New Aspects of Photocurrent Generation at Graphene pn Junctions Revealed by Ultrafast Optical Measurements

TL;DR

This study uses ultrafast optical measurements to explore hot carrier dynamics in graphene pn junctions, revealing their dominant role in photocurrent generation and implications for ultrafast optoelectronic devices.

Contribution

It provides new insights into hot carrier contributions and impact ionization effects in graphene, advancing understanding of non-equilibrium electron dynamics at ultrafast timescales.

Findings

Photocurrent response time of ~1.5 ps at room temperature

Hot carriers dominate energy transport at high frequencies

Multiple polarity-reversals indicate impact ionization

Abstract

The unusual electrical and optical properties of graphene make it a promising candidate for optoelectronic applications. An important, but as yet unexplored aspect is the role of photo-excited hot carriers in charge and energy transport at graphene interfaces. Here, we perform time-resolved (~250 fs) scanning photocurrent microscopy on a tunable graphene pn junction. The ultrafast pump-probe measurements yield a photocurrent response time of ~1.5 ps at room temperature increasing to ~4 ps at 20 K. Combined with the negligible dependence of photocurrent amplitude on environmental temperature this implies that hot carriers rather than phonons dominate energy transport at high frequencies. Gate-dependent pump-probe measurements demonstrate that both thermoelectric and built-in electric field effects contribute to the photocurrent excited by laser pulses. The relative weight of each contribution depends on the junction configuration. A single laser beam excitation also displays multiple polarity-reversals as a function of carrier density, a signature of impact ionization. Our results enhance the understanding of non-equilibrium electron dynamics, electron-electron interactions, and electron-phonon interactions in graphene. They also determine fundamental limits on ultrafast device operation speeds (~500 GHz) for potential graphene-based photon detection, sensing, and communication.