# High performance room temperature p-type injection in few-layered tungsten diselenide films from cobalt and palladium contacts

Jacko Rastikian, Stéphan Suffit, Pascal Martin, Philippe Lafarge, Odile

Bezencenet, Maria Luisa Della Rocca, Clement Barraud

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Jacko Rastikian, Stéphan Suffit, Pascal Martin, Philippe Lafarge, Odile Bezencenet, et al.. High performance room temperature p-type injection in few-layered tungsten diselenide films from cobalt and palladium contacts. Materials Research Express, 2019, 6 (12), 10.1088/2053-1591/ab56d5. hal-02570146

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# High performance room temperature p-type injection in few-layered tungsten diselenide films from cobalt and palladium contacts

To cite this article: Jacko Rastikian et al 2019 *Mater. Res. Express* **6** 126307

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# High performance room temperature p-type injection in few-layered tungsten diselenide films from cobalt and palladium contacts

Jacko Rastikian¹,⁴ Ⓓ, Stéphan Suffit¹ Ⓓ, Salvatore Timpa¹ Ⓓ, Pascal Martin² Ⓓ, Philippe Lafarge¹ Ⓓ, Odile Bezencenet³, Maria Luisa Della Rocca¹ Ⓓ and Clément Barraud¹,⁴ Ⓓ

¹ Université de Paris, Laboratoire Matériaux et Phénomènes Quantiques, CNRS, F-75013 Paris, France

² Université de Paris, Laboratoire ITODYS, CNRS, F-75013 Paris, France

³ Thales Research and Technology, F-91767 Palaiseau, France

⁴ Authors to whom any correspondence should be addressed.

E-mail: clement.barraud@univ-paris-diderot.fr and jacko.rastikian@univ-paris-diderot.fr

**Keywords:** WSe₂, palladium, cobalt, charge injection, holes transport

Supplementary material for this article is available [online](#)

## Abstract

Field-effect electronic devices based on transition metals dichalcogenides (TMDs) have recently received a tremendous interest. Although widely studied, efficient charge injection in those materials is often impeded by the large injection barrier present at the interface between the electrode and the TMD, in particular when holes transport is needed. In this paper, e-beam lithography and e-beam evaporation techniques are used to contact few-layered WSe₂ films with patterned metallic electrodes (Co and Pd). We measured the transport and the injection properties at various gate voltages, source-drain voltages and different temperatures. A quantitative analysis is performed using a two-dimensional version of a thermionic emission model to extract the evolution of the injection barrier height as a function of the gate voltage. Palladium contacts are found to be very transparent in the hole injection regime but weakly tunable under the application of a gate voltage. Cobalt contacts present a low injection barrier which, unlike Pd contacts, can be extremely reduced by electrostatic gating. In both cases low injection barriers with respect to the literature are extracted. These results demonstrate a solid potential for applications implying WSe₂ in the hole transport regime. The results reveal also an excellent agreement when compared to previous theoretical studies by taking into account the role of hybridized interfacial states and of the spin-orbit coupling of the contacting material on the injection barrier.

## Introduction

Two-dimensional (2D) materials are fascinating for the novel physics they offer [1, 2]. Their unique electrical, optical and mechanical properties continuously open a promising panel of new applications such as vertically architectured transistors [3] or highly-tunable opto-electronics devices [4]. In the wide family of existing 2D materials, recently discovered transition metal dichalcogenides (TMDs) [5] exhibit a notable variety of interesting properties for combining opto-electronics [6, 7], spintronics [8, 9] and electronics applications [10–13]. The main reason is that unlike classical materials in thin films such as metals, oxides and semiconductors based on Si, GaAs, and others, TMDs are characterized by a fully controllable electronic structure [14, 15] down to an atomically thin thickness. Tungsten diselenide (WSe₂), a representative TMD, is a semiconductor with a direct band gap of 1.6 eV for one monolayer (ML) and an indirect gap of 1.2 eV in its bulk form [16]. It is also characterized by a large spin-orbit coupling (SOC) making it an interesting material for spin and valley physics [17–19]. Thanks to its large band gap and relatively high carrier's mobility, WSe₂ is also a promising material for field effect transistors (FET) with high On/Off ratio [20, 21].

