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Reconfigurable van der Waals Hetero-Structured Devices with Metal–Insulator Transition Jinseong Heo, Heejeong Jeong, Yeonchoo Cho, Jaeho Lee, Kiyoung Lee, Seunggeol Nam, Eun-Kyu Lee, Sangyeob Lee, Hyangsook Lee, Sung Woo Hwang, and Seongjun Park Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02199 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016

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Reconfigurable van der Waals Hetero-Structured Devices with Metal–Insulator Transition Jinseong Heo, Heejeong Jeong, Yeonchoo Cho, Jaeho Lee, Kiyoung Lee, Seunggeol Nam, EunKyu Lee, Sangyeob Lee, Hyangsook Lee, Sungwoo Hwang, and Seongjun Park* Samsung Advanced Institute of Technology, Samsung Electronics Co., Suwon-si 443-803, Korea.

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ABSTRACT Atomically thin two-dimensional (2D) materials range from semi-metallic graphene to insulating hexagonal boron nitride to semiconducting transition-metal dichalcogenides. Recently, metal–insulator–semiconductor field effect transistors built from these 2D elements were studied for flexible and transparent electronics. However, to induce ambipolar characteristics for alternative power-efficient circuitry, ion-gel gating is often employed for high capacitive coupling, limiting stable operation at ambient conditions. Here, we report reconfigurable MoTe2 optoelectronic transistors with all 2D components, where the device can be reconfigured by both drain and gate voltages. Eight different configurations for each fixed voltage are spatially resolved by scanning photocurrent microscopy. In addition, metal–insulator transition are observed in both electron and hole carriers under 2 V due to strong Coulomb interaction in the system. Furthermore, vertical tunneling photocurrent through multiple van der Waals layers between the gate and source contacts is measured. Our reconfigurable devices offer potential building blocks for system-on-a-chip optoelectronics. KEYWORDS MoTe2, van der Waals hetero-structures, Reconfigurable, Metal-insulator transition, Tunneling photocurrent TEXT In

modern

electronics, various

constituent materials

with characteristic metallic,

semiconducting, or insulating properties are assembled to realize very basic elements in an


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integrated chip (IC) such as field effect transistors (FETs), interlayer dielectrics, interconnects, and so on. As the size of the individual element shrinks to a few tens of nanometers and below, understanding the fundamental aspects of hetero-structures as well as interface engineering is critical, for example, in reducing contact resistance or power consumption in ICs.1–3 Since the discovery of graphene in 2004 and other two-dimensional (2D) materials in subsequent years4–6, individual 2D materials ranging from semimetal to semiconductor and to insulator have been immensely investigated7–9 and considered as next-generation constituents in optoelectronic circuitry.10–12 Moreover, when stacking 2D materials in a pre-designed manner, vertical van der Waals hetero-structures offer the best platform for studying hetero-interfaces13–17 owing to their atomically flat and saturated surfaces with no inter-diffusion and simple fabrication with minimal chemical exposure. However, although ambipolar transistors provide a versatile and efficient building block in ICs18–22, so far, only limited hetero-structured devices with ambipolar characteristics have been reported,23 which often involve ion-gel gate medium for high capacitive coupling24,25 or asymmetricity in transfer curves,23,24 limiting stable device operation or efficient circuit design. In the current study, we present a class of reconfigurable optoelectronic device with all 2D constituents. A flat-band configuration at equilibrium of a fewlayer MoTe2 channel in contact with graphene electrodes and h-BN dielectric enables eight different configurations with an applied voltage of less than 1 V, which is spatially visualized and confirmed by Scanning Photocurrent Microscopy (SPM). In addition, our system belongs to the strong Coulomb interaction regime, leading to the first observation of metal–insulator transition (MIT) for both electron and hole carriers with carrier densities of approximately 1012 cm-2. Finally, our device architecture, including the naturally embedded h-BN barrier in vertical stacks at the contact region, can be implemented to reduce dark current or increase detectivity for


