The scanning ultrafast electron microscope couples a femtosecond pulsed laser with a scanning electron microscope. Photo: Matt Perko.
The scanning ultrafast electron microscope couples a femtosecond pulsed laser with a scanning electron microscope. Photo: Matt Perko.

Researchers at the University of California (UC) Santa Barbara have directly visualized the photocarrier transport properties of cubic boron arsenide, confirming its promise as a next-generation semiconductor material.

“We were able to visualize how the charge 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) setup in operation at a US university, Liao and his team were able to make ‘movies’ showing the generation and transport of a photoexcited charge in cubic boron arsenide. This relatively little-studied III-V semiconductor material has recently been recognized as having extraordinary electrical and thermal properties. Now, the researchers have found another beneficial property that adds to the material’s potential as the next great semiconductor.

They report their research, conducted in collaboration with physics professor Zhifeng Ren’s group at the University of Houston, which specializes in fabricating high-quality single crystals of cubic boron arsenide, in a paper in Matter.

Boron arsenide is being eyed as a potential candidate to replace silicon, the computer world’s staple semiconductor material, due to its promising performance. For one thing, with an improved charge mobility over silicon, it easily conducts current (electrons and their positively charged counterparts, ‘holes’). However, unlike silicon, it also conducts heat with ease.

“This material actually has 10 times higher thermal conductivity than silicon,” Liao said. This heat conducting – and releasing – ability is particularly important as electronic components become smaller and more densely packed, and pooled heat threatens the devices’ performance.

“As your cellphones become more powerful, you want to be able to dissipate the heat, otherwise you have efficiency and safety issues,” he explained. “Thermal management has been a challenge for a lot of microelectronic devices.”

What gives rise to the high thermal conductivity of this material, it turns out, can also lead to interesting transport properties of photocarriers, which are charges excited by light, as produced in a solar cell. If experimentally verified, this would indicate that cubic boron arsenide could also be a promising material for photovoltaic and light-detection applications. Direct measurement of photocarrier transport in cubic boron arsenide has, however, proved challenging, due to the small size of available high-quality samples.

The research team’s study combines two feats: the crystal growth skills of the University of Houston team, and the imaging prowess at UC Santa Barbara. Combining the abilities of the scanning electron microscope and femtosecond ultrafast lasers, the UC Santa Barbara 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’re typically 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 time resolution. We’re talking about one 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 further developed and improved at UC Santa Barbara, which now boasts the only operational SUEM setup at an American university.

“What happens is that we have one pulse of this laser that excites the sample,” explained graduate student researcher Usama Choudhry, the lead author of the paper. “You can think of it like ringing a bell; it’s a loud noise that slowly diminishes over time.” As they ‘ring the bell’, he explained, a second laser pulse is focused onto a photocathode (‘electron gun’) to generate a short electron pulse for imaging the sample. They then scan the electron pulse over time to gain a full picture of the ring.

“Just by taking a lot of these scans, you can get a movie of how the electrons and holes get excited and eventually go back to normal,” he said.

Among the things they observed while exciting their sample and watching the electrons return to their original state is how long high-energy electrons, known as ‘hot’ electrons, persist.

“We found, surprisingly, the ‘hot’ electrons excited by light in this material can persist for much longer times than in conventional semiconductors,” Liao said. These ‘hot’ carriers were seen to persist for more than 200 picoseconds, a property that is related to the material’s high thermal conductivity.

This ability to host ‘hot’ electrons for significantly longer amounts 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 said. “The high-energy electrons have a very short lifetime, and the low-energy electrons have a very long lifetime.”

When it comes to harvesting the energy from a typical solar cell, he continued, only the low-energy electrons are efficiently being collected; the high-energy ones tend to lose their energy rapidly as heat. Because of the persistence of the high-energy carriers in cubic boron arsenide, if this material was used as a solar cell, more energy could efficiently be harvested from it.

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 state-of-the-art material. However, it still faces significant hurdles – particularly the fabrication of high-quality crystals in large quantities – before it can compete with silicon, enormous amounts 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 because of years of investment; people started developing silicon around the 1930s and ‘40s,” he said. “I think once people recognize the potential of this material, there will be more effort put into finding ways to grow and use it. UC Santa Barbara is actually uniquely positioned for this challenge with strong expertise in semiconductor development.”

This story is adapted from material from the University of California, Santa Barbara, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.