Two-dimensional (2D) transition metal dichalcogenides are potential low dissipative semiconductor materials for nanoelectronic devices. Such applications require the deposition of these materials in their crystalline form and with controlled number of monolayers on large area substrates, preferably using growth temperatures compatible with temperature sensitive structures. This paper presents a low temperature Plasma Enhanced Atomic Layer Deposition (PEALD) process for 2D WS2 based on a ternary reaction cycle consisting of consecutive WF6, H2 plasma and H2S reactions. Strongly textured nanocrystalline WS2 is grown at 300 °C. The composition and crystallinity of these layers depends on the PEALD process conditions, as understood by a model for the redox chemistry of this process. The H2 plasma is essential for the deposition of WS2 as it enables the reduction of-W 6+ Fx surface species. Nevertheless, the impact of sub-surface reduction reactions needs to be minimized to obtain WS2 with well-controlled composition (S/W ratio of two).

When two-dimensional (2D) group-VI transition metal dichalcogenides such as tungsten disulfide (WS2) are grown by atomic layer deposition (ALD) for atomic growth control at low deposition temperatures (< 450 °C), they often suffer from a nanocrystalline grain structure limiting the carrier mobility. The crystallinity and monolayer thickness control during ALD of 2D materials is determined by the nucleation mechanism, which is currently not well understood. Here, we propose a qualitative model for the WS2 nucleation behavior on dielectric surfaces during plasma-enhanced (PE-) ALD using WF6, H2 plasma and H2S based on analyses of the morphology of the WS2 crystals. The WS2 crystal grain size increases from ~20 nm to 200 nm by lowering the nucleation density. This is achieved by lowering the precursor adsorption rate on the starting surface using an inherently less reactive starting surface, by decreasing the H2 plasma reactivity, and by enhancing the mobility of the adsorbed species at higher 2 deposition temperature. Since SiO2 is less reactive than Al2O3, and diffusion and crystal ripening is enhanced at higher deposition temperature, WS2 nucleates in an anisotropic island-like growth mode with preferential lateral growth from the WS2 crystal edges. This work emphasizes that increasing the crystal grain size while controlling the basal plane orientation is possible during ALD at low deposition temperatures, based on insight in the nucleation behavior, which is key to advance the field of ALD of 2D materials. Moreover, this work demonstrates the conformal deposition on 3D structures, with WS2 retaining the basal plane orientation along topographic structures.

The rapid cadence of MOSFET scaling is stimulating the development of new technologies and accelerating the introduction of new semiconducting materials as silicon alternative. In this context, 2D materials with a unique layered structure have attracted tremendous interest in recent years, mainly motivated by their ultra-thin body nature and unique optoelectronic and mechanical properties. The development of scalable synthesis techniques is obviously a fundamental step towards the development of a manufacturable technology. Metal-organic chemical vapor deposition has recently been used for the synthesis of large area TMDs, however, an important milestone still needs to be achieved: the ability to precisely control the number of layers and surface uniformity at the nano-to micro-length scale to obtain an atomically flat, self-passivated surface. In this work, we explore various fundamental aspects involved in the chemical vapor deposition process and we provide important insights on the layer-dependence of epitaxial MoS film's structural properties. Based on these observations, we propose an original method to achieve a layer-controlled epitaxy of wafer-scale TMDs.

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Catalytic decomposition of H2S by SnS2, with generation of H2, plays a critical role in the SnCl4/H2S CVD process.

It is difficult to meet all the different material and economical requirements posed to a MEMS structural layer that can be integrated with the electronics on the same substrate using a single layer process. Therefore a multilayer process, which uses a combination of a CVD crystallization layer and a high-growth rate PECVD bulk layer was developed. High-quality films with excellent electrical and mechanical properties can be obtained at low temperature (#450°C) and high deposition rates (~100 nm/min). Fine-tuning of the stress gradient is accomplished by the use of a top stress compensation layer, whose optimal thickness was estimated from an evaluation of the stress gradient profile over thickness. These layers have been used for processing a 10 µm thick poly-SiGe gyroscope on top of a standard 0.35 µm CMOS process.

The structure, crystallinity and properties of as-deposited two-dimensional (2D) transition metal dichalcogenides are determined by nucleation mechanisms in the deposition process. 2D materials grown by atomic layer deposition (ALD) in absence of a template, are polycrystalline or amorphous. Little is known about their nucleation mechanisms.Therefore, we investigate the nucleation behavior of WS2 during plasma enhanced ALD from WF6, H2 plasma and H2S at 300 °C on amorphous ALD Al2O3 starting surface and on monocrystalline, bulk sapphire. Preferential interaction of the precursors with the Al2O3 starting surface promotes fast closure of the WS2 layer. The WS2 layers are fully continuous at WS2 content corresponding to only 1.2 WS2 monolayers. On amorphous Al2O3, (0002) textured and polycrystalline WS2 layers form with grain size of 5 nm to 20 nm due to high nucleation density (~10 14 nuclei/cm 2 ). The WS2 growth mode changes from 2D (layer-by-layer) growth on the initial Al2O3 surface to threedimensional (Volmer-Weber) growth after WS2 layer closure. Further growth proceeds from both WS2 basal planes in register with the underlying WS2 grain, and from or over grain boundaries of the underlying WS2 layer with different in-plane orientation. In contrast, on monocrystalline sapphire, WS2 crystal grains can locally align along a preferred in-plane orientation. Epitaxial seeding occurs locally albeit a large portion of crystals remain randomly oriented, presumably due to the low deposition temperature.The WS2 sheet resistance is 168 MΩµm suggesting that charge transport in the WS2 layers is limited by grain boundaries.3

The increasing scientific and industry interest in 2D MX 2 materials within the field of nanotechnology has made the single crystalline integration of large area van der Waals (vdW) layers on commercial substrates an important topic. The c-plane oriented (3D crystal) sapphire surface is believed to be an interesting substrate candidate for this challenging 2D/3D integration. Despite the many attempts that have been made, the yet incomplete understanding of vdW epitaxy still results in synthetic material that shows a crystallinity far too low compared to natural crystals that can be exfoliated onto commercial substrates. Thanks to its atomic control and in situ analysis possibilities, molecular beam epitaxy (MBE) offers a potential solution and an appropriate method to enable a more in-depth understanding of this peculiar 2D/3D hetero-epitaxy. Here, we report on how various sapphire surface reconstructions, that are obtained by thermal annealing of the as-received substrates, influence the vdW epitaxy of the MBE-grown WSe 2 monolayers (MLs). The surface chemistry and the interatomic arrangement of the reconstructed sapphire surfaces are shown to control the preferential in-plane epitaxial alignment of the stoichiometric WSe 2 crystals. In addition, it is demonstrated that the reconstructions also affect the in-plane lattice parameter and thus the inplane strain of the 2D vdW-bonded MLs. Hence, the results obtained in this work shine more light on the peculiar concept of vdW epitaxy, especially relevant for 2D materials integration on large-scale 3D crystal commercial substrates.