Interlayer coupling in graphene heterostructures can be used to modify their charge transport properties for the use in nanoelectronics applications. To achieve high-performance devices, graphene is typically encapsulated by two thin sheets of hexagonal boron nitride (hBN) and contacted using Cr/Au as electrodes. In this research, Mo was selected as alternative stand-alone electrodes to simplify the fabrication process. We have developed a recipe for the fabrication of field-effect transistor (FET) using twisted bilayer graphene (tBLG) as a transport channel. Mo was employed to make one-dimensional (1D) contacts to the edges of graphene by sputtering. Electronic transport measurement was then performed on the FET devices to investigate the quality of Mo contacts. We achieved ohmic contacts between Mo and graphene. The contact resistivity between Mo and graphene reached a maximum value of around 1,300 Ω⋅µm at charge neutrality point and decreased to 975 Ω⋅µm at a carrier density of ~4×1012 cm-2. However, we have observed that the contact resistivity increases over time likely due to the oxidization of Mo. Despite the increase in contact resistivity, the Mo contacts still exhibit ohmic behavior after 3 months and it can be used to investigate transport phenomena in graphene. In this study, the room-temperature electron and hole mobilities of tBLG reached values as high as 27,000 and 21,000 cm2/V⋅s indicating that Mo electrodes do not hinder the transport characteristics of graphene.
Graphene is an intriguing platform to study exotic quantum transport phenomena due to its intrinsically high mobility and remarkable electronic properties. To achieve high-performance device, graphene is usually encapsulated between thin sheets of hexagonal boron nitride (hBN) to protect graphene layer from extrinsic impurities. Cr/Au is typically employed to make contacts with the edges of the heterostructure. In this research, Mo is used as an alternative electrode for graphene without adhesion layer to simplify the fabrication process. hBN-graphene-hBN heterostructures were fabricated by a pick-up technique and etched in O2/CHF3 gases to expose graphene edges. Mo contacts were deposited onto the substrates by sputtering. We achieved ohmic contacts between graphene and Mo. The contact resistance reaches the maximum of around 1,300 Ω·μm at charge neutrality point and decreases to 975 Ω·μm at the density of 4×1012 cm−2. We observed that the contact resistance increases over time likely due to the oxidation of Mo but remained ohmic after 2 months. The intrinsic transport characteristics of graphene can still be obtained by using four-probe measurement. Here, we realized a high-quality twisted bilayer graphene device with a room-temperature mobility of 27,000 cm2/V·s indicating that Mo can be used as edge-contacts to probe the transport properties of graphene.
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