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Towards scalable van der Waals heterostructure arrays

Fa-Kun Wang Tian-You Zhai

State Key Laboratory of Material Processing and Die and Mould Technology,School of Materials Science and Engineering,Huazhong University of Science and Technology

作者简介：*Tian-You Zhai e-mail:zhaity@hust.edu.cn;

Towards scalable van der Waals heterostructure arrays

Fa-Kun Wang Tian-You Zhai

State Key Laboratory of Material Processing and Die and Mould Technology,School of Materials Science and Engineering,Huazhong University of Science and Technology

Two-dimensional van der Waals heterostructures are mostly created by an arduous micromechanical exfoliation and manual restacking process.In a recent report published in Nature,Li and colleagues reported a chemical vapour deposition (CVD) method with controllable nucleation sites for synthesizing van der Waals heterostructure arrays scalably and controllably,which is a necessary step towards practical integrated circuits.

Two-dimensional (2D) materials have shown great potential applications in modern electronics and optoelectronics in recent years.Compared with traditional silicon and III-V semiconductors,2D materials have been regarding as the optimum transistor channel material for their unique characteristics,including dangling-bondedfree surface,atomically thin thickness and free of shortchannel effects [ 1] .Developing 2D material-based transistors and integrated circuits may be the key to continuing the well-known Moore's law and achieving next-generation high-speed,low power consumption digital electronics.High-efficiency and scalable integration of 2D transistors,a necessary step towards practical integrated circuits,remains a great challenging.

2D van der Waals (vdW) integration is a physical assembly method with great prospects for scalable integration and practical applications [ 2, 3, 4] .It allows unprecedented flexibility to combine highly distinct materials to form a new-generation vdW heterostructures(vdWHs) without the requirement of lattice matching.However,most vdWHs to date were created by an arduous exfoliation and mechanical restacking process,which shows low yield and is clearly not scalable for practical technologies.In contrast to many new discoveries and exciting advancements in fundamental physics and device demonstrations based on various 2D heterostructures,the synthetic effort has received far insufficient attention and the progress in synthetic control is lagging far behind in the field.Writing in Nature,Li et al. [ 5] report a method that allows the controllable and scalable synthesis of periodic arrays of 2D vdWH between metallic transition metal dichalcogenides (m-TMDs) and semiconductor TMDs (sTMDs) via chemical vapour deposition (CVD) system(Fig.1).This work marks an important milestone in rational synthesis of 2D vdWHs.The idea in this work should be quite helpful in pushing the controllable synthesis of 2D community field further in great attention and advance.

In recent years,some researches [ 6, 7, 8, 9] about the synthesis of vdWHs have been reported based on the vdW epitaxial growth,which allow a second TMD to grow at the surface of another pre-grown TMD crystal.However,due to the extreme delicacy of these atomically thin materials and difficulties in precisely controlling their nucleation and growth process,many of these efforts rely on chance nucleation and growth with rather limited synthetic control.Li and colleagues'method overcomes those constraints by direct laser cauterization patterning approach,creating pattern periodic arrays of defects on s-TMDs as the exclusive nucleation sites for site-specific growth of m-TMDs (Fig.1a,b).One major advantage of laser processing is operational simplicity and clean without introducing any exotic contamination,thus minimizingundesired random nucleation in subsequent growth,which is hard to avoid during the traditional lithography process.

Fig.1 Method for synthesizing scalable,controllable vdWH array and corresponding structure and electrical characteristics.Schematic of growth process:a large area monolayer or bilayer s-TMDs (for example,WSe₂,MoS₂,WS₂) were first grown using a CVD process and then selectively patterned to create periodic defect arrays that function as the exclusive nucleation sites for site-specific growth of m-TMDs (for example,VSe₂,VS₂,CoTe₂,NiTe₂ and NbTe₂) to form m-TMD/s-TMD vdWH arrays;b schematic of laser-patterning process via a focused laser (488 nm) irradiation combined with raster scan in a confocal laser system;c three-dimensional schematic of a m-TMD/s-TMD vdWH(blue:W;yellow:S;green:V;and red:Se).d Optical microscopy images of large-scale periodic VSe₂/MoS₂ vdWH arrays grown on continuousMoS₂ thin films.Electron microscopy characterizations of e NiTe₂/WSe₂ and f-k VSe₂/WSe₂ vertical heterostructure.I Typical optical microscopy image of back-gated WSe₂ transistors with neighbouring VSe₂ nanoplates as synthetic vdW contacts and edge-to-edge gap defining channel length.m Output characteristics of a bilayer WSe₂ transistor with synthetic VSe₂ vdW contacts on 70-nm SiN_x/Si substrate.Reproduced with permission[5]

