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

Engineering Wafer-Scale Epitaxial Two-Dimensional Materials through Sapphire Template Screening for Advanced High-Performance Nanoelectronics

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 (MX₂) 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 MX₂ monolayer. Therefore, molybdenum disulfide (MoS₂) 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 MoS₂ monolayer on both sapphire templates. MoS₂ 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 MoS₂ conducts the current more homogeneously. Moreover, these effects propagate into the extrinsic MoS₂ 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 MX₂ growth behavior and capture the influence on its structural and electrical properties.

Covalent functionalization of molybdenum disulfide by chemically activated diazonium salts

Covalent functionalization is one of the most efficient ways to tune the properties of layered materials in a highly controlled manner. However, molecular chemisorption on semiconducting transition metal dichalcogenides remains a delicate task due to the inertness of their surface. Here we perform covalent modification of bulk and single layer molybdenum disulfide (MoS₂) using chemical activation of diazonium salts. A high level of control over the grafting density and yield on MoS₂ basal plane can be achieved by this approach. Using scanning probe microscopies and X-ray photoelectron spectroscopy we prove the covalent functionalization of MoS₂.

Tailoring physical properties of WS2 nanosheets by defects control

The controllable growth of high-quality transition metal dichalcogenides (TMDs) is crucial for their device applications, which rely on the atomic and quantitative understanding of the growth mechanism of TMDs. In this work, we propose a comprehensive picture of the growth of WS₂ nanosheets via Monte Carlo simulation, and an extension of diffusion-limited growth under transition state theory is developed to describe heteroepitaxy growth of WS₂. Theoretical results are in good agreement with the results of chemical vapor deposition that growth temperature dominates growth processes leading to samples with various densities of vacancy defects. The vacancy defects modify the photoluminescence and ferromagnetic behavior. Our work provides a pathway toward realizing controllable physical properties in 2D materials.

Understanding ambipolar transport in MoS2 field effect transistors: the substrate is the key

2D materials offer a pathway for further scaling of CMOS technology. However, for this to become a reality, both n-MOS and p-MOS should be realized, ideally with the same (standard) material. In the specific case of MoS2 field effect transistors (FETs), ambipolar transport is seldom reported, primarily due to the phenomenon of Fermi level pinning (FLP). In this study we identify the possible sources of FLP in MoS2 FETs and resolve them individually. A novel contact transfer technique is used to transfer contacts on top of MoS2 flake devices that results in a significant increase in the hole branch of the transfer characteristics as compared to conventionally fabricated contacts. We hypothesize that the pinning not only comes from the contact-MoS2 interface, but also from the MoS2-substrate interface. We confirm this by shifting to an hBN substrate which leads to a 10 fold increase in the hole current compared to the SiO2 substrate. Furthermore, we analyse MoS2 FETs of different channel thickness on three different substrates, SiO2, hBN and Al2O3, by correlating the p-branch I ON/I OFF to the position of oxide defect band in these substrates. FLP from the oxide is reduced in the case of Al2O3 which enables us to observe ambipolar transport in a bilayer MoS2 FET. These results highlight that MoS2 is indeed an ambipolar material, and the absence of ambipolar transport in MoS2 FETs is strongly correlated to its dielectric environment and processing conditions.

On the van der Waals Epitaxy of Homo-/Heterostructures of Transition Metal Dichalcogenides

Layered materials held together by weak van der Waals (vdW) interactions are a promising class of materials in the field of nanotechnology. Besides the potential for single layers, stacking of various vdW layers becomes even more promising since unique properties can hence be precisely engineered. The synthesis of stacked vdW layers, however, remains to date, hardly understood. Therefore, in this work, the vdW epitaxy of transition metal dichalcogenides (TMDs) on single-crystalline TMD templates is investigated in depth. It is demonstrated that the role of lattice mismatch is insignificant. More importantly is the role of surface energy, calculated using density functional theory, which plays an essential role in the activation energy for adatom diffusion, hence nucleation density. This in turn correlates with defect density since the stacking sequence in vdW epitaxy is generally poorly controlled. Moreover, the vapor pressure of the transition metal is also found to correlate with adatom diffusion. Consequently, the proposed study enables important and new insight in the vdW epitaxy of multilayer 2D homo-/heterostructures.