In a study that confirms its promise as a next-generation semiconductor material, UC Santa Barbara researchers directly visualized the photocarrier transport properties of cubic boron arsenide single crystals.
“We were able to visualize how charge moves through our sample,” said Bolin Liao, assistant professor of mechanical engineering at the College of Engineering. Using the only ultrafast scanning electron microscopy (SUEM) setup in operation at an American university, he and his team were able to make “movies” of the processes of generating and transporting photoexcited charge in this semiconductor material. III-V relatively little studied. , which has recently been recognized as having extraordinary electrical and thermal properties. In the process, they found another beneficial property that adds to the material’s potential as the next great semiconductor.
Their research, conducted in collaboration with physics professor Zhifeng Ren’s group at the University of Houston that specializes in making high-quality cubic boron arsenide single crystals, appears in the journal Question.
‘Ring the bell’
Boron arsenide is considered a potential candidate to replace silicon, the basic semiconductor material of the computing world, due to its promising performance. On the one hand, with improved charge mobility compared to silicon, it easily conducts current (the electrons and their positively charged counterpart, the “holes”). However, unlike silicon, it also conducts heat with ease.
“This material actually has 10 times higher thermal conductivity than silicon,” Liao said. This ability to conduct and release heat is particularly important as electronic components become smaller and denser, and the accumulated heat threatens device performance, he explained.
“As your cell phones get more powerful, you want to be able to dissipate heat, otherwise you have efficiency and safety issues,” he said. “Thermal management has been a challenge for many microelectronic devices.”
It turns out that what gives rise to the high thermal conductivity of this material can also lead to interesting transport properties of photocarriers, which are the charges excited by light, for example, in a solar cell. If verified experimentally, this would indicate that cubic boron arsenide may also be a promising material for photovoltaic and light-sensing applications. However, direct measurement of photocarrier transport in cubic boron arsenide has been difficult due to the small size of high-quality samples available.
The research team’s study combines two feats: the crystal growth skills of the University of Houston team and the imaging prowess of UC Santa Barbara. Combining the capabilities of the scanning electron microscope and ultrafast femtosecond lasers, the UCSB team built what is essentially an extremely fast, exceptionally high-resolution camera.
“Electron microscopes have very good spatial resolution — they can resolve single atoms with their sub-nanometer spatial resolution — but they are generally very slow,” Liao said, noting that this makes them excellent for capturing static images.
“With our technique, we couple this very high spatial resolution with an ultrafast laser, which acts as a very fast shutter, for extremely high temporal resolution,” Liao continued. “We’re talking about a picosecond – a millionth of a millionth of a second. So we can make movies of these microscopic energy and charge transport processes.” Originally invented at Caltech, the method was developed and improved from the ground up at UCSB and is now the only operational SUEM setup at a US university.
“What happens is that we have a pulse from this laser that excites the sample,” explained graduate student researcher Usama Choudhry, lead author of the Matter paper. “You can think of it like ringing a bell; it’s a loud noise that slowly fades over time.” As they “ring the bell,” he explained, a second laser pulse is focused on a photocathode (“electron gun”) to generate a short electron pulse to image the sample. They then scan the electron pulse over time to get a full picture of the ring. “Just by taking a lot of these scans, you can get a movie of how electrons and holes get excited and eventually go back to normal,” he said.
Among the things they observed as they excited their sample and watched the electrons return to their original state was how long the “hot” electrons lasted.
“We have found, surprisingly, that ‘hot’ electrons excited by light in this material can persist much longer than in conventional semiconductors,” Liao said. These “hot” carriers persisted for more than 200 picoseconds, a property related to the same characteristic that is responsible for the high thermal conductivity of the material. This ability to host “hot” electrons for much longer durations has important implications.
“For example, when you excite electrons in a typical solar cell with light, not all of the electrons have the same amount of energy,” Choudhry explained. “High-energy electrons are very short-lived and low-energy electrons are very long-lived.” When it comes to harvesting energy from a typical solar cell, he continued, only low-energy electrons are efficiently harvested; high-energy ones tend to lose their energy quickly as heat. Due to the persistence of high energy carriers, if this material were used as a solar cell, more energy could be efficiently harvested.
With boron arsenide beating silicon in three relevant areas – charge mobility, thermal conductivity, and hot photocarrier transport time – it has the potential to become the electronics world’s next cutting-edge material. However, it still faces significant hurdles – manufacturing high-quality crystals in large quantities – before it can compete with silicon, huge quantities of which can be manufactured relatively cheaply and with high quality. But Liao doesn’t see too much of a problem.
“Silicon is now routinely available thanks to years of investment; people started developing silicon around the 1930s and 1940s,” he said. “I think once people recognize the potential of this material, there will be more effort to find ways to develop it and use it. UCSB is actually uniquely positioned to take on this challenge. with strong expertise in semiconductor development.”
Work at UCSB was partially supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under award number DE-SC0019244 for the development of SUEM, and by the Office of Research from the United States Army under award number W911NF-19-1-0060 to study the dynamics of photocarriers in emergent materials. The growth of boron arsenide crystals at the University of Houston was supported by the United States Office of Naval Research under award number N00014-16-1-2436.
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