of graphene is effectively aligned to the conduction band of MoS 2 with an increase in the gate voltage (Fig. 3c , inset). Such a behaviour is similarly observed from the DD counterparts (Supplementary Fig. 10 ). Further, R c is determined as \(R_{{{\mathrm{c}}}} = \frac{1}{2}\left( {R_{2{{{\mathrm{p}}}}} - R_{4{{{\mathrm{p}}}}}\frac{{l_{{{{\mathrm{tot}}}}}}}{{l_{{{{\mathrm{in}}}}}}}} \right)\) , where R 2p and R 4p are the two- and four-probe resistances, respectively, and l tot and l in are the total and inner channel lengths, respectively. Both MD and DD samples clearly show low R c ranging from 6 to 15?kΩ?μm at V BG ?=?0?V and low temperatures (Supplementary Fig. 11 ). These values of R c and ? B are comparable with or smaller than those of the previously reported graphene-contacted MoS 2 FETs 28 , 30 . Fig. 3: Low-temperature transport measurements of MD MoS 2 FETs. a , Four-probe sheet conductivity ( σ 4p ) as a function of V BG for the MD FET for temperatures varying from 300 to 10?K. Inset: I D ? V D curves at V BG ?=?0?V for the devices under consideration. MoS 2 is doped twice by the PPh 3 solution with a concentration of 45%. b , Arrhenius plots for the FETs as V BG varies from ?60 to 60?V with an increment of 10?V. The symbols and lines represent the experimentally measured values and linear fittings, respectively. c , Schottky barrier height ( ? B ) as a function of V BG extracted from the Arrhenius plot. Error bars are determined from the standard deviation of the linear fitting. For the extraction, V D ?=?0.1?V and η ?=?4.1 calculated from the ln( I D )? V D plot are used. The inset shows the energy band diagram under a high gate voltage; E C , E V and E F denote the conduction band edge, valence band edge and Fermi level, respectively. d , Four-probe sheet conductivity as a function of temperature at different V BG values ranging from ?70 to 70?V with an increment of 10?V. The red- and blue-shaded regions shown in a and d indicate the metallic and insulating states, respectively. Full size image Once reasonably good contacts were achieved, we could verify the metal–insulator transition (MIT) in our samples, which is shown in the σ 4p curves as a function of temperature for various V BG values (Fig. 3d ). Here σ 4p increases as the temperature decreases for V BG ?>??50?V, which is a characteristic of the metallic phase, and vice versa for V BG ?electron density and k F is the 2D Fermi wavevector 22 , 44 , 45 . In a 2DEG, this criterion, namely, k F l e ?=?1, is met at half the conductance quantum ( e 2 / h ), and the phase becomes metallic and insulating above and below the critical point, respectively. According to our transport data (Fig. 3d ), the MD system can, therefore, be considered as a 2DEG because the MIT occurs near a conductivity value of e 2 / h . Impurity-scattering-suppressed charge transport in MoS 2 MODFETs . To further understand the scattering mechanism of electrons, we investigated the temperature-dependent mobility characteristics of four-probe devices. Figure 4a shows the plots of four-probe mobility ( μ 4p ) as a function of temperature for both MD and DD devices without (left) and with (right) doping. The field-effect μ 4p values are extracted from the I D ? V BG curves at various gate voltages (Fig. 3a and Supplementary Fig. 10a ). For the undoped cases as a control, the devices were additionally doped by electrostatic gating to minimize the influence of contact resistance (Fig. 4a , left) 28 , 30 . Before molecular doping, the two devices exhibit similar temperature-dependent mobility behaviour at V BG ?=?70?V. The mobility increases as the temperature decreases from 300 to 100?K and saturates below 100?K, implying that the charge transport is limited by phonon scattering. Fig. 4: Temperature-dependent mobility behaviour and the related scattering mechanism. a , Four-probe mobility ( μ 4p ) as a function of temperature ( T ) for the MD and DD FETs without and with doping. The red open circles (blue open squares) represent μ 4p extracted at V BG ?=?70?V for the MD (DD) FETs before doping (left). The light blue (light red) and blue (red) filled squares (circles) represent μ 4p extracted at V BG ?