Ultrafast Terahertz Photoconductivity and Near-Field Imaging of Nanoscale Inhomogeneities in Multilayer Epitaxial Graphene Nanoribbons (2511.05014v1)

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.

• The paper demonstrates that quasi-neutral graphene layers primarily drive ultrafast THz photoconductivity, yielding ultrahigh carrier mobility.
• It employs a combination of far-field and near-field THz spectroscopy with optical pump-probe techniques to map nanoscale conductivity variations.
• The study reveals that temperature-dependent carrier scattering and defects critically modulate local transport, guiding future graphene-based optoelectronics design.

Ultrafast Terahertz Photoconductivity and Nanoscale Inhomogeneities in Multilayer Epitaxial Graphene Nanoribbons

Introduction

This paper investigates the broadband terahertz (THz) conductivity and ultrafast photoconductivity in multilayer epitaxial graphene (MEG) nanoribbons, patterned via lithography on the carbon-face of 6H-SiC substrates. By utilizing both far-field and near-field THz spectroscopy, the authors distinguish the distinct electronic transport mechanisms in doped and quasi-neutral graphene layers and evaluate the influence of nanoscale structural irregularities such as wrinkles and grain boundaries on local conductivity. Theoretical modelling, supported by comprehensive temperature-dependent and pump-probe experiments over a wide frequency range (0.15-16 THz), enables extraction of fundamental transport parameters and elucidation of hot carrier relaxation dynamics across multiple graphene subsystems.

Experimental Approach

MEG films are synthesized by high-temperature thermal decomposition of SiC, followed by ribbon patterning through electron-beam lithography and oxygen plasma etching. The resulting stacks consist of 14 graphene layers as determined by AFM, with average MEG thicknesses of ~5 nm. Far-field THz time-domain and multi-THz spectroscopy, complemented by optical pump-THz probe schemes, are conducted to capture steady-state and transient complex conductivity spectra across lattice temperatures spanning 6-300 K. Near-field conductivity mapping is performed using THz-SNOM, providing ~50 nm spatial resolution to correlate topographical features with local electronic transport.

Theoretical Modelling

The paper adopts a bilayer model segmenting MEG into "doped layers" (DLs, substrate-adjacent, high Fermi energy) and "quasi-neutral layers" (QNLs, outer layers, low Fermi energy). Conductivity contributions in each subsystem are captured via generalized Drude and inter-band terms, with carrier temperature $T_c$ dictated by experimental conditions:

• DLs: $E_{F,D} \gg k_BT_c$, yielding temperature-independent intra-band conductivity and suppressed inter-band transitions.
• QNLs: $E_{F,QN} \ll k_BT_c$, resulting in strong temperature-dependent intra-band conductivity, variable inter-band processes, and short carrier scattering times under optical excitation.

The 2D conductivity per subsystem is given via

$$\sigma(\omega) = N_{D} \left[\sigma^{\text{inter}}_D(\omega) + \sigma^{\text{intra}}_D(\omega)\right] + N_{QN} \left[\sigma^{\text{inter}}_{QN}(\omega) + \sigma^{\text{intra}}_{QN}(\omega)\right]$$

with patterning effects modelled through surface coverage correction and plasmonic resonance derived from equivalent circuit approaches.

Results: Steady-State Conductivity and Nanoscale Inhomogeneity

Steady-state measurements show that the total THz conductivity rises with increasing temperature, almost exclusively due to QNLs. The DLs contribute a nearly invariant intra-band term, with suppressed inter-band conductivity owing to Pauli exclusion at THz energies. Global model fits yield $N_{D}=3$ (DLs, $E_{F,D}\sim 200$ meV) and $N_{QN}=12$ (QNLs, $E_{F,QN}\sim 8$ meV).

Carrier mobility in QNLs reaches $4.8\times 10^5$ cm$^2$V$^{-1}$s$^{-1}$, exceeding that of DLs ($2300$ cm$^2$V$^{-1}$s$^{-1}$), a direct consequence of weak substrate interaction for outer layers. The width of the ribbons ($\sim$ few $\mu$m) is well above the carrier diffusion length, ensuring Drude transport validity.

THz-SNOM near-field imaging reveals conductivity reductions across wrinkles and grain boundaries, directly correlating topographical features with impeded local charge transport. These nanoscale inhomogeneities, unresolved in far-field experiments, are critical to device performance for applications leveraging MEG nanostructures.

Ultrafast Photoconductivity and Carrier Dynamics

Optical pump-THz probe measurements demonstrate that photoconductivity is dominated by intra-band processes in QNLs, with DL contributions marginal due to limited Fermi level shift upon photoexcitation. Upon excitation, the chemical potential in QNLs approaches the neutrality point, and ultrafast carrier heating causes:

• Drude weight increases approximately linearly with carrier temperature $T_c$.
• Scattering time $\tau_{QN}$ drops sharply with increasing $T_c$, reaching $\sim 10$ fs at $T_c \sim 1300$ K.

This drop in $\tau_{QN}$ is attributed to increased electron-electron and electron-phonon scattering, as well as additional pathways via midgap states induced by atomic-scale defects. Notably, the photoconductivity exhibits a decay lifetime of several ps, mirroring ultrafast carrier cooling dynamics established in related literature.

Plasmonic resonance in multi-THz spectra is blue-shifted at $\sim 4$ THz, four-fold higher than similar structures based on Si-face-grown graphene, reflecting the increased stack conductivity. Excitonic and inter-band contributions become prominent at frequencies above $2E_F$, especially in QNLs.

Modelling Approach and Validation

Attempts to model carrier density depth-profile via exponential decay failed to match experiment, particularly for temperature-dependent conductivity and photoconductivity transients. The stepwise bilayer model—discriminating sharply between QNLs and DLs—provides substantially improved agreement, both for layer counts and fitting accuracy over the probed frequency and temperature ranges.

Practical and Theoretical Implications

The identification of QNLs as the primary channel for tunable ultrafast THz photoconductivity and exceptionally high carrier mobility positions multilayer epitaxial graphene nanoribbons as promising candidates for ultrafast optoelectronics, THz modulators, and tunable plasmonics. Control over nanoscale inhomogeneities and grain boundaries is critical for optimizing device performance, as local conductivity can be reduced by an order of magnitude at structural defects.

From a theoretical perspective, the results underscore the necessity of incorporating spatial and electronic inhomogeneities in modelling multilayer graphene transport. The strong temperature dependence of carrier scattering in QNLs—dominated by reduced screening and enhanced phonon interactions—should be considered in future device designs and fundamental studies of 2D materials.

Future Directions

Potential future developments include:

• Engineering the number and arrangement of QNLs for tailored optoelectronic response and THz photonic applications.
• Employing defect engineering or substrate modification to minimize grain boundary scattering and wrinkles.
• Exploring dynamical interlayer energy and charge transfer for stacks exceeding 30 layers, where transient carrier temperature gradients may yield new transport regimes.
• Integrating ultrafast THz spectroscopy with scanning probe techniques for comprehensive 3D mapping of carrier dynamics in patterned MEG architectures.

Conclusion

This work presents an in-depth analysis of THz conductivity and ultrafast photoconductivity in multilayer epitaxial graphene nanoribbons, combining broad-band, near-field, and time-resolved spectroscopic approaches with detailed theoretical modelling. The two-component MEG system—consisting of doped inner layers and quasi-neutral ultrahigh-mobility outer layers—exhibits tunable ultrafast conductivity, robust against localization effects, and sensitive to nanoscale inhomogeneities. The insights provided are directly relevant for future nanoscale graphene-based optoelectronic and plasmonic devices operating on picosecond timescales.