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photodetector application. Figure 1a shows a schematic of a MoTe2 FET with all 2D constituents. The device fabrication included graphene transfer and patterning as source and drain electrodes underneath, followed by multiple stacking of 2D materials. Briefly, graphene grown by chemical vapor deposition was first transferred and patterned on a SiO2 (100 nm)/Si substrate.26,27 Subsequently, 2D materials, including MoTe2, h-BN, and graphene, were mechanically cleaved on polydimethylsiloxane (PDMS) in a similar manner as that in Ref. 28. MoTe2, h-BN, and graphene on the PDMS were subsequently stamped on pre-patterned graphene electrodes to complete a top-gated FET, as shown in Figure 1a. A cross-sectional transmission electron microscope (TEM) image of a MoTe2 FET in the channel region is shown in Figure 1b with atomically flat and clearly distinguished layered structures of each material. We note that no chemical process was performed after the graphene patterning, which ensured clean interfaces among the different 2D materials on top of graphene. Elementary atomic analysis of the elemental mapping image shown in the lower panel of Figure 1b confirmed the stacked constituents in the right order. The number of layers of each stacked 2D materials ranged from three to eight, but we particularly discuss the main data from the four layers of MoTe2 (Figure S1 of Supporting Information). From the first principles simulation (Figure S2 and Table S1 of Supporting Information), the electronic band structure of semiconducting 2H-MoTe2 depends on the layer [1.6 eV of the direct bandgap (Eg) for a monolayer and 1.1 eV of the indirect bandgap for a bulk], and we found that four-layer MoTe2 had a 1.2-eV indirect bandgap, as shown in Figure 1c. The maximum values of the electron affinity and the ionization potential of four-layer MoTe2 relative to the vacuum were 3.9 and 5.1 eV, respectively, in which the graphene Fermi energy (4.5 eV) was positioned around the center of the bandgap.29–31 For comparison with the experimental result, we measured


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wavelength dependent photocurrent and estimated Eg of 1.12 eV for four layer MoTe2 (Figure S3 of Supporting Information). To estimate the gate efficiency in our device, the drain current (Id) versus gate voltage (Vg) characteristic at a low source–drain bias (Vd) was measured, as shown in Figure 1d, to display the symmetric ambipolar behavior. Because Eg was given by Eg = α ∆Vg, where α is the capacitive coupling factor and ∆Vg between the linearly extrapolated points at Id = 0 is extracted as 1.4 V in Figure 1d, α was estimated to be 0.8 for Eg of 1.12 eV. Figure 2a and 2b show the Id versus Vd characteristics for various fixed Vg’s in linear (Figure 2a) and log scales (Figure 2b), respectively. First, diode-like current rectification was observed for all Vg’s except at 0 V. Surprisingly, the rectification behavior is Vg-dependent; thus, both the rectification directionality or polarity and the degree can be modulated. For example, at Vg = -0.5 V, a positive forward current flows at positive Vd, whereas at Vg = 0.5 V, the sign of the current becomes negative, and the direction is reversed (a more detailed spectrum is shown in Figure S7 of Supporting Information). We analyzed the diode characteristics shown in Figure 2e by extracting the rectification ratio [r = Id (Vd = ± 1 V)/Id (Vd = ∓ 1 V)] and the ideality factor [η: = exp ( ⁄ ), where Isat is the saturation current, kB is the Boltzmann constant, and T is the temperature] as a function of Vg. The highest r value of 103 and the lowest η value of 1.67 were obtained when Vg was equal to -0.5 and -0.3 V, respectively. These Vg values were located approximately at the middle between │Vd│= 0 and 1 V, resulting in M and W shapes for R and η, respectively (Figure 2e). From the │Id│ versus Vg characteristics shown in Figs. 2c and 2d, the symmetric ambipolarity at low Vd (Figure 1d) was modulated to electron- or holedominant polarities as the increased Vd shifted to the minimum conductance point. A minimum subthreshold swing of 180 mV/dec was observed for low Vd. At a reduced capacitance equivalent thickness (CET < 5 nm) or small Eg (1.2 eV), the Vd and Vg values matched or were in the same


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voltage scale. At this point, we hypothesize that matched Vd and Vg can reconfigure our device depending on a specified set of Vd and Vg values, which results in tunable diode characteristics or polarity. Before our detailed explanation, we show one practical application of our device as a crucial element in a low-noise amplifier (LNA). Figure 2f shows intrinsic gain | ⁄ |, where =

