Silicon is one of the most abundant elements on Earth, and in its pure form the material has become the basis of much of modern technology, from solar cells to computer chips. But the properties of silicon as a semiconductor are far from ideal.
For one thing, although silicon allows electrons to flow easily through its structure, it is much less susceptible to “holes” — the positively charged counterparts of electrons — and using both is important for some types of chips. Moreover, silicon is not a very good conductor of heat, which is why overheating problems and expensive cooling systems are common in computers.
Now, a team of researchers from MIT, the University of Houston and other institutions has conducted experiments showing that a material known as cubic boron arsenide overcomes both limitations. It provides high mobility of both electrons and holes and has excellent thermal conductivity. Researchers say it’s the best semiconductor material ever discovered, and perhaps the best possible.
Until now, cubic boron arsenide has only been produced and tested in small, non-uniform, laboratory-scale batches. The researchers had to use special methods originally developed by former MIT postdoc Bai Song to test small regions in the material. More work will be needed to determine whether cubic boron arsenide can be made in a practical, economical form, much less replace the ubiquitous silicon. But even in the near future, the material may find some applications where its unique properties would make a significant difference, the researchers say.
The findings are reported today in the journal Science, in a report by MIT postdoc Jungwoo Shin and MIT mechanical engineering professor Gang Chen; Zhifeng Ren at the University of Houston; and 14 others at MIT, the University of Houston, the University of Texas at Austin, and Boston College.
Earlier research, including work by David Broido, who co-authored the new paper, theoretically predicted the material would have high thermal conductivity; subsequent work proved this prediction experimentally. This latest work adds to the analysis by experimentally confirming a prediction made by Chen’s group in 2018: that cubic boron arsenide will have very high mobilities for both electrons and holes, “which makes this material really unique,” says Chen .
Earlier experiments showed that the thermal conductivity of cubic boron arsenide was almost 10 times that of silicon. “So this is very attractive just for heat dissipation,” says Chen. They also showed that the material has a very good bandgap, a property that gives it great potential as a semiconductor material.
Now new work completes the picture, showing that with its high mobility for both electrons and holes, boron arsenide has all the essential qualities needed for an ideal semiconductor. “This is important because, of course, in semiconductors we have both positive and negative charges equivalently. So if you build a device, you want to have a material where both electrons and holes move with less resistance,” Chen says.
Silicon has good electron mobility but poor hole mobility, and other materials such as gallium arsenide, widely used for lasers, also have good electron but not hole mobility.
“Heat is now a major bottleneck for a lot of electronics,” said Shin, the paper’s lead author. “Silicon carbide is replacing silicon for power electronics in major EV industries, including Tesla, because it has three times the thermal conductivity of silicon despite its lower electrical mobility. Imagine what boron arsenides can achieve with 10 times the thermal conductivity and much higher mobility than silicon. It could be a game changer.”
Shin adds, “The critical cornerstone that makes this discovery possible is the advances in ultrafast laser grating systems at MIT,” originally developed by Song. Without this technique, he says, it would not have been possible to demonstrate the material’s high mobility for electrons and holes.
The electronic properties of cubic boron arsenide were originally predicted based on quantum mechanical density function calculations by Chen’s group, he says, and those predictions have now been confirmed by experiments conducted at MIT using optical methods to detect samples. made by Wren and members of the University of Houston team.
Not only is the material’s thermal conductivity the best of all semiconductors, the researchers say, it has the third-best thermal conductivity of any material — after diamond and isotopically enriched cubic boron nitride. “And now we have predicted the quantum mechanical behavior of electrons and holes, also from first principles, and that has also been shown to be true,” says Chen.
“It’s impressive because I don’t really know of any material other than graphene that has all these properties,” he says. “And it’s a bulk material that has those properties.”
The challenge now, he says, is to come up with practical ways to produce this material in usable quantities. Current manufacturing methods produce very non-uniform material, so the team had to find ways to test only small local patches of the material that were uniform enough to provide reliable data. Although they have demonstrated the great potential of this material, “we don’t know if and where it will actually be used,” says Chen.
“Silicon is the workhorse of the entire industry,” says Chen. “So, okay, we have material that’s better, but is it actually going to compensate the industry? We don’t know.” Although the material appears to be a near-perfect semiconductor, “I think it’s still to be proven whether it can actually go into a device and replace part of the current market.”
And while the thermal and electrical properties have been shown to be excellent, there are many other properties of the material that have yet to be tested, such as its long-term stability, Chen says. “In order to create devices, there are many other factors that we don’t know yet.”
He adds, “This could potentially be really important, and people haven’t really even looked at this material.” Now that the desirable properties of boron arsenide have become clearer, suggesting that the material is “in many ways the most good semiconductor,” he says, “maybe more attention will be paid to this material.”
For commercial purposes, Shin says, “one big challenge would be how to produce and purify cubic boron arsenide as efficiently as silicon. … Silicon took decades to claim the crown, with purity exceeding 99.99999999 percent, or ’10 nines’ for mass production today.”
To make it practical on the market, Chen says, “really requires more people to develop different ways of creating better materials and characterizing them.” Whether there will be the necessary funding for such development remains to be seen, he says.
The research was supported by the US Office of Naval Research and used facilities at MIT’s MRSEC Shared Experiment Facility, supported by the National Science Foundation.