Recently, Professor Xuewen Fu's team from the Laboratory of Ultrafast Electron Microscopy at the School of Physics, Nankai University, successfully developed the world's first multimodal carrier dynamics detection system that combines scanning ultrafast electron microscopy (SUEM) imaging and ultrafast time-resolved cathodoluminescence (TRCL) detection. Under the femtosecond ultrafast electron mode, this system has a spatial resolution better than 10 nm and a time resolution better than 500 fs and 4.5 ps for SUEM imaging and TRCL detection, respectively. All technical performance indicators and parameters reach the international leading level.

The research team used the multimodal carrier dynamics detection system to directly track the complex dynamics of photo-generated carriers in n-GaAs semiconductors at femtosecond and nanosecond spatiotemporal resolution. Using the SUEM imaging and TRCL detection, the research team successfully distinguished the dynamic behavior of surface carriers from that of bulk carriers, providing a comprehensive and intuitive physical picture of photo-generated carrier dynamics in n-GaAs. The successful development of this instrument system fills the gap in China's technology field, providing a powerful high spatiotemporal resolution measurement platform for studying and decoupling the complex dynamics of photo-generated carriers in semiconductors, and also providing important support for the development of new semiconductor materials and high-performance optoelectronic devices. This study, entitled A Femtosecond Electron-based Versatile Microscopy for Visualizing Carrier Dynamics in Semiconductors across Spatiotemporal and Energetic Domains, was recently published in a leading international academic journal, Advanced Science.

Figure 1. Schematic diagram of the instrument system and characterization of its spatiotemporal resolution. (a) Schematic diagram of the multimodal carrier dynamics detection system integrated with SUEM imaging and TRCL detection. The system includes femtosecond optical subsystem, scanning electron microscope, CL collection subsystem, streak camera, and liquid nitrogen/helium cryogenic stage. The upper left corner of the figure shows the SEM image, CL mapping image, and CL spectrum of diamond microcrystals, as well as the streak camera image of n-type GaAs at 77 K. (b) The SEM image of tin ball standard sample in traditional field emission mode. (c) and (d) Electron images of femtosecond electron pulses for the tin ball standard sample under different magnifications, indicating good imaging quality in the femtosecond electron mode with a spatial resolution better than 10 nm. (e) The pulse width of the initial infrared femtosecond laser pulse. (f) The time resolution test of the SUEM imaging, with the instrument response function (IRF) of approximately 500 fs. (g) The time resolution test of the ultrafast cathodoluminescence detection, with the IRF of approximately 4.5 ps.

With the vigorous development of ultrafast electron microscopy, scanning ultrafast electron microscopy (SUEM) and ultrafast time-resolved cathodoluminescence (TRCL) have rapidly emerged, both of which simultaneously possess the ultrafast time resolution of ultra-short pulsed laser and the ultra-high spatial resolution of electron microscopy. Based on the pump-probe principle, the SUEM uses a visible femtosecond laser to excite the surface of the sample to generate carriers (photo-generated carriers), and another synchronized ultraviolet femtosecond laser to excite the photocathode of the scanning electron microscope to generate femtosecond pulse electrons for scanning imaging. The scanning electron microscopy mainly collects secondary electron signals from within a few nanometers of the sample surface, so the ultrafast scanning electron microscopy technology has unique surface sensitivity and can give a direct picture of the spatiotemporal evolution dynamics of photo-generated carriers (electrons and holes) on the surface or interface of semiconductor materials. However, this technology cannot directly distinguish the dynamic process of radiative recombination from that of non-radiative recombination. TRCL technology uses a focused femtosecond pulse electron beam to excite a sample to generate transient fluorescence and uses a streak camera or a time-dependent single photon counter to measure transient fluorescence, so this technology has energy sensitivity and can directly reveal radiative recombination behavior of excited carriers. Therefore, SUEM and TRCL are functionally complementary, with their organic combination it is expected to achieve comprehensive analysis of the dynamic information of surface/interface and bulk carriers in semiconductors with ultra-high spatiotemporal and energy resolutions. By combining femtosecond laser, field emission scanning electron microscope, and transient cathodoluminescence detection module, Professor Fu Xuewen and his team successfully developed the world's first multimodal carrier dynamics detection system that has SUEM imaging and TRCL detection functionalities (as shown in the schematic diagram in Figure 1 and the photo in Figure 2), achieving detection and analysis of dynamic processes of surface/interface carriers and bulk carriers in semiconductors with high spatiotemporal resolution.

Figure 2. Photo of the SUEM and TRCL multimodal carrier dynamics detection system

Figure 3. Results of SUEM imaging and TRCL measurements on the surface of n-type GaAs single crystal. (a) Temporal evolution of SUEM images obtained on the surface of n-type GaAs. (b) Time-dependent SE intensity and corresponding carrier evolution time constant extracted from the excitation region of Figure (a). (c) Time-dependent evolution of the spatial distribution of the surface carriers. (d) Temperature-dependent evolution of the normalized time-integrated cathodoluminescence spectra from 297 K to 77 K. (e) and (g) The streak camera images detected in the SUEM test area of Figure (a) at 297 K and 77 K, respectively. (f) and (h) Decay traces of the TRCL signals and the corresponding fluorescence lifetimes of the near band emission extracted from (e) and (g), respectively.

To demonstrate the unique advantages of the SUEM imaging and TRCL detection in directly visualizing and decoupling the complex dynamics of excited carriers in semiconductors at ultra-high spatiotemporal and energy resolutions, the team used the independently developed multimodal instrument to study carrier dynamics in n-type GaAs. As shown in Figure 3, the SUEM image indicates that due to the surface energy band bending effect, photo-generated carriers in the surface undergo rapid separation after femtosecond laser action, leading to hole enrichment towards the surface. By analyzing the SUEM images at different delay times, the decay time constants of the photo-generated carriers at different stages were extracted. At the same time, by calculating the root mean square displacement of the surface hole distribution, the processes of super-diffusion, localization, and sub-diffusion of the surface holes corresponding to different stages over time were revealed. By further analyzing the corresponding non-equilibrium carrier recombination dynamics and lifetime in streak camera images measured at room temperature and liquid helium temperature, they not only distinguished the surface and bulk carrier dynamics, but also revealed that the hot carrier cooling, defect trapping, and interband/defect-assisted radiative recombination processes of the above-mentioned surface carriers correspond to their super-diffusion, localization, and sub-diffusion in the real space. This work elucidates the important effects of surface and defect states on the dynamics of surface/interface carriers in semiconductors, demonstrating the unique advantages of the multimodal carrier dynamics detection system that combines SUEM imaging and TRCL detection in decoupling dynamics of surface/interface carriers and bulk carriers in semiconductors at ultra-high spatiotemporal resolution.

Nankai University is the first completion institution and communication institution for this paper. PhD student Yaqing Zhang and postdoctoral researcher Xiang Chen from the School of Physics at Nankai University are the co-first authors, and Professor Xuewen Fu from Nankai University is the corresponding author.

Paper Link:

https://doi.org/10.1002/advs.202400633 

（Edited and translated by Nankai News Team.)