Quantum Hall Resistance Standards Based on Epitaxial Graphene with Different Doping Layer Sequence

Epitaxial graphene-based quantum Hall resistance standards (QHRS) can be operated at relaxed working conditions compared to conventional GaAs heterostructure devices. To reach the desired magnetic field working point, exact and reliable control of the charge carrier density of graphene plays a crucial role. This can be achieved by post-growth deposition of F4-TCNQ molecular dopants embedded within a polymer host layer. Here, we investigate QHRS with different sequences of such doping and undoped spacer layers to find a structure that is as simple and optimal as possible. By high-accuracy resistance measurements we found excellent metrological quality for all types of devices in a broad magnetic field range starting at 5 T and at 4.2 K. The resistance quantization is maintained over two years, so far, which gives further confidence in the robustness and long-term stability of such molecular-doped graphene-based QHRS.