(Invited) Bottom up Synthesis of Few-Layer Van-Der Waals Heterostructures on Multifarious Semiconducting Substrates
Authors: Nyakiti Luke O. , Hite Jennifer K , Robinson Zachary , Sridhara Karthik , Myers-Ward Rachael L. , Currie Marc , Eddy Charles R , Gaskill Kurt 
Published: February 2020

MoSe2, MoS2 and WSe2 are direct bandgap semiconducting transition metal dichalcogenides (TMDs). They present a material system that is formed when the metal is sandwiched between dichalcogenides atoms, and forms either a trigonal prismatic or octahedral atomic structure. Theoretical studies of TMDs show optical band structure dependence on layer thickness, a property that makes the material system a promising candidate for high performance, low cost tunable materials for flexible electronics, high electron mobility transistors and field tunneling transistors. Most of the reported 2D TMDs reported in the literature are primarily investigated from square micrometer area flakes and many 100 µm2 area CVD growths some with subsequent transfer onto different semiconducting substrate types. This synthesis technique is only useful for laboratory-based, proof-of-concept studies that cannot be scaled for industrial production. It is therefore prudent that a direct approach for the formation of integrated 2D heterostructure with tunable properties be studied.

This presentation will highlight the nucleation and evolution of TMDs, specifically MoSe2 , MoS2 & WSe2. Studies were performed on substrates such as SiO2/Si, Epitaxial Graphene(EG)/SiC and III-Nitrides. Lateral growth was observed on several samples, and corresponding optical as well as compositional variation across the step-bunched terrace widths of the templated layers of EG/6H-SiC (0001) was observed. 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 TMDs on a template of monolayer EG/6H-SiC (0001) was carried out in horizontal CVD reactor at 700°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 MoSe2/EG/6H-SiC heterostructure shows a strong planar and axial acoustic active modes of the TMDs material system. In addition, the out-of-plane vibrations of Se atoms and in-plane vibrational modes of Mo and Se atoms observed. Initial partial syntheses of WSe2 show random formation of W particle-nucleation sites on the terrace of EG template. The surface density of the nucleation sites was ~ 3.0 x109cm-2. The RMS roughness of 0.5 x 0.5 μm2 AFM scans of the terraced WSe2
structures was ~ 0.213 nm. The Raman spectroscopy shows a convoluted A1’ & E’ signature peak at 220cm-1, which contrast previous reported modes that assumes degeneracy at 248-251cm-1. Raman spectroscopy indicates that graphene is present (2D and D peaks) with minimal disruption to the lattice. PL of WSe2 /EG/6H-SiC (0001) show the presence of two (A and B) low exciton peak energy at 1.68eV and 2.05eV.

The successful synthesis of wafer-scale formation of WSe2 or MoSe2
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 2D heterostructures on multifarious substrate eliminates the need to transfer the 2D material to another substrate, consequently reducing segregation of impurities at the interfaces thereby mitigation against the short channel effects in nano-device structures.