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Two-dimensional transition metal dichalcogenide (TMD) semiconductors have risen as an important material class for novel nanoelectronic applications. Molybdenum disulfide (MoS2) is the most representative TMD compound due to its superior stability and attractive properties for (opto-) electronic devices. However, the synthesis of single-crystalline and functional MoS2 across large-area substrates remains crucial for its successful integration in semiconductor industry platforms. Therefore, this work focuses on the study of MoS2 epitaxy via two well-established industry-compatible synthesis methods, promising for the large-area and single-crystalline integration of van der Waals (vdW) materials. These methods are molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) and have studied MoS2 quasi-vdW heteroepitaxy on reconstructed sapphire substrates and MoS2 vdW homoepitaxy on exfoliated MoS2 flakes. By examining the MoS2 structural properties using diffraction and spectroscopy techniques, the epitaxial relation and crystal quality are assessed, which reveals insights into the prevalence of inter- and intragrain defects such as grain boundaries and sulfur vacancies. The MBE method yields superior epitaxial MoS2 registry on both sapphire and MoS2 surfaces as compared to MOVPE, although inferior defectivity arises from the typical lower MBE growth temperature and chalcogen partial pressure. Moreover, both synthesis methods generate high densities of twinned MoS2 grain boundaries, which hamper defect-free integration. As a result, this challenging integration might become an important bottleneck for industrial TMD-based applications with a low tolerance for material defects. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Implementing two-dimensional materials in field-effect transistors (FETs) offers the opportunity to continue the scaling trend in the complementary metal-oxide-semiconductor technology roadmap. Presently, the search for electrically active defects, in terms of both their density of energy states and their spatial distribution, has turned out to be of paramount importance in synthetic transition metal dichalcogenides layers, as they are suspected of severely inhibiting these devices from achieving their highest performance. Although advanced microscopy tools have allowed the direct detection of physical defects such as grain boundaries and point defects, their implementation at the device scale to assess the active defect distribution and their impact on field-induced channel charge modulation and current transport is strictly restrained. Therefore, it becomes critical to directly probe the gate modulation effect on the carrier population at the nanoscale of an FET channel, with the objective to establish a direct correlation with the device characteristics. Here, we have investigated the active channel in a monolayer MoS 2 FET through in situ scanning probe microscopy, namely, Kelvin probe force microscopy and scanning capacitance microscopy, to directly identify active defect sites and to improve our understanding of the contribution of grain boundaries, bilayer islands, and defective grain domains to channel conductance.

Recently, a step-flow growth mode has been proposed to break the inherent molybdenum disulfide (MoS 2 ) crystal domain bimodality and yield a single-crystalline MoS 2 monolayer on commonly employed sapphire substrates. This work reveals an alternative growth mechanism during the metal-organic chemical vapor deposition (MOCVD) of a single-crystalline MoS 2 monolayer through anisotropic 2D crystal growth. During early growth stages, the epitaxial symmetry and commensurability of sapphire terraces rather than the sapphire step inclination ultimately govern the MoS 2 crystal orientation. Strikingly, as the MoS 2 crystals continue to grow laterally, the sapphire steps transform the MoS 2 crystal geometry into diamond-shaped domains presumably by anisotropic diffusion of ad-species and facet development. Even though these MoS 2 domains nucleate on sapphire with predominantly bimodal 0 and 60° azimuthal rotation, the individual domains reach lateral dimensions of up to 200 nm before merging seamlessly into a single-crystalline MoS 2 monolayer upon coalescence. Plan-view transmission electron microscopy reveals the single-crystalline nature across 50 μm by 50 μm inspection areas. As a result, the median carrier mobility of MoS 2 monolayers peaks at 25 cm 2 V -1 s -1 with the highest value reaching 28 cm 2 V -1 s -1 . This work details synthesis-structure correlations and the possibilities to tune the structure and material properties through substrate topography toward various applications in nanoelectronics, catalysis, and nanotechnology. Moreover, shape modulation through anisotropic growth phenomena on stepped surfaces can provide opportunities for nanopatterning for a wide range of materials.

