Surface proximity effect enables layer-by-layer growth of MoS2

Two-dimensional (2D)monolayer semiconductors have shown great potential for continuous downward scaling to a 2nm technology node [1]. However, the monolayer characteristics make it quite challenging to completely manifest their intrinsic high performance. At the interface between semiconductors and dielectrics, the carriers are scattered by extrinsic impurities and remote optical phonons, which seriously degrade their carrier mobility; at the electrical contact, the relatively wide band gap of monolayer MoS2 hinders efficient carrier injection. Few-layer (bilayer or trilayer) MoS2 have been suggested with higher mobility and lower contact resistance, and can still retain excellent electrostatic control at sub-5-nm nodes [2]. However, precise control of growing waferscale bilayer or trilayer MoS2 remains a grand challenge from a thermodynamic perspective. According to the criteria defined by E. Bauer and J.H. vanderMerwe [3], the 2D growth mode requires that the substrate surface energy (γ s) is larger than the sum of freestandingMoS2 surface energy (γ o) and theMoS2/substrate interface energy (γ i), i.e. γ s > γ o + γ i. The surface energy of freestanding MoS2 increases with the number of layers, which results in the self-limiting characteristics of monolayer MoS2 growth. Different from the very recent edge-aligned bilayer strategy [4], Zhang and his colleagues analyze the surface proximity effect and successfully demonstrate controllable growth of wafer-scale bilayer and trilayer MoS2 in a layer-by-layer mode, by optimizing both thermodynamic and kinetic factors (Fig. 1a and b) [5]. Thermodynamically, they adopt sapphire (0001) with very high substrate surface energy (∼3.3 J/m2). By analyzing the energy relationship of MoS2/sapphire as a new substrate, they identify that it is still energetically feasible to grow additional MoS2 layers on monolayer and bilayer MoS2/sapphire substrate. This surface proximity effect enables the growth of bilayer and trilayer MoS2 in a layer-by-layer manner on


MATERIALS SCIENCE
Surface proximity effect enables layer-by-layer growth of MoS 2

Yang Chai
Two-dimensional (2D) monolayer semiconductors have shown great potential for continuous downward scaling to a 2nm technology node [1]. However, the monolayer characteristics make it quite challenging to completely manifest their intrinsic high performance. At the interface between semiconductors and dielectrics, the carriers are scattered by extrinsic impurities and remote optical phonons, which seriously degrade their carrier mobility; at the electrical contact, the relatively wide band gap of monolayer MoS 2 hinders efficient carrier injection. Few-layer (bilayer or trilayer) MoS 2 have been suggested with higher mobility and lower contact resistance, and can still retain excellent electrostatic control at sub-5-nm nodes [2]. However, precise control of growing waferscale bilayer or trilayer MoS 2 remains a grand challenge from a thermodynamic perspective.
According to the criteria defined by E. Bauer and J.H. van der Merwe [3], the 2D growth mode requires that the substrate surface energy (γ s ) is larger than the sum of freestanding MoS 2 surface energy (γ o ) and the MoS 2 /substrate interface energy (γ i ), i.e. γ s > γ o + γ i . The surface energy of freestanding MoS 2 increases with the number of layers, which results in the self-limiting characteristics of monolayer MoS 2 growth. Different from the very recent edge-aligned bilayer strategy [4], Zhang and his colleagues analyze the surface proximity effect and successfully demonstrate controllable growth of wafer-scale bilayer and trilayer MoS 2 in a layer-by-layer mode, by optimizing both thermodynamic and kinetic factors ( Fig. 1a and b) [5].
Thermodynamically, they adopt sapphire (0001) with very high substrate surface energy (∼3.3 J/m 2 ). By analyzing the energy relationship of MoS 2 /sapphire as a new substrate, they identify that it is still energetically feasible to grow additional MoS 2 layers on monolayer and bilayer MoS 2 /sapphire substrate. This surface proximity effect enables the growth of bilayer and trilayer MoS 2 in a layer-by-layer manner on MoS 2 /sapphire substrate with relatively high surface energy. With the increase of the layer number, the growth mode evolves from 2D to 3D because of the weakened proximity effect, which makes it thermodynamically unfavorable for growing thicker MoS 2 .
To achieve full coverage and a controlled layer number, researchers are also required to optimize kinetic growth factors. They increase Mo-source flux with C The Author(s) 2022. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. high source temperature for high nucleation density on the substrate. By adopting oxygen-assisted chemical vapor deposition, they also achieve ultra-high edge growth rate (∼5-10 μm/min) compared to that without the oxygen assistance (<0.1 μm/min). With the optimization of the nucleation density and the edge growth rate, the diffusion mean free path is larger than the domain size, allowing uniform layer-by-layer growth at a wafer scale (Fig. 1b).
Furthermore, they fabricate longand short-channel field-effect transistors using mono-, bi-and trilayer MoS 2 . Thick-layer field-effect transistors produce significant improvements in device performance. For long-channel devices (channel length of 5 to 50 μm), the average field-effect mobility is ∼80 cm 2 ·V −1 ·s −1 for monolayers, to ∼110/145 cm 2 ·V −1 ·s −1 for bilayer/trilayer devices (Fig. 1c). The high mobility of >100 cm 2 ·V −1 ·s −1 uncovers the great potential of bilayer and trilayer MoS 2 for high-performance transistors. For 100-nm short-channel devices, the current density increases with the layer number, i.e. 0.40 (monolayer), 0.64 (bilayer) and 0.81 (trilayer) mA/μm. Remarkably, the short-channel trilayer device with 40-nm channel length exhibits a record-high on-current density of 1.70 mA/μm at V ds = 2 V and a high on/off ratio exceeding 10 7 . These device characteristics show high potential, with excellent electrostatic control and high drive current for end-of-roadmap transistors (Fig. 1d).