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Open Access Publications from the University of California

Two-dimensional semiconducting materials for next-generation electronics and optoelectronics

  • Author(s): Wang, Chen
  • Advisor(s): Huang, Yu
  • et al.

My Ph.D research interest is applying standard micro/nano synthesis, fabrication, and processing equipment to creatively develop next-generation nanoelectronics and nanophotonics devices by employing two-dimensional (2D) semiconducting materials and novel device concepts.

In details, TMDs, such as WS2, WSe2, MoS2, MoSe2, represent a large family of layered materials, many of which exhibit tunable band gaps that can transition from an indirect band gap in bulk crystals to direct a band gap in monolayer nanosheets. Besides, the black phosphorus and monolayer phosphorene attracted considerable recent interest as an alternative layered semiconductor expected to exhibit high carrier mobility and layer-number tunable electronic properties. An important feature of these layered materials is the van der Waals interactions between neighboring layers that may allow much more flexible integration of distinct materials without the limitation of lattice mismatch, the similar lattice structure but with distinct properties that enable lateral epitaxial heterogeneous integration, thus opening up vast possibilities to nearly arbitrarily combine and control different properties at the atomic scale.

My first research project focused on the FETs performance optimization using WSe2, a new 2D transition metal dichalcogenides (TMDs) (Nano Lett 15, 709-713 (2015)). By tuning the substrate temperature gradient, carrier gas flow, and relative position of the targeted substrate in a home-made CVD system, I succeeded in producing high quality and large scale WSe2 flake with controllable layer number, single crystal size, and even stacking mode, which in turn achieved exceptional transistors performance with record hole mobility of over 350 cm2V-1s-1 and on/off ratio of above 108. This experience gave me a unique perspective on the optimization of CVD parameters and device contact engineering.

Driven by this pioneering study of WSe2, I further developed an in situ source-switch CVD system to synthesize the first atomically thin WSe2/WS2 heterostructure with 1D interface. Based on this specially designed heterostructure, I made the first WSe2/WS2 heterojunction featuring the photovoltaic effect. (Nat. Nanotechnol. 9, 1024 (2014)). My research with this promising material is an important advance in the development of layered semiconductor heterostructures and an essential step towards achieving functional 2D electronics. I also did deep research on the synthesis and electronic properties of the WS2xSe2-2x alloy nanosheets, which realized the full optical response and electronic properties tuning of the two-dimensional material. Additionally, our pioneering method realized the controllable doping of TMDs by the modulation of atomic ratio (Nano Lett. 16, 264 (2016)).

Fascinated by 2D materials, I extended this research by developing a novel intercalation method to obtain the monolayer phosphorene molecular superlattice (MPMS), based on black phosphorus (BP), an emerging 2D materials with great electronics application potential. With this creative method, I can now control BP from few-layer down to monolayer without properties degradation, and achieve exceptionally high performance with the MPMS transistors to 107 on/off ratio and a lifetime of 15X than that of BP. I also developed a novel device concept, BP-MPMS heterojunction with typical p+/p++ junction properties.

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