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Superb semiconductor keeps its cool

Measurements reveal cubic boron arsenide has electrical and thermal properties that are better than silicon’s—but it’s still very difficult to make the material

by Katherine Bourzac
July 27, 2022


Ball and stick structure in the shape of a cube
Credit: Christine Daniloff/MIT
Cubic boron arsenide has both high charge mobility and high thermal conductivity, promising qualities for making electronic devices (black = B, orange = As).

Heat accumulation sets a speed limit on today’s computers and shortens the lifetime of telecommunications equipment. Electrical charges zipping through devices tend to lose some of their energy to heat, and in integrated circuits and other very dense systems, that heat can build up. The problem comes down to the basic material properties of the semiconductors, like silicon, used to make these devices. So researchers have been on the hunt for promising novel semiconductors that can keep things cool.

Now, thanks to the use of painstaking measurement techniques, researchers have proven that cubic boron arsenide has a unique combination of properties: it’s a speedy conductor of both electrons and positive charges and also one of the best thermal conductors ever measured (Science 2022, DOI: 10.1126/science.abn4290). This combination of properties could help lift the limits on device function caused by heat, leading to faster, longer-lasting electronics.

Cubic boron arsenide (c-BAs) drew researchers’ attention in 2013, when modeling work predicted it would have thermal conductivity second only to diamond, says Gang Chen, a mechanical engineer at the Massachusetts Institute of Technology. Experiments a few years later bore this out. In 2018, Gang and other scientists predicted that c-BAs should also have ambipolar mobility, meaning it should rapidly conduct both electrons and positive charges, or holes. Most semiconductors have high mobility for only one or the other—silicon, for example, provides smoother sailing for electrons than for holes, which is part of the reason silicon electronics heat up.

The combination of properties predicted for c-BAs is ideal for making high-performance electronic devices, says Chen. Today’s integrated circuits are made up of transistors or switches that conduct both electrons and holes. A semiconductor with high ambipolar mobility could be used to make efficient versions of both types of devices, ensuring that less energy is lost to heat. And the material’s high thermal conductivity means that any heat generated in future c-BAs devices should dissipate rapidly.

But predicting such properties is one thing—making this material and demonstrating it in the real world is quite another, says Chen. “The measurements are hard because the material still has impurities and defects” such as contamination with carbon or silicon, Chen says. Those defects mean c-BAs’ average electrical properties don’t match what the models predict for pristine crystals of this semiconductor. Chen wanted to know whether he could observe those superb electrical properties if he was able to measure local points within the crystal without defects.

Chen and his collaborators used optical techniques to carry this out. To take a measurement, they aim two laser beams at a point on the material’s surface. The interfering beams excite the material in an irregular fashion, creating patches with more or fewer electrons and holes. This gradient drives the electrons and holes to flow, and the group uses a third beam to measure those movements. High-quality parts of the crystal had electrical properties in line with the theoretical predictions—showing highly efficient conductivity of both electrons and holes. Areas with defects did not.

Ali Shakouri, an electrical and computer engineer at Purdue University, says the ambipolar mobility and thermal conductivity of c-BAs would be a good match for high-current electronics such as the equipment that sends telecommunications signals and the circuits that control and convert power in electric vehicles. But, he adds, “we have some way to go” before c-BAs is in any electronic devices. He notes that the researchers have yet to make a transistor or other device from c-BAs.

Chen hopes the experimental measurements will encourage more researchers to jump in and make those devices—and to work on making high-quality crystals more reliably. “C-BAs is an excellent example where theoretical prediction actually inspired measurements,” says Chen. “Now that we’ve demonstrated this, we hope more researchers will improve this material and you will find it in the real world.”


This article was updated on Aug. 4, 2022, to correct the DOI number of the Science study. It is 10.1126/science.abn4290, not 10.1126/science.abn2490.


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