Imaging Hot Photocarrier Transfer across a Semiconductor Heterojunction with Ultrafast Electron Microscopy

TL;DR

This paper uses ultrafast electron microscopy to visualize and quantify how semiconductor heterojunctions influence hot photocarrier transfer and dynamics, revealing significant modifications in diffusivity and surface effects.

Contribution

It introduces the application of scanning ultrafast electron microscopy (SUEM) to directly image charge dynamics across semiconductor heterojunctions with high spatial-temporal resolution.

Findings

Heterojunctions modify hot photocarrier diffusivities by up to 300%.

Charge dynamics are spatially mapped across the Si/Ge interface.

Surface effects and band offsets influence carrier behavior.

Abstract

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.