Li and co-workers demonstrated their approach is general and not limited to a material with specific chemical composition or lattice structures,realizing the synthesis of a wide range of 2D m-TMD/s-TMD vdWHs,including 1T-VSe₂/WSe₂,NiTe₂/WSe₂,CoTe_2/WSe₂,NbTe₂/WSe₂ and VS ₂/WSe₂.The author further showed that the periodicity,lateral width and edge-to-edge gap between the VSe₂/WSe₂-WSe₂-VSe₂/WSe₂ heterostructure arrays can be precisely controlled by the growth time and designed periodicity.Such controlled growth of heterostructure array represents a clear manifestation of a high degree of synthetic control and may open up a reliable pathway to 2D transistors with synthetically controllable channel length.

Specifically,by using highly oriented continuous monolayer MoS₂ thin film,the author showed much larger periodic arrays (Fig.1d) of VSe₂/MoS₂ 2D vdWHs(>10,000 separated VSe₂/MoS₂ 2D vdWHs) can be achieved with a high yield up to～99%,which unambiguously confirms the scalability of their approach.Li and co-authors'method provides a scalable pathway to highperformance devices,fuelling the practical application of2D transistors and integrated circuits.

Li and co-workers further used systematic scanning transmission electron microscopy studies to reveal the atomically sharp,nearly ideal vdW interfaces with widely tunable Moire superlattices of the synthesized vdWHs(Fig.1e-k).Based on these atomically clean heterostructures interface,the authors further demonstrated that the m-TMDs can function as highly reliable synthetic vdW contacts for the underlying WSe₂ with excellent device performance and yield,delivering a high ON-current density of up to 900μA·μm^-1 in a bilayer WSe₂ transistors with a 1.8-μm-long channel.This represents the highest room-temperature ON-current ever achieved in monolayer or bilayer TMD semiconductor (either exfoliated or CVD grown) transistors [ 10] .Such device configuration can avoid exposure of the functional contact interface to any lithography process and thus avoid the lithography-induced contaminations (e.g.polymer resist residue) and deposition-induced damages to the atomically thin 2D semiconductors.This result suggests that researchers could explore the full potentials for using other types of synthetic vdW contacts to fabricate high-performance 2D materials transistors that would minimize interface damage.Furthermore,the ultrahigh ON-current density obtained in the 2D WSe₂ transistors makes people see the hope to replace the traditional Si transistors with 2D transistors.

Recently,the formation of Moire superlattice structures with tunable periodicity between perse 2D materials holds the significant promise for exploring unique electronic and photonic characteristics that cannot be reached in conventional materials [ 11, 12, 13] .However,the studies on Moire superlattices have been largely limited to mechanically stacked heterostructures.The author demonstrated ability to produce a wide range of 2D heterostructure arrays with well-defined Moire superlattices and widely tunable Moire periodicity could open up exciting opportunities for exploring fundamental physics.

To explore full potentials of this technique,further work is need to do additional synthesis of vertical heterostructures array between s-TMD and other 2D materials with different properties,such as insulator,superconductor and even magnet,to construct new types of device.Finally,Li and co-workers’general concept and approach represents a high degree of synthetic control and establishes the fundamental intellectual underpinning for creating more complex hierarchical 2D heterostructures.This major advancement is critically needed for a new level of fundamental studies“beyond simple exfoliated 2D crystals or manually stacked heterostructures”and for developing practical technologies.