2D-based FETs properties are known to be very dependent on the electronic properties of the microscopic metal/semiconductor interface. The presence of a metal in contact with WSe₂ is known to impact the electronic
properties of both materials [14, 22]. This is due to the formation of chemical bonds and defects that could lead to gap states and thus to Fermi level pinning [23, 24], and to charge transfer and screening [25]. The presence of those effects and their respective amplitude can depend not only on the deposited metals, but also on the deposition techniques and parameters [26]. As a general consequence, an injection barrier such as a Schottky barrier is induced on the TMD's side [27, 28] and is known to also vary with the number of layers composing the connected TMD [29]. Those effects can be probed and quantified thanks to electronic transport measurements. A large contact resistance (i.e. a large injection barrier), for instance, prevents FET's good performance in the high-frequency regime [22, 30]. This contact resistance is related to the density of states and to the work function (WF)

of the metal. More specifically, it is influenced by the relative position between the contact's Fermi level and the energy bands of the semiconductor. In this sense, a precise alignment of the metal's Fermi energy within one of the bands of the semiconductor is generally preferred to promote transparent contacts. Achieving efficient holes carriers' injection through p-type metallic contacts in WSe₂ concentrates a considerable effort for two main reasons [31]. First, WSe₂ prefers p-type doping unlike other TMDs like MoS₂ [21, 32–34] and holes shows high mobilities up to 4 000 cm²/V·s [35]. Second, due to its strong SOC, WSe₂ is characterized by a large spin-splitting of the valence band maximum of the order of 460 meV in a monolayer (ML) WSe₂. This makes such a material highly sought-after for opto-spintronics applications [9, 36].

In this work, we study metal-WSe₂ contact properties for metallic electrodes with high work functions (Co, Pd) within the hole transport regime. We also compare their efficiency at room temperature with an intermediate work-function metal (Ti) showing poor injection properties. We demonstrate a direct correlation between the chosen metal and the injection barrier. To clarify this link, we study both the gate voltage and the temperature dependence of the device resistance including the different contact resistance, showing that devices with Co- and Pd-based contacts are the best candidates to implement holes transport in WSe₂.

## Results and discussions

### WSe₂-based device's micro-fabrication

Figure 1 highlights different characterizations performed on WSe₂ deposited on a Si/SiO₂ commercial wafer demonstrating its excellent physical properties. Figure 1(a) shows an optical image of mechanically exfoliated WSe₂ flakes from a bulk p-doped crystal over a SiO₂ (280 nm)/Si wafer. The thickness of the SiO₂ is chosen to maximize the contrast between WSe₂ sheets of different thicknesses down to a monolayer [37, 38]. Then we performed atomic force microscopy (AFM) in the tapping mode to determine their thickness and their surface topography, as shown in figure 1(b). The lateral profile plotted in figure 1(c) is obtained along a single line cut shown on the AFM image. The thickness of the WSe₂ is identified in this particular case to be 3.5 nm, which corresponds to a 5-layers film. The surface roughness of WSe₂ is of the same order of magnitude of that of SiO₂ (i.e. <1 nm) emphasizing the excellent surface quality. Some recent methods of fabrication, for instance by encapsulating WSe₂ between relatively thick BN sheets, could help reducing this surface roughness down to the angstrom level [39–41]. We also performed Raman spectroscopy measurements to get a better insight in the electronic properties [42, 43]. On the Raman spectra shown in figure 1(d), a large resonance can be seen around 255 cm⁻¹. This peak corresponds to the labelled LA(M) peak containing both the A₁g and the E₂g transitions [44]. The A₁g peak is very sharp (<5 cm⁻¹) and is located around 251 cm⁻¹ and the E₂g peak is centered on 248 cm⁻¹, the resolution of our measurements does not allow to fully split them. The position of the different Raman transition's peaks confirm the material's high quality and its few-layered character [45]. A photoluminescence spectra of a multilayer WSe₂ flake is also shown in figure 1(e). It provides the energy of the direct band gap (1.6 eV) and the energy of the indirect band gap (1.4 eV) which are in perfect agreement with previous measurements [46, 47]. The field-effect device is finally fabricated by defining electrical contacts by e-beam lithography as described in the methods section. Figure 2 shows an AFM image of a final device made of few layered WSe₂ of typical lateral sizes of 10 µm. The metallic contacts serve as source and drain, while the gate voltage is applied to the Si back-gate. Both Ti and Pd based devices were fabricated over a 280 nm thick SiO₂ and the Co based device was fabricated over a 100 nm thick SiO₂ in order to get a larger electrostatic coupling. In all the studied devices, the WSe₂ flakes are composed of few layers with thicknesses around 3.5–4 nm.

