Improving the device performances of two-dimensional semiconducting transition metal dichalcogenides: Three strategies

Abstract

Two-dimensional (2D) semiconductors are emerging as promising candidates for the next-generation nanoelectronics. As a type of unique channel materials, 2D semiconducting transition metal dichalcogenides (TMDCs), such as MoS₂ and WS₂, exhibit great potential for the state-of-the-art field-effect transistors owing to their atomically thin thicknesses, dangling-band free surfaces, and abundant band structures. Even so, the device performances of 2D semiconducting TMDCs are still failing to reach the theoretical values so far, which is attributed to the intrinsic defects, excessive doping, and daunting contacts between electrodes and channels. In this article, we review the up-to-date three strategies for improving the device performances of 2D semiconducting TMDCs: (i) the controllable synthesis of wafer-scale 2D semiconducting TMDCs single crystals to reduce the evolution of grain boundaries, (ii) the ingenious doping of 2D semiconducting TMDCs to modulate the band structures and suppress the impurity scatterings, and (iii) the optimization design of interfacial contacts between electrodes and channels to reduce the Schottky barrier heights and contact resistances. In the end, the challenges regarding the improvement of device performances of 2D semiconducting TMDCs are highlighted, and the further research directions are also proposed. We believe that this review is comprehensive and insightful for downscaling the electronic devices and extending the Moore’s law.

Graphical abstract

Keywords

2D semiconductor / transition metal dichalcogenides / wafer-scale single crystal / ingenious doping / interfacial contact / device performance

1 Introduction

Recent years have witnessed the great breakthrough of semiconductor technology due to the introduction of silicon transistors [1]. Following the Moore’s law, the number of transistors in chips is exponentially increasing and the device performances are improved significantly, in consideration of the continuous miniaturization of silicon-based process nodes [2,3]. However, the Moore’s law is becoming unsustainable due to the physical limits of bulk silicon materials, the short-channel effects and high heat dissipation phenomena are inevitably appeared, and further reducing the size of silicon-based field-effect transistors (FETs) is facing a major challenge [4]. In this context, two-dimensional (2D) materials, such as graphene [5,6], boron nitride [7,8], and transition metal dichalcogenides (TMDCs) [9,10], have attracted extensive attention due to their novel physical properties and excellent device performances, such as anomalous Hall effect [11], room temperature quantum Hall effect [12], ultrahigh carrier mobility [13], excellent thermal conductivity [14], and light transmission [15]. The sizes of FETs can be further reduced by replacing the silicon channels with 2D materials [16,17].

As a unique member of 2D materials, the semiconducting TMDCs have exhibited great potentials in the state-of-the-art FETs [18]. The layered structure allows the channel to be thinned to the monolayer and exhibits excellent electrostatic control capacity, and thus the operating voltage/current and energy consumption can be reduced effectively [19,20]. The smooth surface without dangling bond reduces the degradation of carrier mobility induced by the scattering [21,22]. The abundant band structure enables the design of novel logic and memory device [23,24]. For example, by using the exfoliated few-layer and monolayer MoS₂ nanosheets as the channels, the ultrahigh electron mobility (~34 000 cm²·V⁻¹·s⁻¹) [25] and excellent on/off current ratio (~10⁸) [26] was obtained, respectively. The ultralow contact resistance (~0.2 kΩ·μm) was achieved in monolayer MoS₂ transistor by constructing the 1T-2H MoS₂ lateral heterostructure [27]. By using a physically assembled silicon nanowire as the lift-off mask, the ultrashort channel device with few-layer MoS₂ channel was created and showed remarkable conduction current density (~0.83 mA·μm⁻¹) at room temperature [28]. The abovementioned results reveal that the 2D semiconducting TMDCs offer a promising playground for the construction of high performance electronic devices.

However, the device performances of 2D semiconducting TMDCs are still unsatisfactory, and which are limited by the following bottlenecks. First, the intrinsic defects (e.g., vacancies, grain boundaries, and impurities, etc.) in 2D TMDCs increase the carrier scattering sites and reduce the device performances [29-32]. Second, although the band structures of 2D TMDCs can be modulated and new physical phenomena can be introduced by means of a doping strategy [33-37], the excessive doping should bring in the deep impurity levels and reduce the device performances [38,39]. Third, the difference between the work function of metal electrodes and the electronic affinity energy of 2D semiconducting TMDCs results in high Schottky barrier height and contact resistance [40-43]. Therefore, the controlled synthesis of high-quality samples with few defects, the development of ingenious doping strategy, and the optimization design of interfacial contacts between electrodes and channels are crucial for improving the device performances of 2D semiconducting TMDCs.

