Ultrafast In-Plane Photo-Dember Ring at Buried Semiconductor Heterointerfaces Visualized by Scanning Ultrafast Electron Microscopy.

The ultrafast transport dynamics of photogenerated carriers at buried semiconductor heterointerfaces govern the operation of modern optoelectronic devices, yet direct real-space visualization of in-plane carrier motion at such interfaces has remained an outstanding experimental challenge. Here, using scanning ultrafast electron microscopy (SUEM), we directly image, with nanometer and picosecond spatiotemporal resolution, the in-plane photocarrier dynamics at a shallow-buried cadmium sulfide (CdS)/CdSe heterointerface. Upon focused fs laser excitation, we observe that electrons and holes undergo picosecond-scale radial separation along the interface plane, orthogonal to the built-in electric field, forming a ring-shaped charge distribution that persists for nanoseconds. Through quantitative spatiotemporal analysis and theoretical modeling, we reveal that this behavior originates from a heterointerface-mediated lateral photo-Dember (LPD) effect, driven by the intrinsic mobility asymmetry between separated electrons confined in the upper CdS layer and holes confined in the bottom CdSe layer. The analysis yields a pronounced difference (∼241.8 cm2/s) in subdiffusive transport coefficients between the two carrier populations, providing a quantitative measure of the asymmetric in-plane mobility that underpins the ultrafast radial charge separation. Our work provides direct real-space imaging evidence of the LPD effect in semiconductor heterostructures, which holds implications for optoelectronic devices including on-chip terahertz emitters, high-speed photodetectors, and photocatalyst architectures. These results also establish SUEM as a powerful tool for visualizing and quantifying hidden carrier dynamics at buried interfaces with high spatiotemporal resolution.