Ohmic Contacts to 2D Semiconductors through van der Waals Bonding

The extremely high carrier mobility and the unique band structure, make graphene very useful for field-effect transistor applications. According to several works, the primary limitation to graphene based transistor performance is not related to the material quality, but to extrinsic factors that affect the electronic transport properties. One of the most important parasitic element is the contact resistance appearing between graphene and the metal electrodes functioning as the source and the drain. Ohmic contacts to graphene, with low contact resistances, are necessary for injection and extraction of majority charge carriers to prevent transistor parameter fluctuations caused by variations of the contact resistance. The International Technology Roadmap for Semiconductors, toward integration and down-scaling of graphene electronic devices, identifies as a challenge the development of a CMOS compatible process that enables reproducible formation of low contact resistance. However, the contact resistance is still not well understood despite it is a crucial barrier towards further improvements. In this paper, we review the experimental and theoretical activity that in the last decade has been focusing on the reduction of the contact resistance in graphene transistors.We will summarize the specific properties of graphene-metal contacts with particular attention to the nature of metals, impact of fabrication process, Fermi level pinning, interface modifications induced through surface processes, charge transport mechanism, and edge contact formation.

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“…As possible buffer layers are reported graphene, h-BN, NbS2, and MoO3. The figure is reproduced from [265] with permission.…”

Section: Contacting 2d Materialsmentioning

confidence: 99%

The role of contact resistance in graphene field-effect devices

Giubileo¹,

Bartolomeo

2017

Progress in Surface Science

217

144

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“…An example DOS plot is shown in Figure a. We have adopted the reference‐energy method to estimate the p‐SBH, by aligning the semi‐core levels of the adsorbed TMD with those of a free‐standing TMD layer (see Figure S6 in the Supporting Information for an example). The p‐SBH values thus computed are shown in Figure b.…”

Section: Resultsmentioning

confidence: 99%

“…One way to enhance the hole injection efficiency is to heavily dope the semiconductor in the contact region, but it is difficult to locally control the doping, that could eventually deteriorate with time . An alternative solution would be to insert a buffer layer between the semiconductor and the metal, thereby suppressing the interface states and de‐pinning the Fermi level.…”

Section: Introductionmentioning

confidence: 99%

Engineering Efficient p‐Type TMD/Metal Contacts Using Fluorographene as a Buffer Layer

Musso

Kumar

Grossman

et al. 2017

Adv Elect Materials

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structurally stable and largely lack dangling bonds. The production processes of TMDs are currently well established, ranging from top-down exfoliation of the bulk material using mechanical exfoliation, solution-based approaches and the bottom-up synthesis methods using chemical vapor deposition. [7,8] TMDs have gained significant importance as excellent candidates for nanoelectronic applications. [9,10] MoS 2 is one of the most commonly studied TMD in this regard, which demonstrates a high mobility (in the range 1-50 cm 2 V −1 s −1 at room temperature [11,12] ), comparable to that of silicon. In addition, field-effect transistors (FETs) based on MoS 2 show low power dissipation [1] and efficient control over switching, [2] leading to widespread research interest in this topic.While these properties are certainly encouraging, one major limitation of such FETs is that the carrier transport in the semiconductor channel is mostly electron-mediated, [13] resulting in n-type FETs (n-FETs). Despite attempts to employ high workfunction metal contacts to obtain hole-based transport, the resulting devices have instead widely shown n-character. [13] This intrinsic behavior of the unmodified MoS 2 -metal interface hinders the construction of fully integrated circuits, [7] because of the difficulty in obtaining a CMOS (complementary metal oxide semiconductor) device, where the building block of logic gates and digital circuits require both n-and p-type MOS architectures.The fabrication of p-FETs based on monolayer MoS 2 is challenging [13] because of a particular interfacial phenomenon between the TMD and the metal contact, namely Fermi level pinning. [14,15] It is commonly believed that the interfacial gap states between MoS 2 and the metal contacts are responsible for pinning the Fermi level close to the conduction band, even upon using a high work-function metal. These gap states may be surface states (Bardeen's theory), metal-induced gap states (MIGS) or defect/disorder-induced gap states. [14,15] Guo, Francois and co-workers [16,17] consider the MIGS theory the best candidate to explain the origin of the gap states. A different point of view is assumed by McDonnell et al., [18,19] wherein they attribute the difficulty in producing hole-based MoS 2 devices to the intrinsic low work-function defects present in MoS 2 , responsible for the variability of electronic properties across the samples. These native defects such as vacancies present in MoS 2 and other TMDs like WSe 2 may actually result in variations of the TMDs work function, as observed in experiments. [20] P-type transistors based on high work function transition metal dichalcogenide (TMD) monolayers such as MoS 2 are to date difficult to produce, owing to the strong Fermi level pinning at the semiconductor/contact metal interfaces. In this work, the potential of halogenated graphenes is demonstrated as a new class of efficient hole injection layers to TMDs such as MoS 2 and WSe 2 by taking fluorographene (or GF) as a model buffer layer. Using first-principles c...

