Sergej Pasko

Metalorganic chemical vapor deposition is a promising synthesis technique for two-dimensional materials such as MoS ₂ . In this study, we controlled the growth mode of MoS ₂ on SiO ₂ using Mo(CO) ₆ and di-tert-butyl sulfide as precursors by adjusting the process conditions. The growth was directed from amorphous deposition at 400 ^∘ C and 500 ^∘ C, to crystalline MoS ₂ at 600 ^∘ C and higher. From 750 ^∘ C, not only MoS ₂ grains were deposited, but Mo metal nuclei were also formed during the process. An enhancement of the grain size was achieved by increasing the S/Mo precursor ratio. A more effective method to enlarge the grains and to lower the number density of crystals was to anneal the SiO ₂ substrate in Ar atmosphere prior to deposition. The reduced number density suggested that the pretreatment increased the diffusion length of Mo adatom species on the surface. Furthermore, addition of H ₂ to the N ₂ carrier gas had two effects on the growth mode, without altering the amount of deposited Mo. On one hand, due to a higher fraction of H ₂ in the carrier gas, the grain size slightly increased, and on the other hand, a change towards Mo metal deposition was observed. Control of the process conditions offers the opportunity to deposit large MoS ₂ grains without co-depositing Mo metal.

Two-dimensional (2D) films, such as tungsten disulfide (WS2), are being considered by the microelectronics industry as promising barrier and liner systems for the back-end-of-line (BEOL) metal interconnects. To maximize their efficiency, processes to integrate large grain size 2D barriers in BEOL should be explored. In this work, we investigate the use of large grain WS2 films, which are obtained by the epitaxy of the 2D film on sapphire substrates and then transferred into the dielectric. After demonstrating that a hydrogen disulfide passivation technique can help overcome film transfer challenges, we show that large grain size 2D films have enhanced barrier efficiency. Our results are benchmarked against commercially available TaN systems, proving that thin 2D films are advantageous in reducing metal diffusion in BEOL dielectrics.

2D materials have received a lot of interest over the past decade. Especially van der Waals (vdW) 2D materials, such as transition metal dichalcogenides (TMDCs), and their heterostructures exhibit semiconducting properties that make them highly suitable for novel device applications. Controllable mixing and matching of the 2D materials with different properties and precise control of the in‐plane twist angle in these heterostructures are essential to achieve superior properties and need to be established through large‐scale device fabrication. To gain fundamental insight into the potential control of these twist angles, 2D heterostructures of tungsten disulfide (WS₂) and graphene (Gr) grown by bottom‐up synthesis via metal‐organic chemical vapor deposition (MOCVD) are investigated using a scanning transmission electron microscope (STEM). Specifically, the combination of conventional high‐resolution imaging with scanning nanobeam diffraction (SNBD) using advanced 4D STEM techniques is used to analyze moiré structures. The latter technique is used to reveal the epitaxial alignment within the WS₂/Gr heterostructure, showing a direct influence of the underlying Gr layers on the moiré structure in the subsequent WS₂ layers. In particular, the importance of grain boundaries (GBs) within the underlying WS₂ and Gr layers on the structure of moiré patterns with rotation angles below 2° is discussed.

Epitaxial growth has become a promising route to achieve highly crystalline continuous two-dimensional layers. However, high-quality layer production with expected electrical properties is still challenging due to the defects induced by the coalescence between imperfectly aligned domains. In order to control their intrinsic properties at the device scale, the synthesized materials should be described as a patchwork of coalesced domains. Here, we report multi-scale and multi-structural analysis on highly oriented epitaxial WS₂ and WSe₂ monolayers using scanning transmission electron microscopy (STEM) techniques. Characteristic domain junctions are first identified and classified based on the detailed atomic structure analysis using aberration corrected STEM imaging. Mapping orientation, polar direction and phase at the micrometer scale using four-dimensional STEM enabled to access the density and the distribution of the specific domain junctions. Our results validate a readily applicable process for the study of highly oriented epitaxial transition metal dichalcogenides, providing an overview of synthesized materials from large scale down to atomic scale with multiple structural information.