"Movies" of electrons as they move across a semiconductor junction have been made by researchers at the Okinawa Institute of Science and Technology in Japan using a new imaging technique. Combining photoemission electron microscopy with femtosecond laser pump-probe methods, the technique tracks the motion of electrons on timescales shorter than 1 ps. The researchers say it could provide a better understanding of how semiconductor devices work and lead to more efficient solar cells.
Solar cells – along with diodes, transistors and other semiconductor devices – rely on the flow of electrons across "heterojunctions" between two different types of semiconductor. While this motion is crucial to just about every modern technology, it is not easy to image in real time. The problem is that pulsed laser techniques – which can measure the energy of electrons on very short time scales – do not offer high enough spatial resolution to track the electrons. On the other hand, electron microscopy techniques offer high spatial resolution, but cannot keep up with the fast-moving electrons.
Now Keshav Dani, Michael Man and colleagues have unveiled a new technique that combines high spatial resolution with high temporal resolution. Their method uses the well-established "pump-probe" technique involving laser pulses that are only 200 fs in length. It involves first firing a relatively intense probe pulse at a sample in the form of an indium selenide/gallium arsenide heterojunction that functions as a solar cell. The probe pulse plays the role of sunlight, putting electrons into excited energy states from which electrical energy can be extracted.
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