Establishing ohmic contact to van der Waals semiconductors such as MoS2 is crucial to unlocking their full potential in next-generation electronic devices. Encapsulation of few layer MoS2 with hBN preserves the material’s electronic properties but makes electrical contacts more challenging. Progress toward high quality edge contact to encapsulated MoS2 has been recently reported. Here, we evaluate a contact methodology using sputtered MoRe, a type II superconductor with a relatively high critical field and temperature commonly used to induce superconductivity in graphene. We find that the contact transparency is poor and that the devices do not support a measurable supercurrent down to 3 K, which has ramifications for future fabrication recipes.

Soon after the isolation of monolayer graphene, it was found that mono- and few-layer crystals could be isolated from transition metal dichalcogenides (TMDs).1 TMDs host an array of interesting phenomena such as superconductivity, charge density waves, and quantum spin Hall states.2 Among the library of TMDs, molybdenum disulfide (MoS2) has attracted attention due to its layer-dependent band structure,3,4 high mobility,5–7 large spin–orbit interaction,8–10 and gate-induced superconductivity.11–14 Encapsulation of MoS2 with hexagonal boron nitride (hBN) both protects it from atmosphere and separates it from sources of disorder.15,16 However, due to Schottky barriers, a readily formed oxide layer, and the fabrication challenges that come along with encapsulation, ohmic contact to hBN/MoS2/hBN heterostructures has proven difficult.

The low temperature ohmic contact of normal metals to encapsulated MoS2 has been achieved through workfunction engineering17 as well as intervening graphene layers.15,18 Recently, progress has been made in one-dimensional edge contact to MoS2 with normal metal through in situ Ar+ sputtering.19,20 It would be highly desirable to develop superconducting edge contact to MoS2, which could enable the study of the Josephson junction physics taking advantage of MoS2’s spin–orbit and spin–valley couplings.

In this work, we make one-dimensional edge contact to encapsulated MoS2 using molybdenum-rhenium (MoRe), a type II superconductor known to form high transparency contact to MoS2 for a 2D interface.21 We utilize a recipe known to make ohmic edge contacts to hBN-encapsulated graphene.22,23 Our measurements show low transparency contact to MoS2 that is improved neither by Ar+ sputtering pre-treatment of the contact interfaces nor by annealing. These results indicate the probable presence of interfacial tunnel barriers. This result may prove informative for groups developing hybrid samples made of van der Waals heterostructures with superconducting contacts.

We study two MoS2 devices encapsulated within hBN. Both samples are contacted by several MoRe electrodes, which define a series of Josephson junctions of different lengths. The first device uses bilayer MoS2, while the second device uses monolayer MoS2. Figure 1 shows an optical image of the first device as well as a schematic view of the one-dimensional edge contact between MoS2 and MoRe, created via reactive ion etching (RIE) and sputtering. The second device underwent an in situ Ar+ sputtering pre-treatment immediately before MoRe deposition.

FIG. 1.

(a) Optical image of the first sample. The black outline shows the location of the encapsulated MoS2. Scale bar: 5 μm. (b) Schematic side view of the one-dimensional edge contact between the encapsulated MoS2 and the sputtered MoRe (not to scale).

FIG. 1.

(a) Optical image of the first sample. The black outline shows the location of the encapsulated MoS2. Scale bar: 5 μm. (b) Schematic side view of the one-dimensional edge contact between the encapsulated MoS2 and the sputtered MoRe (not to scale).

Close modal

Both van der Waals heterostructures were assembled from mechanically exfoliated flakes using a dry transfer technique utilizing a polyethylene terephthalate stamp. The polymer residue was removed by immersion in dichloromethane for 1 h at 70 °C followed by several hours at room temperature.

The one-dimensional interface between MoS2 and MoRe was prepared via standard electron-beam lithography techniques, reactive ion etching (RIE), and sputtering. RIE consisted of three steps, all carried out with a process pressure of 10−1 Torr. First, a 10 s CHF3/O2 (10:1 flow rate ratio) step removed leftover e-beam resist (PMMA) residues from the top surface of the heterostructure. This was followed by a 10 s SF6 process to etch through the top hBN. Finally, a 10 s CF4 step was used to etch MoS2 in the contact region. While a CF4 etch is a typical process for MoS2, SF6 may itself be sufficient.19 In order to limit the device’s exposure to atmosphere, and so the formation of MoOx along the interface, the device was not removed from the system and imaged between these steps.