InGaZnO (IGZO)-based thin-film transistors and selector diodes are increasingly investigated for a broad range of applications such as high-resolution displays, high-density memories, and high-speed computing. However, its potential to be a key material for next-generation devices is strongly contingent on developing patterning processes with minimal damage at nanoscale dimensions. IGZO can be etched using CH 4 -based plasma. Although the etched by-products are volatile, there remains a concern that passivation─an associated effect arising from the use of a hydrocarbon etchant─may inhibit the patterning process. However, there has been limited discussion on the CH 4 -based etching of IGZO and the subsequent patterning challenges arising with pitch scaling (<200 nm). In this work, we systematically investigate dry chemical etching schemes to pattern an IGZO film into densely packed nanostructures using CH 4 . Straight IGZO lines, ∼45 nm in width at a pitch of ∼135 nm, are produced by employing the traditional reactive ion etching method. While the passivating effect of CH 4 does not impede the etching process, any further shrinkage of feature and pitch dimensions amplifies reactive ion etching-induced damage in the form of profile distortion and residue redeposition. We show that this is efficiently addressed via atomic layer etching (ALE) of IGZO with CH 4 using a pulsed plasma. The unique combination of ALE and plasma pulsing enables controlled reduction of ion-assisted sputtering and redeposition of residues on the patterned IGZO features. This approach is highly scalable and is successfully applied here to achieve well-separated IGZO lines, with critical dimensions down to ∼20 nm at a dense pitch of ∼36 nm. These lines exhibit steep profiles (∼80°) and no undesirable change in IGZO composition post-patterning. Finally, ALE of IGZO under pulsed plasma, reproduced on 300 mm wafers, highlights its suitability in large-scale manufacturing for the intended applications.

In view of its epitaxial seeding capability, c -plane single crystalline sapphire represents one of the most enticing, industry-compatible templates to realize manufacturable deposition of single crystalline two-dimensional transition metal dichalcogenides (MX 2 ) for functional, ultrascaled, nanoelectronic devices beyond silicon. Despite sapphire being atomically flat, the surface topography, structure, and chemical termination vary between sapphire terraces during the fabrication process. To date, it remains poorly understood how these sapphire surface anomalies affect the local epitaxial registry and the intrinsic electrical properties of the deposited MX 2 monolayer. Therefore, molybdenum disulfide (MoS 2 ) is deposited by metal-organic chemical vapor deposition (MOCVD) in an industry-standard epitaxial reactor on two types of c -plane sapphire with distinctly different terrace and step dimensions. Complementary scanning probe microscopy techniques reveal an inhomogeneous conductivity profile in the first epitaxial MoS 2 monolayer on both sapphire templates. MoS 2 regions with poor conductivity correspond to sapphire terraces with uncontrolled topography and surface structure. By intentionally applying a substantial off-axis cut angle (1° in this work), the sapphire terrace width and step height-and thus also surface structure-become more uniform across the substrate and MoS 2 conducts the current more homogeneously. Moreover, these effects propagate into the extrinsic MoS 2 device performance: the field-effect transistor variability reduces both within and across wafers at higher median electron mobility. Carefully controlling the sapphire surface topography and structure proves an essential prerequisite to systematically study and control the MX 2 growth behavior and capture the influence on its structural and electrical properties.

Following an extensive investigation of various monolayer transition metal dichalcogenides (MX 2 ), research interest has expanded to include multilayer systems. In bilayer MX 2 , the stacking order strongly impacts the local band structure as it dictates the local confinement and symmetry. Determination of stacking order in multilayer MX 2 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 MoS 2 . Machine learning based data segmentation is employed to obtain useful statistics on grain orientation of monolayer and stacking in bilayer MoS 2 .

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 in-plane 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.

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 2 film's structural properties. Based on these observations, we propose an original method to achieve a layer-controlled epitaxy of wafer-scale TMDs.