Ambipolar and Robust WSe₂ Field‐Effect Transistors Utilizing Self‐Assembled Edge Oxides

Abstract

Transition metal oxides (TMOs) with high work function (WF) show promising properties as unipolar p-type contacts for transition metal dichalcogenides. Here, ambipolar field-effect transistors (FETs) enabled by bilayer WSe2 with self-assembled TMOs (WO2.57) as contacts are reported. Systematic material characterizations demonstrated the formation of WO2.57/WSe2 heterojunctions around nanoflake edges with Se atoms substituted by O atoms after air-exposure, while pristine properties of WSe2 almost sustained in inner domains. As-fabricated FETs exhibited both polarities, implying WO2.57 with lowered WF at edges can serve as both the p-type and n-type contact for inner WSe2. Noteworthy, greatly reduced contact resistance and enhanced channel current were achieved, compared to the devices without WO2.57 contacts. Linear drain-source current relationship from 77 K to 300 K indicated the ohmic contact between edge WO2.57 and inner WSe2. Density functional theory calculations further revealed the WO2.57/WSe2 heterojunction formed a barrier-less charge distribution. These nm-scale FETs possessed remarkable electrical conductivity up to ~ 2600 S/m, ultra-low leakage current down to ~ 10-12 A, robustness for high voltage operation and air stability, which even outperformed pristine WSe2 FETs. Theoretical calculations revealed the high conductivity was exclusively attributed to the air-induced WO2.57 and its further carrier injection to WSe2.

Recessed-Channel WSe2 Field-Effect Transistor via Self-Terminated Doping and Layer-by-Layer Etching

“…The ongoing development of advanced Si microelectronics is being hindered by the scaling limits . To overcome this restriction, van der Waals (vdW) semiconductor materials with reduced dimensionality have been proposed. − Among them, two-dimensional (2D) transition-metal dichalcogenides (TMDs) and black phosphorus with thickness-dependent energy bandgaps present intriguing electrical and optoelectronic properties with the potential for diverse application in devices such as diodes, field-effect transistors (FETs), heterojunction bipolar transistors, chemical/optical/mechanical sensors, and photovoltaic devices. − Direct-to-indirect bandgap transitions and quantum confinement structures can be fabricated using 2D materials. Existing semiconductor materials include limitations in the heteroepitaxial growth due to a lattice mismatch. − However, these limitations can be overcome in 2D vdW materials, which do not require the use of high-vacuum facilities for building heterostructures.…”

Morphotaxy of Layered van der Waals Materials

“…In related work, Xu et al utilized WO x as contacts in bilayer WSe 2 FETs, where they argued that the substoichiometric, high work function WO 2.57 offered decreased contact resistance and enhanced channel current, with conductivities as high as 2600 S m –1 . In line with these experimental results, DFT calculations showed that the substoichiometric oxide formed through air exposure gives rise to a defect band near the Fermi energy, resulting in gap-state-assisted carrier transport and enhanced conductivities …”

The Photodetectors Based on Lateral Monolayer MoS2/WS2 Heterojunctions

“…Considering the almost half-a-trillion-dollar semiconductor-chip market, two-dimensional (2D) materials are currently one of the most feasible and promising candidates for extending Moore's law [1][2][3][4][5]. As a representative member of the 2D family, transition metal dichalcogenides (TMDs) have been intensively studied due to their distinctive optoelectronic properties and potential applications [6][7][8][9][10][11][12] in photodetection and lightemitting devices [13,14]. Notably, the tunable bandgap, high carrier mobility, high optical absorption and atomically thin thickness, making TMDs appropriate channel materials for photodetectors, play a crucial role in optoelectronic or electronic devices [15,16].…”