The devices had minimal exposure to air before being transferred to the sputtering system. The second sample was treated with Ar+ sputtering before metal deposition to refresh the contact interface. The chamber was pumped to a pressure of ∼10−8 Torr, and 100 nm of MoRe (50%–50% by weight) was sputtered on both devices. To minimize processing, the Josephson junctions were not shaped with further etching, so the flakes of MoS2 continue beyond the boundaries of the junctions. This is visible in Fig. 1(a), which shows an optical image of the first device.

The samples are cooled in a closed-cycle cryocooler with a base temperature of 3 K. Unless otherwise noted, a voltage Vapplied is applied to the junction in series with a protective RS = 10 MΩ resistor. The drain current, ID, is measured, and the source–drain voltage is calculated as VSD = VappliedRSID; as a result, the curves in Figs. 2 and 3 have different horizontal extent.

FIG. 2.

(a) Gate voltage dependence of the IV characteristics in a 200 nm long, 5 μm wide junction on the first device (J1). Inset: resistance at high VSD for each gate voltage. The junction is seen to be highly resistive across applied gate and bias voltages, and no signs of superconducting behavior are visible. (b) IV curves for junctions J1–3 of the first sample at VBG = 42 V. There is no significant difference between the 200 and 500 nm long junctions, indicating that the current is limited by the contacts. Inset: top-down schematic of the sample with J1–3 labeled.

FIG. 2.

(a) Gate voltage dependence of the IV characteristics in a 200 nm long, 5 μm wide junction on the first device (J1). Inset: resistance at high VSD for each gate voltage. The junction is seen to be highly resistive across applied gate and bias voltages, and no signs of superconducting behavior are visible. (b) IV curves for junctions J1–3 of the first sample at VBG = 42 V. There is no significant difference between the 200 and 500 nm long junctions, indicating that the current is limited by the contacts. Inset: top-down schematic of the sample with J1–3 labeled.

Close modal
FIG. 3.

Temperature dependencies measured in the 200 nm long, 5 μm wide junction in the first device. (a) Post-anneal IV characteristics. (b) Low bias (VSD = 0.05 V) resistance R, plotted in linear and (inset) log scale, which shows R decaying with temperature. VBG = 42 V throughout. (c) ln(ID) vs (VSD/V)1/2 plot of the same data showing an approximately linear relationship in the intermediate temperature range. This is consistent with thermionic transport across the contact interfaces. (d) ln(G/T1/2) plotted vs 1/kBT (dots connected by the blue line). The orange line shows a linear fit with a slope corresponding to the extracted ϕB ≈ 30 meV.

FIG. 3.

Temperature dependencies measured in the 200 nm long, 5 μm wide junction in the first device. (a) Post-anneal IV characteristics. (b) Low bias (VSD = 0.05 V) resistance R, plotted in linear and (inset) log scale, which shows R decaying with temperature. VBG = 42 V throughout. (c) ln(ID) vs (VSD/V)1/2 plot of the same data showing an approximately linear relationship in the intermediate temperature range. This is consistent with thermionic transport across the contact interfaces. (d) ln(G/T1/2) plotted vs 1/kBT (dots connected by the blue line). The orange line shows a linear fit with a slope corresponding to the extracted ϕB ≈ 30 meV.

Close modal

Figure 2(a) shows the effects of electrostatic gating on the IV curves of a 200 nm long and 5 μm wide junction made on the first device. The gate voltage (VBG) increases the Fermi level in MoS2, causing it to approach the conduction band. We observe that for increasing VBG, the threshold of VSD required to achieve a linear slope decreases. Figure 2(b) demonstrates the I–V curves measured for three junctions of different lengths at the maximal gate voltage of 42 V (see the schematic in the inset: J1 is 200 nm long, and J2,3 are 500 nm long). It is clear that (1) the curves show no significant length dependence, indicating that the current is limited by the contact barriers, and (2) the measurements are consistent between the three junctions, indicating uniform properties of the contacts. These initial measurements are consistent with the presence of barriers (such as Schottky barriers) at the interfaces.19 At the highest gate voltage (42 V), the resistance is 2.4 MΩ corresponding to the contact resistance of Rc ≈ 6 MΩ μm.

