Long-lived hot electrons spotted in ‘wonder’ semiconductor

The SUEM instrument in Santa Barbara
Hot electrons: the scanning ultrafast electron microscope at the University of California, Santa Barbara. (Courtesy: Matt Perko/UCSB)

By combining scanning electron microscopy with ultrashort laser pulses, researchers in the US have shown that cubic boron arsenide has an important property that could be used to create better solar cells and photodetectors. Usama Choudhry and colleagues at the University of California, Santa Barbara, and the University of Houston used scanning ultrafast electron microscopy (SUEM) to confirm that “hot” electrons in the semiconductor material have long lifetimes – something that could be useful in a wide range of applications in electronics.

Sometimes dubbed a “wonder material”, cubic boron arsenide is a semiconductor material with several promising properties that could lead to its widespread commercial use. It is a much better conductor of heat than silicon, so it could be used to create integrated circuits that are packed together at higher densities and run at higher frequencies. The material has an electron mobility that is on par with silicon, but it has a much higher hole mobility than silicon – a property that would be useful in designing electronic devices.

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Now, Choudhry and colleagues have shown that cubic boron arsenide has another useful property: long-lived “hot” electrons. When light falls on a semiconductor it can cause the excitation of electrons with a range of energies. The lower energy electrons can persist for long enough so that they can be collected to create an electrical current – which is the basis for solar cells and light detectors. However, in most semiconductors the higher-energy hot electrons have very short lifetimes and are therefore lost before they can be collected.

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Long lived hot electrons

Calculations done in 2017 suggested that hot electrons have relatively long lifetimes in cubic boron arsenide. However, limitations in fabricating and studying cubic boron arsenide crystals had made it difficult to confirm this prediction.

In their study Choudhry’s team used SUEM, which combines the temporal resolution of ultrashort laser pulses with the spatial resolution of scanning electron microscopy. The technique involves splitting the laser pulse into two parts. The first part of the pulse is used to excite hot electrons in a high-quality sample of cubic boron arsenide that was made by the Houston team. After a carefully controlled delay, the second part of the pulse is focused onto a photocathode. This generates an electron pulse that is just a few picoseconds long. This pulse is used by an electron microscope to characterize the electrons in the cubic boron arsenide.

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By changing the delay, the team could measure the lifetime of the fast electrons in the sample, revealing that they persist for over 200 ps, which is far longer than the hot charge carriers in most semiconductors used in solar cells. The researchers say that the long lifetime suggests cubic boron arsenide could be used to make better solar cells, but much more work is needed to improve fabrication techniques.

The research is described in Matter.

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