This article hereby aims to provide an up-to-date summary or discussion on the aforementioned issues as described in Fig.1. Specifically, the chemical vapor deposition (CVD) synthesis of wafer-scale 2D semiconducting TMDCs single crystals will be introduced, and the internal mechanisms will be discussed. The accurate doping of 2D semiconducting TMDCs by the metal atoms and organic molecules will be reviewed. Furthermore, some new methods including the electrode transfer, the introduction of semimetal electrodes, and the construction of 2D metallic/semiconducting TMDCs heterostructures will also be summarized to optimize the interfacial contact.

Fig.1 Three strategies for improving the device performances of 2D semiconducting TMDCs.

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2 Controllable synthesis of wafer-scale 2D semiconducting TMDCs single crystals

The ultrahigh carrier mobility, excellent on/off current ratio, and distinguished subthreshold swing of 2D semiconducting TMDCs provide a wide application prospect in the high performance electronic devices [5,44-46]. However, the presence of grain boundaries in polycrystalline TMDCs films increases the carrier scattering sites and reduces the carrier mobilities and conductance [47-49]. Therefore, controllable synthesis of wafer-scale 2D semiconducting TMDCs single crystals is essential for the electronic device applications. Recently, fruitful achievements have been attained in growing 2D single crystals [50–56]. For example, inch-sized graphene single crystals were synthesized on Cu−Ni alloys by controlling the nucleation [50]. Through the seamless stitching of unidirectional domains, wafer-scale 2D single crystals were obtained, such as graphene on Ge(110) [51], h-BN on liquid Au substrate [54], vicinal Cu(110) [55], and Cu(111)/sapphire [56]. Notably, there are two distinct routes for synthesizing wafer-scale 2D single crystals: (i) the nucleation controlling: allowing only one nucleation site on a wafer-scale substrate, and then the single nucleus further grows to a wafer-scale single crystal; (ii) the orientation controlling: a substantial number of unidirectional domains epitaxially grow on a single-crystalline substrate, and further generate a wafer-scale single crystal by the seamlessly stitching [57].

2.1 Synthesis of 2D semiconducting TMDCs single crystals by the nucleation controlling

The nucleation controlling is extremely difficult for synthesizing wafer-scale 2D semiconducting TMDCs single crystals in view of the complicated growth kinetics. Nevertheless, the phase transition design is a plausible method to solve such an issue. Xu et al. [58] developed a solid-to-solid phase transition and recrystallization strategy to grow 2D semiconducting 2H-MoTe₂ single crystals on amorphous insulating substrates. As illustrated in Fig.2(a), a single-crystalline 2H-MoTe₂ nanoflake was deliberately implanted on the center of 1T′-MoTe₂ wafer and used as the seed crystal to trigger the phase transition and recrystallization. Subsequently, a dense Al₂O₃ film (~30 nm) was deposited on the wafer to isolate the 1T′-MoTe₂ from Te precursors and prevent the random nucleation of 2H-MoTe₂. A small hole was introduced into the seed region and served as the only channel of Te precursors supply.

Fig.2 The nucleation controlling method for synthesizing wafer-scale 2D semiconducting TMDCs single crystals. (a) Schematic diagram of the seed growth of wafer-scale 2H-MoTe2 single crystal [58]. (b) Cross-section STEM image of the seed region [58]. (c) HAADF-STEM image captured from the seed edge [58]. (d) Photograph of the 1-inch 2H-MoTe2 single crystal [58]. (e) Optical image of the coplanar heterophase of 1T′/2H/1T′-MoTe2 FET arrays [58]. (f, g) Statistical distribution of the field-effect mobility and on/off current ratio of 2H-MoTe2 FETs [58].

