Ohmic Behavior in Metal Contacts to n/p-Type Transition-Metal Dichalcogenides: Schottky versus Tunneling Barrier Trade-off

Abstract

High contact resistance (R C ) between 3D metallic conductors and single-layer 2D semiconductors poses major challenges toward their integration in nanoscale electronic devices. While in experiments the large R C values can be partly due to defects, ab initio simulations suggest that, even in defect-free structures, the interaction between metal and semiconductor orbitals can induce gap states that pin the Fermi level in the semiconductor band gap, increase the Schottky barrier height (SBH), and thus degrade the contact resistance. In this paper, we investigate, by using an in-house-developed ab initio transport methodology that combines density functional theory and nonequilibrium Green's function (NEGF) transport calculations, the physical properties and electrical resistance of several options for ntype top metal contacts to monolayer MoS 2 , even in the presence of buffer layers, and for ptype contacts to monolayer WSe 2 . The delicate interplay between the SBH and tunneling barrier thickness is quantitatively analyzed, confirming the excellent properties of the Bi− MoS 2 system as an n-type ohmic contact. Moreover, simulation results supported by literature experiments suggest that the Au−WSe 2 system is a promising candidate for p-type ohmic contacts. Finally, our analysis also reveals that a small modulation of a few angstroms of the distance between the (semi)metal and the transition-metal dichalcogenide (TMD) leads to large variations of R C . This could help to explain the scattering of R C values experimentally reported in the literature because different metal deposition techniques can result in small changes of the metal-to-TMD distance besides affecting the density of possible defects.

Mobility Enhancement of Strained MoS2 Transistor on Flat Substrate

“…Here we need to clarify our carrier mobility is extracted using two-probe methods, and the contact resistance ( R c ) is not excluded, which could impact the evaluation of device behavior. ,, To confirm the mobility improvement is attributed to the strain-induced channel resistance change, rather than modulation of contact resistance, the R c is further extracted from the Y-function approach . As shown in Supporting Information Figure 5a, the R c (red line) is 10.8 kΩ μm at 0% strain, which is over 1 order of magnitude smaller compared to the channel resistance (135.2 kΩ μm), indicating the device is channel-dominated and the contribution of R c is negligible.…”

Fermi level depinning via insertion of a graphene buffer layer at the gold–2D tin monoxide contact

“…The insertion of graphene greatly increases the layer separation (graphene-mSnO: 3.45 Å; graphene-Au: 3.07 Å), but it is still shorter than the reported value for the Al-graphene-monolayer MoS 2 (mMoS 2 ) structure (∼7 Å) [45]. No covalent bond is formed between graphene and either of the other two materials, and the structure becomes a van der Waals (vdW) structure, i.e., physiosorption occurs instead.…”

Reinterpreting Low Resistance in Sb–MoS2 Ohmic Contacts by Means of Ab Initio Transport Simulations

“…The Sb(01 12)-MoS 2 VHJ is simulated with Quantum ESPRESSO (QE) [16], [17] and with an orthorhombic topological symmetry of the VHJ, which is a natural choice for transport calculations [15]. Given the noncommensurate Sb and MoS 2 lattices, we defined a commensurate cell for the VHJ resorting to a procedure well established in the literature [12], [18]. Namely, we matched a strained Sb 2 × 2 unit cell onto an unstrained MoS 2 [Fig.…”

“…work function (WF) is arguably the most promising option to reduce R C in n-type FETs with 2-D semiconductors [7], [8], [9], [10], [11], [12]. This is attributed to a combination of low WF and low density of states (DoS) at the Fermi level (E F ), which leads to a suppression of metal-induced-gap-states (MIGS) close to the MoS 2 conduction band (CB) [12]. The lowest experimentally reported R C extracted in a 2-D FET device using semimetallic Bi to contact MoS 2 is 123 • µm [7].…”

“…In this work, for the first time, we present an ab initio transport study of the Sb(01 12)-MoS 2 system as shown in Fig. 1(a), based on an in-house developed simulator [10], [12], [14], [15]. This article is organized as follows.…”