Ultralow contact resistance between semimetal and monolayer semiconductors

McClellan C.J., Yalon E., Smithe K.K., Suryavanshi S.V., Pop E.

Semiconductors require stable doping for applications in transistors, optoelectronics, and thermoelectrics. However, this has been challenging for two-dimensional (2D) materials, where existing approaches are either incompatible with conventional semiconductor processing or introduce time-dependent, hysteretic behavior. Here we show that low temperature (< 200$^\circ$ C) sub-stoichiometric AlO$_x$ provides a stable n-doping layer for monolayer MoS$_2$, compatible with circuit integration. This approach achieves carrier densities > 2x10$^{13}$ 1/cm$^2$, sheet resistance as low as ~7 kOhm/sq, and good contact resistance ~480 Ohm.um in transistors from monolayer MoS$_2$ grown by chemical vapor deposition. We also reach record current density of nearly 700 uA/um (>110 MA/cm$^2$) in this three-atom-thick semiconductor while preserving transistor on/off current ratio > $10^6$. The maximum current is ultimately limited by self-heating and could exceed 1 mA/um with better device heat sinking. With their 0.1 nA/um off-current, such doped MoS$_2$ devices approach several low-power transistor metrics required by the international technology roadmap

Wang Q., Shao Y., Gong P., Shi X.

Thickness-dependent performance of metal–two-dimensional semiconductor junctions in electronics/optoelectronics have attracted increasing attention but, currently, little knowledge about the micro-mechanism of this thickness dependence is available.

Cao Z., Lin F., Gong G., Chen H., Martin J.

The semiconductor MoS2 has attracted much attention owing to its sizable energy bandgap, significant spin–orbit coupling, and quantum effects such as the valley Hall effect and gate-induced superconductivity. However, in electronic devices, the energy bandgap usually gives rise to the formation of Schottky barriers at the interface to the contact metal, which may render devices intended for quantum transport inapplicable at low temperature. Therefore, the fabrication of Ohmic contacts operational at low temperature is crucial. Yet, it currently remains a substantial challenge to produce low resistive contacts with a simple process. We manifest that low temperature Ohmic contacts to mono- and few-layer MoS2 can be achieved with Tin (Sn) as the contact metal. Sn is directly evaporated onto MoS2, and hence, this establishes a much easier fabrication method than tunneling barriers, for example. We provide detailed device characterization, extract Schottky barrier heights, demonstrate multiterminal measurements, and propose a possible explanation: strain induced deformation of MoS2 imposed by Sn.

Smets Q., Groven B., Caymax M., Radu I., Arutchelvan G., Jussot J., Verreck D., Asselberghs I., Mehta A.N., Gaur A., Lin D., Kazzi S.E.

We show that downscaling the top-contact length to 13nm induces no penalty on the electrical characteristics for CVD MoS 2 FETs. We demonstrate this for devices with different gate-oxides and operating in both channel and contact-limited regimes, thus confirming carrier injection at the edge of the contact metal. Consequently, we have scaled the device footprint achieving an I on =250μA/μm and excellent SS min =80mV/dec for 50nm SiO 2 and 4nm HfO 2 gate oxides, respectively.

Razavieh A., Zeitzoff P., Nowak E.J.

Scaling trends of FinFET architecture, with focus on Front-End-of-Line (FEOL), and Middle-of-Line (MOL) device parameters, is systematically investigated. It is concluded that the combined requirements of device electrostatics together with the demands on contact resistance, presents a Contacted-Gate-Pitch (CGP) scaling limit for horizontal-transport FETs. FET drive is expected to significantly degrade below this CGP ~ 40 nm as a result. Good agreement between hardware data and TCAD simulations is achieved and employed to estimate the contact resistance values for aggressively scaled FinFETs. These observations show that FinFETs scaled below CGP of 40 nm will require the contact resistivity (ρ C ) of ~8 × 10 -10 Ω-cm 2 , while fully ohmic contacts i.e., ρ C of ~1 × 10 -10 Ω-cm 2 will be required if FinFETs are to extend performance below CGP of 30 nm. Ultimately, transition to new device architectures in which contact area is independent of CGP and/or Fin-Pitch will be necessary.

Akinwande D., Huyghebaert C., Wang C., Serna M.I., Goossens S., Li L., Wong H.-., Koppens F.H.

