National Institute of Standards and Technology (NIST), USA
Yanfei Yang is a Postdoctoral Fellow at Joint Quantum Institute, University of Maryland. Her current research interests include: - Simple and efficient synthesis method of large area (centimeter and larger scale) monolayer graphehe for both fundamental research and commercial market. - Controllable synthesis of graphene nanoribbon. - Surface characterization, molecular doping, and functionalization of grpahene. - Passivation and packing technique for graphene-based devices, especially for metrology application. - New nanofabrication strategies for low-dimensional devices.
Graphene is a highly promising material for both quantum resistance metrology and wafer-scale electronics. Great progress has been achieved in producing monolayer epitaxial graphene on silicon carbide substrates. However, the resulting material is typically found to be heavily n-doped,and this causes the Fermi energy to be shifted far away from the Dirac point. The carrier densities reported for epitaxial graphene are usually in the range of 1011~1013 cm-2, due to the charge exchange between the graphene and the non-conducting buffer layer beneath it that is covalently bonded to the SiC substrate. Various gating methods have been developed to reduce the carrier density, but require organic chemical lithography processes that increase the probability of contamination that degrades the performance of the devices. Recently, we fabricated high-quality quantum Hall devices based on epitaxial graphene on diced semi-insulating SiC wafers, obtaining carrier densities in the range of 1010~1011 cm-2and mobility above104 cm2V-1s-1without gating. Our graphene is grown on the Si face of 7.6 mm x 7.6 mm large SiC substrates by a controlled sublimation method at NIST. The epitaxial graphene is characterized by AFM and Raman microscopy, which show that they are homogeneous across 95% of the center area of the samples. Well-developed quantum Hall effect plateaus with the filling factorν=2, the fingerprint for monolayer graphene,are measured at low magnetic field below 2 T at liquid helium temperature.I will discuss our recent progress in quantum resistance metrology, as well as other quantum phenomena observed in these clean, high quality graphene devices.