Using ultrafast scanning electron microscopy, researchers reveal hot photocarrier transport properties of cubic boron

In a study that confirms its promise as the 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 the load moves in our sample,” said Bolin Liao, an assistant professor of mechanical engineering in the College of Engineering. Using the only scanning ultrafast electron microscopy (SUEM) facility operating at a US university, he and his team were able to make “movies” of the production and transport processes of a photoexcited charge in this relatively little-studied III-V semiconductor material. , which has recently been recognized to have excellent electrical and thermal properties. Along the way, they found another beneficial property that adds to the material’s potential as the next big semiconductor.

Their research, conducted in collaboration with University of Houston physics professor Zhifeng Ren’s team, which specializes in making high-quality cubic boron arsenide single crystals, appears in the journal Material.

‘Ring the bell’

Boron arsenide is considered a possible candidate to replace silicon, the main semiconductor material in the computer world, because of its promising performance. First, with improved charge mobility relative to silicon, it easily carries current (electrons and their positively charged counterpart, “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 especially important as electronic components become smaller and more densely packed, and accumulated heat threatens the performance of devices, he explained.

“As your mobile phones get more powerful, you want to be able to dissipate the heat, otherwise you have performance and security issues,” he said. “Thermal management has been a challenge for many microelectronic devices.”

What causes the high thermal conductivity of this material, it turns out, 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 available high-quality samples.

The research team’s study combines two feats: the crystal-growing skills of the University of Houston team and the imaging capability at UC Santa Barbara. By combining the capabilities of scanning electron microscopy and ultrafast femtosecond lasers, the UCSB team built what is essentially an ultra-fast, ultra-high-resolution camera.

“Electron microscopes have very good spatial resolution — they can separate individual atoms with their sub-nanometer spatial resolution — but they’re usually very slow,” Liao said, noting that this makes them great for taking static images.

“With our technique, we combine this very high spatial resolution with an extremely fast laser, which acts as a very fast shutter, for extremely high temporal resolution,” continued Liao. “We’re talking about a picosecond—a millionth of a millionth of a second. So we can make movies of this tiny energy and charge transfer processes.” Originally invented at Caltech, the method was further developed and refined at UCSB from scratch and is now the only operational SUEM facility at an American university.

“What happens is that we have a pulse of 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 pulse of electrons to image the sample. They then scan the electron pulse over time to get a complete picture of the ring. “Just by doing a lot of these scans, you can see a movie of how the electrons and holes get excited and eventually go back to normal,” he said.

Among the things they noticed while exciting their sample and watching the electrons return to their original state is how long the “hot” electrons persist.

“We found, surprisingly, ‘hot’ electrons excited by light in this material can remain for much longer than in conventional semiconductors,” Liao said. These “hot” carriers appeared to persist for more than 200 picoseconds, a property related to the same characteristic responsible for the material’s high thermal conductivity. This ability to host “hot” electrons for significantly longer periods of time has important implications.

“For example, when you excite the electrons in a typical solar cell with light, not every electron has the same amount of energy,” Choudhry explained. “High energy electrons have very short lifetimes and low energy electrons have very long lifetimes.” When it comes to harvesting the 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. Because of the tenacity of high-energy carriers, if this material were used as a solar cell, more energy could be efficiently harvested from it.

With boron arsenide beating silicon in three relevant areas—charge mobility, thermal conductivity, and hot carrier transfer time—it has the potential to become the next state-of-the-art material in the world of electronics. 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 made relatively cheaply and with high quality. But Liao doesn’t see much of a problem.

“Silicon is now commonly available because of a lot of investment; people started developing silicon around the 1930s and 1940s,” he said. “I think once people recognize the potential of this material, more effort will be put into finding ways to develop and use it. UCSB is really uniquely positioned for this challenge with strong expertise in semiconductor development.”