The development of silicon semiconductor technology has produced breakthroughs in electronics—from the microprocessor in the late 1960s to early 1970s, to automation, computers and smartphones—by downscaling the physical size of devices and wires to the nanometre regime. Now, graphene and related two-dimensional (2D) materials offer prospects of unprecedented advances in device performance at the atomic limit, and a synergistic combination of 2D materials with silicon chips promises a heterogeneous platform to deliver massively enhanced potential based on silicon technology. Integration is achieved via three-dimensional monolithic construction of multifunctional high-rise 2D silicon chips, enabling enhanced performance by exploiting the vertical direction and the functional diversification of the silicon platform for applications in opto-electronics and sensing. Here we review the opportunities, progress and challenges of integrating atomically thin materials with silicon-based nanosystems, and also consider the prospects for computational and non-computational applications. Progress in integrating atomically thin two-dimensional materials with silicon-based technology is reviewed, together with the associated opportunities and challenges, and a roadmap for future applications is presented.

Jung Y., Choi M.S., Nipane A., Borah A., Kim B., Zangiabadi A., Taniguchi T., Watanabe K., Yoo W.J., Hone J., Teherani J.T.

Two-dimensional semiconductors have a number of valuable properties that could be used to create novel electronic devices. However, creating 2D devices with good contacts and stable performance has proved challenging. Here we show that transferred via contacts, made from metal embedded in insulating hexagonal boron nitride and dry transferred onto 2D semiconductors, can be used to create high-quality 2D transistors. The approach prevents damage induced by direct metallization and allows full glovebox processing, providing a clean, stable and damage-free platform for 2D device fabrication. Using the approach, we create field-effect transistors (FETs) from bilayer p-type tungsten diselenide (WSe2) that exhibit high hole mobility and low contact resistance. The fabricated devices also exhibit high current and stability for over two months of measurements. Furthermore, the low contact resistance and clean channel allow us to create a nearly ideal top-gated p-FET with a subthreshold swing of 64 mV per decade at 290 K. Bilayer WSe2 field-effect transistors with near ideal device characteristics can be created using transferred via contacts made from metal-embedded hexagonal boron nitride.

Wang Y., Kim J.C., Wu R.J., Martinez J., Song X., Yang J., Zhao F., Mkhoyan A., Jeong H.Y., Chhowalla M.

As the dimensions of the semiconducting channels in field-effect transistors decrease, the contact resistance of the metal–semiconductor interface at the source and drain electrodes increases, dominating the performance of devices1–3. Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS2) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors4,5. However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition-metal dichalcogenide3,5–9. Recent studies have shown that van der Waals contacts formed by transferred graphene10,11 and metals12 on few-layered transition-metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nanometre-thick indium metal capped with 100-nanometre-thick gold electrodes and monolayer MoS2. Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold-capped indium and the MoS2 is atomically sharp with no detectable chemical interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS2. The contact resistance of the indium/gold electrodes is 3,000 ± 300 ohm micrometres for monolayer MoS2 and 800 ± 200 ohm micrometres for few-layered MoS2. These values are among the lowest observed for three-dimensional metal electrodes evaporated onto MoS2, enabling high-performance field-effect transistors with a mobility of 167 ± 20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220 ± 50 ohm micrometres on ultrathin niobium disulfide (NbS2) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS2) and tungsten diselenide (WSe2) contacted with indium alloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors. Ultraclean van der Waals bonds between gold-capped indium and a monolayer of the two-dimensional transition-metal dichalcogenide molybdenum disulfide show desirably low contact resistance at the interface, enabling high-performance field-effect transistors.

Sotthewes K., van Bremen R., Dollekamp E., Boulogne T., Nowakowski K., Kas D., Zandvliet H.J., Bampoulis P.

Understanding the electron transport through transition-metal dichalcogenide (TMDC)-based semiconductor/metal junctions is vital for the realization of future TMDC-based (opto-)electronic devices. Despite the bonding in TMDCs being largely constrained within the layers, strong Fermi-level pinning (FLP) was observed in TMDC-based devices, reducing the tunability of the Schottky barrier height. We present evidence that metal-induced gap states (MIGS) are the origin for the large FLP similar to conventional semiconductors. A variety of TMDCs (MoSe2, WSe2, WS2, and MoTe2) were investigated using high-spatial-resolution surface characterization techniques, permitting us to distinguish between defected and pristine regions. The Schottky barrier heights on the pristine regions can be explained by MIGS, inducing partial FLP. The FLP strength is further enhanced by disorder-induced gap states induced by transition-metal vacancies or substitutionals at the defected regions. Our findings emphasize the importance of defects on the electron transport properties in TMDC-based devices and confirm the origin of FLP in TMDC-based metal/semiconductor junctions.

