Abstract
Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. Well-resolved narrow excitonic resonances can be observed for such monolayers although only for materials of sufficient crystalline quality and so far mostly available in the form of micrometer-sized flakes. A further significant improvement of optical and electrical properties can be achieved by transferring the TMD on hexagonal boron nitride (hBN). To exploit the full potential of TMDs in future applications, epitaxial techniques have to be developed that not only allow the growth of large-scale, high-quality TMD monolayers but also allow the growth to be performed directly on large-scale epitaxial hBN. In this work, we address this problem and demonstrate that MoSe2 of high optical quality can be directly grown on epitaxial hBN on an entire 2 in. wafer. We developed a combined growth theme for which hBN is first synthesized at high temperature by metal organic vapor phase epitaxy (MOVPE) and as a second step MoSe2 is deposited on top by molecular beam epitaxy (MBE) at much lower temperatures. We show that this structure exhibits excellent optical properties, manifested by narrow excitonic lines in the photoluminescence spectra. Moreover, the material is homogeneous on the area of the whole 2 in. wafer with only ±0.14 meV deviation of excitonic energy. Our mixed growth technique may guide the way for future large-scale production of high quality TMD/hBN heterostructures.
Highlights
Transition metal dichalcogenides (TMDs), representatives of 2D layered materials, are intensively studied as promising candidates for future realizations of optoelectronic devices,[1,2] photodetectors,[3−5] sensors,[6−8] energy and memory storages,[9−11] or transistors.[12−14] Such realizations are commonly demonstrated on micrometer-sized flakes obtained by mechanical exfoliation from bulk crystals and consecutively stacked to a heterostructure by time-consuming deterministic transfer processes.[1,15−18] This approach allows the production of high quality samples in terms of electronic properties such as carrier mobility or conductivity.[19]
We show that the grown material is of excellent, homogeneous optical quality, confirmed by Raman spectroscopy and photoluminescence mapping of the 2 in. wafer
The properties of the obtained Hexagonal boron nitride (hBN) layers depend on a set of growth parameters including growth time, temperature, III−V ratio, or the employed growth mode.[52]
Summary
Transition metal dichalcogenides (TMDs), representatives of 2D layered materials, are intensively studied as promising candidates for future realizations of optoelectronic devices,[1,2] photodetectors,[3−5] sensors,[6−8] energy and memory storages,[9−11] or transistors.[12−14] Such realizations are commonly demonstrated on micrometer-sized flakes obtained by mechanical exfoliation from bulk crystals and consecutively stacked to a heterostructure by time-consuming deterministic transfer processes.[1,15−18] This approach allows the production of high quality samples in terms of electronic properties such as carrier mobility or conductivity.[19]. Molecular beam epitaxy (MBE) appears to be a promising candidate for the development of large-area samples, as it allows the production of high purity materials with atomic precision.[36−42] the optical quality of the samples is still far behind the best results obtained for mechanically exfoliated flakes. A way to overcome this obstacle is to combine TMD layers with hBN Successful realizations of this approach such as MoSe2 growth by MBE or CVD on initially exfoliated hBN flakes[43,44] or an entirely grown hBN/MoS2 heterostructure by CVD45 have been already presented. The proposed approach provides a reliable template for the growth of high optical performance TMD layers by utilizing large area epitaxial hBN as a substrate. Our method of combined MBE−MOVPE growth may pave the way for future applications of van der Waals heterostructures on the wafer scale
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