### Influence of the contacts on charge injection in WSe₂

Figure 3(a) shows the WSe₂ band gap indicating the positions of the top of the valence band (E_v) and of the bottom of the conduction band (E_c) [48] and the work functions of the metals used as electrical contacts in this study [49]. By comparing the relative positions of the different work functions to the position of the valence band maximum, it is expected that Co and Pd greatly favor holes injection and transport. We now focus on the two-terminal I<sub>SD</sub>–V<sub>SD</sub> characteristics measured at V<sub>G</sub> = 0 V and at room temperature for Ti (black dots), Co (red
<figure><img src="image_7.png" /><figcaption>**Figure 1.** (a) Optical microscope image of multiple WSe₂ flakes exfoliated over a Si/SiO₂ (280 nm) substrate. (b) Atomic force microscope image obtained in the tapping mode of an individual WSe₂ flake. (c) Profile obtained along the line cut indicated in blue over the WSe₂ flake. The thickness of the flake is measured to be 3.5 nm. (d) Raman spectra obtained on the WSe₂ flake (λ = 633 nm, P = 2 mW). Resonances attributed to A₁g, E₂g and 2LA are identified by red, green and yellow lines respectively. Unassigned resonances are also marked (they are also reported in [44]). (e) Photoluminescence spectra (P = 0.1 mW and λexc = 514 nm) of a multilayer WSe₂ flake deposited on a SiO₂ substrate. The indirect band gap peak is centered on 1.4 eV and the direct band gap peak is centered on 1.6 eV.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 2.** Three-dimensional atomic force image of a final device with four Ti (5 nm)/Au (95) electrodes. The wiring of the device is also schematized with the two different voltage sources (source-drain voltage V<sub>SD</sub> and gate voltage V<sub>G</sub>).</figcaption></figure>