and =

=

as a function of Vg at various fixed Vd values. The

gain is greater than one for Vg > 0.5 V and otherwise less than one for Vg < 0.5 V, implying that an input signal above 0.5 V is amplified, whereas a signal below 0.5 V or the noise is reduced. Thus, our single device can operate both as a signal amplifier and a noise reducer, which can be useful in achieving smaller footprint of LNA circuits.32–34 We further discuss other applications in Figure S8 of Supporting Information such as multi-functional device and ternary inverter as multi-valued logic. Figure 3 shows the band diagram configurations for each set of Vd and Vg, and we assign each set to a few representative points indicated by the arrows in the device characteristics shown in Figure 2. As a starting point, Figure 3a shows an equilibrium state when Vd and Vg are equal to 0 V. The band alignment of the Fermi energy of the graphene relative to the conduction and valence bands of MoTe2 is estimated to be in the middle (intrinsic) of the symmetric low-bias characteristic shown in Figure 1d. Following the red curve shown in Figure 2b, when Vg is fixed at 0.5 V, Vd varies from -1 to 0. 5 and to 1 V, as shown in Figs. 3b–3d. Because the sign and magnitude of Vg,eff (x) = Vg – V (x), which is the effective gate field along the channel where V (x) is the channel voltage, determine the carrier type [n type, i (intrinsic), or p type) and density, Figure 3b (where Vg,eff < 0 V throughout the channel), shows an overall n-type configuration. Similarly, Figure 3c shows n–i and Figure 3d shows n–i–p if a constant voltage drop along the channel is assumed for simplicity. Because channel conductance is dependent on how an energy


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barrier is formed at the contacts where a dominant carrier is injected, a thinner barrier width or a higher tunneling probability at the right side contact (shown in Figure 3b) than that at the left sides (shown in Figs. 3c and 3d) leads to higher conductance state, as shown in Figure 2b. On the other hand, when Vd is fixed at -1 V, as shown in Figs. 3e–3h, Vg varies from -1 to 0.5, 0, and 1 V, which can be assigned to p–i, p–i–n, i–n, and n, respectively, by considering Vg,eff (x) along the channel (indicated by arrows in the black curve shown in Figure 2c). Finally, when Vd is fixed at 1 V, as shown in Figs. 3i–3l, Vg varies from 1 to 0.5, 0, and -1 V, which correspond to n– i, n–i–p, i–p, and p, respectively (marked in the black curve in Figure 2d). As a result, for each set of Vd and Vg, eight different configurations (Figure 3) can be obtained from a single device (More possible configurations for four-terminal devices are discussed in Supporting Information). We will directly prove later this hypothesis by visualizing the device configuration from the SPM. The T-dependent transport shown in Figure 4 provides better physical understanding of our device. Figure 4a shows the conductivity (σd) versus Vg plots for various fixed T values from 1.8 to 300 K for both hole and electron carriers. MIT is clearly observed in both carriers for the first time. Transition Vg occurs at -0.66 V (hole carrier density, nh ~6.2 × 1011 cm-2) and at 0.69 V (electron carrier density, ne ~9.0 × 1011 cm-2) for the hole and electron carriers, respectively [from the capacitance–voltage (C–V) measurement shown in Figure S5 of Supporting Information]. The observed quantum critical point in Vg or n separates the metallic state, which shows that the conductivity decreases with T; the insulating state that exhibits conductivity increases with T. The MIT in our device can be explained in terms of a strongly correlated 2D electron gas system.35–38 The insulating behavior below the transition point is characterized by strong localization in a low carrier concentration, whereas the metallic phase above the transition


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point is dominated by weak localization. A measure of the strength of Coulomb interaction between carriers in a 2D system can be characterized by dimensionless Wigner–Seitz radius rs, which is the ratio of the Coulomb energy ("# ) and kinetic energy ("$ ):). % =

"# &' &' .∗ / , = ∗ = "$ ( √+ &,- 4+1ℏ, √+ &,-

where &' is the number of degenerate valleys, (∗ is the effective Bohr radius, 1 is the dielectric constant of the material, .∗ is the effective mass, and &,- is the carrier density of the 2D system. ∗ In our device, &' = 1 or 2 for the electron or hole carrier, respectively. .3,4 = 0.64 .8 and

0.78 .8 for the electron and hole carrier types, respectively, 1 ~818 (adopted from Ref. 39), and &,- is 9.0 × 1011 cm-2 for the electrons and 6.2 × 1011 cm-2 for the holes, which correspond to % ~9.1 for the electrons and % ~ 27 for the holes. We note that although no hysteresis is observed in the Vg sweeping, we estimate a charge impurity density &?@ of 3.5 × 1011 cm-2 on the substrate (SiO2) from the hysteresis of the back-gate voltage (Vbg) sweeping (Figure S6 of ∗ Supporting Information). Thus, increased transition carrier density &,= &,- + &?@ still

remains less than 1.3 × 1012 cm-2; therefore, % ≫ 1. For a system with % ≫ 1 or a highCoulomb interaction regime, the scaling theory of localization is not valid; rather, a 2D system is predicted to manifest the MIT as the carrier density increases,40,41 and such strong interaction systems with % > 4 have been reported in MoS229,37 or Si FET.42 However, in our device, unlike the previously reported strong interaction systems, much reduced voltages with both Vd and Vg of less than 2 V can induce the transition, which are very competitive in terms of power consumption for a new type of fast optoelectronic switches with gate controllability in electrical conductivity as well as in optical transmission. The external two-terminal mobility values of 10–20 cm2/(V·s) at room temperature (Figure S6


