[Paper Review] Imaging ultrafast carrier transport in nanoscale devices using femtosecond photocurrent microscopy

Byung-Hee Son, Jaeku Park+4 more|arXiv (Cornell University)|Jun 2, 2014

Nanowire Synthesis and Applications39 references19 citations

TL;DR

This study introduces a femtosecond photocurrent microscopy technique combining time-resolved pump-probe methods with scanning photocurrent imaging to visualize ultrafast carrier transport in one-dimensional nanoscale devices such as semiconductor nanowires and carbon nanotubes. The key finding is that carrier transit times decrease with increasing negative gate bias due to enhanced electric fields at Schottky barriers, enabling direct measurement of carrier velocities up to 10^6 m/s on sub-100 fs timescales.

ABSTRACT

One-dimensional nanoscale devices, such as semiconductor nanowires (NWs) and single- walled carbon nanotubes (SWNTs), have been intensively investigated because of their potential application of future high-speed electronic, optoelectronic, and sensing devices. To overcome current limitations on the speed of contemporary devices, investigation of charge carrier dynamics with an ultrashort time scale is one of the primary steps necessary for developing high-speed devices. In the present study, we visualize ultrafast carrier dynamics in nanoscale devices using a combination of scanning photocurrent microscopy and time- resolved pump-probe techniques. We investigate transit times of carriers that are generated near one metallic electrode and subsequently transported toward the opposite electrode based on drift and diffusion motions. Carrier dynamics have been measured for various working conditions. In particular, the carrier velocities extracted from transit times increase for a larger negative gate bias, because of the increased field strength at the Schottky barrier.

Motivation & Objective

• To visualize ultrafast carrier transport dynamics in nanoscale electronic devices with sub-100 fs temporal resolution.
• To investigate the influence of external gate bias on carrier transit times and velocities in one-dimensional nanostructures.
• To measure drift and diffusion-dominated carrier transport in semiconductor nanowires and single-walled carbon nanotubes.
• To establish a direct experimental method for probing carrier dynamics at the nanoscale with high spatiotemporal resolution.
• To correlate electric field modulation via gate bias with changes in carrier velocity and transport efficiency.

Proposed method

• The study employs a time-resolved pump-probe configuration with femtosecond laser pulses to excite carriers in nanoscale devices.
• Scanning photocurrent microscopy is used to map the spatial distribution of photocurrent with nanoscale lateral resolution.
• A pump pulse generates electron-hole pairs near a metallic electrode, while a probe pulse measures the resulting transient photocurrent at varying time delays.
• The gate voltage is varied to modulate the electric field at the Schottky barrier, enabling control over carrier drift velocity.
• Transit times are extracted from the time delay between the pump excitation and the peak photocurrent response.
• The carrier velocity is calculated from the measured transit time and the known device length, revealing field-dependent transport dynamics.

Experimental results

Research questions

• RQ1 How do carrier transit times vary in nanoscale devices under different gate biases?
• RQ2 What is the role of the Schottky barrier electric field in modulating carrier drift velocity in nanowires and carbon nanotubes?
• RQ3 Can ultrafast carrier transport dynamics be spatially and temporally resolved using femtosecond photocurrent microscopy?
• RQ4 To what extent do drift and diffusion mechanisms contribute to carrier transport in one-dimensional nanostructures?
• RQ5 How does the applied gate voltage influence the effective carrier velocity in nanoscale field-effect devices?

Key findings

• Carrier transit times in nanoscale devices decrease with increasing negative gate bias, indicating enhanced carrier transport efficiency.
• The measured carrier velocities reach up to approximately 10^6 m/s, consistent with drift-dominated transport under high electric fields.
• The increase in carrier velocity with gate bias is attributed to the enhanced electric field at the Schottky barrier, which accelerates carriers.
• The technique successfully resolves carrier dynamics with sub-100 fs temporal resolution, enabling direct observation of ultrafast transport processes.
• The spatial mapping of photocurrent reveals asymmetric carrier transport behavior, dependent on the position of optical excitation relative to the electrodes.
• The results demonstrate that gate voltage can be used as a tunable parameter to control carrier velocity in nanoscale field-effect devices.