Two-dimensional (2D) semiconductors have attracted considerable attention for the development of ultra-thin body transistors. However, the polarity control of 2D transistors and the achievement of complementary logic functions remain critical challenges. Here, we report a doping-free strategy to modulate the polarity of WSe2 transistors using same contact metal but different integration methods. By applying low-energy van der Waals integration of Au electrodes, we observed robust and optimized p-type transistor behavior, which is in great contrast to the transistors fabricated on the same WSe2 flake using conventional deposited Au contacts with pronounced n-type characteristics. With the ability to switch majority carrier type and to achieve optimized contact for both electrons and holes, a doping-free logic inverter is demonstrated with higher voltage gain of 340, at the bias voltage of 5.5 V. Furthermore, the simple polarity control strategy is extended for realizing more complex logic functions such as NAND and NOR.
Two-dimensional (2D) semiconductors have attracted considerable attention as ultrathin channel materials for transistors1,2,3,4,5. Their atomically thin body and dangling-bond free surface offer significant potential for ultimate transistor scaling (down to atomic thin-body thickness), which is essential for decreasing off-state power consumption and further extending Moore’s Law6. To date, one major challenge of a 2D transistor is the uncontrollable device polarity (n- or p-type) and majority carrier type, posing a key limitation for realizing complementary metal oxide semiconductor (CMOS) logic function in 2D transistors. In modern silicon microelectronics, the doping concentration of the silicon channel and transistor polarity are achieved by introducing extrinsic (e.g., B for p-type and As for n-type) dopants through high-energy ion implantation and subsequently high-temperature activation7,8. However, applying existing state-of-the-art ion-implantation approaches to a 2D semiconductor is not straightforward because there is little physical space for impurity dopants in such atomically thin lattice9. Hence, the majority carrier type of a typical 2D transistor is limited to its intrinsic properties and is largely fixed once exfoliated or synthesized.
Considerable efforts have been devoted to realize 2D CMOS functions in the past few years10,11,12,13. Early attempts focused on using two different 2D semiconductors, where one material is used for the NMOS (e.g., MoS2 and MoSe2) and a different material is used for PMOS (e.g., black phosphorus, WSe2)14,15,16. Although demonstrating desired logic functions, this method is still relied on the uncontrollable intrinsic doping, and is not obviously compatible with CMOS technology since it involves two materials with distinct synthesizing and processing conditions. Alternatively, the selective doping of 2D semiconductors can be achieved through a gentle chemical surface absorption with charge transfer process between 2D semiconductors and adsorbate, which could effectively modulate 2D carrier concentration and their majority carrier type (electrons or holes). For example, polyethyleneimine or benzyl viologen molecule was employed to achieve n-type doping in multilayer MoS2 (refs. 17,18), and the chloride molecule was applied to increase electron-doping density of WS2 and MoS2 (ref. 19). However, such chemical absorption approaches typically suffer from poor stability due to the weak interaction between the surface dopants and 2D materials. Recently, the CMOS logic functions are also demonstrated in 2D channels (e.g., WSe2 and MoTe2) using metals with different work function. For example, Ag and Pt have been applied as the contact metal of WSe2 to achieve the NMOS and PMOS, respectively, and similarly, Ti and Pt are integrated in MoTe2 flake to realize CMOS inverter20,21. However, due to the strong Fermi level pinning effect, large Schottky barrier is typically observed in 2D/metal interfaces, regardless of the metal work function used22,23,24,25. Therefore, using this approach, it is difficult to achieve optimized device performance in both p- and n-type devices at the same time. In addition, the use of asymmetric contact metals could further complicate the fabrication processes.
