Role of Metal Contacts in Designing High-Performance Monolayer n-Type WSe₂ Field Effect Transistors

Abstract

This work presents a systematic study toward the design and first demonstration of high-performance n-type monolayer tungsten diselenide (WSe2) field effect transistors (FET) by selecting the contact metal based on understanding the physics of contact between metal and monolayer WSe2. Device measurements supported by ab initio density functional theory (DFT) calculations indicate that the d-orbitals of the contact metal play a key role in forming low resistance ohmic contacts with monolayer WSe2. On the basis of this understanding, indium (In) leads to small ohmic contact resistance with WSe2 and consequently, back-gated In-WSe2 FETs attained a record ON-current of 210 μA/μm, which is the highest value achieved in any monolayer transition-metal dichalcogenide- (TMD) based FET to date. An electron mobility of 142 cm(2)/V·s (with an ON/OFF current ratio exceeding 10(6)) is also achieved with In-WSe2 FETs at room temperature. This is the highest electron mobility reported for any back gated monolayer TMD material till date. The performance of n-type monolayer WSe2 FET was further improved by Al2O3 deposition on top of WSe2 to suppress the Coulomb scattering. Under the high-κ dielectric environment, electron mobility of Ag-WSe2 FET reached ~202 cm(2)/V·s with an ON/OFF ratio of over 10(6) and a high ON-current of 205 μA/μm. In tandem with a recent report of p-type monolayer WSe2 FET ( Fang , H . et al. Nano Lett. 2012 , 12 , ( 7 ), 3788 - 3792 ), this demonstration of a high-performance n-type monolayer WSe2 FET corroborates the superb potential of WSe2 for complementary digital logic applications.

High Mobility WSe2 p- and n-Type Field-Effect Transistors Contacted by Highly Doped Graphene for Low-Resistance Contacts

“…Variable temperature electrical measurements performed on WSe 2 FETs with low-resistance graphene contacts reveal that as the temperature decreases from 160 to 77 K, the extrinsic field-effect mobility increases from ∼196 to ∼330 cm 2 V −1 s −1 for the electron channel and from ∼204 to ∼270 cm 2 V −1 s −1 for the hole channel. These mobility values are comparable to the highest electron and hole mobilities reported on thin film WSe 2 FETs as well as bulk WSe 2 , 8,10,12,34 indicating that our graphene-contacted few-layer WSe 2 devices approach the intrinsic phonon-limited mobility for both electrons and holes. The most significant finding of our study is that highly "doped" graphene is an excellent contact electrode material for high-performance p-and n-type TMD FETs.…”

“…Low-resistance contacts have been achieved by surface doping of WSe 2 (e.g., with NO 2 and K) in order to reduce the SB thickness, or by using low work function contact metals such as indium in order to lower the height of the SB to the conduction band. 8,10,12 However, …”

Low-Resistance 2D/2D Ohmic Contacts: A Universal Approach to High-Performance WSe2, MoS2, and MoSe2 Transistors

“…Furthermore, the 1T phase MoS 2 is thermally unstable above 100 o C. The availability of a variety of semiconducting TMDs such as MoSe 2 , WS 2 and WSe 2 with different band structures and charge neutrality levels offers additional distinct properties and opportunities for device applications. 5,6,8,9,13,[26][27][28][29][30][31][32][33][34][35][36][37] However, the variation of electron affinity, band gap, and band alignments also presents significant challenges to contact engineering. To unlock the full potential of TMDs as channel materials for high-performance thin-film transistors, highly effective and versatile contact strategies for making low-resistance ohmic contacts are needed.…”

Graphene: The quest for supercarbon

“…Doping graphene with other materials or slicing it into narrow ribbons can open up a small band gap, but this also slows the flow of electrons. So researchers are trying to tune its electrical properties by combining graphene with other monolayer materials such as boron nitride or creating transistors from molybdenum disulphide and tungsten diselenide [9][10][11] .…”