Lateral Integration of SnS and GeSe van der Waals Semiconductors: Interface Formation, Electronic Structure, and Nanoscale Optoelectronics

Abstract

The emergence of atomically thin crystals has allowed extending materials integration to lateral heterostructures where different 2D materials are covalently connected in the plane. The concept of lateral heterostructures can be generalized to thicker layered crystals, provided that a suitably faceted seed crystal presents edges to which a compatible second van der Waals material can be attached layer by layer. Here, we examine the possibility of integrating multilayer crystals of the group IV monochalcogenides SnS and GeSe, which have the same crystal structure, small lattice mismatch, and similar bandgaps. In a two-step growth process, lateral epitaxy of GeSe on the sidewalls of multilayer SnS flakes (obtained by vapor transport of a SnS 2 precursor on graphite) yields heterostructures of laterally stitched crystalline GeSe and SnS without any detectable vertical overgrowth of the SnS seeds and with sharp lateral interfaces. Combined cathodoluminescence spectroscopy and ab initio calculations show the effects of small band offsets on carrier transport and radiative recombination near the interface. The results demonstrate the possibility of forming atomically connected lateral interfaces across many van der Waals layers, which is promising for manipulating optoelectronics, photonics, and for managing charge-and thermal transport.

Photonics in Multimaterial Lateral Heterostructures Combining Group IV Chalcogenide van der Waals Semiconductors

“…Multimaterial multilayer lateral van der Waals heterostructures were prepared in several consecutive steps, starting with i) synthesis of SnS seed flakes that are used as the core of the heterostructures. The growth of SnS flakes is well established on a variety of substrates [ 21c,31 ] and with different thicknesses. [ 32 ] Here, SnS growth was performed by vapor transport from a SnS 2 precursor on HOPG.…”

“…[ 32 ] Here, SnS growth was performed by vapor transport from a SnS 2 precursor on HOPG. [ 21c ] In the following steps, the precursor vapor was changed to ii) a mixture of GeS and GeSe in order to obtain a laterally attached band (I) of S‐rich GeS 1− x Se x alloy; iii) GeSe to attach a pure GeSe band; and finally iv) again a mixture of GeS and GeSe to terminate the heterostructure with another band (II) of GeS 1− x Se x alloy. The growth conditions used were suitable for the preferential incorporation of GeS [ 21a ] and GeSe [ 21c ] into the straight‐side facets of SnS seeds (see Experimental Section for details).…”

“…Figure 2 shows a STEM image and EDS maps of the elemental distribution across a section of a heterostructure encompassing part of the center and the laterally attached bands. The STEM image (Figure 2b) shows that the SnS is bounded by slanted facets, typical for the growth on HOPG, [ 21c ] while the interfaces between GeS 1− x Se x and GeSe are vertical. EDS mapping detects Sn only in the central SnS seed of the heterostructure (Figure 2d).…”

“…Compared with monolayers, heterostructures involving multilayer van der Waals crystals have remained far less explored even though they offer unique structural degrees of freedom, such as different layer stacking arrangements [ 19 ] and interlayer twist, [ 20 ] as well as straightforward lateral integration of different materials. Multilayer lateral heterostructures have only been demonstrated for the group IV chalcogenide system [ 21 ] and WSe 2 ‐MoSe 2 . [ 22 ] Group IV chalcogenide multilayer heterostructures showed high‐quality lateral interfaces that are both sharp and spatially coordinated across the entire thickness, [ 21c ] and which are transparent to the transfer of intact electron–hole pairs in SnS‐GeS heterostructures [ 21b ] while driving carrier separation near interfaces in SnS‐SnS 2 heterostructures.…”

“…Multilayer lateral heterostructures have only been demonstrated for the group IV chalcogenide system [ 21 ] and WSe 2 ‐MoSe 2 . [ 22 ] Group IV chalcogenide multilayer heterostructures showed high‐quality lateral interfaces that are both sharp and spatially coordinated across the entire thickness, [ 21c ] and which are transparent to the transfer of intact electron–hole pairs in SnS‐GeS heterostructures [ 21b ] while driving carrier separation near interfaces in SnS‐SnS 2 heterostructures. [ 19 ] Multilayer lateral heterostructures—taking advantage of an enhanced optical thickness, the anisotropic structure and properties of the group IV chalcogenide semiconductor layered crystals combined with facile intralayer carrier transport at lateral interfaces—have the potential to contribute to the photonics of 2D/layered crystals, [ 23 ] adding to phenomena such as single‐photon emission, [ 24 ] strong light‐matter coupling, [ 25 ] gate‐tunable plasmons, [ 26 ] formation of hybrid states such as exciton‐polaritons, [ 18,27 ] guided modes in ultrathin waveguides (WGs), [ 28 ] and the realization of integrated multifunctional photonic systems.…”