Atomic Layer Deposition of Large-Area Polycrystalline Transition Metal Dichalcogenides from 100 °C through Control of Plasma Chemistry

Abstract

Two-dimensional transition metal dichalcogenides, such as MoS 2 , are intensely studied for applications in electronics. However, the difficulty of depositing large-area films of sufficient quality under application-relevant conditions remains a major challenge. Herein, we demonstrate deposition of polycrystalline, wafer-scale MoS 2 , TiS 2 , and WS 2 films of controlled thickness at record-low temperatures down to 100 °C using plasma-enhanced atomic layer deposition. We show that preventing excess sulfur incorporation from H 2 S-based plasma is the key to deposition of crystalline films, which can be achieved by adding H 2 to the plasma feed gas. Film composition, crystallinity, growth, morphology, and electrical properties of MoS x films prepared within a broad range of deposition conditions have been systematically characterized. Film characteristics are correlated with results of field-effect transistors based on MoS 2 films deposited at 100 °C. The capability to deposit MoS 2 on poly(ethylene terephthalate) substrates showcases the potential of our process for flexible devices. Furthermore, the composition control achieved by tailoring plasma chemistry is relevant for all low-temperature plasma-enhanced deposition processes of metal chalcogenides.

Toolbox of Advanced Atomic Layer Deposition Processes for Tailoring Large-Area MoS2 Thin Films at 150 °C

“…An example of this from our earlier work is shown Figure 1c, illustrating how increasing H 2 flow ratio (eq 1) from 0.20 to 0.80 at 150 °C changes the resulting film from amorphous MoS 3.5 to crystalline MoS 2 . 39 However, the increase in the H 2 flow ratio also leads to incorporation of H dopants into the films, resulting in high carrier densities, a drawback for many semiconductor applications. In addition, high H 2 flow ratios, which we denote as a subscript of B (e.g., B 0.80 ) lead to rough morphology (Figure 1c).…”

“…38 Recently, we showed that tailoring the plasma deposition chemistry enables a decrease of deposition temperature of both semiconducting and metallic TMDCs to record low 100 °C. 39 The deposition of stoichiometric and crystalline MoS 2 , TiS 2 , and WS 2 films was achieved by addition of sufficient hydrogen to the H 2 S plasma feed gas, which prevents excess S incorporation from otherwise happening at these low temperatures. However, tailoring the chemistry of a single plasma step in such a manner offers limited control over film properties, as the plasma species play multiple simultaneous roles including removal of ligands of the metal precursor, deposition of sulfur, and supplying energy in the form of low-energy ions and other reactive species for creation and healing of defects.…”

Synthesis of Large-Area Crystalline MoS2 by Sputter Deposition and Pulsed Laser Annealing

“…The current levels are admittedly not optimized; as stated before, the aim of this transistor study was not to focus on a device figure of merit but rather to extract insight into the quality and current carrying capability of the material thin films. However, the current levels are comparable with FETs fabricated using ALD thin films processed only at low temperatures, namely, 100–300 °C . Performing a high-temperature annealing or synthesis can undoubtedly and impressively improve the current levels and FET device figures of merit , through better crystallinity and reduced defect concentrations, but that would limit the use to front-end-of-line applications or relying on risky layer transfer.…”

A New Year and 35 Years of Chemistry of Materials

“…Our “35 Voices” initiative will also begin so that we may hear what motivates materials researchers from all over the world. To kick off our 35th year, however, we start by highlighting 35 of the most cited and downloaded manuscripts published in 2022. − …”

“…MXenes and layered dichalcogenides (Table ) − as well as covalent organic framework (COF) materials (Table ) − were also well-represented. The contributions on MXenes include a Methods/Protocols paper that provides practical insights for the synthesis and processing of these exciting materials .…”