Researchers have found a material that can perform much better than silicon. The next step is finding practical and economic ways to make it.
By David L. Chandler at MIT News Office
July 21, 2022 -- Silicon is one of the most abundant elements on Earth,
and in its pure form the material has become the foundation of much of modern
technology, from solar cells to computer chips. But silicon’s properties as a
semiconductor are far from ideal.
For one thing, although silicon lets electrons whizz through its
structure easily, it is much less accommodating to “holes” — electrons’
positively charged counterparts — and harnessing both is important for some
kinds of chips. What’s more, silicon is not very good at conducting heat, which
is why overheating issues and expensive cooling systems are common in
computers.
Now, a team of researchers at MIT, the University of Houston, and other
institutions has carried out experiments showing that a material known as cubic
boron arsenide overcomes both of these limitations. It provides high mobility
to both electrons and holes, and has excellent thermal conductivity. It is, the
researchers say, the best semiconductor material ever found, and maybe the best
possible one.
So far, cubic boron arsenide has only been made and tested in small,
lab-scale batches that are not uniform. 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, economical form, much less
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 reported today in the journal Science,
in a paper by MIT postdoc Jungwoo Shin and MIT professor of mechanical engineering
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 is a co-author of
the new paper, had theoretically predicted that the material would have high
thermal conductivity; subsequent work proved that prediction experimentally.
This latest work completes the analysis by confirming experimentally a
prediction made by Chen’s group back in 2018: that cubic boron arsenide would
also have very high mobility for both 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 10 times greater than that of silicon. “So, that is
very attractive just for heat dissipation,” Chen says. They also showed that
the material has a very good bandgap, a property that gives it great potential
as a semiconductor material.
Now, the 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. “That’s 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 travel 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, similarly have good
mobility for electrons but not for holes.
“Heat is now a major bottleneck for many electronics,” says Shin, the
paper’s lead author. “Silicon carbide is replacing silicon for power
electronics in major EV industries including Tesla, since it has three times
higher thermal conductivity than silicon despite its lower electrical
mobilities. Imagine what boron arsenides can achieve, with 10 times higher
thermal conductivity and much higher mobility than silicon. It can be a
gamechanger.”
Shin adds, “The critical milestone that makes this discovery possible is
advances in ultrafast laser grating systems at MIT,” initially developed by
Song. Without that 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 based on quantum mechanical density function calculations made by
Chen’s group, he says, and those predictions have now been validated through
experiments conducted at MIT, using optical detection methods on samples made
by Ren and members of the team at the 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 predicted the electron and hole quantum mechanical
behavior, also from first principles, and that is also proven to be true,” Chen
says.
“This is impressive, because I actually don’t 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 figure out practical ways of making
this material in usable quantities. The current methods of making it produce
very nonuniform material, so the team had to find ways to test just small local
patches of the material that were uniform enough to provide reliable data.
While they have demonstrated the great potential of this material, “whether or
where it’s going to actually be used, we do not know,” Chen says.
“Silicon is the workhorse of the entire industry,” says Chen. “So, OK,
we’ve got a material that’s better, but is it actually going to offset 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 that still 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. “To make devices, there are
many other factors that we don’t know yet.”
He adds, “This potentially could be really important, and people haven’t
really even paid attention to this material.” Now that boron arsenide’s
desirable properties have become more clear, suggesting the material is “in
many ways the best semiconductor,” he says, “maybe there will be more attention
paid to this material.”
For commercial uses, Shin says, “one grand challenge would be how to
produce and purify cubic boron arsenide as effectively as silicon. … Silicon
took decades to win the crown, having purity of over 99.99999999 percent, or
‘10 nines’ for mass production today.”
For it to become practical on the market, Chen says, “it really requires
more people to develop different ways to make better materials and characterize
them.” Whether the necessary funding for such development will be available
remains to be seen, he says.
The research was supported by the U.S. Office of Naval Research, and used
facilities of MIT’s MRSEC Shared Experimental Facilities, supported by the
National Science Foundation.
https://news.mit.edu/2022/best-semiconductor-them-all-0721
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