Chemical vapor deposition of monolayer-thin WS2 crystals from the WF6 and H2S precursors at low deposition temperature

Challenges of Wafer-Scale Integration of 2D Semiconductors for High-Performance Transistor Circuits

Large-area 2D-material-based devices may find applications as sensor or photonics devices or can be incorporated in the back end of line (BEOL) to provide additional functionality. The introduction of highly scaled 2D-based circuits for high-performance logic applications in production is projected to be implemented after the Si-sheet-based CFET devices. Here, a view on the requirements needed for full wafer integration of aggressively scaled 2D-based logic circuits, the status of developments, and the definition of the gaps to be bridged is provided. Today, typical test vehicles for 2D devices are single-sheet devices fully integrated in a lab environment, but transfer to a more scaled device in a fab environment has been demonstrated. This work reviews the status of the module development, including considerations for setting up fab-compatible process routes for single-sheet devices. While further development on key modules is still required, substantial progress is made for MX₂ channel growth, high-k dielectric deposition, and contact engineering. Finally, the process requirements for building ultra-scaled stacked nanosheets are also reflected on.

Wafer-sized WS2 monolayer deposition by sputtering

We demonstrate that tungsten disulphide (WS₂) with thicknesses ranging from monolayer (ML) to several monolayers can be grown on SiO₂/Si, Si, and Al₂O₃ by pulsed direct current-sputtering. The presence of high quality monolayer and multilayered WS₂ on the substrates is confirmed by Raman spectroscopy since the peak separations between the A_1g-E_2g and A_1g-2LA vibration modes exhibit a gradual increase depending on the number of layers. X-ray diffraction confirms a textured (001) growth of WS₂ films. The surface roughness measured with atomic force microscopy is between 1.5 and 3 Å for the ML films. The chemical composition WS_x (x = 2.03 ± 0.05) was determined from X-ray Photoelectron Spectroscopy. Transmission electron microscopy was performed on a multilayer film to show the 2D layered structure. A unique method for growing 2D layers directly by sputtering opens up the way for designing 2D materials and batch production of high-uniformity and high-quality (stochiometric, large grain sizes, flatness) WS₂ films, which will advance their practical applications in various fields.

Thioether complexes of WSCl4, WOCl4 and WSCl3 and evaluation of thiochloride complexes as CVD precursors for WS2 thin films

Thioether complexes of WECl₄ and WECl₃ (E = S, O) are prepared and characterised; the dinuclear [(WSCl₄)₂{ⁱPrS(CH₂)₂SⁱPr}] is shown to be an effective single source precursor for low pressure CVD of WS₂ thin films.

Gas-Source CVD Growth of Atomic Layered WS2 from WF6 and H2S Precursors with High Grain Size Uniformity

Two-dimensional (2D) transition-metal dichalcogenides have attracted a considerable amount of attention because of their potential for post-silicon device applications, as well as for exploring fundamental physics in an ideal 2D system. We tested the chemical vapour deposition (CVD) of WS₂ using the gaseous precursors WF₆ and H₂S, augmented by the Na-assistance method. When Na was present during growth, the process created triangle-shaped WS₂ crystals that were 10 μm in size and exhibited semiconducting characteristics. By contrast, the Na-free growth of WS₂ resulted in a continuous film with metallic behaviour. These results clearly demonstrate that alkali-metal assistance is valid even in applications of gas-source CVD without oxygen-containing species, where intermediates comprising Na, W, and S can play an important role. We observed that the WS₂ crystals grown by gas-source CVD exhibited a narrow size distribution when compared with crystals grown by conventional solid-source CVD, indicating that the crystal nucleation occurred almost simultaneously across the substrate, and that uniform lateral growth was dominant afterwards. This phenomenon was attributed to the suppression of inhomogeneous nucleation through the fast and uniform diffusion of the gas-phase precursors, supported by the Na-assisted suppression of the fast reactions between WF₆ and H₂S.