Source: This article was compiled from UCSB by Semiconductor Industry Watch (ID: icbank). Thank you.

In a study confirming its potential as a next-generation semiconductor material, researchers at the University of California, Santa Barbara directly visualized the photo carrier transport characteristics of cubic boron arsenide single crystals.

We are able to visualize the movement of charges in the sample, "said Bolin Liao, assistant professor of mechanical engineering at the School of Engineering. He and his team used the only operating scanning ultrafast electron microscope (SUEM) device at a university in the United States to create "movies" of the generation and transport of photoexcited charges in this relatively less studied III-V semiconductor material, which has recently been recognized for its extraordinary electrical and thermal performance. During this process, they discovered another beneficial characteristic that increased the potential of the material as the next great semiconductor.

Their research was conducted in collaboration with the team of physics professor Zhifeng Ren at the University of Houston, who specializes in manufacturing high-quality cubic boron arsenide single crystals, published in the journal Matter.

Boron arsenide is considered a potential candidate to replace silicon, the main semiconductor material in the computer world, due to its excellent performance. On the one hand, as the charge mobility of silicon increases, it is easy to conduct current (electrons and their positively charged counterparts, "holes"). However, unlike silicon, it is also easily thermally conductive.

The thermal conductivity of this material is actually 10 times that of silicon, "Liao said. He explained that as electronic components become smaller and denser, this heat conduction and release capability becomes particularly important, and the collected heat threatens the performance of the device.

As your phone becomes increasingly powerful, you want to be able to dissipate heat, otherwise you will encounter efficiency and safety issues, "he said. For many microelectronic devices, thermal management has always been a challenge

It has been proven that the high thermal conductivity of this material may also be due to the interesting transport characteristics of photo carriers, namely the photoexcited charges, such as in solar cells. If experimentally validated, this will indicate that cubic boron arsenide can also be a promising material for photovoltaic and photodetection applications. However, due to the small size of available high-quality samples, directly measuring the transport of photo carriers in cubic boron arsenide has always been challenging.

The research team's study combines two remarkable achievements: the crystal growth skills of the University of Houston team and the imaging capabilities of the University of California, Santa Barbara. By combining the capabilities of scanning electron microscopy and femtosecond ultrafast lasers, the UCSB team has built a camera that is essentially extremely fast and has high resolution.

Electron microscopes have very good spatial resolution - they can distinguish individual atoms with sub nanometer spatial resolution - but they are usually very slow, "Liao said, pointing out that this makes them very suitable for capturing static images.

Through our technology, we combine this very high spatial resolution with ultrafast laser as a very fast shutter to achieve extremely high temporal resolution, "Liao continued. What we're talking about is one picosecond - one millionth of a second. So we can make movies of these microscopic energy and charge transfer processes. "This method was originally invented at the California Institute of Technology, and later further developed and improved from scratch at UCSB. It is now the only operational SUEM setup in American universities.

What happened was that we had a laser pulse that excited the sample, "explained Usama Choudhry, the first author and graduate researcher of the Matter paper. You can imagine it as ringing a bell; it's a huge noise that gradually diminishes over time, "he explained. When they" Ringed the Bell, "the second laser pulse focused on the photocathode (" electron gun ") to generate a short electron pulse to image the sample. Then they scan the electronic pulses over time to obtain the full picture of the ring. By conducting a large number of such scans, you can obtain a movie of how electrons and holes are excited and eventually return to normal, "he said.

One of the things they observed when exciting the sample and observing the electrons returning to their original state was how long the "hot" electrons lasted.

We found that surprisingly, the 'hot' electrons excited by light in this material can last longer than traditional semiconductors, "Liao said. These "hot" charge carriers are believed to last for over 200 picoseconds, a characteristic that is related to the material's high thermal conductivity. The ability to carry "hot" electrons for a longer period of time is of great significance.

For example, when you excite electrons in a typical solar cell with light, not every electron has the same energy, "explained Choudhury. The lifespan of high-energy electrons is very short, while the lifespan of low-energy electrons is very long, "he continued. When collecting energy from typical solar cells, only low-energy electrons are effectively collected. High energy often rapidly loses energy in the form of heat. Due to the persistence of high-energy carriers, if this material is used as a solar cell, it can effectively extract more energy from it.

With boron arsenide surpassing silicon in three related fields - charge mobility, thermal conductivity, and hot photo carrier transport time - it has the potential to become the next most advanced material in the electronic world. However, before it competes with silicon, it still faces a huge obstacle - producing high-quality crystals in large quantities - a large amount of silicon can be relatively inexpensive and of high quality. But Liao didn't see much of a problem.

Due to years of investment, silicon can now be used routinely; people began developing silicon around the 1930s and 1940s, "he said. I believe that once people recognize the potential of this material, they will put in more effort to find ways to grow and use it. UCSB has strong expertise in semiconductor development and is actually in a unique position to address this challenge. ”

The work carried out at UCSB has received partial support from the Office of Basic Energy Sciences of the US Department of Energy, with award number DE-SC0019244 for the development of SUEM, and award number W911NF-19-1-0060 from the US Army Research Office for the study of photocarrier dynamics in emerging materials. The growth of boron arsenide crystals at the University of Houston has received support from the Office of Naval Research in the United States, with award number N00014-16-2436.