Yue D., Kim C., Lee K.Y., Yoo W.J.

Chee S., Seo D., Kim H., Jang H., Lee S., Moon S.P., Lee K.H., Kim S.W., Choi H., Ham M.

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS2 devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS2 film and a Ag electrode as an interfacial layer. The MoS2 field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm2 V−1 s−1, an on/off current ratio of 4 × 108, and a photoresponsivity of 2160 A W−1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS2 and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS2 and graphene is a key step toward the practical application of atomically thin TMDC‐based devices with low‐resistance contacts for high‐performance large‐area electronics and optoelectronics.

Moe Y.A., Sun Y., Ye H., Liu K., Wang R.

Strain usually exists in two-dimensional (2D) materials and devices, and its presence drastically modulates their properties. When 2D materials interface with noble metals, local strain and surface plasmon can couple at the metal-2D material boundaries, delivering a lot of intriguing phenomena. Current studies are mostly focused on the explanations of these strain-related phenomena based on a static point of view. Although strain can typically be relaxed in many environments, the time evolution of strain at metal-2D material interfaces remains largely unknown. In this work, we investigate the evolution of local strain at Ag-MoS2 boundaries by surface-enhanced Raman scattering. With the split of MoS2 Raman peaks as an indicator of local strain, it is found that the originally localized strain at Ag-MoS2 boundaries evolves and relaxes with time into a delocalized strain in MoS2 plane. The time to start the strain relaxation depends on the number of layers of MoS2 flakes, suggesting that the relaxation may result from the mechanical instability of the interface between the topmost MoS2 layer and the underlying materials. The relaxation occurs in a certain period of time, i.e., ∼70 days for 1L and ∼30 days for 3L. Accompanying the strain relaxation, surface sulfurization of Ag also occurs, a process that reduces the strength of locally enhanced electric field. Our results not only provide a deep understanding of strain evolution at metal-MoS2 interfaces but also shed light on the optimization of MoS2-based device fabrications.

Liu Y., Guo J., Zhu E., Liao L., Lee S., Ding M., Shakir I., Gambin V., Huang Y., Duan X.

The junctions formed at the contact between metallic electrodes and semiconductor materials are crucial components of electronic and optoelectronic devices 1 . Metal–semiconductor junctions are characterized by an energy barrier known as the Schottky barrier, whose height can, in the ideal case, be predicted by the Schottky–Mott rule2–4 on the basis of the relative alignment of energy levels. Such ideal physics has rarely been experimentally realized, however, because of the inevitable chemical disorder and Fermi-level pinning at typical metal–semiconductor interfaces2,5–12. Here we report the creation of van der Waals metal–semiconductor junctions in which atomically flat metal thin films are laminated onto two-dimensional semiconductors without direct chemical bonding, creating an interface that is essentially free from chemical disorder and Fermi-level pinning. The Schottky barrier height, which approaches the Schottky–Mott limit, is dictated by the work function of the metal and is thus highly tunable. By transferring metal films (silver or platinum) with a work function that matches the conduction band or valence band edges of molybdenum sulfide, we achieve transistors with a two-terminal electron mobility at room temperature of 260 centimetres squared per volt per second and a hole mobility of 175 centimetres squared per volt per second. Furthermore, by using asymmetric contact pairs with different work functions, we demonstrate a silver/molybdenum sulfide/platinum photodiode with an open-circuit voltage of 1.02 volts. Our study not only experimentally validates the fundamental limit of ideal metal–semiconductor junctions but also defines a highly efficient and damage-free strategy for metal integration that could be used in high-performance electronics and optoelectronics. In metal–semiconductor junctions, interfacial bonding and disorder cause deviations from theoretical predictions for the energy barrier, but delicately transferring pre-fabricated metal films onto two-dimensional semiconductors can overcome this challenge.

Smithe K.K., Suryavanshi S.V., Muñoz Rojo M., Tedjarati A.D., Pop E.

