Low-Temperature Ohmic Contact to Monolayer MoS<sub>2</sub> by van der Waals Bonded Co/<i>h</i>-BN Electrodes

Xu, Chonghai; Shih, En-Min; Jauregui, Luis A.; Chae, Sang Hoon; Kim, Young Duck; Li, Baichang; Seo, Dongjea; Pistunova, Kateryna; Yin, Jun; Park, Jihoon; Choi, Heon‐Jin; Lee, Young Hee; Watanabe, Kenji; Taniguchi, Takashi; Kim, Philip; Dean, Cory; Hone, James

doi:10.1021/acs.nanolett.7b01536

Cited by 253 publications

(267 citation statements)

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“…It is noteworthy that the roughness of the oxidized InSe surface is comparable to that of h-BN (i.e., 90 pm), which exhibits an atomically smooth surface 30 and can be used in tunneling contacts for 2D-semiconductor-based devices. 31,32 This ultra-uniform surface thus suggests the viability of employing the OML as a tunneling barrier. As a comparison, nucleation sites are observable in the InSe samples not subjected to the UV-ozone treatment, which may be attributed to the native oxide that arises during exposure to the ambient atmosphere ( Supplementary Fig.…”

Section: Resultsmentioning

confidence: 86%

Oxidized-monolayer tunneling barrier for strong Fermi-level depinning in layered InSe transistors

et al. 2019

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In 2D-semiconductor-based field-effect transistors and optoelectronic devices, metalsemiconductor junctions are one of the crucial factors determining device performance. The Fermilevel (FL) pinning effect, which commonly caused by interfacial gap states, severely limits the tunability of junction characteristics, including barrier height and contact resistance. A tunneling contact scheme has been suggested to address the FL pinning issue in metal-2D-semiconductor junctions, whereas the experimental realization is still elusive. Here, we show that an oxidizedmonolayer-enabled tunneling barrier can realize a pronounced FL depinning in indium selenide (InSe) transistors, exhibiting a large pinning factor of 0.5 and a highly modulated Schottky barrier height. The FL depinning can be attributed to the suppression of metal-and disorder-induced gap states as a result of the high-quality tunneling contacts. Structural characterizations indicate uniform and atomically thin surface oxidation layer inherent from nature of van der Waals materials and atomically sharp oxide-2D-semiconductor interfaces. Moreover, by effectively lowering the Schottky barrier height, we achieve an electron mobility of 2160 cm 2 /Vs and a contact barrier of 65 meV in two-terminal InSe transistors. The realization of strong FL depinning in high-mobility InSe transistors with the oxidized monolayer presents a viable strategy to exploit layered semiconductors in contact engineering for advanced electronics and optoelectronics.3

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Section: Resultsmentioning

confidence: 86%

Oxidized-monolayer tunneling barrier for strong Fermi-level depinning in layered InSe transistors

et al. 2019

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“…Due to this, the MoS 2 conduction band can be readily modulated by the back gate voltage and pushed below the metal Fermi level, thereby making itself accessible for electrons tunneling through the oxide.This lowers the transfer resistance from Ti to MoS 2 . The usage of such Fermi level de-pinning layer has been recently demonstrated for Co/h-BN electrodes 44.…”

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Electron Transport through Metal/MoS₂ Interfaces: Edge- or Area-Dependent Process?

et al. 2019

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In ultra-thin two-dimensional (2-D) materials, the formation of ohmic contacts with top metallic layers is a challenging task that involves different processes than in bulk-like structures. Besides the Schottky barrier height, the transfer length of electrons between metals and 2-D monolayers is a highly relevant parameter. For MoS 2 , both short (≤30 nm) and long (≥0.5 µm) values have been reported, corresponding to either an abrupt carrier injection at the contact edge or a more gradual transfer of electrons over a large contact area. Here we use ab initio quantum transport simulations to demonstrate that the presence of an oxide layer between a metallic contact and a MoS 2 monolayer, for example TiO 2 in case of titanium electrodes, favors an area-dependent process with a long transfer length, while a perfectly clean metal-semiconductor interface would lead to an edge process. These findings reconcile several theories that have been postulated about the physics of metal/MoS 2 interfaces and provide a framework to design future devices with lower contact resistances. 1 Keywords: 2-D materials, metal-semiconductor interfaces, contact physics, transfer length, Fermi level pinning, ab initio device simulations Transistors made of novel two-dimensional (2-D) materials beyond graphene such as single-layer MoS 2 1 have generated considerable excitement among the scientific community for their potential as active components of future integrated circuits. Transition metal dichalcogenides (TMDs), 2 black phosphorus, 3,4 and hundreds of other presumably exfoliable 2-D monolayers 5 appear as excellent candidates to outperform Si FinFETs, the current workhorse of the semiconductor industry, for next-generation ultra-scaled logic switches. 6 The advantages of 2-D materials over competing technologies reside in their naturally passivated surfaces, their planar geometry providing an excellent electrostatic control, 7 their exceptionally high carrier mobilities as compared to 3-D compounds with the same subnanometer thickness, 8-10 and the possibility of stacking them on top of each other to form van der Waals heterostructures. 11-13Before MoS 2 field-effect transistors (FETs) with a monolayer channel can reach their full potential and deliver the expected performance, 14 several key challenges remain to be solved.

