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

Abstract

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.

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

“…The observation of these inequivalent streaks is in agreement with the threefold in‐plane rotational symmetry of the Bi 2 Se 3 crystal structure, and confirms the preferred and unique stacking of Bi 2 Se 3 on sapphire and hence the reduced formation of 60° twins. [ 65 ] In summary, Bi 2 Se 3 quasi‐vdW heteroepitaxy on sapphire is less prone to stacking fault formation compared to WSe 2 (and in general TMDs [ 21,34 ] ), despite the equivalent in‐plane crystal structure symmetry and presence of vdW gap in both compounds. Consequently, Bi 2 Se 3 shows significantly more promise for defect‐free epitaxial integration.…”

“…The larger grain size is linked with the larger vapor pressure of the elemental bismuth, since adatom diffusion is previously reported to correlate with vapor pressure in vdW epitaxy of TMDs by MBE. [ 34,66 ] The Bi 2 Se 3 vdW compound does not suffer from the fundamental limitation of stacking fault formation in vdW homoepitaxy as generally observed in TMD vdW compounds. [ 21,33,34 ] This opens a window for defect‐free integration of Bi 2 Se 3 through the growth process of vdW homoepitaxy, and possibly also for other related vdW compounds.…”

“…[ 23 ] The virtual WSe 2 (0001) and Bi 2 Se 3 (0001) substrates were fabricated relying on mechanical exfoliation on silicon substrates. [ 33,34 ] The epitaxies were performed using plasma‐assisted (PA‐)MBE with H 2 X radio frequency (RF) plasma sources [ 61 ] and electron‐beam evaporation of elemental W transition metals (Figure 1a) and thermal evaporation of elemental Bi metals (Figure 1b). For the WSe 2 compound, the growths occurred at a temperature of 450 °C with low growth rates of ≈0.1–1.3 ML h −1 , driven by the W evaporation flux.…”

“…[ 15 ] Therefore, the quasi‐vdW and vdW epitaxy (2D‐on‐3D and 2D‐on‐2D, respectively) of layered chalcogenides is extensively being researched in the literature. [ 15–20 ] One of the major concerns in (quasi‐)vdW epitaxy of these materials is the systematic formation of stacking faults like 60° twins, as observed in either molecular beam epitaxy (MBE) (quasi‐vdW [ 21–29 ] and vdW [ 21,30–36 ] ), metalorganic vapor phase epitaxy (quasi‐vdW [ 21,37–42 ] and vdW [ 21,43,44 ] ), and chemical vapor epitaxy (quasi‐vdW [ 45–47 ] and vdW [ 48–53 ] ). To mitigate the formation of these defects, several approaches are being reported that rely on optimized growth conditions, [ 54–56 ] buffer layer growth, [ 57 ] growth on h‐BN templates, [ 54,58 ] or the introduction of a 3D aspect in the growth surface like surface roughness [ 59 ] or surface step edges.…”

Molecular Beam Epitaxy growth of MoTe$_{\tiny{\textrm{2}}}$ on Hexagonal Boron Nitride

“…In order to quantitatively compare the influence of the growth conditions on the quality of the samples we use the diffusion coefficient estimation [30]. It relies on the activation energy of adatom diffusion [31] and so, uniquely describes each specific epitaxial system.…”