Despite much interest in applications of two-dimensional (2D) fabrics such as MoS2, to date most studies have focused on single or few devices. Here we examine the variability of hundreds of transistors from monolayer MoS2 synthesized by chemical vapor deposition. Ultraclean fabrication yields low surface roughness of ∼3 Å and surprisingly low variability of key device parameters, considering the atomically thin nature of the material. Threshold voltage variation and very low hysteresis suggest variations in charge density and traps as low as ∼1011 cm-2. Three extraction methods (field-effect, Y-function, and effective mobility) independently reveal mobility from 30 to 45 cm2/V/s (10th to 90th percentile; highest value ∼48 cm2/V/s) across areas >1 cm2. Electrical properties are remarkably immune to the presence of bilayer regions, which cause only small conduction band offsets (∼55 meV) measured by scanning Kelvin probe microscopy, an order of magnitude lower than energy variations in Si films of comparable thickness. Data are also used as inputs to Monte Carlo circuit simulations to understand the effects of material variability on circuit variation. These advances address key missing steps required to scale 2D semiconductors into functional systems.

Cui X., Shih E., Jauregui L.A., Chae S.H., Kim Y.D., Li B., Seo D., Pistunova K., Yin J., Park J., Choi H., Lee Y.H., Watanabe K., Taniguchi T., Kim P., et. al.

Monolayer MoS2, among many other transition metal dichalcogenides, holds great promise for future applications in nanoelectronics and optoelectronics due to its ultrathin nature, flexibility, sizable band gap, and unique spin-valley coupled physics. However, careful study of these properties at low temperature has been hindered by an inability to achieve low-temperature Ohmic contacts to monolayer MoS2, particularly at low carrier densities. In this work, we report a new contact scheme that utilizes cobalt (Co) with a monolayer of hexagonal boron nitride (h-BN) that has the following two functions: modifies the work function of Co and acts as a tunneling barrier. We measure a flat-band Schottky barrier of 16 meV, which makes thin tunnel barriers upon doping the channels, and thus achieve low-T contact resistance of 3 kΩ.μm at a carrier density of 5.3 × 1012/cm2. This further allows us to observe Shubnikov-de Haas oscillations in monolayer MoS2 at much lower carrier densities compared to previous work.

Kwon S., Ma P.J., Lum C., Hajarian A., Seo J., Nam S.

Chen Z., Mu X., Wang Y., Zhou Y., Gao J., Li X.

Chang C., Huang P., Lin K., Chang T., Tu W., Kaun C., Lin S.

Yao Q., Li S., Dong L., Gu P., Liu X., Wang F., Yuan G., Yu Z.

Tang J., Li S., Zhan L., Li S.

Lan H., Tan Y., Yang S., Liu X., Shang Z., Appenzeller J., Chen Z.

Pan C., Sun D., Gong W., Xu L., Zhang X., Shi A., Li Y., Niu X.

AbstractRising 2D metal‐semiconductor junctions (MSJs) greatly advance nanoelectronics. Currently, evaluating the performance of MSJs mainly relies on determining the formation of Ohmic or Schottky contacts through static electronic structures. However, when MSJs are integrated into optoelectronic devices, dynamic transport and excited state carrier dynamics become crucial, yet the impact of the interface remains unclear. Herein, taking 2H‐WTe2 based vertical and lateral MSJs as examples, their performances are evaluated in the perspective of ground and excited states. Faced with the significant band hybridization induced by strong interfacial coupling, identifying Ohmic contact only by analyzing electronic structures possibly becomes defunct. Through quantum transport simulation, the junction with the largest on‐state current can be further filtered. Meanwhile, non‐adiabatic molecular dynamics simulation uncovers the carrier lifetime can be shortened or extended as the interfaces are formed, which depends on the competitive relationship between electron‐phonon coupling and trap states. In these MSJs studied, the photoexcited carriers with the nanosecond lifetime can be rapidly conducted away by electrodes, indicating a high‐effect utilization rate. The work advances the all‐around routine for rational designing and characterizing 2D MSJs.

Jia Z., Zhang X., Jiang Z., Zhang Z.

Hu H., Wang Z., Pan M., Chen Y., Han Y., Wang J.