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of Supporting Information) increase up to the saturated values of 50–60 cm2/(V·s) at low temperatures, as shown in Figure 4b. The sharp decrease in the mobility as the temperature increases from 100 to 400 K or the strong temperature dependence of the mobility can be attributed to carrier scattering from phonons. We can fit the high-temperature (> 100 K) mobility to D ∝ FG , where H is the exponent and found to be approximately one for both carriers, which is comparable with the reported values for MoS2 (H ~0.55–2.12)29,37 and ReS2 (~1.6–2.5).38 We note that a more theoretical work is needed to provide a more accurate scattering strength or mechanism in detail of our top-gated device geometry (h-BN for top dielectric and SiO2 for bottom dielectric) as well as back-gated MoTe2 FETs. We attribute the relatively low-saturation external mobility at low temperatures to the high contact resistance due to the flat-band contact alignment. Although most efforts so far in 2D devices have been made at reducing the energy barrier at the contacts,43 namely, contact resistance by chemical treatment,44 phase engineering,45 or Fermi level de-pinning,29 we exploit the intrinsic properties to propose reconfigurable optoelectronic devices. We can estimate the minimum contact resistance (RC,min) by fitting the Id–Vd characteristics into a Lambert W function from the Schockley diode equation with a series resistance (Rs).46,47 This equation characterizes a Schottky diode formed at one contact with η and considers Rs to include both the channel resistance and the other side-contact resistance. Because we only consider the overall n- or p-doping configuration when evaluating the external mobility, for example, when Vg is equal to 1 V and Vd < 0 V or forward biased, the two asymmetric Schottky contacts (Figure 3h) plus channel resistance correspond to the total device resistance (RT). Hence, RC,min is given by RT – RS. When the mobility is recalculated by eliminating RC,min, it goes up to 66 cm2/(V·s) at 100 K and 27 cm2/(V·s) at 300 K for the hole carriers (open red circles in Figure 4b). To investigate the MIT in detail, the resistivity (ρ) as a


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function of T is plotted for a few representative Vg’s in Figure 4c. As Vg varies from positive (negative) to negative (positive) for the hole and electron, respectively, transition is smoothly developed from highly insulating state in the bandgap region to the metallic state in a high carrier density of over 1012 cm-2. Figure 4d shows the plot of ln(Isat/T3/2) versus e/kT for the electron side, where Isat is the reverse saturation current in the Id–Vd curve shown in Figure S7 of Supporting Information. Because = II∗ J/, exp L

FMN OP Q

R in the diode equation for 2D hetero-

structures,48–50 where A is the area of the Schottky junction, A* is the effective Richardson constant, e is the elementary charge, and Φ is the barrier height, Φ for each Vg can be obtained from the linear fit shown in Figure 4d. Figure 4e shows that the barrier linearly changes with Vg from 0.26 to 0.20, 0.12, 0.06, and 0.01 eV. One possible scenario is that Φ is modulated by Vg due to the change in the Fermi energy of graphene by an unscreened gate field through four layers of MoTe2. The change in Φcon of over 0.26 eV at 0.8 V in Vg represents greatly enhanced gate efficiency compared with previous works on van der Waals hetero-structures.13,14 One way of visualizing and directly proving the device configuration is SPM. Figure 5a shows an optical microscopy image of a MoTe2 FET with all 2D components where the dotted black lines indicate the underlying graphene source and drain electrodes and the top-contact graphene electrode. The dotted orange lines represent the MoTe2, and the dotted white lines represent the h-BN. The top graphene covers only half of the MoTe2 channel to view the initial band alignment and bending at equilibrium with or without a top graphene and to confirm that the measured photocurrent comes from the photovoltaic effect due to band bending when applying Vg. When the laser illuminates and scans over the boxed area shown in Figure 5a for each set of (Vd, Vg) or in the configuration, we observe nine different visual maps with photocurrent in the color scale, as shown in Figure 5b. Each map has different characteristic activation spots as well