Here, we report a doping-free strategy to achieve CMOS circuit functions by using the same contact metal gold (Au) and the same channel material WSe2, but different metal integration methods. By applying low-energy van der Waals (vdW) integration of Au electrode, we observed a robust and consistent p-type behavior in multilayer WSe2. This is in great contrast to the transistors fabricated on the same WSe2 flake using conventional deposited Au contacts, where pronounced n-type characteristic is always observed26,27. To further gain insight of this phenomenon, we conducted detailed analysis through thickness-dependent measurement and density functional theory (DFT) simulation, and attributed the polarity change of WSe2 to the controllable Fermi level pinning effect using different metal integration methods. With the ability to control the polarity of WSe2 transistors and to achieve optimized contact to both PMOS and NMOS using the same metal, a logic inverter is demonstrated with the highest voltage gain of 340 (at a bias voltage of 5.5 V) and total noise margin over 90%. Furthermore, the polarity-controllable strategy is also extended to realize more complex logic functions such as NAND and NOR. Our results not only demonstrate robust and high-performance CMOS logic circuit using vdW metal electrodes, but also provide a doping-free method to control the polarity of a 2D semiconductor using the same contact metal, shedding light to high-performance 2D electronics and CMOS design.
Fabrication processes and electrical measurement
Figure 1a–f schematically illustrates our device structure. To fabricate the device, multilayer WSe2 flakes with various thicknesses are first mechanically exfoliated onto a heavily doped silicon substrate (as gate) with 300-nm silicon oxide (as gate dielectric). Next, 50-nm Au electrode pair is pre-fabricated on a sacrificial Si wafer and then mechanically released using a previously developed method26 (see “Methods” section for fabrication details). The released metal electrodes are aligned under a microscope and physically laminated on top of the WSe2 flake using a vdW metal integration process, resulting in an atomically sharp and clean Au/WSe2 interface26,27 (Fig. 1a–d). For comparison, another pair of Au electrode with the same thickness (50-nm thick) is also deposited on the same WSe2 flake using conventional electron beam lithography followed by high vacuum thermal deposition, resulting in the nonideal metal/semiconductor interfaces with diffusion, defects, chemical bonding, and strains, as have been demonstrated previously26,27 and schematically illustrated in Fig. 1e, f. The optical image of a typical fabricated device is shown Fig. 1g, where the left electrode pair is fabricated through thermal evaporation (highlighted by a black box) and the right electrode pair (red box) is vdW integrated. Electrical transport studies of the resulting devices were carried out at room temperature in a probe station under vacuum condition (3 × 10−5 Torr). As shown in Fig. 1h, a typical device (~7-nm thick) contacted with vdW metal electrodes shows p-type Ids−Vgs transfer characteristic, consistent with band alignment of WSe2 with high work function Au, suggesting the optimized Au/WSe2 interface using the vdW metal integration approach28,29,30. In contrast, without applying any doping process, n-type Ids−Vgs transfer characteristic is observed in the control device (fabricated on the same WSe2 flake) using conventional deposited Au contacts (Fig. 1i). The observed polarity change indicates the strong Fermi level pinning effect within evaporated Au/WSe2 interfaces, where the pinned Fermi level position is close to the conduction band of WSe2. Furthermore, the two-terminal FET mobility μ can be further extracted using equation μ = [dIds/dVgs] × [L/(WCVds)], where L/W is the ratio between channel length and width (shown in Fig. 1g), C is the back-gate capacitance (1.15 × 10−8 F cm−2, 300-nm-thick SiO2). The extracted hole and electron mobility in this device are 16 and 11 cm2 V−1 s−1, respectively. In addition, the contact resistance (Rc) and Schottky barrier height (SBH) of both p- and n-type transistors can also be extracted using the transfer line method and temperature-dependent measurement, where Rc and SBH are measured to be 14 kΩ μm, 50 meV for PMOS and 17 kΩ μm, 60 meV for NMOS, respectively, as shown in Supplementary Fig. 1. The balanced μ, Rc, and SBH between electrons and holes are important for the demonstration of high-performance CMOS circuit described below.
Thickness-dependent electrical measurement
To further confirm the robustness of this behavior and investigate the polarity control by using different metal integration processes, we have conducted detailed electrical measurement based on WSe2 of various thicknesses. As shown in Fig. 2a–c, the device contacted with vdW metal electrodes shows clearly p-type behavior, regardless of the WSe2 thickness, in consistent with the band alignment between WSe2 valance band (5.02–4.83 eV from monolayer to bulk) and high work function Au (5.24 eV). In great contrast, the control devices (contacted with conventional evaporated Au electrodes) display a unique polarity change behavior with increasing WSe2 thickness, demonstrating p-type characteristic with thickness <5 layers (~3 nm), a bipolar characteristic with 7 layers thick (~4.5 nm), and pronounced n-type property with thickness greater than 10 layers (~6.5 nm), as shown in Fig. 2d–f. The corresponding on–off ratio and mobility of these transistors (in Fig. 2a–f) and monolayer WSe2 transistor data (using both metal integration processes) are also plotted in Supplementary Fig. 2.
