UC Santa Barbara researchers created a first-ever “movie” of electric charges traveling across the interface of two different semiconductor materials. Employing scanning ultrafast electron (SUEM) techniques developed in the Bolin Liao lab, the research team’s visualization of the phenomenon was a first. The ability to visualize the process will allow semiconductor materials scientists to benchmark theories and indirect measurements. The research is published in the Proceedings of the National Academy of Sciences.
You can see photocarriers in action within a solar cell. Sunlight hits a semiconductor material, exciting electrons so that they move. The movement and separation from their opposite-charged ‘holes,’ creates a current we can harness to power electronic devices.
However, the photocarriers lose most of their energy within picoseconds. This means we can only harvest a fraction of these carriers’ energy in their “hot” state before they cool down and release excess energy as waste heat. This hot state holds a lot of potential for things like energy efficiency; however, the heat within the semiconductor material may affect device performance.
To visualize the hot carriers, Liao and his group focused on a heterojunction of silicon and germanium fabricated by collaborators at UCLA, a combination of standard semiconductor materials with promise in photovoltaics and telecommunications.
Their process uses ultrafast laser pulses to act as a picosecond-scale shutter as they fire an electron beam to scan the surface of the materials through which the hot photocarriers travel, excited by an optical pump beam. Able to visualize how the charges actually transfer across the junction, the images show the photocarriers as they diffuse from one semiconductor material to the other.
When a charge is excited near the junction, a fraction of the carriers is trapped by the junction potential, slowing them down and resulting in reduced carrier mobility, which can negatively affect the performance of devices that separate and collect these hot charges.
Noted Liao, “We didn’t expect to be able to image this effect directly,” he said, adding that this phenomenon might be something semiconductor device designers may want to address.