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as different amplitudes of the photocurrent at the source and drain contacts. To understand the characteristic features shown in Figure 5b, the simplified band diagrams shown in Figure 3 corresponding to different configurations for each set are shown in Figure 5d. We note that all photocurrents come from the contact areas between MoTe2 and the underlying graphene electrodes, which in turn implies that band bending happens only at the contacts. This result can be explained by the characteristic length scale λc over which the band bending occurs in the metal–semiconductor

interface

in

an

ultra-thin

body

device.51

Given

that

ST = UV1WXY3Z ⁄14F[ \ ]4F[ ]WXY3Z , if 14F[ = 4.2 18 (which is the dielectric constant of hBN, ]4F[ ≈ 5 nm (which is the thickness of h-BN, Figure S1 in Supporting Information), and ]WXY3Z ≈ 2.7 nm (which is the thickness of MoTe2, then ST ≈ 5.3 nm in our device, which is much smaller than the channel length (3 µm) and the laser spot size (~1 µm). This result implies that the reconfigurable characteristics of our device are scalable up to a few nanometer range in the lateral dimension because ST can be reduced to 2–3 nm when the thicknesses of h-BN and MoTe2 are decreased by half. At equilibrium, (0 V, 0 V), a few nanoamperes of photocurrent is observed, which is negligible compared with the other non-equilibrium configurations that are almost two orders of magnitude less than the full current scale. This result directly proves the neutral arrangement of the band alignment of graphene on MoTe2 in our device. As a general rule, the sign of the photocurrent is determined by the slope of the band bending. For example, for (1 V, 0.45 V) set at the top middle map shown in Figure 5b, almost equal and positive currents (electron flow to the left is defined as positive in our convention) at both contacts are measured, consistent with the p–i–n configuration or the positive slope of the band bending at the top middle diagram shown in Figure 5d. If only one junction exists such as the i–n configuration at the top right band in Figure 5d, only a positive current at the right contact flows at the top right


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map shown in Figure 5b, whereas no measurable current is detected at the left contact corresponding to a flat-band configuration. Finally, for the overall p- or n-dominant configuration in the middle left or right diagrams shown in Figure 5d, we measure currents at opposite directions at each contact shown in Figure 5b. Similarly, all nine configurations in the band diagram shown in Figure 5d can be matched to the SPM images shown in Figure 5b. If light is focused on one of the contact regions indicated by the arrows in the middle panel (Vd = 0 V) in Figure 5b, the responsivity (R in Figs. 5c and 5e) or the response-time characteristics (Figure 5f) as figures of merit of the photodetectors can be evaluated. Figure 5c shows that R approaches zero at Vg = 0 V, as expected, from the flat-band configuration, and it becomes larger at lower laser power. The power dependence of R, namely, R ~ P

-0.9

, is clearly shown in Figure

5e and Figure S10 of Supporting Information at Vg = 0.8 V when the laser spot is focused at the left contact. An R value of up to 88 A/W is measured at the lowest power of 0.92 nW, but it dramatically decreases to 0.048 A/W at 4.6 µW. For the other device shown in Figure S11 of Supporting Information, we observe R and D values of up to 610 A/W and 3.3 × 1011 Jones, respectively, with extreme Vg tunability of R over 81 dB due to the band reconfiguration at the contact. Such non-linear response of R is reported in most 2D or hetero-structure devices.52–55 Thus, the poor linear dynamic range needs to be improved for further photodetector application. The Id upon on and off of the laser is recorded as a function of time where rise time τR and fall time τF are both evaluated as 330 µs (see Table S2 of Supporting Information for comparison with reported devices in the literature). Although our device, which shows submillisecond response, is among the fastest 2D hetero-structured photodetectors,56,

57

we attribute the

degradation to be mainly due to the residue at the top of the transferred underlying graphene on the contact regions, and it can be further improved when optimal process is developed.


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Not only the lateral photoresponse but also the vertical hot carrier photoresponse of our device can be detected through the van der Waals tunneling barrier (see the band diagram of Figure S12 of Supporting Information). Although previous studies focus on vertical graphene–2D-graphene junctions for both fast response time and large junction area, these devices suffer from large dark current, preventing achievement of high detectivity.17,54,58,59 One approach in decreasing the dark current is inserting a barrier layer between the photoactive 2D layer and the top or bottom graphene electrode. In our device architecture, h-BN layer is naturally embedded as gate dielectric; simultaneously, it can be employed as an inserted barrier in 2D hetero-structured vertical photodetector geometry. Figure 6a shows current density Jd versus vertical electric field E under dark (black) and illuminated (red) conditions. After the onset of appreciable tunneling current at 0.4 V/nm, a vertical photocurrent of over 0.02 pA/µm2 (at E > 0.6 V/nm) is observed (Figure 6b). We calculate the tunneling photocurrent using Y,@4 = [ ∙ @4 , where Y,@4 is the tunneling photocurrent, [ is the tunneling probability in the Fowler–Nordheim regime, and @4 is the photocurrent generated in few-layer MoTe2 between h-BN and the bottom graphene. Y,@4 ~2.3 pA (or photocurrent density of 0.023 pA/µm2 for 100 µm2 area) at vertical electric field EV = 5.8 MV/cm and λ = 633 nm with P = 4.6 µW (see Figure S12 of Supporting Information for details), which shows good agreement with the experimental value. However, the relatively low Y,@4 can be increased to 5.6 nA for λ = 413 nm with P = 4.6 µW. This proofof-concept device presents the lowest dark current of below 10-2 pA/µm2 (detection limit) in vertical 2D hetero-structured photodetectors with measurable photocurrents. In summary, we have discussed reconfigurable van der Waals hetero-structured FETs as versatile optoelectronic devices. By applying only 1 V, eight different configurations led to unique tunable ambipolar characteristics, which can be applied to LNA using only one active