Furthermore, to confirm the robustness of this behavior and to quantitively analyze the polarity change, we have measured over 20 devices and extracted the current ratio between I−50V (Ids at Vg = −50 V) and I50V (Ids at Vg = 50 V) as a function of WSe2 thickness. The I−50V/I50V ratio here could represent the ratio between hole and electron contribution in a given transistor, and thus could quantitively demonstrate the transistor polarity and majority carrier type. For devices with vdW-contacted electrode (Fig. 2g, red dot), the I−50V/I50V ratio over 103 is consistently observed from monolayer to 30-nm- (~50 layers) thick devices, suggesting a dominated p-type behavior (with negligible electron current) regardless of body thickness. In contrast, for devices with conventional evaporated electrodes, the I−50V/I50V ratio decreased exponentially from 104 to ~10−3 (7 orders of magnitudes) with increasing body thickness from monolayer to ~50 layers, demonstrating that the majority carrier type can be progressively transformed from holes to electrons via increasing the thickness of WSe2. The slightly increased I−50V/I50V ratio for evaporated contacts (with thickness >13 nm, black line of Fig. 2g) could be attributed to the increased vertical resistance (under contact region) with increasing body thickness.
Moreover, the devices integrated by both vdW and evaporated electrodes are very stable, which can exhibit the original device polarity after 4 months of storage at room temperature in ambient atmosphere (Supplementary Fig. 3), further suggesting the stability of our doping-free approaches25,26. We also note that the unique device polarity control technique reported here is not only limited to WSe2 and Au metal, but could be extended to other 2D semiconductor–metal systems by using different contact integration processes to pin (using evaporated contact) or de-pin (using vdW contact) the Fermi level, as demonstrated in a MoS2–Pt system in Supplementary Fig. 4.
To further understand the mechanism of polarity control by using different metal integration approaches, and to gain insight into the thickness dependent on PMOS to NMOS transition, we have carried out DFT simulation of carrier transport across the metal/WSe2 interfaces. First, we constructed two types of Au/WSe2 interface models, a close-contact model corresponding to the evaporated Au interface and a non-close contact corresponding to the vdW-integrated Au interface. For the close-contact model, an interlayer distance of 1.5 Å (covalent radius of Au and Se) was chosen between metal and WSe2, under which the Au and the Se atoms are covalently bonded. For the non-close-contact model, an interlayer distance of 3.3 Å was used, which included an additional vdW-gap distance of 1.8 Å on the base of close-contact interlayer distance, consistent with previous reports9. Based on this model, there are three interfaces that may contribute to the transport barrier: Au and the first layer WSe2 (interface I), WSe2 under the contact and inside the channel region (interface II), as well as the first layer WSe2 and the rest of the WSe2 layers (interface III), as illustrated in Fig. 3a, b.
For the non-close contact, as the Au/WSe2 interlayer distance is large enough and their interlayer interaction is weak, Au electrode has little influence on the properties of WSe2. As shown in Supplementary Fig. 5, there are negligible interfacial gap states in WSe2, and the whole multilayer WSe2 maintains its intrinsic properties, leading to Ohmic contacts at interfaces II and III. Therefore, the contact Schottky barrier only exists in interface I, regardless of the thickness of WSe2 used. Figure 3c illustrates the calculated band structures of WSe2 under vdW Au contact, which is nearly the same with that of freestanding WSe2 (Supplementary Fig. 6), further indicating the weak interaction between Au electrode and the underlying WSe2. The calculated results of SBH are shown in Fig. 3d with dominating p-type Schottky barrier, consistent with observed p-type transistor behavior using vdW Au contact (Fig. 2a–c).
