For the first time, physicists captured the movement of charges crossing the boundary of semiconductors.

The principle behind solar panels is based on generating photocharges from sunlight: light hits the semiconductor material, excites electrons—transferring energy from photons to them, causing them to move. The movement of electrons and their separation from positively charged "holes" generates the current that can power electronic devices.

Photoelectrons in the semiconductor lose most of their energy in trillionths of a second, which is why solar cells generate only a fraction of the energy that these charges contain in their "hot" state, before their "cooling"—the loss of excess energy as heat.

Researchers need to understand how these "hot" charges behave when moving through different semiconductor materials, especially at their boundary—the heterojunction. Heterojunctions are used everywhere in semiconductor devices, from lasers to sensors.

This time, scientists focused on the heterojunction between silicon and germanium, which is commonly used in semiconductor electronics. They were able to visualize the transfer of charges across the heterojunction from one semiconductor material to another immediately after generation.

The key to their visualization technique is the use of ultrafast laser pulses as a gate for an electron beam scanning the surface of the material with "hot" photocharges. Each laser pulse separates two images of the sample being studied, and the microscope can take up to a trillion images per second, which can later be compiled into a video.

If charges are excited in homogeneous regions of silicon or germanium, the "hot" charges in each will move rapidly. However, if a charge is excited near the heterojunction, some of the charges will be captured and slowed down by the potential at the boundary. The capture of "hot" charges decreases their mobility, which can negatively affect the performance of heterojunction-based devices.

The process of charge capture at silicon/germanium heterojunctions can be explained by semiconductor theory, but direct experimental observation was a surprise for scientists. Observing this phenomenon could be a crucial detail for designing semiconductor devices.

The ability to visualize this process in practice will allow scientists working with semiconductor materials to test their theories and confirm indirect measurements.

The study was published in the journal Proceedings of the National Academy of Sciences.