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Notably, during the annealing process, the Te vacancies were reduced and the rearrangement of Mo and Te atoms of the bottom 1T′-MoTe₂ layers triggered the transition from 1T′ to 2H phase. The lattice structures, crystal orientations, and stacking orders of the bottom MoTe₂ were identical to the seed crystal after the recrystallization process, as proved by the aberration-corrected scanning transmission electron microscopy (AC-STEM) in Fig.2(b). Interestingly, with increasing the annealing time, the phase transition expanded to the outside seed regions and then the wafer-scale 2H-MoTe₂ single crystal was formed, as shown in Fig.2(c) and (d). In addition, a large-scale coplanar heterophase of 1T′ /2H/1T′-MoTe₂ FET arrays were then fabricated to verify the device performance uniformity of such a single crystal film [Fig.2(e)]. As shown in Fig.2(f) and (g), the excellent room temperature carrier mobility (45 ± 2 cm²·V^–1·s^–1) and on/off current ratio [(1.8 ± 0.3) × 10⁴] were obtained, comparable to the reported values of single-crystalline 2H-MoTe₂ nanoflake transistors. And such results suggested the ultrahigh device performance and its spatial uniformity of 2H-MoTe₂ single crystal [59,60].

In short, the phase transition design provides an innovative route for synthesizing wafer-scale 2D semiconducting 2H-MoTe₂ single crystals. However, the uniform and atomically flat few-layer 1T′-MoTe₂ film is required to serve as the precursor of phase transition, and it is difficult to synthesize monolayer 2H-MoTe₂ single crystal. Moreover, this method is also inapplicable for the growth of other 2D TMDCs with large energy differences between different phases.

2.2 Synthesis of 2D semiconducting TMDCs single crystals by the orientation controlling

The orientation controlling was proven to be a reasonable method for the direct synthesis of wafer-scale 2D single crystals [51,54]. However, it remains challenging to obtain the unidirectional domains of 2D TMDCs due to the symmetry mismatch between TMDCs and substrate [57]. Chen et al. [61] proposed that the parallel steps of sapphire (0001) surfaces could be served as the nucleation sites of monolayer WS₂ and further induced the formation of unidirectional alignment domains. Recently, Li et al. [62] realized the epitaxial growth of 2-inch monolayer MoS₂ single crystal on a c-plane sapphire with a miscut angle towards a-axis. As presented in Fig.3(a) and(b), the parallel steps of c/a sapphire were used as the nucleation sites for growing monolayer MoS₂ with unidirectional alignments and single crystal films [Fig.3(c) and (d)].

Fig.3 The orientation controlling method for synthesizing wafer-scale 2D semiconducting TMDCs single crystals. (a) Step orientation on c/a sapphire wafer and the corresponding epitaxially aligned MoS2 domains [62]. (b) AFM image of an annealed c/a sapphire [62]. (c) Optical image of MoS2 domains on c/a sapphire [62]. (d) Photography of the 2-inch monolayer MoS2 single crystal on c/a sapphire [62]. (e) Calculated formation energy of different edge configurations [62]. (f) Optical image of monolayer MoS2 FET arrays [62]. (g) Statistical distribution of carrier mobilities and subthreshold swings [62]. (h) Photography of the 2-inch monolayer WS2 single crystal on vicinal a-plane sapphire [67]. (i) Corresponding LEED pattern [67]. (j) SEM image of the as-grown WS2 film after O2 etching [67].

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The first-principles calculations were then performed to clarify the growth mechanism, as revealed in Fig.3(e). The presence of parallel steps lowered the symmetry of c-plane sapphire and broke the degeneracy of formation energy of two possible edge configurations (ZZ−Mo−S₂ and ZZ−S₂), which resulted in the most energetically favorable configuration of ZZ−Mo−S₂ for the unidirectional growth of monolayer MoS₂. Furthermore, the monolayer MoS₂ FET arrays were also fabricated to evaluate the device performance and its uniformity, as presented in Fig.3(f). The statistical results of 151 monolayer FETs showed that both the carrier mobilities and subthreshold swings subjected to the Gaussian distribution with the variation of ~15% and ~11%, respectively, suggested the high device performance uniformity of wafer-scale monolayer MoS₂ single crystal [Fig.3(g)]. The average and maximum carrier mobility was obtained to be ~77.6 and ~102.6 cm²·V^–1·s^–1, respectively, much higher than that of the monolayer polycrystalline MoS₂ film [63, 64]. Notably, the terrace height of sapphire substrate is also crucial for synthesizing wafer-scale bilayer MoS₂ [65, 66].

