Ultrafast Terahertz Photoconductivity and Near-Field Imaging of Nanoscale Inhomogeneities in Multilayer Epitaxial Graphene Nanoribbons

TL;DR

This study investigates broadband terahertz conductivity and ultrafast photoconductivity in multilayer epitaxial graphene nanoribbons, revealing nanoscale inhomogeneities and the effects of temperature and photoexcitation on carrier dynamics.

Contribution

It provides new insights into local conductivity variations and ultrafast photoconductivity in multilayer graphene nanoribbons using near-field and far-field THz spectroscopy.

Findings

Local conductivity varies across nanoscale inhomogeneities.

QNLs exhibit high mobility and positive photoconductivity.

Carrier scattering time decreases with increasing temperature.

Abstract

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 quasi-neutral layers (QNLs) further from the substrate. Temperature-dependent THz conductivity spectra are dominated by intra-band transitions both in the doped and QNLs. Photoexcitation then alters mainly the response of the QNLs: these exhibit a very high carrier mobility and a 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.