Low-Temperature Synthesis of Wafer-Scale MoS₂–WS₂ Vertical Heterostructures by Single-Step Penetrative Plasma Sulfurization

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention owing to their synergetic effects with other 2D materials, such as graphene and hexagonal boron nitride, in TMD-based heterostructures. Therefore, it is important to understand the physical properties of TMD–TMD vertical heterostructures for their applications in next-generation electronic devices. However, the conventional synthesis process of TMD–TMD heterostructures has some critical limitations, such as nonreproducibility and low yield. In this paper, we synthesize wafer-scale MoS2–WS2 vertical heterostructures (MWVHs) using plasma-enhanced chemical vapor deposition (PE-CVD) via penetrative single-step sulfurization discovered by time-dependent analysis. This method is available for fabricating uniform large-area vertical heterostructures (4 in.) at a low temperature (300 °C). MWVHs were characterized using various spectroscopic and microscopic techniques, which revealed their uniform nanoscale polycrystallinity and the presence of vertical layers of MoS2 and WS2. In addition, wafer-scale MWVHs diodes were fabricated and demonstrated uniform performance by current mapping. Furthermore, mode I fracture tests were performed using large double cantilever beam specimens to confirm the separation of the MWVHs from the SiO2/Si substrate. Therefore, this study proposes a synthesis mechanism for TMD–TMD heterostructures and provides a fundamental understanding of the interfacial properties of TMD–TMD vertical heterostructures.

Controllable Preparation of 2D Vertical van der Waals Heterostructures and Superlattices for Functional Applications

“…[19,40] Compared to direct vapor phase synthesis, sulfurization (or selenization) of metal (or metal oxide) thin films has been widely used to synthesize large-scale TMD/TMD vdWHs and their arrays via successive deposition of metal film precursors followed by sulfurization (or selenization). [88,[92][93][94][95][96] In contrast to the synthesis of a single component, the growth of vdWHs often involves multistep deposition of metal precursors, which require a more gentle deposition method to avoid damage to the predeposited metal films or pregrown 2D TMDs. [97] The sulfurization (or selenization) processes include two main categories: high-temperature (>600 °C) thermal CVD [92] and low-temperature (≈300 °C) PECVD.…”

“…MoS 2 /WS 2 vdWH, and the synthesis mechanism was confirmed using time-dependent crosssectional high-resolution transmission electron micro scopy (HRTEM) and Raman spectroscopy analyses (Figure 5j). [88] The intrinsic scalability and controllability of this approach open up a variety of opportunities for generating new heterojunctions and realizing their electric and optoelectronic applications. For example, Group 10 metal-based TMD (PtSe 2 / PtS 2 ) heterojunctions can also be synthesized by altering the corresponding metal precursors and substituting the second selenization for sulfurization.…”

“…Reproduced with permission. [ 88 ] Copyright 2021, American Chemical Society. k) The schematic of seeded growth of the single‐crystalline 2H MoTe 2 wafers induced by phase transition.…”

“…[ 97 ] The sulfurization (or selenization) processes include two main categories: high‐temperature (>600 °C) thermal CVD [ 92 ] and low‐temperature (≈300 °C) PECVD. [ 88 ] Owing to the presence of active species (such as accelerated energetic electrons, excited molecules, and free radicals) in the plasma, PECVD has advantages such as a relatively low substrate temperature, better control of nanostructure formation, and uniform thickness. Recently, PECVD was used to fabricate a 4 in.…”

“…j) Illustration of the mechanism for the synthesis of MoS 2 /WS 2 vdWHs by PECVD and crosssectional HRTEM images (scale bar: 10 nm). Reproduced with permission [88]. Copyright 2021, American Chemical Society.…”