Fabrication of Graphene/WC/SiC Heterostructures and its Electronic Properties

Graphene can be grown by thermal decomposition of SiC and other carbides, which means that we can easily fabricate graphene/carbide heterostructure ^[1]. In this study, we focus on tungsten carbide (WC) as the carbide. Recently, it is found that Hexagonal WC is one of the topological semimetals, and it has attractive properties such as Fermi-arc surface states, negative magnetoresistance, and dHvA oscillations ^{[2, 3]}. In graphene/topological semimetal heterostructures, nonlocal resistance originating from spin-polarized carriers has been observed, which is expected to be applied to spintronics ^[4]. Therefore, fabrication of graphene/WC heterostructures is expected to enable us to observe new electronic properties.<br/>In this study, we fabricated graphene/WC/SiC heterostructure and investigate the electronic properties. Highly crystalline WC thin films were formed on 4H-SiC single-crystal substrates by pulsed laser deposition (PLD). Then, graphene was fabricated by thermal decomposition of the films. The fabricated graphene/WC/SiC samples were characterized by some measurements. The electronic state of the samples was measured by angle-resolved photoemission spectroscopy (ARPES). Transport measurements of WC/SiC and graphene/WC/SiC samples were also performed at low temperatures.<br/><br/>4H-SiC (000-1) single crystal substrate (5×5 mm², on-axis, CREE) was used as a substrate. The WC thin film was fabricated by the PLD method. Then, the substrate was heated at 1100°C for 10 minutes. The resulting WC thin film was heated at 1750°C for 10 minutes in a vacuum to fabricate graphene. Reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), ARPES, and low-temperature transport measurements were performed on the prepared samples.<br/><br/>From the High-resolution TEM images after WC deposition, we observed a WC thin film deposited on the SiC substrate with a thickness of about 7 nm. Analysis of the electron diffraction showed that the relationship of the orientation between hexagonal WC and SiC substrate was (0001)_WC//(000-1)_SiC and [11-20]_WC//[11-20]_SiC.<br/>ARPES measurements of the graphene/WC/SiC sample showed band dispersions of graphene and bulk WC. Furthermore, we observed a band near the Γ point of WC, which we cannot explain by the band structure of graphene and bulk WC. We think the band originate from the surface state of WC or the interface interaction between graphene and WC.<br/>Transport measurements indicate that superconductivity of WC/SiC and graphene/WC/SiC sample. We estimated the transition temperature (Tc) of WC/SiC is about 1.8-2.2 K. We found that graphene/WC/SiC has smaller Tc than WC/SiC.<br/><br/>[1] W. Norimatsu, et al., Nanotechnology <b>31</b>, 145711 (2020).<br/>[2] J. Z. Ma, et al., Nat. Phys. <b>14</b>, 349 (2018).<br/>[3] J. B. He, et al., Phys. Rev. B <b>95</b>, 195165 (2017).<br/>[4] Y. Wu, et al., Adv. Mater. <b>30</b>, 34 (2018).