dots) and Pd (blue dots) contacts plotted in figure 3(b). Because of this specific configuration, the measured signal can include contributions of the contact resistances and the channel resistance. As reported, for Ti contacts, the current level is very weak reaching a few hundreds of pA, while a current up to 20 nA is observed for the device with Co contacts at V<sub>SD</sub> > 2 V. On the other hand, the blue curve on figure 3(b) shows for the Pd contacts a current two orders of magnitude higher than that for the devices with Co contacts. These measurements illustrate also asymmetric and non-linear characteristics at high bias voltages (>0.2 V). As illustrated in figures 4(a)–(f), we also observe that the current remains rather weak around V<sub>SD</sub> = 0 V for Co contacts in contrast to Pd contacts at any explored temperature. As an example, at 300 K and for V<sub>SD</sub> = 0.2 V, I<sub>SD</sub> is ~0.5 nA for Co contacts and ~31 nA for Pd contacts as extracted from figure 3(b). Zooms at low bias voltages (<0.2 V) are also plotted for Pd contacts in figure S1 is available online at stacks.iop.org/MRX/6/126307/mmedia highlighting the linear variation of I<sub>SD</sub> with respect to V<sub>SD</sub>. For both devices based on Pd and Co contacts, I<sub>SD</sub> increases drastically for |V<sub>SD</sub>| > 2 V when applying negative gate voltages. The behavior of devices based on Pd and Co contacts are in agreement with expectations in the case of holes injection [32]. This is also confirmed by the two I<sub>SD</sub>-V<sub>G</sub> characteristics (in figure 3(c)) obtained at 3 K shown for Co contacts (in red) and Pd contacts (in blue) based devices. Interestingly, we clearly notice the valence band edge for Co contacts at V<sub>G</sub> = -8 V whereas it is very diffuse for the Pd contacts.
<figure><img src="image_9.png" /><figcaption>**Figure 3.** (a) Band alignment with respect to the vacuum level of the bottom of the conduction band (orange line at 3.5 eV) and top of the valence band (blue line at 4.9 eV) for WSe₂ and the Fermi energy positions for Ti (4.3 eV), Co (5 eV) and Pd (5.4 eV) [48, 49]. (b) Room temperature *I*<sub>SD</sub>–*V*<sub>SD</sub> characteristics obtained at zero gate voltage on devices with Ti (black), Co (red) and Pd (blue) contacts over a 280 nm SiO<sub>2</sub> substrate. (c) *I*<sub>SD</sub>–*V*<sub>G</sub> characteristics measured for *V*<sub>SD</sub> = –2 V at 3 K for Co contacts (red) and Pd contacts (blue).</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 4.** *I*<sub>SD</sub>–*V*<sub>SD</sub> characteristics obtained at different gate voltages on a device with Pd (and Co) contacts over a 280 nm (100 nm) SiO<sub>2</sub> substrate for temperatures of a, (d) 200 K (up), b, (e) 100 K (middle) and c, (f) 3 K (down).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Figure 5. (a) $I_{SD}-V_{SD}$ characteristics in semi-log scale measured at zero gate voltage at four different temperatures ranging from 3 K to 295 K for the Co-based device. (b)–(e) $\ln(I_{SD}/T^{3/2}) = f(1000/T)$ plots performed at $V_G = 0$ V, $-4$ V, $-8$ V and $-12$ V respectively. (f) Evolution of the injection barrier height for holes ($\phi_B$) as a function of the gate voltage ($V_G$).</figcaption></figure>

To summarize this part, in devices with Pd contacts a stronger current (i.e. more transparent contacts) is observed with respect to devices with Co or Ti contacts. This can be understood by looking at the band diagram presented in figure 3(a). The Ti Fermi level falls deep in the band gap of $WSe_2$, which indeed creates a large injection barrier, while the Co and Pd Fermi levels fall deep inside the valence band of the $WSe_2$, strongly promoting hole injection.

## Electrostatic gating effect on the injection barrier height

To investigate in more details the charge injection properties, we present in figures 5(a) and 6(a) the $I_{SD}-V_{SD}$ characteristics measured at different temperatures ranging from room temperature to 50 K at $V_G = 0$ V for the Co-based device and the Pd-based device respectively in a two-terminal configuration. Given the poor transparency of the Ti contacts, we discharge these devices from the following analysis. The plots reveal a general decrease of the current when decreasing the temperature, meaning that the charge transport process is thermally activated. From figure 5(a) we remark that the symmetry of the Co two-terminal $I_{SD}-V_{SD}$ characteristics plot is preserved at all temperatures. For the Pd contacts, the symmetry is broken at room temperature as shown in figure 6(a), due to the presence of a negative differential resistance (NDR) signature at negative polarization. NDR has already been reported in $WSe_2$-based FET with Pt contacts [35], even if not systematically. It has been shown to be strongly gate voltage dependent and it has been interpreted as the effect on charge transport of the $V_G$-induced change of the vertical carrier distribution across the different layers composing the connected $WSe_2$ flake.

In order to get a deeper understanding of the charge injection process with Co and Pd contacts, we perform a full temperature study. Figures 5(b) to (e) and 6(b) to (e) present the temperature and the gate dependence of the current ($I_{sd}$) applied to the device through Co and Pd contacts respectively for different applied voltage bias ($V_{sd} > 0.5$ V).

