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.

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.

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.

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.