Competing Surface and Subsurface Carrier Dynamics Resolved by Voltage-Tunable Scanning Ultrafast Electron Microscopy

Distinguishing surface recombination from subsurface transport is vital for optoelectronics but remains challenging in scanning ultrafast electron microscopy (SUEM) because of signal convolution. Here, we demonstrate that the detector bias (V_f) enables effective depth-selective probing to spatially disentangle these competing dynamics within the near-surface region. Experiments on p-type silicon reveal a striking voltage-tunable contrast inversion, marking a transition from surface-dominated to subsurface-dominated regimes. We attribute this to a mechanistic competition between surface potential restoration governing collection and subsurface band flattening modulating emission. Multiphysics simulations confirm this framework by linking depth-dependent charge distributions to contrast evolution. We thus achieve independent visualization of spatially entangled processes, specifically isolating surface trapping from subsurface diffusion and providing a physical basis for resolving vertical carrier stratification.