Carrier Type Control of WSe₂ Field‐Effect Transistors by Thickness Modulation and MoO₃ Layer Doping

Abstract

Control of the carrier type in 2D materials is critical for realizing complementary logic computation. Carrier type control in WSe2 field‐effect transistors (FETs) is presented via thickness engineering and solid‐state oxide doping, which are compatible with state‐of‐the‐art integrated circuit (IC) processing. It is found that the carrier type of WSe2 FETs evolves with its thickness, namely, p‐type (<4 nm), ambipolar (≈6 nm), and n‐type (>15 nm). This layer‐dependent carrier type can be understood as a result of drastic change of the band edge of WSe2 as a function of the thickness and their band offsets to the metal contacts. The strong carrier type tuning by solid‐state oxide doping is also demonstrated, in which ambipolar characteristics of WSe2 FETs are converted into pure p‐type, and the field‐effect hole mobility is enhanced by two orders of magnitude. The studies not only provide IC‐compatible processing method to control the carrier type in 2D semiconductor, but also enable to build functional devices, such as, a tunable diode formed with an asymmetrical‐thick WSe2 flake for fast photodetectors.

The ambipolar transport behavior of WSe2 transistors and its analogue circuits

“…In this study, the lower work function of Ti (~4.3 eV) 35 and the higher Schottky barrier height in the hole transport region can induce the preferential electron dominant conduction in thinner WSe 2 . This phenomenon is in line with the previous reported preferential n-type in thinner WSe 2 with low-work-function metals, such as Ni, In, and Ag 24,35 . The requirement for real application of ambipolar semiconductor is not only the balanced hole and electron carrier but also the higher carrier mobility.…”

“…Among them, tungsten diselenide (WSe 2 ), which consists of one layer of W atoms sandwiched between two layers of Se atoms, has many excellent properties providing potential applications, including valley-based electronics 15,16 , spin-electronics, and optoelectronics 17,18 . More significantly, different from the unipolar n-type semiconductor MoS 2 with the presence of sulfur vacancy and the strong Fermi level pinning near the conduction band [19][20][21] , WSe 2 as an ambipolar semiconductor has been demonstrated as having Fermi level effectively shifting between the valence band and the conduction band under application of an external field [22][23][24] . Recently, the optical and hole dominant transport properties of exfoliated WSe 2 have been explored [24][25][26][27] .…”

“…More significantly, different from the unipolar n-type semiconductor MoS 2 with the presence of sulfur vacancy and the strong Fermi level pinning near the conduction band [19][20][21] , WSe 2 as an ambipolar semiconductor has been demonstrated as having Fermi level effectively shifting between the valence band and the conduction band under application of an external field [22][23][24] . Recently, the optical and hole dominant transport properties of exfoliated WSe 2 have been explored [24][25][26][27] . To facilitate the potential application of WSe 2 in CMOS, it is essential to understand the underlying ambipolar hole and electron transport mechanisms.…”

Modulation doping of transition metal dichalcogenide/oxide heterostructures

“…[1][2][3][4][5] The modulation of carrier density in the TMDs provides a way to change their physical properties in a controllable manner. For typical TMD materials, the pristine carrier type is predominated by their intrinsic characteristics.…”

“…For example, pristine MoS 2 is of the n-type while WSe 2 shows p-type or ambipolar characteristics (the carrier type of WSe 2 is dependent on its thickness). 2,[6][7][8] Controllable injection and extraction of electrons/holes are important for effective n-and p-doping of TMDs, which are crucial for building complementary logic circuits in the future. On the other hand, the interaction between carriers and excitons enables an additional path for the PL modulation of 2D layered materials.…”