  Semiconductor heterojunctions have gained significant attention for efficient
optoelectronic devices owing to their unique interfaces and synergistic
effects. Interaction between charge carriers with the heterojunction plays a
crucial role in determining device performance, while its spatial-temporal
mapping remains lacking. In this study, we employ scanning ultrafast electron
microscopy (SUEM), an emerging technique that combines high spatial-temporal
resolution and surface sensitivity, to investigate photocarrier dynamics across
a Si/Ge heterojunction. Charge dynamics are selectively examined across the
junction and compared to far bulk areas, through which the impact of the
built-in potential, band offsets, and surface effects is directly visualized.
In particular, we find that the heterojunction drastically modifies the hot
photocarrier diffusivities by up to 300%. These findings are further elucidated
with insights from the band structure and surface potential measured by
complementary techniques. This work demonstrates the tremendous effect of
heterointerfaces on charge dynamics and showcases the potential of SUEM in
characterizing realistic devices.