Abstract2D transition metal dichalcogenides (2D TMDs) have emerged as promising candidates in electrocatalysis due to their unique band structures and tunable electronic properties. Nevertheless, establishing robust, low‐resistance contacts between TMDs layers and conductive supports has remained a challenge. Their atomically thin nature makes these layers prone to structural disruption and undesired chemical interactions, hampering charge transfer and diminishing catalytic efficiency. Recently, the visualization of microscopic interface behaviors and atomic layer interactions between metals and 2D TMDs has led to the introduction of ohmic contact metal‐TMDs electrocatalysts to address these challenges. Specifically, synergy at the metal‐2D TMDs interface endows the catalyst with new functionalities, including enhanced redox activity and selective reactant immobilization, thus helping address core challenges in energy conversion and storage. This work first examines the fundamental structural traits of 2D TMDs and introduces design principles and strategies for ohmic metal‐TMDs composites in electrocatalysis. The discussion covers methods for adjusting work function differences, constructing edge contacts in TMDs, incorporating interface doping/insertion, and engineering orbital hybridization or bonding interfaces. Additionally, this work analyzes the advantages, limitations, and future prospects of each approach, offering valuable insights for the development of efficient metal‐semiconductor catalysts, electrodes, and energy conversion and storage devices.

Hassan Y., Sabbtain Abbas M., Sup Choi M.

This chapter provides an in-depth analysis of advanced contact and doping techniques to optimize the performance of two-dimensional (2D) semiconductors, which are promising candidates for next-generation electronic and optoelectronic devices. Contact engineering techniques, including top and bottom contacts, transferred contacts, van der Waals (vdW), edge contacts, and semi-metallic contacts, are systematically explored. These approaches address critical issues such as Schottky barrier formation, Fermi level pinning, and interface damage caused by conventional deposition techniques. For instance, bottom and transferred contacts minimize interfacial defects, while vdW, edge, and semi-metallic contacts mitigate metal-induced gap states and ensure clean, defect-free interfaces, enhancing charge injection efficiency and device performance. In parallel, the chapter examines doping strategies to modulate the electronic properties of 2D materials. Oxidation doping introduces oxygen atoms to substitute sulfur vacancies, enabling p-type doping with controlled structural integrity. Laser-induced doping leverages precision energy delivery to tailor carrier concentration and reduce contact resistance, while charge transfer doping achieves either n-type or p-type behavior through surface-adsorbed dopants like benzyl viologen, Magic Blue, and metal oxides without compromising the crystal structure. Each method is discussed with its mechanisms, advantages, and challenges, including issues like contamination, scalability, and stability.

Mousavi Khaleghi S.S., Wei J., Liu Y., Wang Y., Fan Z., Li K., Chen J., Kudrawiec R., Yang R., Crozier K.B., Dan Y.

Chou S., Chang C., Wu B., Chuu C., Kuo P., Pan L., Huang K., Lai M., Chen Y., Lee C., Chen H., Shiue J., Chang Y., Li M., Chiu Y., et. al.

Wong H.

Samizadeh Nikoo M., Chu C., Lin B., Liu Y., Kim Y., Wang H.

Gao B., Yan Y., Zhang S., Wu Z., Meng Y., Zhang Y., Wang W., Shen Y., Hu S., Li B., Shao H., Xie P., Yip S., Ho J.C.

AbstractVan der Waals (vdWs) p–n junctions assembled from 2D materials offer enhanced flexibility for creating versatile electronic and optoelectronic devices, attracting significant interest. However, the lack of reliable methods to produce high‐quality p‐type 2D semiconductors, especially patterned p‐type channels, remains a major challenge for progress in the field. Here, a precise substitutional doping strategy for 2D semiconductors is presented, enabling the production of millimeter‐scale WS2 single‐crystal thin films with tailored p‐type and n‐type properties. This advancement supports the fabrication of high‐performance WS2‐based p‐type and n‐type field‐effect transistor (FET) miniaturized arrays with near‐ohmic contact. Building on this progress, a WS2 van der Waals homojunction p‐n array demonstrating distinct anti‐ambipolar behavior and excellent rectification characteristics is developed. In self‐powered photodetection mode, leveraging the strong coupling of the vdWs homojunction interface, the device achieves an exceptional photovoltaic effect with a high specific detectivity of 3.4 × 1010 Jones and a fast response time of 400 µs. The development of WS2 p‐n homojunction arrays presents immense potential for advancing next‐generation logic electronics and optoelectronic devices, opening new avenues for large‐scale industrial applications.