Silicon is one of the most abundant elements on Earth, and in its pure form the material has become the basis of much modern technology, from solar cells to computer chips. But the properties of silicon as a semiconductor are far from ideal.
For one thing, while silicon allows electrons to pass easily through its structure, it is much less accommodating of “holes” (positively charged counterparts of electrons), and harnessing both is important for some types of chips Also, silicon is not a very good conductor of heat, so 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 of these limitations. It provides high mobility for both electrons and holes, and has excellent thermal conductivity. Researchers say it’s the best semiconductor material ever found, and perhaps the best possible.
To date, cubic boron arsenide has only been manufactured 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 within the material. More work will be needed to determine whether cubic boron arsenide can be made in a practical and economical way, let alone replace the ubiquitous silicon. But even in the near future, the material could find some uses where its unique properties would make a significant difference, the researchers say.
The findings are published today in the journal science, in a paper 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.
Previous research, including work by David Broido, who is a co-author of the new paper, had theoretically predicted that the material would have high thermal conductivity; later work experimentally demonstrated this prediction. This latest work completes the analysis by experimentally confirming a prediction made by Chen’s group in 2018: that cubic boron arsenide would also have very high mobility for both electrons and holes, “which makes this material is really unique,” says Chen.
Previous experiments showed that the thermal conductivity of cubic boron arsenide is 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 fills in the picture, showing that with its high mobility for both electrons and holes, boron arsenide has all the main qualities needed for an ideal semiconductor. “This is important because, of course, in semiconductors we have positive and negative charges equally. So if you’re building a device, you want to have a material where electrons and holes travel with less resistance,” says Chen.
Silicon has good electron mobility but poor hole mobility, and other materials such as gallium arsenide, widely used for lasers, have good electron mobility but not hole mobility.
“Heat is now a major bottleneck for many electronic products,” says Shin, the paper’s lead author. “Silicon carbide is replacing silicon for power electronics in major electric vehicle industries, including Tesla, as it has three times the thermal conductivity of silicon despite its lower electrical mobilities. Imagine what they can do achieve boron arsenides, with 10 times greater thermal conductivity and much greater mobility than silicon. It could be a game changer.”
Shin adds, “The critical milestone 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 initially predicted from quantum mechanical density functional calculations by Chen’s group, he says, and those predictions have now been validated by experiments at MIT, using methods of optical detection in samples made by Ren and members of the University of Houston team.
The researchers say that not only is the material’s thermal conductivity the best of any semiconductor, it has the third-best thermal conductivity of any material, next to diamond and isotopically enriched cubic boron nitride. “And now, we predicted the quantum mechanical behavior of electrons and holes, also from first principles, and it has also been shown to be true,” says Chen.
“This is impressive, because I don’t really know of any other material, other than graphene, that has all these properties,” he says. “And this is a bulk material that has these properties.”
The challenge now, he says, is to find practical ways to make this material in usable quantities. Current methods of making it 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, “if or where it will actually be used, we don’t know,” says Chen.
“Silicon is the workhorse of the entire industry,” says Chen. “So, OK, we have a material that’s better, but is it really going to make up for the industry? We don’t know.” While the material appears to be almost an ideal semiconductor, “whether it can go into a device and replace some of the current market, I think that has yet to be proven.”
And while the thermal and electrical properties have been shown to be excellent, there are many other properties of a material that have yet to be tested, such as its long-term stability, Chen says. “In making devices, there are many other factors that we don’t know about yet.”
He adds: “This could be very important, and people haven’t even paid attention to this material.” Now that the desirable properties of boron arsenide have become clearer, suggesting that the material is “in many ways the best semiconductor,” he says, “perhaps more attention will be paid to this material.”
For commercial uses, Shin says, “a big challenge would be how to produce and purify cubic boron arsenide as efficiently as silicon. … It took decades for silicon to win the crown, with a purity of more than 99.99999999 percent, or “10 new” for today’s mass production.”
For it to be practical in the market, Chen says, “it really requires more people to develop different ways of making better materials and characterizing them.” It remains to be seen whether the necessary funding for such a development will be available, he says.
The research was supported by the US Office of Naval Research and used facilities at MIT’s MRSEC Shared Experimental Facility, supported by the National Science Foundation.