A quantitative description of the injection properties in ultrathin 2D materials is given by a 2D thermionic emission equation [50], where the expression of $I_{SD}$ employs a reduced power-law $T^{3/2}$:

$$I_{SD} = A_{SD}^* ST^{3/2} \exp \left[ -\frac{1}{k_B T} \left( \varphi_B - q \frac{V_{SD}}{n} \right) \right],$$

where $A_{SD}^*$ is the 2D equivalent Richardson constant, $q$ the elementary charge, $S$ the contact area, $T$ the temperature, $k_B$ the Boltzmann constant and $n$ is the ideality factor. This model has been developed for symmetric metal/2D semiconductor/metal devices and is commonly employed, when the device is sufficiently polarized, for characterizing only one over two injection barriers in FET devices [30] based on a TMD [51–53] channel. The model is also used even in the limit of very transparent metal/TMD or metal/h-BN/graphene contacts [54, 55], where a substantial part of the potential drop could occur within the semiconducting channel itself. Reference [30] fully details the validity of this method. In accordance with this method, we restrain this
<figure><img src="image_12.png" /><figcaption>Figure 6. (a) $I_{SD}-V_{SD}$ characteristics in ln scale measured at zero gate voltage at four different temperatures ranging from 50 K to 295 K for the Pd-based device. (b), (c), (d) and (e) $\ln(I_{SD}/T^{3/2}) = f(1000/T)$ plots performed at $V_G = 0$ V, $-11$ V, $-22.5$ V and $-32$ V respectively. (f) Evolution of the injection barrier height for holes ($\phi_B$) as a function of the gate voltage ($V_G$).</figcaption></figure>

study to $V_{SD} > 0.5$ V in order to ensure that only one barrier mainly contribute to the measured signal as it is discussed in [30]. To determine the height of the injection barrier ($\phi_B$) as a function of $V_G$, Richardson plots of $\ln(I_{SD}/T^{3/2})$ against $1000/T$ at different applied $V_G$ for both Co and Pd based devices are shown in figures 5 and 6, from b to e, respectively. For each value of $V_{SD}$ we realize a linear fit in the high temperature regime (between 300 K and 100 K for the Co based devices and between 200 K and 50 K for the Pd based devices). We exclude the room temperature measurement for the Pd-based device where the NDR effect dominates. The slope extracted from the linear fit is given by:

$$a = -\frac{1}{1000k_B}\left(\phi_B - q\frac{V_{SD}}{n}\right)$$

By plotting the slopes as a function of $V_{SD}$, we can extract the injection barrier height from the y-intercept as [56]:

$$a_0 = -\frac{1}{1000k_B}(\phi_B).$$

The extracted injection barrier heights with their standard deviations are plotted as a function of the applied gate voltage $V_G$ and on figures 5(f) and 6(f) for the Co and Pd based devices respectively. The barrier height decreases from 90 meV down to 20 meV in the Co-based device when the gate voltage is lowered from 0 V to -32 V. In the Pd-based device, the barrier height is already lower at low gate voltages and remains stable around 40 meV at negative gate voltages. The extracted values of the injection barrier at zero gate voltage are plotted and compared to the literature for different contact materials in figure 7. The comparison with experimental data reported at zero gate voltage by Kim et al [57] reveals a large discrepancy but those data have been measured with chemical vapor deposited WSe$_2$ for which the intrinsic doping might be very different than after exfoliation from a bulk crystal. Note that a perfect agreement exists between our observations and the theoretical predictions made by Wang et al [31] for the Pd contacts based devices, including the influence of spin-orbit coupling which tends to shift the valence band closer to the metal's Fermi energy. The Schottky-Mott rule ($\phi_B = |WF - 4.9|$) is also plotted as a reference.

