Raman Mapping Analysis of Graphene-Integrated Silicon Micro-Ring Resonators

Siham M. Hussein, Iain F. Crowe, Nick Clark, Milan Milosevic, Aravind Vijayaraghavan, Frederic Y. Gardes, Goran Z. Mashanovich, Matthew P. Halsall

We present a Raman mapping study of monolayer graphene G and 2D bands, after integration on silicon strip-waveguide-based micro-ring resonators (MRRs) to characterize the effects of the graphene transfer processes on its structural and optoelectronic properties. Analysis of the Raman G and 2D peak positions and relative intensities reveal that the graphene is electrically intrinsic where it is suspended over the MRR but is moderately hole-doped where it sits on top of the waveguide structure. This is suggestive of Fermi level 'pinning' at the graphene-silicon heterogeneous interface, and we estimate that the Fermi level shifts down by approximately 0.2 eV from its intrinsic value, with a corresponding peak hole concentration of ~ 3 × 1012 cm-2. We attribute variations in observed G peak asymmetry to a combination of a 'stiffening' of the E 2g optical phonon where the graphene is supported by the underlying MRR waveguide structure, as a result of this increased hole concentration, and a lowering of the degeneracy of the same mode as a result of localized out-of-plane 'wrinkling' (curvature effect), where the graphene is suspended. Examination of graphene integrated with two different MRR devices, one with radii of curvature r = 10 μm and the other with r = 20 μm, indicates that the device geometry has no measureable effect on the level of doping.