Query: Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform

Candidate papers:

abstract: Atomically thin two dimensional semiconductors such as MoS2 hold great promise for electrical, optical and mechanical devices and display novel physical phenomena. However, the electron mobility of mono and few layer MoS2 has so far been substantially below theoretically predicted limits, which has hampered efforts to observe its intrinsic quantum transport behaviours. Potential sources of disorder and scattering include defects such as sulphur vacancies in the MoS2 itself as well as extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, we have developed here a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride and electrically contacted in a multi terminal geometry using gate tunable graphene electrodes. Magneto transport measurements show dramatic improvements in performance, including a record high Hall mobility reaching 34,000 cm(2) V( 1) s( 1) for six layer MoS2 at low temperature, confirming that low temperature performance in previous studies was limited by extrinsic interfacial impurities rather than bulk defects in the MoS2. We also observed Shubnikov de Haas oscillations in high mobility monolayer and few layer MoS2. Modelling of potential scattering sources and quantum lifetime analysis indicate that a combination of short range and long range interfacial scattering limits the low temperature mobility of MoS2.
author_names: Xu Cui

Gwan-Hyoung Lee

Young Duck Kim

Ghidewon Arefe

Pinshane Y Huang

Chul-Ho Lee

Daniel A Chenet

Xian Zhang

Lei Wang

Fan Ye

Filippo Pizzocchero

Bjarke S Jessen

Kenji Watanabe

Takashi Taniguchi

David A Muller

Tony Low

Philip Kim

James C Hone
title: Multi terminal transport measurements of MoS2 using a van der Waals heterostructure device platform.
venue: Nature nanotechnology
year: 2015

abstract: Atomically thin two dimensional (2D) semiconductors such as molybdenum disulphide (MoS2) hold great promise in electrical, optical, and mechanical devices and display novel physical phenomena such as coupled spin valley physics and the valley Hall effect. However, the electron mobility of mono and few layer MoS2 has so far been substantially below theoretically predicted limits, particularly at low temperature (T) which has hampered efforts to observe its intrinsic quantum transport behaviors. Potential sources of disorder and scattering include both defects such as sulfur vacancies in the MoS2 itself, and extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering and approach the intrinsic limit, we developed a van der Waals (vdW) heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride (hBN) and electrically contacted in a multi terminal geometry using gate tunable graphene electrodes. Multi terminal magneto transport measurements show dramatic improvements in performance, including a record high Hall mobility reaching 34,000 cm2/Vs for 6 layer MoS2 at low T. Comparison to theory shows a decrease of 1 2 orders of magnitude in the density of charged impurities, indicating that performance at low T in previous studies was limited by extrinsic factors rather than defects in the MoS2. We also observed Shubnikov de Haas (SdH) oscillations for the first time in high mobility monolayer and few layer MoS2. This novel device platform therefore opens up a new way toward measurements of intrinsic properties and the study of quantum transport phenomena in 2D semiconducting materials.
author_names: Xu Cui

Gwan-Hyoung Lee

Young Duck Kim

Ghidewon Arefe

Pinshane Y Huang

Chul-Ho Lee

Daniel A Chenet

Xian Zhang

Lei Wang

Fan Ye

Filippo Pizzocchero

Bjarke S Jessen

Kenji Watanabe

Takashi Taniguchi

David A Muller

Tony Low

Philip Kim

James C Hone
title: Multi terminal electrical transport measurements of molybdenum disulphide using van der Waals heterostructure device platform
year: 2014

abstract: Valleytronics is an appealing alternative to conventional charge based electronics that aims at encoding data in the valley degree of freedom, that is, the information as to which extreme of the conduction or valence band carriers are occupying. The ability to create and control valley currents in solid state devices could therefore enable new paradigms for information processing. Transition metal dichalcogenides (TMDCs) are a promising platform for valleytronics due to the presence of two inequivalent valleys with spin valley locking1 and a direct bandgap2,3, which allows optical initialization and readout of the valley state4,5. Recent progress on the control of interlayer excitons in these materials6 8 could offer an effective way to realize optoelectronic devices based on the valley degree of freedom. Here, we show the generation and transport over mesoscopic distances of valley polarized excitons in a device based on a type II TMDC heterostructure. Engineering of the interlayer coupling results in enhanced diffusion of valley polarized excitons, which can be controlled and switched electrically. Furthermore, using electrostatic traps, we can increase the exciton concentration by an order of magnitude, reaching densities in the order of 1012 cm 2, opening the route to achieving a coherent quantum state of valley polarized excitons via Bose Einstein condensation.Engineering the interlayer coupling in a van der Waals heterostructure enables electrical control over the transport and density of valley polarized interlayer excitons.
author_names: Dmitrii Unuchek