In addition, as shown in Fig.3(h), the orientation controlling method was also employed to synthesize wafer-scale monolayer WS₂ single crystals on vicinal a-plane sapphire [67]. The low-energy electron diffraction (LEED) pattern captured from different regions of WS₂ films showed two sets of 3-fold symmetry spots with different diffraction intensities [Fig.3(i)], confirmed the formation of monolayer WS₂ single crystal. Furthermore, the parallelly aligned holes with hexagonal shapes were generated on monolayer WS₂ film during the oxygen etching process, reconfirmed the epitaxial growth behavior and seamless stitching of misorientation-free WS₂ domains on the vicinal a-plane sapphire [Fig.3(j)]. Besides the step-guided strategy, the other methods were also proposed to regulate the orientations of 2D semiconducting TMDCs and synthesize the wafer-scale single crystals. For example, the parallelly aligned monolayer MoS₂ domains were formed as increased the S/Mo precursor ratio, in view of the complete sulfurization of Mo precursors [68, 69]. In addition, the growth temperature is also crucial for growing wafer-scale 2D semiconducting TMDCs single crystals, and the high temperature results in the undulation of step alignment and the misorientation of TMDCs domains [70].

In short, by the nucleation and orientation controlling, the wafer-scale 2D semiconducting TMDCs (e.g., MoS₂, WS₂, 2H-MoTe₂) single crystals have been successfully synthesized and which lays the solid foundation for constructing high performance electronic devices. Nevertheless, the controlled growth of 2D semiconducting single crystals remains challenging, such as the complicated growth process of phase transition and recrystallization, as well as the expensive growth substrates of c/a and a-plane sapphire.

3 The accurate doping of 2D semiconducting TMDCs

The controlled doping of 2D semiconductors is an important strategy for modulating their electronic properties and introducing novel physical phenomena [71–74]. For example, the band structure, carrier density, and photoluminescence intensity of 2D semiconducting TMDCs can be tuned by means of accurate doping. However, the excessive doping should introduce the deep impurity levels and reduce the device performances [38,39]. In this section, we will summarize the recent experimental advances regarding the accurate doping of 2D semiconducting TMDCs and its influence on the device performances.

3.1 The Fe-doping of monolayer MoS₂

As a type of interesting dopants, the magnetic transition metal atoms (e.g., Fe, Co, and Mn, etc.) could introduce the novel physical phenomena in 2D TMDCs [75–78]. Recently, Li et al. [79] reported the CVD synthesis of large-area monolayer Fe-doped MoS₂ on SiO₂/Si. As shown in Fig.4(a), the monolayer Fe-doped MoS₂ triangles were obtained with the maximum domain size of ~250 µm. Notably, the Fermi level of monolayer MoS₂ was moved toward the conduction band minimum (CBM) after the Fe doping due to the additional bands from s and p orbitals of Fe, indicated the n-type doping behavior, as revealed in Fig.4(b).

Fig.4 The Fe-doping of CVD-synthesized monolayer MoS2. (a) Optical microscopy and atomic force microscopy images of CVD-synthesized monolayer Fe-doped MoS2 on SiO2/Si [79]. (b) Band structures of monolayer Fe-doped MoS2 with spin-up (left) and spin-down bands (right) [79]. (c) Contact resistance extraction of monolayer Fe-doped MoS2 by using TLM [79]. (d) Arrhenius plots of the Ohmic-contacted Fe-doped MoS2 and Schottky-contact MoS2 devices [79]. (e) Electron mobility evolution of monolayer Fe-doped MoS2 and pristine monolayer MoS2 devices as a function of temperature [79].

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To evaluate the device performance, the monolayer Fe-doped MoS₂ FETs were thus constructed by using Cr/Au as the electrodes. Excellent room temperature on/off current ratio and electron mobility (~10⁸ and ~54 cm²·V^–1·s^–1) were obtained for monolayer Fe-doped MoS₂, much higher than those of the pristine monolayer MoS₂ (~10⁷ and ~6 cm²·V^–1·s^–1), suggested the enhanced device performance. Furthermore, the contact resistance was also extracted by using the transfer length method (TLM), as shown in Fig.4(c). The contact resistance of monolayer Fe-doped MoS₂ was calculated as ~678 Ω·µm, much smaller than that of the pristine monolayer MoS₂ (~117 kΩ·µm), indicated the outstanding interfacial contacts between monolayer Fe-doped MoS₂ and electrodes. To clarify the internal mechanism, the temperature-dependent electrical measurements were thus performed. The energy barriers of monolayer Fe-doped MoS₂ and pristine monolayer MoS₂ were extracted from the Arrhenius plots in Fig.4(d), and the perfect Ohmic contact was confirmed for Fe-doped MoS₂. Furthermore, the enhanced electron mobility was also observed in monolayer Fe-doped MoS₂ as the temperature decreasing from ~300 K to ~100 K, indicated the suppressed ionized impurity scattering [Fig.4(e)]. Such results reveal that the device performances of 2D semiconducting TMDCs can be improved by the accurate Fe doping, and which provides a new scheme for downscaling the electronic devices.

