Role of Stronger Interlayer van der Waals Coupling in Twin‐Free Molecular Beam Epitaxy of 2D Chalcogenides

Abstract

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.

Anisotropy-Driven Crystallization of Dimensionally Resolved Quasi-1D Van der Waals Nanostructures

“…Altogether, we determined herein that the growth of bulk and nanoscale structures derived from q-1D vdW phases is strongly influenced by the highly anisotropic bonding nature of Sb 2 S 3 and is reliant on growth temperatures and ratios of the Sb and S species in the vapor phase prior to deposition. 71 ■ CONCLUSIONS Overall, we established that the interplay between the strong intrachain covalency and the highly anisotropic interchain vdW interactions across 1D and q-1D vdW structures can be harnessed for the directional growth of nanowires, nanoribbons, and nanosheets at various length scales. The ability to access these diverse nanostructures has enabled the discovery of emergent size-and dimensionality-dependent photophysical properties.…”

“…Since growth along the [010] direction (Figure S13d) involves the formation of more reactive covalent bonds, we anticipate that this process will be the most favorable. In contrast, when considering the growth along the vdW-bound interchain direction, we accounted for the tendency of the facet to harbor growth similar to vdW epitaxy observed in 2D vdW crystals. − Among these interchain directions, the growth along the [100] direction (Figure S13a) involves the least number and weakest vdW interactions to grow the next layer of the material. This suggests that minimal energy is required to initiate growth, making this growth mode more favorable than along directions ([001] or [−101]), which require stronger vdW interactions to grow the next layer (Figure S13b,c).…”

Two-dimensional single crystal monoclinic gallium telluride on silicon substrate via transformation of epitaxial hexagonal phase

“…2b, are schematically highlighted within the plane of reciprocal space parallel to the sample surface: (i) The radial scan intersecting a Si(220) substrate reflection provides information about the in-plane orientation of the layer with GaTe[2110] parallel to Si [110], and the respective in-plane lattice parameter ( ≈ 6.2% larger than the Si(220) net plane distance) which matches the RHEED values; (ii) The angular inspection of the GaTe[2110] layer peak reveals a marked repetition every 60 ∘ but also a minor (1.4% vs. 98.6%) component 30 ∘ off. The sixfold rotational symmetry has been already observed for chalcogenide thin films grown on Si(111) 38 and it is the effect of the weak vdW coupling between the substrate and the epitaxial layer 39 .…”

Hints for a General Understanding of the Epitaxial Rules for van der Waals Epitaxy from Ge‐Sb‐Te Alloys

“…For instance, when using graphene or other 2D crystals as buffer layers, for vdW epitaxy the underlying substrates may still interact with the growing film. [ 8–15 ] An intermediate behavior in the growth between 2D and 3D materials has also been observed, actually allowing strain engineering in these materials. [ 16–21 ] Epitaxial rules for 2D materials are therefore highly desirable to allow the prediction of substrate surface interaction, vdW heterostructures commensurability, and strain relaxation during interface growth.…”