Garming Mathijs W H, Bolhuis Maarten, Conesa-Boj Sonia, Kruit Pieter, Hoogenboom Jacob P
Department of Imaging Physics, Delft University of Technology, 2628 CN Delft, The Netherlands.
Kavli Institute of Nanoscience, Delft University of Technology, 2628 CJ Delft, The Netherlands.
J Phys Chem Lett. 2020 Oct 15;11(20):8880-8886. doi: 10.1021/acs.jpclett.0c02345. Epub 2020 Oct 6.
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.
可视化电荷载流子在界面上或表面附近的流动，在分辨率以及体相和表面动力学的极大不同时间尺度方面面临巨大挑战。使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）规避了光学衍射极限，但尚未在单一测量方案中实现区分表面介导的俘获和超快载流子动力学。在此，我们展示了锁相USEM，它能同时可视化快速的体相复合和缓慢的俘获。作为概念验证，我们表明砷化镓上的表面终止，即镓或砷，会深刻影响超快成像。我们证明这些差异可归因于约100 - 200 mV的俘获诱导表面电压，二次电子粒子追踪计算进一步证实了这一点。这两种竞争过程的同时可视化，为研究层状、纳米结构和二维半导体中的载流子输运开辟了新的视角，其中载流子俘获是器件效率的主要瓶颈。