3.2 The remote modulation doping of few layer MoS₂ by triphenylphosphine

Although the doping is required for modulating the electronic properties of semiconductors, the introduction of impurities should result in the Coulomb scattering and hinders the charge transport, especially for 2D semiconductors. To avoid this issue, a remote modulation doping strategy was developed in the WSe₂/h-BN/MoS₂ heterostructures [80]. The triphenylphosphine (PPh₃) was selected as the dopants and then imposed on the top layers of WSe₂ [Fig.5(a)]. Notably, the electrons from PPh₃ were introduced into WSe₂ layers through the molecular doping and spontaneously transferred to MoS₂ layers in view of the conduction band offset, which resulted in the n-type doping in MoS₂ channel, as shown in Fig.5(b).

Fig.5 The remote modulation doping of few-layer MoS2 by triphenylphosphine. (a) Schematic diagram of the representative WSe2/h-BN/MoS2 heterostructures [80]. (b) Band diagram of the doped WSe2/h-BN/MoS2 FET [80]. (c) Gate voltage dependent electrical sheet conductivity of MD and DD MoS2 FETs before (dashed lines) and after (solid lines) PPh3 doping [80]. (d) Electron concentration dependent carrier mobility before (open symbols) and after (filled symbols) PPh3 doping [80].

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To verify the doping capability of PPh₃, the electrical measurements of modulation-doped (MD) WSe₂/h-BN/MoS₂ and directly doped (DD) MoS₂ FETs were performed. The negative shifts of threshold voltages were clearly observed for both MoS₂ devices after the PPh₃ treatments, indicated the n-type doping behavior, as shown in Fig.5(c). The same variation tendency was also demonstrated in the electron density, which suggested that the PPh₃ molecules could modulate the carrier density of underlying MoS₂ channels through the remote charge transfer across the WSe₂/h-BN layers without a substantial loss in the doping efficiency. Notably, the electron densities of such two type MoS₂ devices could be tuned by the doping concentrations and treatment time. In addition, the carrier mobility of MoS₂ in MD device remained unchanged at ~60 cm²·V^–1·s^–1, even after the electron doping. Nevertheless, a degraded mobility from ~60 cm²·V^–1·s^–1 to ~35 cm²·V^–1·s^–1 was obviously observed for the DD counterparts [Fig.5(d)]. Such results suggested that the charged impurity scattering was suppressed through the remote modulation doping strategy.

3.3 The photo-induced and electrostatic doping strategies of 2D TMDCs

In addition, the other doping methods, such as the photo-induced and electrostatic doping, have also been proposed in 2D TMDCs. For example, Seo et al. [72] reported a reversible photo-induced doping of few-layer 2H-MoTe₂ and 2H-WSe₂ as revealed in Fig.6(a)–(c), and the polarity of channel was reconfigured from n-type to p-type under the different laser light frequencies. The light-lattice interactions, as well as the self-interstitial defects of Te under ultraviolet illumination and the subsequent substitutions of O in Te/Mo vacancies under visible illumination resulted in the reconfigurable doping effect. By using such a method, the complementary metal-oxide-semiconductor (CMOS) device on a single channel was also constructed, and the circuit functions were dynamically reset from the inverter to the switch under the different light frequencies, as shown in Fig.6(d). Furthermore, Wang et al. [81] reported an electrostatic-doping-driven phase transition between the hexagonal and monoclinic phases of monolayer MoTe₂, as demonstrated in Fig.6(e) and proved by the hysteretic loop in Raman spectra [Fig.6(f)]. The electrostatic doping method opens up a new possibility for the construction of phase-change devices at the atomic scale.

Fig.6 Photo-induced and electrostatic doping strategy of 2D TMDCs. (a) Transfer characteristics of reconfigurable doping on few-layer 2H-MoTe2 [72]. (b) Schematic diagram of the in-plane crystal structures of few-layer 2H-MoTe2 [72]. (c) Schematics diagram of the selective doping and corresponding transfer characteristics of few-layer 2H-WSe2 [72]. (d) Transfer characteristics of CMOS inverter [72]. (e) Schematic diagram and measurement configuration of a monolayer MoTe2 transistor [81]. (f) Gate-dependent Raman intensity ratios [81].