=?0 and 70?V for the DD (MD) FETs, respectively, after doping (right). The black dashed line shows the power-law fits ( μ ?=? T – γ ) with γ ?=?2.13 and 0.94 at V BG ?=?0?V for the MD and DD devices, respectively. b , Corresponding colour plots of γ as a function of V BG and temperature for the MD (top) and DD (bottom) devices. The red and blue regions indicate positive (phonon scattering) and negative (Coulomb impurity scattering) values of γ , respectively. Full size image After molecular doping, such features are prominently contrasted for two types of device (Fig. 4a , right). In the high-temperature region from 200 to 300?K, μ 4p of both MD and DD devices increases as the temperature decreases, and it approximately follows the relation μ ?=? T – γ , where γ is the phonon damping factor. Phonon scattering becomes predominant at high temperatures because the phonon population follows the Bose–Einstein distribution \(n_{{{\mathrm{p}}}} = 1/[{{{\mathrm{exp}}}}\left( {\frac{{\hbar \omega }}{{k_{{{\mathrm{B}}}}T}}} \right) - 1]\) , where \(\hbar \omega\) is the energy of the phonon 28 , 31 . At V BG ?=?0?V, γ is fitted to be 2.13 and 0.94 for the MD and DD samples, respectively. Its value for the MD device is comparable to the theoretically predicted values of 1.69 and 2.50 for monolayer and bulk MoS 2 , respectively 46 , 47 . In contrast, a much lower value is obtained for the DD device, indicating that charged impurity scattering is still pronounced due to the Coulomb potential of molecular dopants in close contact with the MoS 2 channel even at high temperatures. The most important feature in temperature-dependent mobility appears in the low-temperature region below 200?K, where charged impurity scattering governs the charge transport. For the MD device, as the temperature continuously decreases, μ 4p monotonically increases and—like the undoped control samples—begins to saturate below 100?K. However, in the case of the DD device, μ 4p reaches a peak at approximately 200?K and gradually decreases as the temperature further decreases. This behaviour is prominently observed at V BG ?=?0?V, where the chemically doped electrons in the MoS 2 channel dominantly contribute to charge transport. The degree of decreasing μ 4p is reduced at V BG ?=?70?V because of the increased carrier screening effects by the additionally induced electrons via electrostatic gating. Nevertheless, as shown in Fig. 4b , such a decrease in μ 4p as the temperature decreases—featured by a negative value of γ —occurs only in the DD sample regardless of the gate voltages. This suggests that ionized impurities from external dopants are the dominant scattering sources in this sample 48 . Furthermore, μ 4p of the MD device is always higher than that of the DD counterpart, which is much more pronounced at lower temperatures (Supplementary Fig. 12 ). Particularly at 10?K, compared with the DD device, the MD device shows an order of magnitude enhancement in μ 4p as its value increases from 63 to 1,100?cm 2 ?V –1 ?s –1 and from 220 to 1,200?cm 2 ?V –1 ?s –1 at V BG ?=?0 and 70?V, respectively. All the mobility behaviours imply the suppression of impurity scattering in the MD device because of the spatial separation of electrons and their parent dopants. It is worth noting that the use of WSe 2 layers in the MD structure is necessary—which cannot be replaced by hBN layers—to achieve efficient charge transfer doping while suppressing charged impurity scattering. When only 1-nm-thick hBN is used without WSe 2 , doping-induced scattering cannot be effectively prevented because the Coulomb potential arising from the dopants can exert influence over the distance of a few nanometres (Supplementary Fig. 13 ) 15 , 31 . In addition, the use of a 4-nm-thick hBN layer—the equivalent thickness of the entire WSe 2 /hBN layers used in this study—reduces the doping efficiency because the tunnelling probability exponentially decreases with the width of the tunnel barrier (Supplementary Fig. 14 ) 49 . Furthermore, the remote modulation doping scheme proposed in this study can be universally expanded to other material combinations with appropriate type-II (or type-I) band alignments and doping polarities, which enables the realization of high-mobility 2D transistors. Conclusions . We have reported remote charge transfer doping in band-modulated WSe 2 /hBN/MoS 2 vdW heterostructures. By spatially separating charge carriers from dopants on the WSe 2 surface, higher mobility can be achieved compared with directly doped FETs via a reduction in dopant-induced scattering while still modulating the carrier density in the 2D channel. The mobility enhancement (up to a factor of 18) and impurity-scattering-suppressed transport characteristics are confirmed via temperature-dependent electrical characterization. Our approach could be used to control both carrier mobility and density in a 2D FET and thus could be of value in the development of high-performance 2D semiconductor electronics, such as HEMTs. Methods . Device fabrication . Two-dimensional material layers (including MoS 2 , WSe 2 , graphene and hBN) were mechanically exfoliated onto 285?nm SiO 2 /p + -Si substrates. The number and size of each layer were identified using a combination of optical contrast and atomic force microscopy. The same number of layers, especially in the case of MoS 2 , was used in the fabricated devices to fairly compare the electrical characteristics, because the electrical properties of 2D materials are considerably affected by the number of layers 28 . To fabricate vdW heterostructures, including WSe 2 /hBN/MoS 2 /hBN and MoS 2 /hBN, the conventional wet transfer method was used 50 . The as-exfoliated hBN on the Si substrate was spin-coated with polypropylene carbonate (PPC) at 1,000?r.p.m. for 1?min, followed by baking on a hot plate at 60?°C for 1?min to achieve intimate adhesion. Subsequently, PPC/hBN was picked up with a polydimethylsiloxane (PDMS) stamp in deionized water. The PDMS/PPC-supported hBN was transferred onto a SiO 2 /Si substrate with pre-patterned Cr/Au (5/35?nm) electrodes and then heated to 100?°C to melt the PPC and remove the PDMS from the PPC. Thereafter, the sample was dipped in acetone for 30?min to dissolve the PPC and then annealed at 300?°C in an argon atmosphere for 10?min to remove the polymer residues on the surface of hBN. This annealing process was repeated after every stacking step. Separately exfoliated MoS 2 sheets were carefully stacked onto hBN on the SiO 2 /Si substrate using the same process as mentioned above. For the MoS 2 FET, patterned graphene is transferred on the MoS 2 surface. To fabricate WSe 2 /hBN/MoS 2 /hBN heterostructures, an additional WSe 2 /hBN stack was precisely transferred onto a graphene-contacted MoS 2 /hBN sample. The as-fabricated devices were finally annealed at 300?°C in an argon atmosphere for 10?min to remove polymer residues as well as contamination between each layer. Charge transfer doping of MoS 2 . Chemical doping of MoS 2 was demonstrated using a PPh 3 solution as the surface charge transfer donor due to its stable and effective electron-doping ability 34 . Several different concentrations of PPh 3 (Sigma Aldrich) solutions were prepared by dissolving PPh 3 powder into toluene. To investigate the chemical doping effect of PPh 3 on MoS 2 , PPh 3 solutions with concentrations of 10%, 30% and 45% were spin-coated onto MoS 2 at 1,000?r.p.m. for 1?min to obtain a uniform distribution of dopants and then the substrates were baked on a hot plate at 300?°C for 3?min to remove the solvent. To compare the temperature-dependent electrical characteristics of the DD MoS 2 and MD WSe 2 /hBN/MoS 2 FETs, MoS 2 is doped twice by the PPh 3 solution with a concentration of 45%. Optical and electrical characterization . Raman and PL measurements were performed using an optical microscope equipped with a 532?nm continuous-wave laser (power, 120?μW) and a monochromator (Andor, Solis 303i). For the scanning Raman and PL measurements, the sample stage was moved in the horizontal plane using a computer-controlled XY stage. Room-temperature electrical measurements were carried out with a semiconductor parameter analyser (HP 4145B). Low-temperature electrical measurements were performed using a source meter (Keithley 2400) in a cryostat (Oxford Instruments) under a high vacuum. Four-probe measurements were performed with lock-in amplifiers (SR830 and SR810, Stanford Research Systems) to measure the contact resistance. Data availability . 