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“…c) Cross‐sectional diagram of metal/TMD interface with insertion of various 2D layers. Reproduced with permission . d) Contact scheme between metallic and semiconducting 2D materials.…”

Section: Fabrication Technologiesmentioning

confidence: 99%

Electronic and Optoelectronic Devices based on Two‐Dimensional Materials: From Fabrication to Application

Shim

Park

Kang

et al. 2017

Adv Elect Materials

156

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promising and important 2D material. Graphene-based electronic and optoelectronic device applications [11][12][13][14][15] have been widely researched to take advantage of its many outstanding properties, such as high carrier mobility (10 000 cm 2 V −1 s −1 at room temperature), [16] excellent optical transparency (97% for monolayer), [17] high Young's modulus (0.5-1 TPa), [18] and wide absorption spectrum (300-1400 nm). [19] The unique properties of graphene have also triggered extensive research in other 2D materials including TMDs, particularly for various electronic [4,[11][12][13] and optoelectronic device applications. [3,5,14,15,24,31, 2D electronic devices have advanced from single junction to heterojunction devices and have recently been used in lateral/vertical field-effect transistors (FETs), [4,[11][12][13][62][63][64]67,68] negative differential resistance (NDR) devices, [42][43][44][45][46][47][48][49][50]66] and memory devices. [51][52][53][54][55][56][57][58][59][60][61]65] Especially, type-II heterojunctions have enabled efficient electron-hole separation during the conversion from light to current, and these have found use in high-performance 2D optoelectronic devices such as photodetectors, [3,5,14,15,24,31,[69][70][71][72][73][74][75][76][77][78][79][80][81][82][83]102,103] photovoltaic devices, [84][85][86][87][88][89][90][91][92][93][94][95] and light-emitting diodes (LEDs). [87,88,[95][96][97][98][99][100][101] The electronic and optoelectronic properties of semiconducting TMDs depend on the number of layers due to quantum confinement effects. [104,105] For example, the transition from an indirect energy bandgap to a direct bandgap was observed for molybdenum-and tungstenbased TMDs as layer thickness decreases and approaches a monolayer. It has been reported that bulk molybdenum disulfide (MoS 2 ) and tungsten diselenide (WSe 2 ) have an indirect bandgap of 1.2 eV, whereas monolayer MoS 2 and WSe 2 have direct bandgaps of 1.88 eV and 1.65 eV, respectively, which lead to a high on/off-current ratio and high quantum efficiency. [106] Meanwhile, rhenium disulfide (ReS 2 ) exhibited a direct bandgap of 1.44 eV independent of its layer thickness, [107] and it is considered a promising candidate for future optoelectronic devices. [24,31,33,82,83] In this review, we first introduce important fabrication techniques of i) doping, ii) contact engineering, and iii) heterojunction formation for improving the performance of 2D-material-based devices in terms of fieldeffect mobility, on-current, responsivity, and temporal response (Section 2). We then discuss promising 2D-material-based electronic (Section 3) and optoelectronic (Section 4) devices,Since the rediscovery of graphene in 2004, this material has attracted an enormous amount of interest owing to its unique structural, mechanical, electronic, and optical properties. Beyond this, the unique properties of graphene have also triggered extensive research on other two-dimensional (2D) materials including transition metal dichalcogenides (TMDs), part...

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Ohmic Contacts to 2D Semiconductors through van der Waals Bonding

Cited by 105 publications

References 61 publications

The role of contact resistance in graphene field-effect devices

The role of contact resistance in graphene field-effect devices

Engineering Efficient p‐Type TMD/Metal Contacts Using Fluorographene as a Buffer Layer

Electronic and Optoelectronic Devices based on Two‐Dimensional Materials: From Fabrication to Application

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