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device. In addition, from the temperature-dependent transport, the MIT for both hole and electron carriers was observed at Vd and Vg < 2 V due to strong Coulomb interaction. Later, all configurations were directly visualized using spatially resolved photocurrent imaging where band bending occurred only at the contact regions. Finally, we propose barrier-inserted vertical photodetectors as a viable approach to reduce dark current or increase detectivity. Thus, the reconfigurable all 2D devices presented in this work provide a method for integrated optoelectronics on a chip.


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FIGURES

Figure 1. MoTe2 FET based on van der Waals hetero-junctions. a, Schematic of a MoTe2 FET with all 2D constituents. Patterned graphene as source and drain contacts (GB) are placed underneath few-layer MoTe2. Few-layer h-BN and graphene (GT) as a gate dielectric and a gate contact, respectively, are subsequently stacked on top of MoTe2. The three-terminal measurement scheme is also shown. b, Cross-sectional (top panel) TEM image and


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corresponding element analysis in (bottom panel) the elemental mapping image of a MoTe2 FET in the channel region between the graphene source drain contacts shown in a. From the bottom: four-layer MoTe2, eight-layer h-BN, and four-layer top graphene. Atomically flat interfaces between each layer are formed. Scale bar: 5 nm. c, Calculated electronic band structure of fourlayer MoTe2 using HSE06 functional. The lowest conduction band and the highest valence band along a few representative symmetric points are shown in black dots for clarity. Indirect bandgap Eg is approximately 1.2 eV. The electron affinity and the ionization potential relative to the vacuum level are found to be 3.9 and 5.1 eV, respectively. The graphene Fermi energy of 4.5 eV is represented by the dashed line. d, Id versus Vg at Vd = 10 mV is plotted in a linear scale. A transport gap of 1.4 V in Vg is extracted from the linear extrapolation of Id. From the estimated bandgap shown in Figure S3 of Supporting Information, the capacitive coupling factor is estimated to be 0.80.


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a

b

0.4

-6

10 Id (µA)

Id (µA)

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Figure 2. Characteristics of MoTe2 FET with all 2D components. a, Id versus Vd curves at various Vg values from -1 to 1 V with a 0.5-V step. The rectifying behavior of Id is highly tunable as Vg and its polarity (forward and reverse bias) are flipped according to the sign of Vg. b, Semilogarithmic plots of │Id│–Vd characteristics shown in a. Nearly symmetrical rectification curves around Vd = 0 V axis are observed (│Id (Vd, Vg)│= │Id (-Vd, -Vg)│). c and d, Semi-logarithmic │Id│–Vg characteristics for (c) negative and (d) positive Vd. Minimum subthreshold swings of 180 mV/dec for both negative and positive Vd are observed. We note that as │Vg│ increases from 0.1 V, the ambipolar characteristics become electron- or holedominant ones, as shown in c and d, respectively, due to the directional shift in the minimum conductance points. e, r obtained at (black square to left axis) Vd = ± 1 V and (blue circles to right axis) η from the │Id│–Vd characteristics shown in Figure S7 of Supporting Information. r of up to 103 and η of up to 1.67 can be gradually tuned by Vg, resulting in a W or an inverted W shape in R and η, respectively. f, Transconductance ratio gm/gd or intrinsic gain as a function of Vg for various Vd values. As Vg varies from 0 to 1 V, the gain becomes larger than one for Vg > 0.5 V and goes up to 100 at Vg = 1 V, whereas the gain is 0.5 V and noise reduction at “low” input for Vg < 0.5 V.


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Vg = 0 V

a 0V

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Gr.