Alberto Ciarrocchi

Ahmet Avsar

Zhe Sun

Kenji Watanabe

Takashi Taniguchi

Andras Kis
title: Valley polarized exciton currents in a van der Waals heterostructure
venue: Nature Nanotechnology
year: 2019

abstract: The promise of high density and low energy consumption devices motivates the search for layered structures that stabilize chiral spin textures such as topologically protected skyrmions. At the same time, layered structures provide a new platform for the discovery of new physics and effects. Recently discovered long range intrinsic magnetic orders in the two dimensional van der Waals materials offer new opportunities. Here we demonstrate the Dzyaloshinskii Moriya interaction and Neel type skyrmions are induced at the WTe2/Fe3GeTe2 interface. Fe3GeTe2 is a ferromagnetic material with strong perpendicular magnetic anisotropy. We demonstrate that the strong spin orbit interaction in 1T' WTe2 does induce a large interfacial Dzyaloshinskii Moriya interaction at the interface with Fe3GeTe2 due to the inversion symmetry breaking to stabilize skyrmions. Transport measurements show the topological Hall effect in this heterostructure for temperatures below 100 K. Furthermore, Lorentz transmission electron microscopy is used to directly image Neel type skyrmions along with aligned and stripe like domain structure. This interfacial coupling induced Dzyaloshinskii Moriya interaction is estimated to have a large energy of 1.0 mJ/m^2, which can stabilize the Neel type skyrmions in this heterostructure. This work paves a path towards the skyrmionic devices based on van der Waals heterostructures.
author_names: Yingying Wu

Senfu Zhang

Junwei Zhang

Wei Wang

Yang Zhu

Jin Hu

Kin Wong

Chi Fang

Caihua Wan

Xiufeng Han

Qiming Shao

Takashi Taniguchi

Kenji Watanabe

Zhiqiang Mao

Xixiang Zhang

Kang L Wang
title: N\'eel type skyrmion in WTe2/Fe3GeTe2 van der Waals heterostructure.
year: 2019

abstract: The optical and electronic properties of van der Waals (vdW) heterostructures depend strongly on the atomic stacking order of the constituent layers. This is exemplified by periodic variation of the local atomic registry, known as moire patterns, giving rise to superconductivity and ferromagnetism in twisted bilayer graphene and novel exciton states in transition metal dichalcogenides (TMD) heterobilayers. However, the presence of the nanometer scale moire superlattices is typically deduced indirectly, because conventional imaging techniques, such as transmission electron microscopy (TEM) require special sample preparation that is incompatible with most optical and transport measurements. Here, we demonstrate a method that uses a secondary electron microscope to directly image the local stacking order in fully hexagonal boron nitride (hBN) encapsulated, gated vdW heterostructure devices on standard Si substrates. Using this method, we demonstrate imaging of reconstructed moire patterns in stacked TMDs, ABC/ABA stacking order in graphene multilayers, and AB/BA boundaries in bilayer graphene. Furthermore, we show that the technique is non destructive, thus unlocking the possibility of directly correlating local stacking order with optical and electronic properties, crucial to the development of vdW heterostructure devices with precisely controlled functionality.
author_names: Andrey Sushko

