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

Area of Science:

• Optoelectronics
• Materials Science
• Surface Science

Background:

• Distinguishing surface recombination from subsurface transport is crucial for optoelectronic devices.
• Scanning ultrafast electron microscopy (SUEM) faces challenges in separating these dynamics due to signal convolution.

Purpose of the Study:

• To develop a depth-selective probing method using detector bias (Vf) in SUEM.
• To spatially disentangle competing surface and subsurface charge dynamics.

Main Methods:

• Utilized detector bias (Vf) in SUEM experiments on p-type silicon.
• Performed multiphysics simulations to model charge distributions and contrast evolution.

Main Results:

• Demonstrated voltage-tunable contrast inversion, indicating a transition between surface- and subsurface-dominated regimes.
• Mechanistically linked contrast changes to competition between surface potential restoration and subsurface band flattening.
• Successfully isolated surface trapping from subsurface diffusion.

Conclusions:

• Detector bias (Vf) provides effective depth-selective probing in SUEM.
• Achieved independent visualization of spatially entangled surface and subsurface processes.
• Established a physical basis for resolving vertical carrier stratification in near-surface regions.