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Briefly, the band structures and electronic properties of 2D semiconducting TMDCs can be modulated by means of ingenious doping, and which provides a new paradigm for improving the device performances and extending the Moore’s law. However, the internal mechanism of metal doping should be further explored and the stability of remote modulation doping should also be enhanced.

4 The optimization design of interfacial contacts between electrodes and 2D semiconducting channels

The optimization design of interfacial contacts between electrodes and channels is crucial for realizing the applications of 2D semiconductors in high performance electronic and optoelectronic devices [41,43,82–84]. Notably, the interfacial contact quality can be evaluated by the contact resistance. For example, the low contact resistance is crucial for achieving high “on” current density, large photoresponse, and high-frequency operation [85]. Nevertheless, the difference between the work function of metal electrodes and the electronic affinity energy of 2D semiconducting TMDCs results in the formation of Schottky barrier [86]. Furthermore, the Fermi-level pinning effect is also induced by the disorder-induced gap state (DIGS) and metal-induced gap state (MIGS) [87], which breaks the Schottky−Mott rule. Therefore, the optimization design of interfacial contacts is significant for improving the device performances of 2D semiconducting TMDCs. In this section, we will review three methods to optimize the interfacial contacts including the electrode transfer, the introduction of semimetal electrodes, and the construction of 2D metallic/semiconducting TMDCs heterostructures.

4.1 The electrode transfer for improving the device performances of 2D semiconducting TMDCs

The defects and disorders should be introduced on the surfaces of 2D semiconducting TMDCs during the lithography and thermal evaporation processes, which degenerate the device performances accordingly. Notably, the van der Waals contacts between electrodes and 2D semiconductors without Fermi-level pinning are theoretically possible. For example, the clean interfaces were created in the van der Waals metal−semiconductor junctions by laminating the metal electrodes with atomically flat surfaces onto the dangling-bond-free 2D semiconductors, as shown inFig.7(a) [88]. The excellent device performances (with the electron and hole mobility up to ~260 and ~175 cm²·V^–1·s^–1, respectively) were clearly observed in the exfoliated few-layer MoS₂ by using the transferred Ag and Pt electrodes [Fig.7(b), (c)]. Furthermore, the S factor, which was characterized the Fermi level pin strength, was obtained to be ~0.96 for the devices with transferred metal electrodes, approached to the Schottky−Mott law limit defined by the electrostatic energy alignment. On the contrary, theS factor was calculated as ~0.09 for the control devices with evaporated metal electrodes, indicated strong Fermi-level pinning effect near the CBM at the electrode/MoS₂ interfaces, as demonstrated in Fig.7(d). Such an interesting electrode transfer establishes a highly efficient and damage-free strategy for constructing high performance electronic devices.

Fig.7 The electrode transfer method for improving the device performances of 2D semiconducting TMDCs. (a) Schematic diagram of the transferred and deposited Au electrodes on MoS2 [88]. (b, c) Electron and hole mobility of few-layer MoS2 by using the transferred Ag and Pt electrodes [88]. (d) Schottky barrier height of few-layer MoS2 by using different transferred and evaporated electrodes [88]. (e) Schematic diagram of the van der Waals and direct Au contacted few-layer WSe2 [89]. (f) Transfer curve of few-layer WSe2 with the van der Waals and direct Au contact, respectively [89]. (g, h) DFT calculation results of WSe2 with the van der Waals and direct Au contact, respectively [89].

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In addition, the interaction- and defect-free van der Waals contacts were formed between the metal electrodes and exfoliated few-layer WSe₂ nanosheets via a metal deposition process that used the Se buffer layer [89]. In detail, the Se layer was used to protect the WSe₂ channel during the metal evaporation process and it could be removed by the annealing process at ~150 ℃ [Fig.7(e)]. The n-type behavior was clearly observed for the direct Au contacted few-layer WSe₂ device, in view of the Fermi-level pinning effect near the conduction band at the Au−WSe₂ interface. However, the p-type characteristic with a large on/off current ratio (>10⁶) and high hole mobility (~135 cm²·V^–1·s^–1) was presented in the van der Waals contacted few-layer WSe₂ due to the perfect interface [Fig.7(f)]. Density functional theory (DFT) calculations were then performed to understand the band structure at the Au−WSe₂ interface [Fig.7(g), (h)]. The new states were formed in the mid-bandgap region of the direct Au contact WSe₂, and changed the band bending direction. Nevertheless, the density of states (DOS) of WSe₂ were formed in the van der Waals contact with Au electrodes and exhibited few mid-gap states. The construction of van der Waals contact using a Se buffer layer provides new route for controlling the Schottky barrier height and can be used for fabricating the large-scale electronic devices.

