Ultrafast terahertz photoconductivity and near-field imaging of nanoscale inhomogeneities in multilayer epitaxial graphene nanoribbons

We study broadband terahertz (THz) conductivity and ultrafast photoconductivity spectra in lithographically fabricated multilayer epitaxial graphene nanoribbons grown on C-face of 6H-SiC substrate. THz near-field spectroscopy reveals local conductivity variations across nanoscale structural inhomogeneities such as wrinkles and grain boundaries within the multilayer graphene. Ultrabroadband THz far-field spectroscopy ( 0.15 – 16 THz ) distinguishes doped graphene layers near the substrate from quasineutral layers (QNLs) further from the substrate. Temperature-dependent THz conductivity spectra are dominated by intraband transitions both in the doped and QNLs. Photoexcitation then alters mainly the response of the QNLs: these exhibit very high carrier mobility and large positive THz photoconductivity with picosecond lifetime. The response of QNLs strongly depends on the carrier temperature T c : the scattering time drops by an order of magnitude down to ∼10 fs upon an increase of T c from 50 K to T c > 1000 K , which is attributed to an enhanced electron-electron and electron-phonon scattering and to an interaction of electrons with mid-gap states.