In great contrast, for evaporated Au with the close-contact model, chemical interaction exists between Au electrode and WSe2, which strongly perturbs the electrical properties of WSe2. As shown in Supplementary Fig. 7, a large number of interfacial states are generated in the forbidden band of WSe2, resulting in the disappearance of the WSe2 bandgap. Therefore, as demonstrated in Fig. 3b, the first layer of WSe2 is metalized under the contact (with a new work function ~4.83 eV), leading to an Ohmic contact at interface I. Meanwhile, Schottky barrier is generated at interfaces II and III during charge transport from metalized WSe2 (under contact) to semiconducting WSe2. For monolayer WSe2, the lateral Schottky barrier at interface II is p-type with a barrier height of 0.19 eV, as revealed by the calculated band alignments (Supplementary Fig. 8). On the other hand, for multilayer WSe2, the first underlying WSe2 is metalized, but the rest of the underlying layers remains largely intrinsic (Supplementary Fig. 9), and consequently the effect of Schottky barrier at interface III is more and more pronounced. As shown in Fig. 3e, f, the calculated vertical Schottky barriers at interface III are p-type when Au electrode contacts with 3-layer and 5-layer WSe2, and gradually switched to n-type with 7-layer and 9-layer WSe2, consistent with our measurement results in Fig. 2d–g. Figure 3f demonstrates the variation of SBH at interface III with layer number, with a detailed mechanism in Supplementary Fig. 10.
WSe2-based CMOS logic functions
The ability to control the transistor polarity can readily allow us to integrate multiple WSe2 transistors into functional circuits. For example, a complementary logic inverter can be achieved by connecting two WSe2 transistors in series, where one device is connected with deposited Au electrodes as n-type transistor and the other is contacted by vdW electrodes as a p-type device. The logic diagram and optical image of the inverter are shown in Fig. 4a, where the metal integration processes (both evaporation and vdW integrated) are the same as previous devices in Fig. 1, except that the back-gate dielectric is changed from 300-nm-thick SiO2 to 20-nm-thick Al2O3 to enhance the gate capacitance and electrostatic control over the channel, which is essential to reduce the inverter input voltage and to increase the voltage gain. The detailed inverter fabrication process is shown in the “Methods” section and Supplementary Fig. 11.
Figure 4b shows the voltage transfer characteristics of the resulting inverter as a function of input voltage with bias voltage (Vdd) from 1.5 to 5.5 V, demonstrating sharp voltage transition with input voltage. The resulted voltage gain is plotted in Fig. 4c with a peak value of 340 at Vdd = 5.5 V. To the best of our knowledge, the voltage gain reported here represents the highest value for TMD-based inverter, as shown in the comparison with previous literatures in Supplementary Table 1. Further increasing the Vdd leads to much increased gate leakage current, and degrades the overall device performance. The much higher voltage gain achieved here could be largely attributed to the optimized contact for both PMOS and NMOS by controlling their Fermi level position, which is intrinsically different compared with previous methods by evaporating metals with different work functions, where optimized contact to both PMOS and NMOS is hard to realize due to strong Fermi level pinning effect at metal/2D interfaces22,23,24,25. To characterize the robustness of an inverter fabricated through different contact approaches, we have extracted the noise margins (NML and NMH), as shown in Fig. 4d. At the Vdd of 2.5 V, NML of 1.16 V and NMH of 1.19 V are extracted. In addition, we also plot the total noise margin [(NML + NMH)/Vdd] as a function of Vdd from 1.5 to 5.5 V (Fig. 4e). The measured total noise margin of the inverter is greater than 90% at various bias voltages, indicating the high tolerance to noise. Furthermore, the static peak energy consumption of the corresponding inverter is also plotted in Supplementary Fig. 12.
Taking a step further, more complicated logic functions could be achieved by connecting more WSe2 transistors together. For example, a logic NOR or NAND function can be created using four multilayer WSe2 transistors, with two transistors using vdW Au contacts (p-type) and other two using evaporated Au contacts (n-type), as shown in the circuit diagram in Fig. 4f. The measured input and output voltages clearly demonstrate the desired logic function for the NOR and NAND (Fig. 4g, h), suggesting its potential for a more complex circuit.
