Mg/MgO/Graphene Tunnel Junctions Made by Dry Transfer of Graphene in Vacuum

Mg/MgO/Graphene junctions were fabricated by dry transfer of single layer graphene film grown by chemical vapor deposition on Cu Mg strips were deposited onto Si/SiO$_{2}$ or glass substrates by thermal evaporation through a shadow mask. The tunnel barrier MgO was formed by exposing deposited Mg for about 10 minutes in air prior to the graphene transfer. To prevent degradation of MgO by liquids, a dry transfer technique is used. First a graphene film was transfer onto a free-standing 4$\mu $m-thick Cu film using the traditional wet method, then pressed onto a transparent and flexible PDMS stamp followed by etching away the Cu film in FeCl$_{3}$ solution, and finally stamped onto the Mg strips in vacuum to prevent any gas bubbles that may form between graphene and Mg strips. The dry-transferrd graphene has similar properties to traditional wet-transferred graphene, characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and transport measurements. It has a sheet resistance of 1.6 $\sim$ 3.4 k$\Omega$/$\Box$, charge carrier density of 4.1 $\sim$ 5.3 $\times$ 10$^{12}$ /cm$^{2}$ and mobility of 460 $\sim$ 760 cm$^{2}$/Vs without doping at room temperature. Mg/MgO/graphene junctions show good tunneling characteristics at temperatures down to 4.2 K. The barrier height and width were obtained by fitting with the Brinkman-Dynes-Rowell trapezoid-shaped barrier model with consideration of graphene electron structure.