Wouter Mortelmans

Large‐area epitaxy of layered materials is one of the cornerstones for a successful exploitation of van der Waals (vdW) materials in the semiconductor industry. The formation of 60° twin stacking faults and 60° grain boundaries is of major concern for the defect‐free epitaxial growth. Although strategies to overcome the occurrence of these defects are being considered, more fundamental understanding on the origin of these defects is highly essential. This work focuses on understanding the formation of 60° twins in (quasi‐)vdW epitaxy of 2D chalcogenides. Molecular beam epitaxy (MBE) experiments reveal the striking difference in 60° twin occurrence between WSe₂ and Bi₂Se₃ in both quasi‐vdW heteroepitaxy and vdW homoepitaxy. Density functional theory (DFT) calculations link this difference to the interlayer vdW coupling strength at the unit cell level. The stronger interlayer vdW coupling in Bi₂Se₃ unit cells compared to WSe₂ unit cells is explained to result in a reduced twin occurrence. Hence, such compounds show significantly more promise for defect‐free epitaxial integration. This interesting aspect of (quasi‐)vdW epitaxy reveals that the strength of interlayer vdW coupling is key for workable 2D materials and opens perspectives for other strongly coupled vdW materials.

Defect-free epitaxial growth of 2D materials is one of the holy grails for a successful integration of van der Waals (vdW) materials in the semiconductor industry. The large-area (quasi-)vdW epitaxy of layered 2D chalcogenides is consequently carefully being researched since these materials hold very promising properties for future nanoelectronic applications. The formation of defects such as stacking faults like 60o twins and consequently 60o grain boundaries is still of major concern for the defect-free epitaxial growth of 2D chalcogenides. Although growth strategies to overcome the occurrence of these defects are currently being considered, more fundamental understanding on the origin of these defects at the initial stages of the growth is highly essential. Therefore this work focuses on the understanding of 60o twin formation in (quasi-)vdW epitaxy of 2D chalcogenides relying on systematic molecular beam epitaxy (MBE) experiments supported by density functional theory (DFT) calculations. The MBE experiments reveal the striking difference in 60o twin formation between WSe2 and Bi2Se3 in both quasi-vdW heteroepitaxy and vdW homoepitaxy, which from our DFT calculations links to the difference in interlayer vdW coupling strength. The stronger interlayer vdW coupling in Bi2Se3 compared to WSe2 results in a striking enhanced control on twin formation and hence shows significantly more promise for defect-free epitaxial integration. This interesting aspect of (quasi-)vdW epitaxy reveals that the strength of interlayer vdW coupling is key for functional 2D materials and opens perspectives for other vdW materials sharing strong interlayer interactions.

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.

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 important is the role of surface energy, calculated using density functional theory, that 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.

As interest in layered van der Waals (vdW) materials keeps increasing, fundamental knowledge about their synthesis is gaining more value. The defect-free heteroepitaxial integration of vdW materials on large-area substrates is currently thoroughly being researched since it might encompass a successful transition to industrial applications. Transition metal dichalcogenides (TMDs) are considered as promising vdW materials. However, the electrical characterization of heteroepitaxially grown TMDs shows inferior performance as compared to exfoliated TMD flakes. This is mainly attributed to the high defect density resulting from the challenging heteroepitaxy. We have investigated in-depth the vdW homoepitaxy of the WSe ₂ TMD compound. We demonstrate that also for homoepitaxy, challenges such as the formation 60° twins need to be addressed. The presence of these stacking faults is associated with their very similar binding energy as revealed by density functional theory (DFT) calculations. Stacking faults are therefore identified as the fundamental limitation of lowly-defective multilayer TMD vdW epitaxy. Furthermore, a generalized model is developed that determines the defect density based on the stacking control and the nucleation density. This model therefore assesses and quantifies for the first time the ultimate defect density level that can be achieved with vdW epitaxially grown 2D materials.

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 (MoS ₂ ) on sapphire substrates at different temperatures. We show that a high temperature leads to a perfect epitaxial alignment of the MoS ₂ 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 H ₂ S environment at different growth temperatures. Structural analyses conducted on the different samples confirm an improved layer quality at high temperatures. MoS ₂ channel-based metal–oxide–semiconductor field-effect transistors are fabricated showing improved device performance with channel layers grown at high temperature.

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.

We present in this paper the use of Gas Source Molecular Beam Epitaxy for the large-scale growth of transition metal dichalcogenides. Fiber-textured MoS₂ co-deposited thin films (down to 1 MLs) are grown on commercially 200 mm wafer size templates where MX₂ crystalline layers are achieved at temperatures ranging from RT to 550 °C. Raman Spectroscopy and photoluminescence measurements along with X-Ray Photoelectron Spectroscopy show that a low growth rate is essential for complete Mo sulfurization during MoS₂ co-deposition. Finally, cross-section Transmission Electron Microscopy investigations are discussed to highlight the influence of SiO₂ and Al₂O₃ used surfaces on MoS₂ deposition.