4.2 The introduction of semimetal electrodes for improving the device performances of 2D semiconducting TMDCs

The Fermi-level pinning effect can be suppressed by constructing van der Waals contact, nevertheless, the tunneling barriers are inevitable because of the interface gaps between electrodes and channels. In addition, the electrode transfer process should be implemented in a sufficiently clean environment, therefore, this approach is vulnerable to the external influence and difficult to apply to industrially relevant large-substrate technology. Herein, the exploitation of new electrode materials is necessary.

Notably, as the semiconductor closed to a metal surface, the extended wavefunction of metal perturbed the environment of semiconductor and resulted in the emergence of MIGS [Fig.8(a)], totally different from the initial state of semiconductor [Fig.8(c)] [90]. Specially, if the Fermi level of electrode lied inside the semiconductor bandgap, the Schottky barrier was thus formed unavoidably. However, for the semimetal Bi electrode, the zero DOS near the Fermi level induced few MIGS. Particularly, its Fermi level closed to the CBM of monolayer MoS₂, and the conduction-band-contributed MIGS were greatly suppressed [Fig.8(b)]. As a result, the degenerate state and free of Schottky barrier at the interface were formed as the semimetal Bi contacted monolayer MoS₂. The schematic diagram of monolayer MoS₂ back-gated FET with Bi contact was shown in Fig.8(d). The ultralow contact resistance (~123 Ω·μm at the carrier density of 1.5×10¹³ cm⁻²) and zero Schottky barrier height was obtained, respectively, as revealed in Fig.8(e) and (f), indicated the ultrahigh device performance of Bi contacted monolayer MoS₂. Notably, the contact resistance values were comparable to those of three-dimensional semiconductors, and approached to the quantum limit [Fig.8(g)]. Even so, in view of the low work function of Bi electrode, it is incapable to reduce the Schottky barrier height of hole. Therefore, searching for high work function semimetal electrodes is crucial for fabricating high performance p-type devices.

Fig.8 The introduction of semimetal electrodes for improving the device performances of 2D semiconducting TMDCs. (a) DOS of normal metal and semiconductor contact, and the corresponding band structure [90]. (b) DOS of semimetal and semiconductor contact, and the corresponding band structure [90]. (c) The reference DOS of semiconductor before contact [90]. (d) Schematic diagram of a monolayer MoS2 FET with Bi contact [90]. (e) Contact resistance of Bi-MoS2 FETs extraction by using the TLM [90]. (f) Arrhenius plots of the Ohmic Bi-MoS2 and Schottky Ni-MoS2 FETs at the carrier density of 1.5 × 1012 cm−2 [90]. (g) The state-of-the-art contact technology of MoS2 FETs plotted as a function of carrier density [90].

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4.3 The construction of 2D metallic/semiconducting TMDCs heterostructures

2D metallic TMDCs have attracted great attention due to their interesting physical properties (e.g., charge density waves order, superconductivity, and magnetism, etc.) [91–108]. In addition, some revolutionary applications such as transparent electrodes and energy conversion/storage have also explored in the 2D metallic TMDCs, by virtue of the ultrahigh conductivities, excellent electrocatalytic activities, and novel layered structures [109–124]. Notably, the 2D metallic TMDCs are considered as ideal electrode materials for improving the device performances of 2D semiconducting TMDCs in view of their similar atomic structures and complementary electronic properties [27,59,125–133].

The VS₂−MoS₂ lateral heterostructures were successfully synthesized on SiO₂/Si by a two-step CVD method, as revealed in Fig.9(a) and (b) [134]. The back-gate monolayer MoS₂ FETs were then fabricated with the lateral VS₂ and vertical Ni contact, respectively [Fig.9(c)]. The contact resistances of such two typical monolayer MoS₂ devices were extracted by the four-probe measurements and plotted in Fig.9(d). Notably, the contact resistance of VS₂−MoS₂ was obtained to be ~520 Ω·μm, more than ten times smaller than that of Ni-MoS₂ contacts (~8640 Ω·μm) [Fig.9(d)], indicated the perfect interfacial contact. Furthermore, the 2D metallic/semiconducting TMDCs heterostructure arrays were also CVD synthesized by using a focused laser irradiation and raster scan method, as shown in Fig.9(e) [135]. The VSe₂/WSe₂ heterostructure array FETs were thus constructed [Fig.9(f)], and demonstrated ultrahigh device performances with the mobility of ~135 cm²·V^–1·s^–1, much higher than that of Cr/Au contacted monolayer WSe₂ (~10 cm²·V^–1·s^–1), which was attributed to the protective effect of VSe₂ on the monolayer WSe₂ channel during the electrode evaporation process [Fig.9(g)].

