Graphene Metrology: Substrate and Environmental Effects on Graphene

摘要

Graphene, a single layer of sp2-bonded carbon atoms, has received significant attention due to its exceptional opto-electronic properties and potentially scalable production processes. However, scalable graphene requires an underlying substrate, which is often a source of strain, doping and carrier scattering, limiting the mobility and quality of graphene. It was shown that by intercalating graphene on SiC by hydrogen, the interfacial layer, associated with n-doping and mobility degradation, is de-coupled from the substrate. The transformations of the H2-intercalation were demonstrated using Raman spectroscopy, while the SiC/interface changes were probed using surface enhanced Raman scattering. The H2-intercalation resulted in carrier type inversion, where the decoupled graphene change from n- to p-type, as well as showing mobility enhancement, up to more than four times, compared to as-grown graphene. Using calibrated Kelvin probe force microscopy, local work function maps were generated, demonstrating the changes in local electronic properties with nanoscale resolution. Furthermore, the layer structure, doping and strain induced by the underlying substrate are compared to CVD grown graphene transferred onto Si/SiO2.;In addition to the substrate effects, the electronic properties of graphene are also significantly affected due to the direct exposure of pi electrons to the environment. For the investigation of the environmental effects on graphene (i.e. H2O and NO2), a custom-built environmental transport properties measurement system was designed and developed, allowing magneto-transport measurements to be conducted in highly controlled environments. Using this system and calibrated local work function mapping, it is demonstrated that water withdraws electrons from graphene on SiC and SiO2 substrates, as well as acting as a source of impurity scattering. However, the sensitivity of graphene to water depends highly on the underlying substrate and substrate-induced doping. Moreover, it is shown that epitaxial graphene can successfully be used as the sensing material with detection down to 10 parts-per-billion molecules. Considering the environmental effects on the electronic properties of graphene, the importance of clearly reporting the measurement environmental conditions is high-lighted, whenever a routine characterisation for carrier concentration and mobility is reported.