Synthesis of MoS2 on Homogeneous Single Layer Epitaxial Graphene on 6H-SiC(0001)
Authors: Nyakiti Luke O. , Robinson Zachary , Myers-Ward Rachael L. , Currie Marc , Hite Jennifer K , Sridhara Karthik , Clancy Edward , Eddy Charles R. , Gaskill D. Kurt 
Published: February 2020

Molybdenum Disulfide (MoS2
) is a semiconducting transition metal dichalcogenide (TMD) formed when Mo is sandwiched between S atoms to form either a trigonal prismatic or octahedral atomic structure. Theoretical studies using different first principles calculations show exceptional optical band structure dependence on layer thickness, a property that makes the nanomaterial system a promising candidate for high performance, low cost tunable materials for flexible electronics, high electron mobility transistors and field tunneling transistors. The 2D heterostructure materials reported in the literature have been primarily investigated from sub-micron size flakes produced by mechanical exfoliation process followed by transfer onto SiO2/Si substrate. This technique is only useful for laboratory-based, proof-of-concept studies that cannot be scaled for industrial production. In addition, device structures fabricated from flakes are known to have interface impurities that degrade the device performance. It is therefore prudent that a direct approach for the formation of integrated 2D heterostructure with tunable properties be studied. This presentation will highlight synthesis of nucleation structures of MoS2
, effects of underlying Epitaxial Graphene (EG) layer on MoS2
lateral growth and corresponding optical as well as composition variation across the step-bunched terrace widths of the templated layers of EG/6H-SiC(0001). µ-Raman spectroscopy, AFM and Photoluminescence (PL) will be used to extract composition, presence, layer thickness, surface morphology variations and optical band structures of the heterostructure.

The synthesis process of MoS2
on a template of monolayer EG/6H-SiC(0001) was carried out in horizontal CVD reactor at 800°C – 950˚C for 10 - 30 min in a flowing UHP Ar/H2 ambient of 80/20 sccm and 10 - 100 mbar. The initial Raman Spectroscopy characterization of MoS2/EG/6H-SiC heterostructure shows planar (E2) and axial (A1) acoustic active modes of underlying 6H-SiC at 250 and 501 wave numbers, respectively. In addition, the out-of-plane vibrations of S atoms (A1) and in-plane vibrational modes of Mo and S atoms (E1
1g) were located at 402 and 378 wave numbers, respectively (refer to Figure 1). The Raman spectrum also show EG Raman 2D mode post MoS2 synthesis, suggesting preservation of underlying EG. The successful synthesis of wafer-scale formation of MoS2
/EG/SiC heterostructures offers the opportunity for a number of applications with significant advantages over the conventional top-down exfoliation and transfer process. For instance, direct growth of MoS2 on n-type 6H-SiC(0001) eliminates the need to transfer the TMD to another substrate, consequently allowing no impurities to be pinned at the interfaces thereby mitigation against the short channel effects in device structures, enabling tunneling devices and variations of hot electron transistor devices.

Figure 1