Abstract
Longitudinal strains in epitaxial monolayer graphene (EMG) grown on SiC substrates were evaluated by z-polarization Raman microscopy. Due to the covalent bonds formed at the interface between graphene and the substrate, strong compressive strains were loaded on the EMG, which were sensitively detected by Raman spectroscopy. Our polarization Raman microscope was specially designed for evaluating the longitudinal (z-polarization) strain, as well as the lateral (xy-polarization). Z-polarization Raman microscopy revealed the relationship between the fluctuation of the local strains and the sample morphology in the SiC-graphene through submicron spatial resolution mapping. The amount of strain estimated through Raman shift and its spatial inhomogeneity have critical influence on the mobility of electrons, which are essential for future device applications of EMG.
Highlights
Longitudinal strains in epitaxial monolayer graphene (EMG) were quantitatively evaluated by z-polarization Raman microscopy
We employed a radial wave plate to produce a strong z-polarization under a highresolution optical microscope while keeping the diffraction-limited spatial resolution of ∼0.4 μm
The results support the existence of longitudinal strain between the substrate and the sample, and indicate that the rapid cooling process successfully quenched the covalent bonds between the substrate and the graphene
Summary
Since the isolation of monolayer graphene in 2004 by Novoselov and Geim, the material has attracted much attention because of its extremely high electron mobility originating from the unique band structures found near the K-point. Graphene, as well as other two-dimensional atomic layered materials, is potentially applicable to a variety of novel devices, such as high-frequency transistors, solar cells, light emitting devices, superconductors etc. the advantage of graphene is that the monolayer can be grown in a wafer size on a substrate and is compatible with conventional silicon device technology.. A single atomic layer is influenced by the properties of the substrate, both chemically and mechanically; for example, unexpected doping, carrier scattering, and modification of the band structures from the ideal linear-dispersion due to the lattice stress, affecting the electron mobility.. The interaction between graphene and the substrate results in lattice strains, followed by modification of the electronic structures, which has been sensitively monitored using Raman spectroscopy.. Since the strain occurs either in the longitudinal or lateral direction with respect to the sample plane, polarization Raman. We employ a z-polarization microscopy technique to evaluate the strain occurring in SiC-graphene. Two kinds of graphene samples were prepared: as-grown (SiC-graphene) and a rapidly cooled sample (RC-graphene) The former had a strong interaction with the substrate, whereas the interaction was quenched in the latter.
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