Data underlying the publication: Lock-in Ultrafast Electron Microscopy Simultaneously Visualizes Carrier Recombination and Interface-Mediated Trapping

Visualizing charge carrier flow over interfaces or near surfaces meets great challenges concerning resolution and vastly different time scales of bulk and surface dynamics. Ultrafast or four-dimensional scanning electron microscopy (USEM) using a laser pump electron probe scheme circumvents the optical diffraction limit, but disentangling surface-mediated trapping and ultrafast carrier dynamics in a single measurement scheme has not yet been demonstrated. Here, we present lock-in USEM, which simultaneously visualizes fast bulk recombination and slow trapping. As a proof of concept, we show that the surface termination on GaAs, i.e., Ga or As, profoundly influences ultrafast movies. We demonstrate the differences can be attributed to trapping-induced surface voltages of approximately 100–200 mV, which is further supported by secondary electron particle tracing calculations. The simultaneous visualization of both competing processes opens new perspectives for studying carrier transport in layered, nanostructured, and two-dimensional semiconductors, where carrier trapping constitutes a major bottleneck for device efficiency.

可视化界面处或表面附近的电荷载流子流动，在分辨率以及体相与表面动力学迥异的时间尺度方面均面临极大挑战。采用激光泵浦-电子探针方案的超快或四维扫描电子显微镜（USEM）可突破光学衍射极限，但在单次测量框架内解耦表面介导的载流子俘获与超快载流子动力学的方案，迄今尚未得到证实。本研究提出锁相USEM技术，可同时可视化快速的体相复合过程与缓慢的载流子俘获过程。作为概念验证，我们证实砷化镓（GaAs）的表面终止基团（即镓或砷端基）会对超快成像结果产生显著影响。我们证明，该差异可归因于俘获效应诱导的约100~200 mV的表面电势，这一结论进一步得到二次电子粒子追踪计算的佐证。对这两种相互竞争的载流子过程的同步可视化，为层状、纳米结构及二维半导体中的载流子输运研究开辟了新视角——在这类材料中，载流子俘获是制约器件效率的主要瓶颈。