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Epitaxial Growth of Two-Dimensional Transition Metal Dichalcogenide Lateral Heterojunctions
MING-YANG LI
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Epitaxial Growth of Two-Dimensional
Transition Metal Dichalcogenide Lateral Heterojunctions

MING-YANG LI2, LAIN-JONG LI1, 2 AND WEN-HAO CHANG3
1 PHYSICAL SCIENCES AND ENGINEERING DIVISION, KING ABDULLAH UNIVERSITY OF SCIENCE AND TECHNOLOGY, KINGDOM OF SAUDI ARABIA
2 RESEARCH CENTER FOR APPLIED SCIENCES, ACADEMIA SINICA, TAIWAN
3 DEPARTMENT OF ELECTROPHYSICS, NATIONAL CHIAO TUNG UNIVERSITY, TAIWAN

Transition metal dichalcogenides (TMDs) have been recognized as a new class of semiconducting two-dimensional (2D) layered materials, which opens up new opportunities in semiconductor technology for developing future 2D electronics and optoelectronics. Monolayer TMDs also notable for their direct energy band gap, good carrier mobility and an excellent ON/OFF current ratio when fabricated into field effect transistors, which are important properties for future low-power electronics and optoelectronics. For further applications in advanced circuits, the development of a two-dimensional (2D) p-n junction is a prerequisite. Earlier studies mainly focus on vertically stacked van der Waals heterojunctions, where the interface properties and stacking angle strongly affect the junction performance. Alternatively, chemical vapor deposition (CVD) has shown its reliability in the growth of wafer scale 2D materials and its feasibility in lateral heterojunction growth. However, the thermodynamically favored formation of TMD alloys at the hetero-interface during the so-called "one-pot" synthetic process is still an obstacle to the formation of lateral heterojunctions via direct CVD growth. Furthermore, the one-pot synthetic process usually only allows the growth of heterostructures with either different metals or different chalcogens, which limits the selection of materials for practical applications.


Fig. 1: (a) The optical image (left) of the WSe2/MoS2 lateral heterojunction and the High-resolution STEM images (right) taken at the interface. (b) Maps of the orientation angle extracted form SHG measurement (left). The insert shows the optical image, and the white dashes indicate the interface of the junction. The right optical images show MoS2 growth from per-patterned WSe2. The scale bar is 5 μm. (c) The optical image (top) and the electrical transport curves (down) of the WSe2/MoS2 p-n junction device with and without light illumination, showing clear rectifying behaviors and photovoltaic effects.

Recently, an international collaboration (including researchers from Saudi Arabia, Taiwan and Japan) led by Prof. Lain-Jong Li of King Abdullah University of Science and Technology, has developed a two-step epitaxial growth method for growing monolayer WSe2/MoS2 lateral heterojunctions with an atomically sharp p-n interface [1]. The monolayer WSe2 single crystal was first synthesized on a sapphire substrate, followed by MoS2 growth in a separate furnace. Cleaning the interface through a precise control of the vapor source is a crucial step to avoid alloy formation during MoS2 growth. The interface structure was carefully examined by scanning transmission electron microscopy (STEM). The annular dark field image of the junction shows atomically sharp interface between WSe2 and MoS2 (Fig. 1a), manifesting the feasibility of the two-step growth process. Polarization-resolved second-harmonic generation (SHG) microscopy reveals that the MoS2 was epitaxially grown out from the edge of WSe2 with the same crystal orientation, rather than following the sapphire substrate (Fig. 1b). The demonstration of MoS2 growth from pre-pattern WSe2 further shows the potential for versatile design in electronics (Fig. 1b). Furthermore, the electrical transport measurements have confirmed that the WSe2/MoS2 lateral heterojunction is a native p-n junction, exhibiting good rectifying behaviors, photoresponses, and photovoltaic effects, which are essential properties of fundamental building blocks for future 2D electronics and optoelectronics (Fig. 1c).

The work by Li et al. [1] not only demonstrated the growth of TMD lateral heterojunctions, but also proposed a two-step growth method that can be applied to construct heterostructures formed by various 2D layered materials for versatile monolayer electronic components. Furthermore, the atomically sharp interface offers an interesting platform for the study of fundamental material science.

References

[1] M. Y. Li, et al., Science 349, 524 (2015).

 

Ming-Yang Li is a postdoctoral fellow at the Research Center for Applied Sciences, Academia Sinica. His research interests include the transport, electronic and optical properties of two-dimensional (2D) materials and related heterostructures. After receiving his doctorate degree from National Tsing Hua University, Taiwan in 2013, he joined the Institute of Atomic and Molecular Sciences, Academia Sinica as a postdoctoral fellow. He moved to the Research Center for Applied Sciences in 2015.

Lain-Jong Li started his associate professorship at King Abdullah University of Science and Technology (KAUST) in 2014. He received a BSc and an MSc in chemistry at National Taiwan University. From 1997 to 2002 he worked on R&D at Taiwan Semiconductor Manufacturing Company, and he obtained his PhD from Oxford University in 2006. He was an assistant professor at Nanyang Tech. Univ. Singapore from 2006-2009. Since 2010, he has been an associate prof. at Academia Sinica Taiwan. In 2010 he was awarded the Career Development Award Taiwan and in 2011 he obtained the Humboldt Research Fellowship for Experienced Researchers. He received an Academia Sinica Research Award and a Wu Ta-Yu Research Award in 2013.

Wen-Hao Chang is a professor of the Department of Electrophysics at National Chiao Tung University (NCTU). He received his PhD (2001) in physics from National Central University (NCU), Taiwan. After his postdoctoral research at NCU, he joined NCTU as an assistant professor in 2005 and has been a professor since 2012. His research interests include semiconductor nanostructures and cavity quantum electrodynamics, quantum optics of nano-photonics/plasmonics, and semiconducting 2D materials. He was awarded the Ta-Yu Wu Memorial Award of the National Science Council of Taiwan in 2010.