Epitaxial registry and crystallinity of MoS2 via molecular beam and metalorganic vapor phase van der Waals epitaxy

Abstract

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.

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

“…This is a similar epitaxial relationship as previously reported for the growths of WSe 2 /MoS 2 on various reconstructed sapphire surfaces. [ 21,23,37 ] The identical and (01) diffraction streaks observed from the diffraction patterns uncover an important limitation of the quasi‐vdW epitaxy experiment. In Figure 2b, the RHEED pattern in the WSe 2 direction is presented where several intensity line profiles are extracted from various “ k z ” positions that give information about the out‐of‐plane ordering of the grown 2D crystal planes.…”

“…[ 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.…”

“…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.…”

“…[ 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.…”

Epitaxy of new layered materials: 2D chalcogenides and challenges of weak van der Waals interactions

“…The orientational replication of vdW layers on vdW substrates through vdW interaction is successfully demonstrated in many different cases [81]. These range from MBE grown MX 2 materials on different 2D substrates (HOPG [82][83][84][85], graphene [86][87][88][89][90][91][92][93], MX 2 [94][95][96], h-BN [97,98], mica [99], ...), to many other 2D materials such as for example M 2 X 3 [100][101][102][103][104][105], h-BN [106] and graphene [107].…”