One of the major advantages of two-dimensional (2D) materials is the enormous versatility of their properties [1]. By combining two or more distinct 2D materials to form van der Waals heterostructures, unprecedented opportunities to explore novel properties and potential applications are provided. Dr. Wang’s group studies 2D semiconductors that have attracted widespread interest recently given their potential for novel functionalities in nanoelectronics. Owing to their atomically thin layers and excellent gate coupling, 2D semiconductors show a promising lack of short-channel effects, which considerably lower device performance. Moreover, the flexibility and transparency of these 2D semiconductors makes them highly suitable for next-generation electronics. By exploring van der Waals heterostructures, Dr. Wang’s group has devised novel electrical contact schemes as well as novel functionalities realized by field-effect transistors based on 2D semiconductors.

In 2D-semiconductor-based field-effect transistors and optoelectronic devices, Fermi level (FL) pinning commonly occurs at the metal–semiconductor junctions, severely limiting the tunability of the barrier height and the contact resistance. 2D semiconductors manifest inherent advantages in terms of suppressing FL pinning through tunneling contact, originating from the atomically thin, layered structure. However, a feasible method to synthesize large-area, high-quality tunneling barriers to address the FL pinning issue in layered-semiconductor-based field-effect transistors has not been realized. Here, we demonstrate an atomically thin surface oxidation layer that functions as an effective tunneling barrier at the contact interface of indium selenide (InSe) transistors [2]. This oxidation layer exhibits a very uniform morphology and atomically sharp, clean interfaces with highly crystalline InSe underneath. With this tunneling contact, we successfully realized pronounced FL depinning in the InSe transistors (Fig. 1), resulting in effectively modulated barrier height and threshold voltage as well as high two-terminal electron mobility with a low contact barrier.

Hexagonal boron nitride has been extensively used to encapsulate various 2D materials, providing chemical stability. Moreover, the dielectric effect resulting from the boron nitride is important for reducing Coulomb scattering and increasing the screening effect, leading to enhanced mobility. However, because the semiconducting channel is embedded in the insulating layers, a viable method to achieve high-quality electrical contact is a pressing need for the encapsulated 2D materials.

Another issue in terms of device fabrication with 2D materials relates to the contact when e-beam lithography is employed. The resist residue and material degradation during contact fabrication usually have a negative effect on the field-effect transistor. Accordingly, we have developed a technique to enable electrical side contact for a 2D semiconductor encapsulated in hexagonal boron nitride based on the use of the transition-metal dichalcogenide MoS2. The chemical bonding and the sloped edge of the contact can be controlled with a novel approach, leading to high-quality device performance (Fig. 2). This method can be applied to other 2D materials where chemical stability and contact properties are critical, thus providing an efficient platform to study their transport properties.

An ionic liquid (IL) can be used in an electrostatic gating technique for effectively achieving high-carrier-density regimes in 2D semiconductors by creating an electric double layer (EDL) at the semiconductor/IL interfaces. The EDL arises when an IL gate voltage is applied and mobile ions accumulate at the interfaces, which attract charge carriers to the interface. However, self-gating without an applied IL gate in IL/2D-semiconductors is highly desirable yet rarely discussed. We observe a greatly enhanced electrostatic coupling at the interface between the layered InSe and the IL. We have also observed an anomalous temperature dependence of the capacitive coupling when the IL undergoes phase transition, indicating a strong self-gating effect (Fig. 3). The phase modulation of this self-gating is further evidenced by the strong correlation between the IL phase and transport characteristics. Interestingly, the self-gating effect can be attributed to the balance between intra- and inter-system Coulomb interactions, suggesting novel functionalities that may be used to control electron transport in 2D-semiconductor/IL hybrid systems.

Figure 1.
Oxidized-monolayer tunneling barrier for strong Fermi-level depinning in layered InSe transistors. Top: A schematic of the InSe device structure at the contact regime. With the insertion the oxidized-monolayer, the tunneling barrier can facilitate Fermi-level depinning. Bottom: The Schottky barrier height as a function of metal work function for the oxidized-monolayer-embedded InSe devices and the control samples, indicating a pronounced Fermi-level depinning.



Figure 2.
Electrical side contact to 2D semiconductors encapsulated by hexagonal boron nitride. A schematic of the MoS2 device structure at the contact regime. The Raman spectra reveals two characteristic peaks of the MoS2 sample which correspond to the E12g and A1g resonance modes. The Id-VG curves of an edge-contacted MoS2 device at room temperature and at 80 K. The edge-contacted MoS2 device exhibits typical n-type semiconducting behavior, high mobility, and metallic temperature dependence, indicating high electrical performance.



Figure 3.
Self-gating effect in IL-functionalized field-effect transistors based on a 2D semiconductor. The interfacial capacitance at the temperature ranging from glass to liquid phases of the IL, showing a prominent peak in the rubber phase with a large enhancement factor. The molecular structures of the trans- and cis- conformer of TFSI− anions. Arrhenius plot of the ln ( r ) vs. 1000/T for the IL-functionalized InSe device, indicating a greater tendency to transform to the trans -TFSI−. The r = Itrans / Icis is Raman peak ratio of the trans- and cis- conformer of TFSI− anions.



[1] “Two-dimensional semiconductors for transistors” M. Chhowalla, D. Jena, and H. Zhang, Nature Reviews Materials, 1 (2016) P. 16052
[2] “Oxidized-monolayer Tunneling Barrier for Strong Fermi-level Depinning in Layered InSe Transistors” npj 2D Materials and Applications, 3 (49) (2019) P. 1–7


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