## Discussion

The method we employed to extract the injection barrier in two-terminal 2D material based devices is standardly used in the literature [58]. Three distinct injection regimes can usually be identified depending on the gate voltage $V_G$. A thermionic emission regime is associated to a linear decrease of $\phi_B$ as a function of $V_G$, a tunneling regime is associated to an almost stable $\phi_B$ as a function of $V_G$ and, in between, a crossover regime is ascribed as
<figure><img src="image_13.png" /><figcaption>**Figure 7.** Holes injection barrier $\phi_B$ (in meV) as a function of work function (in eV) for different materials. Calculated hole injection barriers (purple dots from Wang *et al* [31] and pink triangles from Ouyang *et al* [25]) and measured hole injection barriers (red dots from Prakash *et al* [28], light pink triangles from Kim *et al* [57] and the pink diamonds from this work) are represented. The standard Schottky-Mott rule is plotted (red dotted line) for reference.</figcaption></figure>

the flat band limit for which both thermionic and tunneling injection can occur simultaneously [32, 59]. The intrinsic Schottky barrier height can only be determined at the flat band limit [30]. For the case of the Co contacts based device, we observe a decreasing behavior of the injection barrier while increasing the gate voltages towards negative values, as shown in figure 5(f). This is expected since, based on the energy level alignments, Co contacts are supposed to promote holes injection in WSe₂. In the studied experimental conditions, this injection barrier decreases down to 20 meV at $V_G = -30$ V. Note that no change of linear regime is visible in figure 5(f), meaning that the flat band limit is not yet reached. As a consequence, a value of the order of 20 meV can be considered as an upper limit of the intrinsic Schottky barrier for the Co contacts based device. As shown in figure 6(f), the behavior of $\phi_B$ as a function of $V_G$ for the Pd contacts based device is quite different. We do not report any dependence of the injection barrier as a function of the gate voltage. Note that given the difference of SiO₂ thickness for the two devices (280 nm thick insulating SiO₂ for the Pd contacts based device and 100 nm thick insulating SiO₂ for the Co contacts based devices), a different coupling to the gate electrode is realized in the two cases for the same applied gate voltage, inducing a less effective field effect in the Pd case. In the meanwhile, the barrier height shows an almost constant behavior, within the error bars, that could suggest that the injection process occurs already by tunneling. However, the strong temperature dependence of the current observed in a large range of $V_{sd}$ values (0.1 V < $V_{sd}$ < 1 V) as shown in figures 5(a) and 6(a) is not in favor of this argument. We suggest that the observed invariance of the barrier height could be a specific signature of Fermi level pinning [60] occurring at the Pd/WSe₂ interface. Very weak variations of the injection barrier height with respect to gate voltage were also reported in MoS₂/Au and MoTe₂/Au interfaces where Fermi level pinning have been proved to occur [24]. Several physical mechanisms have been proposed to explain the pinning phenomena at metal/TMD interfaces. Sotthewes and coworkers have put forward the role of metal-induced gap states and of disorder [60]. Gong and coworkers have identified two other contributions in the specific cases of metal/MoS₂ interfaces but that could be potentially transposed to the metal/WSe₂ case: interfacial dipoles and gap states due to the metal/S interaction [61]. Concerning the WSe₂/metal interface, a recent study detailing the chemistry of the Pd/WSe₂ interface demonstrates that WSe₂ remains chemically unperturbed after Pd deposition at room temperature [51]. However, from the electronic structure's point of view, interfacial states dubbed overlap states were predicted by density functional theory to be present at the Fermi energy [31]. They could thus be incriminated in the Fermi level pinning phenomena. Being located at the Fermi energy such hybridized interfacial states can also promote holes injection in WSe₂ by efficiently coupling metallic bands with holes states from the WSe₂. This represents an interesting feature for high performance charge injection in WSe₂. Note that the Fermi level pinning phenomenon is strongly supported by the transfer characteristics of the device with Pd contacts shown in figure 3(c) for which the current varies only weakly with respect to $V_G$ in opposition to the device with Co contacts (unpinned case). An inverted band bending with respect to conventional semiconductor/metal contacts was also predicted at WSe₂/Pd interfaces [31]. Therefore, in the studied Pd devices, one could consider that an unconventional mechanism for the injection occurs. Our results presented in figure 6(f) could be a signature of all those theoretical predictions for transparent contacts [31]. This conclusion is also strongly supported by the linear $I_{SD}-V_{SD}$ characteristics measured at low bias voltages
($<$0.2 V) in the Pd-based devices recalling a quasi-ohmic behavior (see figure S1) as reported for Co/h-BN/MoS₂ contacts [54] or NbₓW₁₋ₓSe₂/WSe₂ contacts [62].

