Fundamental limitation of van der Waals homoepitaxy by stacking fault formation in WSe₂

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 WSe₂ 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. (paper)