Polymer coating and stress test for carrier density stabilization in epitaxial graphene

Homogeneous monolayer epitaxial graphene (EG) is an ideal candidate for the development of a quantum Hall resistance (QHR) standard. A clean fabrication process was used to produce EG-QHR devices with a n-type doping level of order 10$^{\mathrm{11}}$ cm$^{\mathrm{-2}}$, which delivers the metrological accuracy at the $\nu =$2 plateau in a moderate magnetic field (\textless 9 T). However, the $\nu =$2 plateau deviates from h/2e$^{\mathrm{2}}$ quickly as the carrier density shifts close to the Dirac point (\textless 10$^{\mathrm{10}}$ cm$^{\mathrm{-2}})$, and this observation occurs over time as EG is exposed to air, allowing for complexation with p-type molecular dopants. Here we report experimental results on the use of parylene C as an encapsulation layer, whereby EG can maintain its carrier density level under ambient laboratory conditions for a few months. Furthermore, we varied the parylene C thicknesses and the controllable temperatures (up to 85$^{\circ}$ C) and humidities (up to 85{\%}). We monitored the electronic properties of our EG samples by low temperature magnetotransport measurements in a 9 T superconducting magnet cryostat, and room temperature surface conductance in a resonant microwave cavity. We will compare parylene C, Cytop, and PMMA and show that polymer encapsulation may offer a solution to the problem of carrier density instability from atmospheric doping.