The best semiconductor of all? | MIT News

Silicon is one of the most abundant elements on earth, and in its pure form the material has become the basis of many modern technologies, from solar cells to computer chips. But the properties of silicon as a semiconductor are far from ideal.

Although silicon easily zips electrons through its structure, it’s much less adaptable to “holes” — electrons’ positively charged counterparts — and exploiting both is important for some types of chips. Also, silicon doesn’t conduct heat very well, which is why computers often have overheating problems and expensive cooling systems.

Now a team of researchers at MIT, the University of Houston and other institutions have conducted experiments showing that a material known as cubic boron arsenide overcomes both of these limitations. It offers high mobility to both electrons and holes and has excellent thermal conductivity. It is, the researchers say, the best semiconductor material ever found, and perhaps the best possible.

To date, cubic boron arsenide has only been produced and tested in small, non-uniform batches on a laboratory scale. 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 produced in a practical, economical form, let alone replace the ubiquitous silicon. But even in the near future, the material could find some applications where its unique properties would make a significant difference, the researchers say.

The results are published in the journal today Science, in an article 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, a co-author of the new paper, had theoretically predicted that the material would exhibit high thermal conductivity; subsequent work proved this prediction experimentally. This latest work completes the analysis by experimentally confirming a 2018 prediction by Chen’s group: that cubic boron arsenide would also exhibit very high mobility for electrons and holes, “which makes this material really unique,” says Chen.

The earlier experiments showed that the thermal conductivity of cubic boron arsenide is almost ten times higher than that of silicon. “So that’s very attractive for heat dissipation alone,” says Chen. They also showed that the material has a very good band gap, a property that gives it great potential as a semiconductor material.

Now the new work fills the picture, showing that boron arsenide, with its high electron and hole mobility, has all the important properties needed for an ideal semiconductor. “That’s important, because of course we have equally positive and negative charges in semiconductors. So when you’re building a device, you want a material in which both electrons and holes move with less resistance,” says Chen.

Silicon has good electron mobility but poor hole mobility, and other materials such as gallium arsenide, which is widely used for lasers, have similarly good electron mobility but not hole mobility.

“Heat is a major bottleneck for many electronic devices today,” says Shin, the lead author of the publication. “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 thermal conductivity 10 times higher and mobility much higher than silicon. It can be a game changer.”

Shin adds, “The critical milestone making this discovery possible are advances in ultrafast laser grating systems at MIT,” originally developed by Song. Without this technique it would not have been possible to prove the high mobility of the material 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 these predictions have now been validated by experiments conducted at MIT using optical detection methods on samples from Ren and members of the team of University of Houston.

Not only is the material’s thermal conductivity the best of any semiconductor, the researchers say, it has the third-best thermal conductivity of any material — next to 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 turned out to be true,” says Chen.

“That’s impressive because I don’t really know of any other material apart from graphene that has all these properties,” he says. “And this is a bulk material that has these properties.”

The challenge now is to find practical ways to produce this material in usable quantities. Current manufacturing methods produce highly inconsistent material, requiring the team to find ways to test only small local patches of material that were uniform enough to provide reliable data. Although they have demonstrated the great potential of this material, “we don’t know if or where it’s actually being used,” says Chen.

“Silicon is the workhorse of the entire industry,” says Chen. “Okay, we have better material, but will it actually compensate the industry? We don’t know.” While the material appears to be almost an ideal semiconductor, “whether it can actually get into a device and replace some of the current market, I think, remains to be proven.”

And while the thermal and electrical properties have proven excellent, there are many other properties of a material that still need to be tested, such as its long-term stability, says Chen. “To make devices, there are many other factors that we don’t know yet.”

He adds, “That could potentially be really important, and people haven’t even really been paying attention to this material.” Now that the desirable properties of boron arsenide have become clearer, suggesting that the material is “the best in many ways.” Semiconductor,” he says, “maybe more attention will be paid to this material.”

For commercial purposes, Shin says, “a major challenge is to produce and purify cubic boron arsenide as effectively as silicon. … Silicon took decades to take the crown, with purity in excess of 99.99999999 percent, or ’10 nines’ for mass production today.”

For it to become viable in the market, Chen says, “it really needs more people to come up with different ways to make better materials and characterize them.” Whether the necessary funds for such a development will be available remains to be seen, says he.

The research was supported by the US Office of Naval Research and utilized facilities at MIT’s MRSEC Shared Experimental Facilities supported by the National Science Foundation.

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