Due to this high contact resistance, we next anneal the sample at 200 °C for 17 h in a vacuum of 10−6 mbar. Annealing processes have been shown to decrease contact resistance in similar devices. This may be due to a host of phenomena, which change the bonding or structure at the interface.19 In this study, the annealing resulted in higher contact resistance with an increase of as much as 40% at high bias and VBG = 42 V. This decrease in contact quality may be due to MoRe reflowing away from the contact edge, as seen in gold junctions without an additional metal sticking layer.19 

We study the behavior of the junction as a function of temperature to gain insight into the poor contact quality. Figure 3(a) plots the IV characteristics of the same junction from 3 to 290 K. A clear reduction in low-bias resistance spanning more than a decade is seen as the temperature rises [Fig. 3(b)]. Such a behavior is consistent with thermionic transport across a barrier. This interpretation is supported by an approximately linear relation between the log of the current and the square root of the bias voltage in the device [Fig. 3(c)] as expected, e.g., for a triangular Schottky barrier.24 This relation breaks down for low bias voltages at higher temperatures.

To extract the barrier height (ϕB) for the device, we plot ln(G/T1/2) vs 1/kBT in Fig. 3(d), following Ref. 25. In this fit, we limit the source–drain voltage and temperature ranges eV < kBT to access the regime of linear conductance. We find ϕB ≈ 30 meV, significantly smaller than the expected ∼100 meV based on the workfunction of MoRe.25 This reduction is likely explained by the high gate voltage at which our data are taken. The resulting electric field at the contact lowers the apparent ϕB via the Schottky effect.

Due to the contact characteristics of this device, we study a second device utilizing Ar+ sputtering immediately prior to the deposition of MoRe contacts, focusing on a 500 nm long and 5 μm wide junction. Despite this change in deposition parameters and an overnight anneal at 300 °C in 10−6 mbar, this second device also displays high contact resistances at low temperature. In Fig. 4, we utilize a direct voltage biasing scheme without a 10 MΩ series resistor, we measure gate sweeps for different VSD. Even at the highest applied VSD = 5 V, the currents supported by the junction are orders of magnitude lower than comparable or longer junctions made with both top contacts26,27 and high quality normal metal edge contacts.19,20

FIG. 4.

Current vs VBG sweeps measured in a 500 nm long and 5 μm wide junction in the second device following the annealing, which show the induced highly resistive behavior. The three curves correspond to VSD = 1.5, 3, and 5 V. Inset: the same data in log scale.

FIG. 4.

Current vs VBG sweeps measured in a 500 nm long and 5 μm wide junction in the second device following the annealing, which show the induced highly resistive behavior. The three curves correspond to VSD = 1.5, 3, and 5 V. Inset: the same data in log scale.

Close modal

In summary, we tested a methodology for making one-dimensional edge contact to encapsulated MoS2 with MoRe and found high contact resistances on the order of MΩ μm. This contact was not improved by annealing at 200–300 °C. In situ Ar+ sputtering of the interface before the deposition of MoRe also did not improve the contact quality. We conclude that the presence of tunnel barriers limits the performance of these devices. The lack of length dependence, consistency between different junctions, insensitivity to Ar+ pre-cleaning, and the lack of improvement upon annealing all point to the presence of intrinsic Schottky barriers at the interfaces.

Higher transparency contacts may be achieved in the future by replacing MoRe with superconductors having a significantly higher or lower workfunction. Nevertheless, the current contact recipe could support the use of MoS2 in more complex superconducting heterostructures. Namely, TMDs, including MoS2,28 are already used to induce the spin–orbit coupling in graphene.29,30 One can extend these studies to Josephson junctions by making superconducting contacts that would selectively contact the graphene but not the TMD layer. In this context, our work establishes an order of magnitude estimate for the (very small) current expected to be shunted through an MoS2 layer in such a complex van der Waals heterostructure.

Transport measurements conducted by A.S., E.G.A., T.F.Q.L., L.Z., and G.F. were supported by the Office of Basic Energy Sciences, U.S. Department of Energy, Award No. DE-SC0002765. Sample fabrication by A.S. and E.G.A. was supported by the National Science Foundation, Grant No. DMR-2004870. V.Z.C. and A.K.M.N. acknowledge support from the National Science Foundation (Grant No. ECCS-1708907) and the Department of Defense Award (Grant No. 72495RTREP). K.W. and T.T. acknowledge the Elemental Strategy Initiative conducted by the MEXT, Japan, and the CREST (Grant No. JPMJCR15F3), JST. F.A. acknowledges the ARO under Award No. W911NF-16-1-0132. This work was performed, in part, at the Duke University Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant No. ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI).

A.S. and E.G.A. contributed equally to this work.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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