Kristiaan De Greve

Trond I Andersen

Giovanni Scuri

You Zhou

Ji Ho Sung

Kenji Watanabe

Takashi Taniguchi

Philip Kim

Hongkun Park

Mikhail D Lukin
title: High resolution imaging of reconstructed domains and moire patterns in functional van der Waals heterostructure devices.
year: 2019

abstract: The recently discovered two dimensional materials can be used to fabricate multilayer heterostructures. Interlayer charge transfer is a key process in such heterostructures as it can enable emergent optoelectronic properties. Efficient interlayer charge transfer in van der Waals heterostructures has been observed by femtosecond transient absorption and steady state optical spectroscopy measurements, based on measuring the interlayer carrier distribution. Here, we show that a second harmonic generation process allows direct probing of the electric field induced by the charge transfer. An ultrashort laser pulse was used to excite electrons and holes in a MoS2/WS2 heterostructure. The separation of the electrons and holes from the two monolayers generates an electric field, which enables the generation of the second harmonic of an incident fundamental pulse. We further studied the time evolution of this electric field by measuring the second harmonic signal as a function of the time delay between the pump and the fundamental pulses. The result agrees well with the dynamics revealed by a transient absorption measurement. These results provide direct evidence of interlayer charge transfer and demonstrate an all optical method of studying charge transfer and induced electric fields in two dimensional materials. Furthermore, this effect, if large enough, could be utilized in optical devices based on 2D heterostructures with nonlinear optical responses controllable by interlayer charge transfer.
author_names: Peng Yao

Da-wei He

Peymon Zereshki

Yongsheng Wang

Hui Zhao
title: Nonlinear optical effect of interlayer charge transfer in a van der Waals heterostructure
venue: Applied Physics Letters
year: 2019

abstract: Two dimensional (2D) material based heterostructures provide a unique platform where interactions between stacked 2D layers can enhance the electrical and opto electrical properties as well as give rise to interesting new phenomena. Here, the operation of a van der Waals heterostructure device comprising of vertically stacked bilayer MoS2 and few layered WSe2 has been demonstrated in which an atomically thin MoS2 layer has been employed as a tunneling layer to the underlying WSe2 layer. In this way, simultaneous contacts to both MoS2 and WSe2 2D layers have been established by forming a direct metal semiconductor to MoS2 and a tunneling based metal insulator semiconductor contacts to WSe2, respectively. The use of MoS2 as a dielectric tunneling layer results in an improved contact resistance (80 kO mm) for WSe2 contact, which is attributed to reduction in the effective Schottky barrier height and is also confirmed from the temperature dependent measurement. Furthermore, this unique contact engineering and type II band alignment between MoS2 and WSe2 enables a selective and independent carrier transport across the respective layers. This contact engineered dual channel heterostructure exhibits an excellent gate control and both channel current and carrier types can be modulated by the vertical electric field of the gate electrode, which is also reflected in the on/off ratio of 104 for both electron (MoS2) and hole (WSe2) channels. Moreover, the charge transfer at the heterointerface is studied quantitatively from the shift in the threshold voltage of the pristine MoS2 and the heterostructure device, which agrees with the carrier recombination induced optical quenching as observed in the Raman spectra of the pristine and heterostructure layers. This observation of dual channel ambipolar transport enabled by the hybrid tunneling contacts and strong interlayer coupling can be utilized for high performance opto electrical devices and applications.
author_names: Muhammad Atif Khan

Servin Rathi

Changhee Lee

Dongsuk Lim

Yunseob Kim

Sun Jin Yun

Doo-Hyeb Youn

Gil-Ho Kim
title: Tunable Electron and Hole Injection Enabled by Atomically Thin Tunneling Layer for Improved Contact Resistance and Dual Channel Transport in MoS2/WSe2 van der Waals Heterostructure.
venue: ACS applied materials interfaces
year: 2018

abstract: Utilizing spin or valley degree of freedom is one of the promising approaches to realize more energy efficient information processing. In the 2D transition metal dichalcogenide, the spin/valley current can be generated by utilizing the circular photogalvanic effect (CPGE) i.e. the generation of photocurrent by a circularly polarized light. Here we show that an in plane electric field at MoS2/WSe2 heterostructure electrode boundary results in an electrically tunable circular photogalvanic effect (CPGE) under optical excitation with normal incidence. The observed CPGE can be explained by the valence band shift due to the in plane electric field and different effective relaxation times between hole and electron combined with the valley optical selection rule. Furthermore, we show that the CPGE can be controlled by changing the Fermi level using an out of plane electric field. Such phenomena persist even at room temperature. This finding may facilitate the utilization of 2D heterostructure as an opto valleytronics and opto spintronics device platform.
author_names: Abdullah Rasmita

