Peculiar alignment and strain of 2D WSe<sub>2</sub> grown by van der Waals epitaxy on reconstructed sapphire surfaces

technique which can give insight in the trap density inside the MoS 2 bandgap, while being non-invasive and fast. One of the factors that can strongly influence the performance of 2D materials is the substrate interaction. For both graphene and transition metal dichalcogenides (TMDCs), it has been shown [3-8] that amorphous dielectric substrates such as SiO 2 cause increased scattering, while enhanced performance is achieved when atomically flat 2D materials, such as hexagonal boron nitride (hBN), are employed as substrates. Performance, however, is often measured from devices, where mobility, hysteresis, or interface trap density (D IT) are assessed. This requires full device processing before the metrics can be acquired, where such processing can introduce non-idealities in the material which are difficult to deconvolve using electrical measurements. [9-12] This strongly supports the need for characterization techniques and metrics which can correlate material and interface quality before any processing takes place or/and between fabrication steps. For the case of graphene and carbon nanotubes, Raman spectroscopy is a powerful in-line quality assessment tool, with information on defectivity, flatness, thickness, etc. [13-15] The same fast and non-invasive quality metrics are not mature for the case of 2D TMDCs. One technique which can potentially fill this role is photoluminescence (PL) spectroscopy. Several works have used photoluminescence to study the properties of the interface between 2D TMDC and different substrates, with particular focus on the direct photoluminescence peak from monolayers, arising from direct radiative transitions from the conduction band to the valence band. [16-19] The direct photoluminescence peak of MoS 2 , around 1.8 eV, contains information on thickness, doping, and strain. [20-22] Additionally, direct exciton lifetime measurements can give insights on interface scattering and defect dynamics. [23,24] However, as thickness increases above the monolayer limit, the direct bandgap of MoS 2 transitions to an indirect bandgap. Notably, not much attention has been given to the peak originating from indirect bandgap transitions, present in thicker layers (⩾2 monolayers). [20] From a technological perspective, thicker layers have a series of advantages compared to monolayers: their lower volume-to-surface ratio makes them more tolerant towards ambient or process-induced degradation; [25] their smaller bandgap results in higher current density and lower contact resistance. [26-28] Here, we investigate the behavior Defect characterization of 2D materials is a critical aspect for their successful integration in future electronic devices. Here, a simple characterization technique is proposed that opens a path for fast, non-invasive, quality assessment of transition metal dichalcogenide layers, such as MoS 2 , and their interfaces. It relates to the correlation between substrate-induced traps and the indirect-to-direct photoluminescence peak ratio. It is shown that the indirect peak is quenche...

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“…They were characterized by an atomically smooth surface showing the typical step‐and‐terrace‐like structure. [ 53 ]…”

Section: Methodsmentioning

confidence: 99%

Use of the Indirect Photoluminescence Peak as an Optical Probe of Interface Defectivity in MoS₂

Leonhardt

Rosa

Nuytten

et al. 2020

Adv Materials Inter

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“…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

(0 \bar{1})

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

(⟨⟩, 11 \overset{2}{true} 0)

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.…”

Section: Resultsmentioning

confidence: 99%

“…The (1 × 1) reconstructed sapphire surfaces were obtained by thermal annealing at ≈900 °C under H 2 as reported and characterized previously. [ 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).…”

Section: Methodsmentioning

confidence: 99%

“…[ 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.…”

Section: Introductionmentioning

confidence: 99%

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

Mortelmans

Smet

Meng

et al. 2021

Adv Materials Inter

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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.

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“…Similarly as for vdW epitaxy on vdW surfaces, in-plane epitaxial alignment of vdW layers on 3D crystalline substrates is demonstrated in numerous cases. The quasi-vdW heteroepitaxies performed using MBE focus on a wide-ranging set of substrates such as sapphire [138][139][140][141][142][143][144][145], GaAs [146][147][148][149][150][151],…”

Section: Epitaxial Alignmentmentioning

confidence: 99%

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

Mortelmans,

De Gendt,

Heyns

et al. 2020

Preprint

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The application of new materials in nanotechnology opens new perspectives and enables groundbreaking innovations. Two-dimensional van der Waals materials and more specific, 2D chalcogenides are a promising class of new materials awaiting their usage in the semiconductor industry. However, the integration of van der Waals materials relying on industry-compatible manufacturing processes is still a major challenge. This is currently restricting the application of these new materials to the research laboratories environment only. The large-area and single-crystalline growth of van der Waals materials is one of the most important requirements to meet the challenging demands implied by the semiconductor industry. This review contributes to a more generalized understanding on the integration of van der Waals materials -and in more specific 2D chalcogenides -through the growth process of epitaxy. This, can pursue further the aspiration of large-area, single-crystalline and defect-free epitaxial integration of (quasi) van der Waals homo-and heterostructures into the great world of the semiconductor industry.

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Peculiar alignment and strain of 2D WSe₂ grown by van der Waals epitaxy on reconstructed sapphire surfaces

Cited by 19 publications

References 68 publications

Use of the Indirect Photoluminescence Peak as an Optical Probe of Interface Defectivity in MoS₂

Use of the Indirect Photoluminescence Peak as an Optical Probe of Interface Defectivity in MoS₂

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

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

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Peculiar alignment and strain of 2D WSe2 grown by van der Waals epitaxy on reconstructed sapphire surfaces

Cited by 19 publications

References 68 publications

Use of the Indirect Photoluminescence Peak as an Optical Probe of Interface Defectivity in MoS2

Use of the Indirect Photoluminescence Peak as an Optical Probe of Interface Defectivity in MoS2

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

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

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Peculiar alignment and strain of 2D WSe₂ grown by van der Waals epitaxy on reconstructed sapphire surfaces

Use of the Indirect Photoluminescence Peak as an Optical Probe of Interface Defectivity in MoS₂

Use of the Indirect Photoluminescence Peak as an Optical Probe of Interface Defectivity in MoS₂