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“…[2][3][4][5][6] In particular, atomically thin monolayers have proven to be an ideal platform for the exploration of the unique features associating with 2D electronic systems such as ultrahigh mobility, [7] quantum phase transition, [8] indirect exciton condensation, [9] quantum oscillation, [10] and the quantum Hall effect. and other mono-elemental materials (e.g., black phosphorus, tellurene, etc.)…”

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Probing Distinctive Electron Conduction in Multilayer Rhenium Disulfide

Lee

Choi

et al. 2018

Advanced Materials

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WSe 2 , etc.) and other mono-elemental materials (e.g., black phosphorus, tellurene, etc.) have generated significant interest in numerous research fields. [2][3][4][5][6] In particular, atomically thin monolayers have proven to be an ideal platform for the exploration of the unique features associating with 2D electronic systems such as ultrahigh mobility, [7] quantum phase transition, [8] indirect exciton condensation, [9] quantum oscillation, [10] and the quantum Hall effect. [11] Despite the unique merits of monolayers, multilayers, which consist of multiple conducting monolayers, may be a better candidate for most 2D layered materials because of their higher carrier mobility and larger current density compared to monolayers, with respect to applications in electronic devices. [3,6,12,13] Previous theoretical and experimental investigations have clearly demonstrated that the effective mass increases with bandgap energy as thickness reduced. [14] In addition, the adjacent surface phonon and numerous Coulomb scatterers that surround atomically thin channel materials further degrade the intrinsic carrier mobility, [15,16] implying the disadvantage of monolayer platform as a transistor.Among the various families of 2D layered materials, a multilayer rhenium disulfide (ReS 2 ) has recently garnered notable attention because of their three unique properties compared to other TMD materials; i) anisotropic in-plane transport, ii) direct bandgap, and iii) layer-independent electronic band structure. [17] The distorted octahedral (1T′) structure of ReS 2 is responsible for the anisotropic in-plane transport property. [18][19][20] The layer-independent electronic band structure with direct bandgap is linked to a decoupled vdW interaction between adjacent layers, [21] leading to a much higher interlayer resistivity of bulk ReS 2 [22] compared to that of the other TMDs [23] including MoS 2 . [24] This fact implies that few-layered ReS 2 is the potentially a superior platform for studying intrinsic carrier transport features of multilayer systems. This is because the interlayer resistivity governs the charge distribution along the channel thickness [25] and the channel access resistance, in addition to the Thomas-Fermi charge screening length.Herein, we report on the distinctive electron conduction mechanism of few-layered 1T′ ReS 2 by presenting; i) the anisotropic carrier transport, ii) modification of charge distribution Charge carrier transport in multilayer van der Waals (vdW) materials, which comprise multiple conducting layers, is well described using Thomas-Fermi charge screening (λ TF ) and interlayer resistance (R int ). When both effects occur in carrier transport, a channel centroid migrates along the c-axis according to a vertical electrostatic force, causing redistribution of the conduction centroid in a multilayer system, unlike a conventional bulk material. Thus far, numerous unique properties of vdW materials are discovered, but direct evidence for distinctive charge transport behavior in 2D layered materi...

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Low-Temperature Ohmic Contact to Monolayer MoS₂ by van der Waals Bonded Co/h-BN Electrodes

Cited by 253 publications

References 43 publications

Oxidized-monolayer tunneling barrier for strong Fermi-level depinning in layered InSe transistors

Oxidized-monolayer tunneling barrier for strong Fermi-level depinning in layered InSe transistors

Electron Transport through Metal/MoS₂ Interfaces: Edge- or Area-Dependent Process?

Probing Distinctive Electron Conduction in Multilayer Rhenium Disulfide

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Low-Temperature Ohmic Contact to Monolayer MoS2 by van der Waals Bonded Co/h-BN Electrodes

Cited by 253 publications

References 43 publications

Oxidized-monolayer tunneling barrier for strong Fermi-level depinning in layered InSe transistors

Oxidized-monolayer tunneling barrier for strong Fermi-level depinning in layered InSe transistors

Electron Transport through Metal/MoS2 Interfaces: Edge- or Area-Dependent Process?

Probing Distinctive Electron Conduction in Multilayer Rhenium Disulfide

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Low-Temperature Ohmic Contact to Monolayer MoS₂ by van der Waals Bonded Co/h-BN Electrodes

Electron Transport through Metal/MoS₂ Interfaces: Edge- or Area-Dependent Process?