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eee ee e e e ee e e

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ee e

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n

i

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l

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ee e e

h h hh

Figure 3. Band diagrams for reconfigurable device operations. a, Schematic band diagram of a MoTe2 FET with all 2D components at equilibrium (Vd = Vg = 0 V). The flat MoTe2 band configuration at both graphene source and drain contacts is due to the symmetric Id–Vg characteristics for low Vd shown in Figure 1d. The left graphene contact is set to the ground, and the drain and gate voltages as well as the channel type (i: intrinsic) are specified along the channel. b–d, Band configurations for the │Id│–Vd characteristic shown in Figure 2b. When Vg = 0.5 V is fixed, Vd varies from -1 to 0.5 and to 1 V, as shown in b–d, respectively, following the


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arrows in the red curve shown in Figure 2b. Because Vg,eff (x) = Vg – V (x) yields the effective gate field along the channel, Vd = -1 V corresponds to the overall n configuration, as shown in b. Vd = 0.5 V corresponds to the n–i configuration, and Vd = 1 V corresponds to the n–i–p configuration, as shown in c and d, respectively. e–l, Band configurations for the │Id│–Vg characteristics shown in Figs. 2c and 2d. When Vd = -1 V is fixed, Vg varies from -1 V, as shown in e to -0.5, 0, and 1 V, as shown in f–h, respectively, following the arrows in the black curve shown in Figure 2c. By considering Vg,eff (x), the configuration assignments are e: Vg = -1 V to p–i, f: Vg = -0.5 V to p–i–n, g: Vg = 0 V to i–n, and h: Vg = 1 V to n. When Vd = 1 V is fixed, Vg varies from 1 V, as shown in i to 0.5, 0, and -1 V, as shown in j–l, respectively. This result follows the arrows in the black curve shown in Figure 2d. The assignments are i: Vg = 1 V to n–i, j: Vg = 0.5 V to n-i-p, k: Vg = 0 V to i-p, and l: Vg = -1 V to p.


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a

b Hole

6

100

Electron

4

2

µ (cm /V⋅s)

2 T

electron hole

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γ ∼ 1.02

1

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0

10

Vg (V)

electron

hole

d -20

e electron

-0.2 V

10

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0V

2

ln (I/T )

ρ (Ω )

0.4 V

10

6

10

4

0.6 V 1.4 V

-28

0

150 300

- 0.2 V

-26

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-24

0 150 300 T (K)

electron

0.2

0V 0.2 V

0.3

0.2 V

-22

0.6 V 8

100 T (K)

c

10

γ ∼ 1.05

Insulating

-1

0.8 V

−γ

h

300 K Metallic

0

µ∼Τ

Φ (eV)

σd (µS)

1.8 K

e

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0.1

- 0.4 V - 0.6 V

30 35 40 45 e/(kT)

0.0 -0.6 -0.3 0.0 0.3 Vg (V)

Figure 4. Temperature-dependent transport of a MoTe2 FET with all 2D components. a, Tdependent σd–Vg characteristics. T varies from 1.8 to 300 K. MIT is distinctly observed for both hole and electron carriers. The transition Vg’s are -0.66 and 0.69 V for the hole and electron, respectively. The corresponding carrier densities are 6.2 × 1011 and 9.0 × 1011 cm-2, respectively (Figure S5 of Supporting Information). b, Temperature-dependent mobility for (black squares) electron and (red circles) hole carriers in the two-terminal measurements. Power-law dependence with exponents γe ~1.05 for the electrons and γh ~1.02 for the holes above 100 K is observed. The saturated mobility at low temperatures is 50–60 cm2/(V·s). If we subtract Rc,min, which is


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estimated by fitting to the Lambert W function, the mobility increases to 51 cm2/(V·s) at 100 K and 20 cm2/(V·s) at 300 K for (black open squares) the electrons and 66 cm2/(V·s) at 100 K and 27 cm2/(V·s) at 300 K for (red open circles) the holes. c, ρ versus T curve at various Vg values for both electron and hole carriers. Metallic dependence is seen at high Vg,eff at both sides, whereas insulating behavior is developed as Vg,eff decreases. d and e, Thermionic contributions of the insulating behavior in the temperature-dependent transfer curves. d. ln(I/T3/2) versus e/kT plots. e. Extracted barrier heights for electrons. The insulating behavior is dominated by Schottky contacts at the MoTe2–graphene junctions. Increased doping by Vg in the contacts can modulate the Schottky barriers from 0.26 to 0.01 eV by varying Vg from -0.6 to 0.2 V.


