GIST Successfully Tracks Ultrafast Charge Movement in Semiconductor Surface in Real Time

(From the left) Professor Lee Jong-seok of GIST, Choi In-hyuk, a doctoral student. Photo by GIST.

[Asia Economy Honam Reporting Headquarters, Reporter Cho Hyung-joo] Professor Lee Jong-seok’s research team from the Department of Physics and Optical Science at GIST announced on the 6th that they have tracked the ultrafast behavior of photoexcited electrons on semiconductor surfaces and revealed the formation process of surface electric fields accordingly.

Understanding the ultrafast dynamics of electrons on polar semiconductor surfaces greatly aids in improving the performance related to energy generation, transfer, and storage in solar cells, and can contribute to establishing efficiency enhancement strategies for photocatalyst-related devices.

Previous studies tracked electron movement on semiconductor surfaces by reducing the size of semiconductor samples to the nanometer (10^-9 meters) scale to amplify surface effects. However, this inevitably caused quantum confinement effects, which have been an obstacle to understanding the intrinsic properties of the surface.

This study succeeded in real-time tracking of ultrafast electron movement on the surface without reducing the sample size by utilizing time-resolved nonlinear spectroscopy techniques.

The research team observed in real time the surface electric fields ultrafast formed by photoexcited electrons in representative polar semiconductors gallium arsenide (GaAs) and indium arsenide (InAs) through femtosecond time-resolved second harmonic generation technology.

When photons (light) enter a material, hot electrons and holes with high kinetic energy are generated, and these recombine due to surface defects such as dangling bonds on the semiconductor surface. Since the electron mobility is more than a thousand times faster than that of holes, the spatial distribution of electrons and holes differs, thereby forming a surface electric field.

The research team measured this surface electric field formed within a few nanometers thickness near the surface through second harmonic generation. In particular, using femtosecond laser-based time-resolved technology, they revealed that the electric field formed by the movement of these photocharges develops over hundreds of femtoseconds (10^-15 seconds) and disappears over tens of picoseconds (10^-12 seconds).

Professor Lee Jong-seok of GIST stated, “This research achievement is a meaningful result that tracked the ultrafast movement of photoexcited charges on semiconductor surfaces in real time, and it is expected to greatly contribute to the development and performance enhancement of solar cells and photocatalyst-related devices in the future.”