Thickness Trends of Electron and Hole Conduction and Contact Carrier Injection in Surface Charge Transfer Doped 2D Field Effect Transistors

Abstract

One of the main limiting factors in the performance of devices based on two-dimensional (2D) materials is Fermi level pinning at the contacts, which creates Schottky barriers (SBs) that increase contact resistance and, for most transition metal dichalcogenides (TMDs), limit hole conduction. A promising method to mitigate these problems is surface charge transfer doping (SCTD), which places fixed charge at the surface of the material and thins the SBs by locally shifting the energy bands. We use a mild O 2 plasma to convert the top few layers of a given TMD into a substoichiometric oxide that serves as a p-type SCTD layer. A comprehensive experimental study, backed by TCAD simulations, involving MoS 2 , MoSe 2 , MoTe 2 , WS 2 , and WSe 2 flakes of various thicknesses exposed to different plasma times is used to investigate the underlying mechanisms responsible for SCTD. The surface charge at the top of the channel and the gate-modulated surface potential at the bottom are found to have competing effects on the channel potential, which results in a decrease in the dopinginduced threshold shift and an increase in minimum OFF state current with increasing thickness. Additionally, an undoped channel region is shown to mitigate carrier injection issues in sufficiently thin flakes. Notably, the band movements underlying the SCTD effects are independent of the particular semiconductor material, SCTD strategy, and doping polarity. Consequently, our findings provide critical insights for the design of high-performance transistors for a wide range of materials and SCTD mechanisms including TMD devices with strong hole conduction.

Annealing Accelerator for Ising Spin Systems based on In-memory Complementary 2D FETs

“…2a-b, respectively. Undoped WSe2 demonstrates ambipolar transport with both electron (n-type) and hole (p-type) conduction owing to the pinning of metal Fermi-level near the middle of the WSe2 bandgap [39][40][41]. However, for the design of the multiplier module, unipolar p-type WSe2 is preferred.…”

“…However, for the design of the multiplier module, unipolar p-type WSe2 is preferred. Hence, WSe2 is doped p-type using surface charge transfer doping with sub-stochiometric WO3-x [40]. Doping is achieved on a multilayered WSe2 flake by converting its top layers to WO3-x by exposure to mild O2 plasma as discussed in our earlier reports [40,42].…”

“…As shown in Fig. 2a before O2 plasma exposure, the flakes are patterned to obtain a p-ip structure i.e., p-doped source and drain extension regions with the middle region left intrinsic [40] (see Methods section for details on p-type WSe2 FET fabrication). The length of the intrinsic region is designed to be 500 .…”

Annealing Accelerator for Ising Spin Systems based on In-memory Complementary 2D FETs

“…However, for the design of the multiplier module, unipolar p-type WSe2 is preferred. Hence, WSe2 is doped p-type using surface charge transfer doping with sub-stochiometric WO3-x [40]. Doping is achieved on a multilayered WSe2 flake by converting its top layers to WO3-x by exposure to mild O2 plasma as discussed in our earlier reports [40,42].…”

Interface Defect Engineering of a Large‐Scale CVD‐Grown MoS2 Monolayer via Residual Sodium at the SiO2/Si Substrate

“…[18][19][20][21][22] Thus, many methods, which include substitution, charge transfer, intercalation, and electrostatic doping, have been developed to improve the contact properties or modulate the polarity of transition metal dichalcogenides (TMDs). [23][24][25] Meanwhile, the alkali metal halide-assisted CVD methods enable the rapid fabrication of the continuous and waferscale uniform monolayer TMDs directly on several oxide substrates. [26][27][28][29][30][31][32][33][34] Alkali metal cations promote a rapid increase of the single domain size of monolayer TMDs due to dehydration and nucleation suppression, regardless of the halide anions.…”