Fig.9 The construction of 2D metallic/semiconducting TMDCs heterostructures for improving the device performances. (a) Optical image of the MoS2-VS2 lateral heterostructure [134]. (b) Schematic illustration of the stitching growth mechanism of monolayer MoS2 [134]. (c) Schematic diagram of the monolayer MoS2 FETs with the lateral VS2 (upper) and vertical Ni contacts (lower) [134]. (d) Contact resistances of monolayer MoS2 FETs with the VS2 and Ni contacts [134]. (e) The growth process of 2D metallic/semiconducting TMDCs heterostructure arrays [135]. (f) Optical image of back-gated WSe2 transistors with the VSe2 contacts [135]. (g) Carrier mobility distribution of monolayer WSe2 transistors with the VSe2 and Cr/Au contacts [135].

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5 Conclusion and perspectives

2D semiconducting TMDCs have shown great application potentials in the electronic devices due to their novel physical properties, atomically thin thicknesses, and abundant band structures. However, the device performances are still lower than that of the theoretical values, which is attributed to the intrinsic defects, excessive doping, and daunting contacts. From these perspectives, we have summarized three strategies for improving the device performances of 2D semiconducting TMDCs, including (i) the synthesis of wafer-scale 2D semiconducting TMDCs single crystals by the nucleation and orientation controlling to reduce the grain boundary densities, (ii) the accurate doping of 2D semiconducting TMDCs by the metal atoms and remote modulations to tune the band structures and suppress the impurity scatterings of 2D semiconducting TMDCs, and (iii) the optimization design of interfacial contacts between electrodes and 2D semiconducting TMDCs channels by means of transferring metal electrodes, introducing semimetal electrodes, and constructing 2D metallic/semiconducting TMDCs heterostructures.

Although the impressive achievements have been made in such aspects, some daunting challenges are still unresolved. For the synthesis of wafer-scale 2D semiconducting TMDCs single crystals, the high-cost sapphire substrates with a small miscut angle are used for controlling the domain orientations. Moreover, the longtime pretreatment of substrates at extremely high temperature is needed before the synthesis of 2D TMDCs. Thus, it is urgent to develop a facile technology for growing wafer-scale 2D semiconducting TMDCs single crystals on the commercially available substrates. For the controllable doping, the organic molecules are unstable in the atmosphere, and which are incompatible with the device construction. Furthermore, the intrinsic mechanism of metal doping is still unclear, and more experimental and theoretical explorations are required. For the optimization design of interfacial contact, the electrode transfer method is complicated, and the gap between electrode and channel will introduce the tunneling barrier. In addition, the small work function of semimetal electrode is disadvantageous to reduce the Schottky barrier height of p-type semiconductor. The controllable synthesis of wafer-scale 2D metallic/semiconducting TMDCs heterostructures is still unaddressed.

Although there are some challenges, many breakthroughs will be made in the near future regarding the improvement of device performances of 2D semiconducting TMDCs. For instance, the interactions between 2D TMDCs domains and substrates can be tuned by the metal doping during the CVD growth process, and it is feasible to obtain the wafer-scale 2D semiconducting TMDCs single crystals on the commercially available substrates. Meanwhile, the metal doping is also beneficial for modulating the band structures of 2D TMDCs and reducing the contact resistances between electrodes and channels. In addition, some newfangled physical properties, such as magnetism, can be introduced by the metal doping, and which provides a new route for the construction of multifunctional electronic devices.

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Acknowledgements

This work was supported by the National Key R&D Program of China (Grant Nos. 2018YFA0703700 and 2021YFA1200800), the National Natural Science Foundation of China (Grant Nos. 91964203 and 92164103), the Beijing National Laboratory for Molecular Sciences (Grant No. BNLMS202001), and the Fundamental Research Funds for the Central Universities (Grant No. 2042021kf0029).

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