Chongyun Jiang

Hui Ma

Zhurun Ji

Ritesh Agarwal

Wei-bo Gao
title: Circular photogalvanic effect in 2D van der Waals heterostructure
year: 2019

abstract: Van der Waals heterostructures (vdWHs) based on two dimensional (2D) materials are being studied extensively for their prospective applications in photodetectors. As the pristine WSe2/MoTe2 heterostructure is a type I (straddling gap) structure, it cannot be used as a photovoltaic device theoretically, although both WSe2 and MoTe2 have excellent photoelectric properties. The Fermi level of p doped WSe2 is close to its valence band. The p doped WSe2/MoTe2 heterostructure can perform as a photovoltaic device because a built in electric field appears at the interface between MoTe2 and p doped WSe2. Here, a 633 nm laser was used for scanning the surface of WSe2 in order to obtain the p doped WSe2. X ray photoelectron spectroscopy (XPS) and electrical measurements verified that p type doping in WSe2 is produced through laser treatment. The p type doping in WSe2 includes substoichiometric WOx and nonstoichiometric WSex. A photovoltaic device using p doped WSe2 and MoTe2 was successfully fabricated. The band structure, light matter reactions, and carrier transport in the p doped WSe2/MoTe2 heterojunction were analyzed. The results showed that this photodetector has an on/off ratio of 104, dark current of 1 pA, and response time of 72 ms under the illumination of 633 nm laser at zero bias (Vds 0 V) The proposed p doping method may provide a new approach to improve the performance of nanoscale optoelectronic devices.
author_names: Jing Chen

Yabing Shan

Qiyuan Wang

Junqiang Zhu

Ran Liu
title: P type laser doped WSe2/MoTe2 van der Waals heterostructure photodetector.
venue: Nanotechnology
year: 2020

abstract: Two dimensional van der Waals heterostructures (vdWHs) have attracted considerable interest 1 4 However, most vdWHs reported so far are created by an arduous micromechanical exfoliation and manual restacking process 5 which although versatile for proof of concept demonstrations 6 16 and fundamental studies 17 30 is clearly not scalable for practical technologies. Here we report a general synthetic strategy for two dimensional vdWH arrays between metallic transition metal dichalcogenides (m TMDs) and semiconducting TMDs (s TMDs) By selectively patterning nucleation sites on monolayer or bilayer s TMDs, we precisely control the nucleation and growth of diverse m TMDs with designable periodic arrangements and tunable lateral dimensions at the predesignated spatial locations, producing a series of vdWH arrays, including VSe 2 /WSe 2 NiTe 2 /WSe 2 CoTe 2 /WSe 2 NbTe 2 /WSe 2 VS 2 /WSe 2 VSe 2 /MoS 2 and VSe 2 /WS 2 Systematic scanning transmission electron microscopy studies reveal nearly ideal vdW interfaces with widely tunable moire superlattices. With the atomically clean vdW interface, we further show that the m TMDs function as highly reliable synthetic vdW contacts for the underlying WSe 2 with excellent device performance and yield, delivering a high ON current density of up to 900 microamperes per micrometre in bilayer WSe 2 transistors. This general synthesis of diverse two dimensional vdWH arrays provides a versatile material platform for exploring exotic physics and promises a scalable pathway to high performance devices. A general strategy for the synthesis of two dimensional van der Waals heterostructure arrays is used to produce high performance electronic devices, showing the potential of this scalable approach for practical technologies.
author_names: Jia Li

Xiangdong Yang

Yang Liu

Bolong Huang

Ruixia Wu

Zhengwei Zhang

Bei Zhao

Huifang Ma

Weiqi Dang

Zheng Wei

Kai Wang

Zhaoyang Lin

Xingxu Yan

Mingzi Sun

Bo Li

Xiaoqing Pan

Jun Luo

Guangyu Zhang

Yuan Liu

Yu Huang

Xidong Duan

Xiangfeng Duan
title: General synthesis of two dimensional van der Waals heterostructure arrays
venue: Nature
year: 2020