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Boosting the photocatalytic hydrogen evolution performance via an atomically thin 2D heterojunction visualized by scanning photoelectrochemical microscopy. Nano Energy 65 , 104053 (2019). Article ? Google Scholar ? Download references Acknowledgements . This work was supported by the National Research Foundation (NRF) of Korea (2021M3H4A1A01079471, 2020R1A2C2009389, 2017R1A5A1014862 (SRC Program: vdWMRC center) and 2020M3H3A1105796) and the KU-KIST School Project. D.L. acknowledges support from the Basic Science Research Program through the NRF of Korea funded by the Ministry of Education (2020R1I1A1A01071872). Y.D.K. and J.J.L. acknowledge support from the NRF of Korea (2021M3H4A1A03054856). Low-temperature measurements were supported by a grant from Kyung Hee University in 2019 (KHU-20192441). Author information . Affiliations . KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, Republic of Korea Donghun Lee,?Yoon Seok Kim,?Yeon Ho Kim,?Woong Huh,?Jaeho Lee,?Sungmin Park?&?Chul-Ho Lee Department of Physics, Kyung Hee University, Seoul, Republic of Korea Jea Jung Lee?&?Young Duck Kim Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, Republic of Korea Jong Chan Kim UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology (UNIST), Ulsan, Republic of Korea Hu Young Jeong Department of Information Display, Kyung Hee University, Seoul, Republic of Korea Young Duck Kim KHU-KIST Department of Converging Science and Technology, Kyung Hee University, Seoul, Republic of Korea Young Duck Kim Department of Integrative Energy Engineering, Korea University, Seoul, Republic of Korea Chul-Ho Lee Advanced Materials Research Division, Korea Institute of Science and Technology, Seoul, Republic of Korea Chul-Ho Lee Authors Donghun Lee View author publications You can also search for this author in PubMed ? Google Scholar Jea Jung Lee View author publications You can also search for this author in PubMed ? Google Scholar Yoon Seok Kim View author publications You can also search for this author in PubMed ? Google Scholar Yeon Ho Kim View author publications You can also search for this author in PubMed ? Google Scholar Jong Chan Kim View author publications You can also search for this author in PubMed ? Google Scholar Woong Huh View author publications You can also search for this author in PubMed ? Google Scholar Jaeho Lee View author publications You can also search for this author in PubMed ? Google Scholar Sungmin Park View author publications You can also search for this author in PubMed ? Google Scholar Hu Young Jeong View author publications You can also search for this author in PubMed ? Google Scholar Young Duck Kim View author publications You can also search for this author in PubMed ? Google Scholar Chul-Ho Lee View author publications You can also search for this author in PubMed ? Google Scholar Contributions . C.-H.L. and D.L. conceived the idea and supervised the project. D.L. fabricated the devices and performed the measurements and data analysis. Y.S.K., W.H., J.L., S.P. and Y.H.K. assisted with the device fabrications. J.C.K. and H.Y.J. performed the cross-sectional high-resolution transmission electron microscopy analysis. D.L., J.J.L. and Y.D.K. carried out the low-temperature measurements. D.L. and C.-H.L. wrote the manuscript. All the authors contributed to discussions. Corresponding authors . Correspondence to Donghun Lee or Chul-Ho Lee . Ethics declarations . Competing interests . The authors declare no competing interests. Additional information . Peer review information Nature Electronics thanks Du Xiang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Supplementary information . Supplementary Information . Supplementary Figs. 1–14. Rights and permissions . Reprints and Permissions About this article . Cite this article . Lee, D., Lee, J.J., Kim, Y.S. et al. Remote modulation doping in van der Waals heterostructure transistors. Nat Electron (2021). https://doi.org/10.1038/s41928-021-00641-6 Download citation Received : 27 May 2020 Accepted : 03 August 2021 Published : 13 September 2021 DOI : https://doi.org/10.1038/s41928-021-00641-6 .
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