Understanding noninvasive charge transfer doping of graphene: a comparative study

In this work, we systematically investigate and compare noninvasive doping of chemical vapor deposition graphene with three molecule dopants through spectroscopy and electrical conductivity techniques. Thionyl chloride shows the smallest improvement in conductivity with poor temporal and thermal stability and nitric acid induces the biggest sheet resistance reduction with modified stability. Molybdenum trioxide doping stands out, after thermal annealing, with both causing a significant sheet-resistance reduction and having superior temporal and thermal stability. These properties make it ideal for applications in advanced electronics. Theoretical studies based on the van der Waals density functional method suggest that cluster formation of molybdenum trioxide underpins the significant reduction in sheet resistance, and the stability, that arises after thermal annealing. Our comparative study clarifies charge transfer doping of graphene and brings understanding of the weak-interaction nature of such non-destructive doping of graphene. Our work also shows that we can use weak chemisorption to tailor the electronic properties of graphene, for example, to improve conductivity. This ability open up possibilities for further use of graphene in electronic interconnects, field effect transistors and other systems

Nucleation Mechanism during WS2 Plasma Enhanced Atomic Layer Deposition on Amorphous Al2O3 and Sapphire Substrates

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 (~1014 nuclei/cm2). The WS2 growth mode changes from 2D (layer-by-layer) growth on the initial Al2O3 surface to three-dimensional (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.status: publishe

Reactive plasma cleaning and restoration of transition metal dichalcogenide monolayers

The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique physical, optical, and chemical properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of “monolayer” TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temperature processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temperature in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temperature H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of H2 plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H2 and OCS is very reproducible, fast (completed in a few minutes) and uses a 300 mm industrial plasma etch system qualified for standard semiconductor pilot production. This process is, therefore, expected to enable the industrial scale-up of 2D-based devices, co-integrated with silicon technology

Atomic-Scale Characterization of Structure and Defects in Synthetic 2D Layered Chalcogenides

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Unravelling stacking order in epitaxial bilayer MX(2)using 4D-STEM with unsupervised learning

Following an extensive investigation of various monolayer transition metal dichalcogenides (MX2), research interest has expanded to include multilayer systems. In bilayer MX2, the stacking order strongly impacts the local band structure as it dictates the local confinement and symmetry. Determination of stacking order in multilayer MX2 domains usually relies on prior knowledge of in-plane orientations of constituent layers. This is only feasible in case of growth resulting in well-defined triangular domains and not useful in-case of closed layers with hexagonal or irregularly shaped islands. Stacking order can be discerned in the reciprocal space by measuring changes in diffraction peak intensities. Advances in detector technology allow fast acquisition of high-quality four-dimensional datasets which can later be processed to extract useful information such as thickness, orientation, twist and strain. Here, we use 4D scanning transmission electron microscopy combined with multislice diffraction simulations to unravel stacking order in epitaxially grown bilayer MoS2. Machine learning based data segmentation is employed to obtain useful statistics on grain orientation of monolayer and stacking in bilayer MoS2.status: publishe

Formation mechanism of 2D SnS2 and SnS by chemical vapor deposition using SnCl4 and H2S

© 2018 The Royal Society of Chemistry. SnS2 and SnS are two-dimensional (2D) semiconductors with distinct properties, as they exhibit a different type of conduction. They are of interest for applications in nanoelectronics, optoelectronics and sensors. To enable these applications, the deposition of SnS2 and SnS layers with a well-controlled phase, crystallinity and thickness at the nanometer level is required on large-area substrates. Chemical vapor deposition (CVD) of SnS2 and SnS using SnCl4 and H2S has previously been reported to give micrometer level polycrystalline SnS2 and SnS layers, which were insulating due to the uncontrolled grain orientations. In this work, we investigate the formation mechanism and phase control of nanometer level 2D SnS2 and SnS by SnCl4/H2S CVD. Nanometer level and phase-pure 2D hexagonal SnS2 and orthorhombic SnS layers are obtained. The SnSx phase depends on both the temperature and the H2S/SnCl4 concentration ratio. Compared to the formation of the SnS2 phase, the formation of the SnS phase is favorable at higher temperature and, surprisingly, at a higher H2S/SnCl4 concentration ratio. This is explained by the catalytic decomposition of H2S by SnS2 with the formation of H2, where the as such generated H2 reduces SnS2 to SnS at temperatures equal to or higher than 350 °C. By adjusting the conditions of the CVD process, the product can be tuned to either n-type SnS2 or p-type SnS, as demonstrated by back-gated field effect transistors.status: Published onlin

Chemical Vapor Deposition of 2D Semiconducting SnS2 and SnS using SnCl4 and H2S

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Importance of the substrate's surface evolution during the MOVPE growth of 2D-transition metal dichalcogenides

In this paper, we explore the impact of changing the growth conditions on the substrate surface during the metal-organic vapor phase epitaxy of 2D-transition metal dichalcogenides. We particularly study the growth of molybdenum disulfide (MoS2) on sapphire substrates at different temperatures. We show that a high temperature leads to a perfect epitaxial alignment of the MoS2 layer with respect to the sapphire substrate underneath, whereas a low temperature growth induces a 30° epitaxial alignment. This behavior is found to be related to the different sapphire top surface re-arrangement under H2S environment at different growth temperatures. Structural analyses conducted on the different samples confirm an improved layer quality at high temperatures. MoS2 channel-based metal-oxide-semiconductor field-effect transistors are fabricated showing improved device performance with channel layers grown at high temperature.status: publishe

Epitaxial registry and crystallinity of MoS2 via molecular beam and metalorganic vapor phase van der Waals epitaxy

file: :C$$:/Users/mortel43/Desktop/Literature/5.0013391.pdf:pdfstatus: publishe