## Concluding remarks

In conclusion, we have characterized holes injection in WSe₂ from two different metals, a metal with strong spin-orbit coupling (Pd) and a ferromagnetic metal (Co). We demonstrated that Pd guarantees very transparent contacts and efficient holes injections in WSe₂ even when no gate voltage is applied, with an almost constant injection barrier and higher current values as a function of the source-drain and the gate voltages. One possible explanation is related to the presence of interfacial states coupling metallic and holes bands from the Pd and the WSe₂ respectively and inducing Fermi level pinning. On the contrary we also showed that Co/WSe₂ contacts can be strongly tuned by applying a back-gate voltage, shifting the injection barrier height down to 20 meV. These results pave the ways for further studies concerning the fundamental aspect of charge and spin injection in WSe₂ in the promising holes transport regime for probing the strong spin-orbit coupling physics.

## Materials and methods

### Sample preparation

WSe₂ is purchased from HQ Graphene. Bulk crystals are mechanically exfoliated and flakes are deposited on a Si/SiO₂ substrate (100 nm or 280 nm thick) to maximize the optical contrast. The devices are fabricated by defining electrical contacts by e-beam lithography using a double layered PMMA process: a 50 K lower layer of PMMA and a 950 K upper layer of PMMA both spin-coated at 4000 RPM. The metallic contacts are deposited under high vacuum (a few $10^{-7}$ mBar) by e-beam evaporation. They are deposited at very low evaporation rates (0.1 nm s⁻¹). The thicknesses are 5 nm of Ti covered by 95 nm of Au for the device with Ti contacts, and 20 nm of Co and Pd covered with 30 nm of Au for samples with Co contacts and Pd contacts respectively. Scanning electron microscope images of Co (2 nm) deposited on WSe₂ and Pd (2 nm) deposited on WSe₂ are shown in figures S2 and S3. They reveal that the metal film is already continuous at 2 nm of nominal thicknesses. In all the studied devices, the WSe₂ flakes are composed of few layers leading to thicknesses around 3.5–4 nm. In a separate device with Pd contacts supported on h-BN, the field-effect mobility has been measured to be around 21 cm²/V·s at 300 K and for a charge density around 1.10¹³ cm⁻² (see figure S4).

### Devices measurements

All the devices are measured with a Cryostation from Montana Instruments. All the measurements are performed in dark conditions and under vacuum. Current-voltage measurements are realized by applying to the device a DC voltage from a GS200 from Yokogawa. After passing through the sample, the output DC current is amplified by an I-V converter. The DC output is measured by a digital voltmeter (34401A from Keysight). The gate voltage is a DC voltage and is applied from a GS200 from Yokogawa.

## Acknowledgments

We acknowledge C. Manquest and P. Filloux for technical supports within the clean-room of the Laboratoire Matériaux et Phénomènes Quantiques (UMR 7162) at the Université de Paris. This work is supported by the 2DSPIN project from the ‘Ville de Paris’ Emergence program. This work has been also supported by the Region Ile-de-France in the framework of DIM Nano-K (SMS project).

## Conflict of interest

The authors declare that there are no competing interests.

## Author contribution

JR, ST and CB performed the experiments, analyzed the data and wrote the manuscript. S S, M L D R and P M participated to the development of the fabrication process. All authors contributed to the interpretation of the data.
ORCID iDs

Jacko Rastikian https://orcid.org/0000-0003-0764-7045

Stéphan Suffit https://orcid.org/0000-0002-7979-5138

Salvatore Timpa https://orcid.org/0000-0001-5133-9707

Pascal Martin https://orcid.org/0000-0003-1010-8421

Philippe Lafarge https://orcid.org/0000-0002-5107-1058

Maria Luisa Della Rocca https://orcid.org/0000-0002-9801

Clément Barraud https://orcid.org/0000-0002-5675-9927

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