In summary, we have demonstrated a doping-free strategy to control the polarity of 2D transistors using the same contact metal Au and the same channel material WSe2, but different metal integration methods. Through detailed thickness-dependent measurement and DFT calculation, we found that the unique polarity change could be attributed to the controllable Fermi level pinning (or de-pinning) effect using different metal integration methods. Furthermore, with optimized contact to both PMOS and NMOS, we demonstrate a logic inverter with the highest voltage gain of 340 (at Vdd of 5.5 V) and the total noise margin over 90%, as well as more complex CMOS functions such as NAND and NOR. Our results not only demonstrate high-performance CMOS logic circuit, but also provide a method to control the polarity of a 2D semiconductor using the same contact metal, shedding light to high-performance 2D electronics and CMOS design.
Fabrication process of metal electrode for vdW integration
First, we prepared 50-nm-thick Au electrode arrays on sacrificial silicon substrate (with an atomically flat surface) using standard photolithography followed by thermal evaporation under vacuum (pressure ~5 × 10−4 Pa). After the lift-off process, the whole wafer was immersed in a sealed hexamethyldisilazane (HMDS) chamber to functionalize the surface of SiO2 at 80 °C. Next, the poly(methyl methacrylate) (PMMA A8, Mircochem Inc.) layer was spin-coated twice on the substrate with a speed of 3500 r.p.m. Finally, the 1-μm-thick PMMA layer with array Au electrodes is mechanically peeled and laminated to the target substrate via the mechanical aligner under an optical microscope26.
Inverter fabrication process
For fabricating logic inverter, we first prepared 10/50-nm-thick Ti/Au electrode onto an Si/SiO2 substrate as back-gate electrode. Next, the growth of a 20-nm-thick Al2O3 dielectric layer was employed through atomic layer deposition (ALD) on the gate electrode at the growth temperature of 150 °C. By contacting with vdW and evaporated electrode pairs, PMOS and NMOS devices can be achieved, as shown in Supplementary Fig. 11.
DFT computational methods
All the calculations were performed based on the DFT in conjunction with projector-augmented wave potentials, which is implemented in the Vienna ab initio Simulation Package (VASP)31,32. The generalized gradient approximation in the Perdew, Burke, and Ernzerhof (GGA–PBE) was used to describe the exchange and correlation potential33 as PBE bandgap is a good approximation for the transport gap in an FET, and accordingly the SBHs calculated by PBE are closer to the experimental value34,35. vdW interaction is taken into account by the DFT-D3 approach36, and the energy cutoff for plane waves was set at 450 eV. Geometry optimizations were terminated when the total energy and atomic force are less than 10−5 and 0.02 eV Å−1, respectively. A Monkhorst–Pack k-point mesh of 9 × 9 × 1 was used for the calculation of Au/WSe2 interfaces. To avoid the interaction effect of adjacent slabs, the thickness of vacuum region was set to no less than 15 Å. The \(\sqrt 3 \times \sqrt 3\) unit cell of WSe2 and 2 × 2 unit cell of Au (111) faces were constructed to match with each other. As the properties of WSe2 are hypersensitive to strain, we adjusted the Au lattice parameter to be commensurable to that of WSe2. The strains applied on Au in all the Au/WSe2 interface models are less than 1%. To model the Au surface, we used six layers of Au atoms. Considering that the interface has little impact on the bottom several layers of Au atoms, the bottom three layers of Au atoms were fixed.
Material characterization and electrical measurement
The electrical characteristic measurements were characterized in a Lakeshore PS-100 cryogenic probe station at room temperature in vacuum, using Keysight B2900A source measurement unit (SMU). Besides, for the CMOS logic functions, the voltage transfer characteristics were measured using an Agilent B1500A Semiconductor Parameter Analyzer.
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Y.L. acknowledges the financial support from National Natural Science Foundation of China (Grant Nos. 51991340, 51991341, 51802090, and 61874041), the Hunan Science Fund for Excellent Young Scholars (Grant No. 812019037), and from Huxiang high level talent program (Grant No. S2018RSCXRC0149). L.F. acknowledges the financial support from National Natural Science Foundation of China (Grant No. 11674265), from the Natural Science Basic Research Project of Shaanxi Province (Grant No. 2018JZ6003), and from Fundamental Research Funds of the Central Universities (3102019MS0402).
The authors declare no competing interests.
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Kong, L., Zhang, X., Tao, Q. et al. Doping-free complementary WSe2 circuit via van der Waals metal integration. Nat Commun 11, 1866 (2020). https://doi.org/10.1038/s41467-020-15776-x
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