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Figure 5. Reconfigurable scanning photocurrent microscopy with band diagrams and photocurrent measurements in a contact region. a, Optical microscopic image of a MoTe2 FET with all 2D components. Photocurrent measurement scheme is also illustrated upon laser illumination on the device. The scale bar is 5 µm. b, Scanning photocurrent maps of a MoTe2 FET inside the dotted black box in a with color scale of the photocurrent for nine different configurations. Each configuration is indicated by different combinations of Vd and Vg [(Vd, Vg) = (1 V, 0 V), (1 V, 0.45 V), (1 V, 1 V), (0 V, -1 V), (0 V, 0 V), (0 V, 1 V), (-1 V, -1 V), (-1 V, -0.7 V), and (-1 V, 0 V)]. The wavelength and power of the laser diode are 614 nm and 3.1 µW, respectively. c, R versus Vg curves when Vd = 0 V at the contact region indicated by dotted arrows in b. the negligible photoresponse at Vg = 0 V directly proves the flat-band assumption at equilibrium configuration (Vd = Vg = 0 V) shown in Figure 3a. The 633-nm laser with two different power values (black: 4.6 µW, red: 9.2 nW) is used. d, Schematic band diagrams of the nine different configurations corresponding to the scanning photocurrent maps in b. The configuration assignments are as follows: (Vd = 1 V, Vg = 0 V) to p–i, (1 V, 0.45 V) to p–i–n, (1 V, 1 V) to i–n, (0 V, -1 V) to p, (0 V, 0 V) to i, (0 V, 1 V) to n, (-1 V, -1 V) to i–p, (-1 V, -0.7 V) to n–i–p, and (-1 V, 0 V) to n–i. e, R versus Vd curve when Vg = 0.8 V (n configuration) at the contact region in a. R increases up to 88 A/W as Vg,eff or the band bending is enhanced at 0.92 nW. The R versus P plot at Vd = -1 V is shown in Figure S10 of Supporting Information. f, Time response of the Id upon illumination of the laser at a left contact region in a. The device configuration is n–i (Vd = -1 V, Vg = 0 V). Both rise and fall times are 330 µs. See Table S2 of Supporting Information for comparison with reported devices in the literature.


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Figure 6. Vertical tunneling photocurrent measurements. a, Current density Jd versus vertical field EV for (black) dark and (red) laser illumination. A schematic of the contact region of our device with the photocurrent measurement scheme is shown. Vertical tunneling photocurrent between the top and bottom graphene electrodes across the insulating hBN layers is observed where the dark tunneling current starts to increase above the noise current level, which is E = 0.4 V/nm. The wavelength and power of the laser diode are 633 nm and 4.6 µW, respectively. b, Photocurrent response for various E values with time. Photocurrent density ∆J = 0.020 pA/µm2 from 0.086 pA/µm2 (dark) to 0.106 pA/µm2 (when illuminated) at E = 0.63 V/nm (red squares) and the photocurrent density ∆J = 0.022 pA/µm2 from 0.093 pA/µm2 (dark) to 0.115 pA/µm2 (when illuminated) at E = 0.65 V/nm (blue squares) are shown.


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ASSOCIATED CONTENT Supporting Information. Thickness measurement of MoTe2 and h-BN by Atomic Force Microsocpe (AFM). Layerdependent band structure simulation of MoTe2. Bandgap estimation of 4 layer MoTe2 by wavelength dependent photo-response. Raman spectroscopy of MoTe2. Capacitance–voltage (C– V) measurement of a MoTe2 FET. Hysteresis in Id–Vbg plot and field effect mobility calculation. Electrical characteristics (Id–Vg and Id–Vd). Reconfigurable Device applications: Multifunctional device and Ternary inverter. Configurations of Four-terminal device. Device stability over time. Responsivity versus power plot. Photoresponse of a MoTe2 FET. Comparison table for 2D photodetectors. Band diagram for tunneling photocurrent measurement. This material is available free of charge via the Internet at http://pubs.acs.org.free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Seongjun Park (Address: Samsung Advanced Institute of Technology, Samsung Electronics Co., Suwon-si 443-803, Korea. Phone: +82-31-8061-4340; fax: +82-31-8061-4673; E-mail: [email protected]) Author Contributions J. H. and S. P. conceived this work. J. H. designed the experiment. J. H. carried out the device fabrication, characterization, and data analysis. H. J. conducted the SPM mapping experiment. Y. C. executed the band structure simulation. H. L. carried out TEM analysis. H. J., J. L., K. L., E.K. L., S. L., S. N., and S. P. contributed to the analysis and interpretation of the results. J. H.


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wrote the manuscript. S. P. and S. H. supervised the project. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank our colleagues at the Samsung Advanced Institute of Technology: Nano Fabrication group for the process assistance they provided.


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