Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Abstract

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.

Mobility Deception in Nanoscale Transistors: An Untold Contact Story

“…We refer to this V BG as the aforementioned contact threshold voltage, V TB‐C . The difference in the magnitude of V TB‐CH and V TB‐C can originate from intrinsic effects such as reduced band movement due to metal induced doping of the channel extension underneath the contacts or from extrinsic effects such as channel doping introduced by surface adsorption which are prevalent for atomically thin materials . Our dual‐gated 2D FETs encounter a similar effect introduced via electrostatic doping using the top‐gate.…”

“…Figure a shows the schematic of a standard back‐gated (BG) FET geometry based on 2D layered semiconductors. Compared to a conventional BG Si FET shown in Figure b, there are two key differences: 1) in Si FETs, the metal/Si interfaces are mostly Ohmic in nature owing to the formation of metal silicides, whereas, metal/2D interfaces are mostly Schottky in nature, and 2) the Si channel underneath the metal contacts are degenerately doped, whereas, the 2D channels are mostly intrinsic or unintentionally lightly doped. The contact resistance ( R C ) in Ω µm in such BG FET geometries can be expressed through Equation , where ρ C is the specific contact resistivity in Ω µm 2 and ρ S is the channel sheet resistance under the contact in Ω ◻ −1 .…”

Atomic Layer Deposition of Al-Doped MoS2: Synthesizing a p-type 2D Semiconductor with Tunable Carrier Density

“…, intentional) doping are essential. 1 , 2 , 8 , 9 TMD-based field-effect transistors (FETs) have already been realized with intrinsic materials. However, to fabricate truly high-performance FETs, locally doped regions and control over the carrier density are essential.…”

“…However, to fabricate truly high-performance FETs, locally doped regions and control over the carrier density are essential. 1 , 2 , 8 , 9 Extrinsically doped TMDs will also play a role in other transistor designs. For example, the operating principle of bipolar junction transistors is based on junctions between p - and n -type materials and cannot be realized until TMDs with both carrier types can be synthesized.…”

Giant proximity exchange and valley splitting in transition metal dichalcogenide/ hBN /(Co, Ni) heterostructures

“…For spin injection, one can either contact the TMDC with metal or metal/insulator interfaces or inject them optically 62,63 . In the former case, studying the dependence of the Schottky barrier on the used electrode is crucial [64][65][66][67][68][69][70][71] . It turns out that a hBN tunnel barrier is also a good choice here, preserving the intrinsic properties of the TMDC while enormously reducing the contact resistance 65,[72][73][74] .…”