Physicists develop a method to detect single-atom defects in semiconductors

One of the challenges of cramming smarter and more powerful electronics into ever-smaller devices is developing tools and techniques to analyze the materials that make them up with increasingly intimate precision.

Physicists at Michigan State University have taken a long-awaited step on that front with an approach that combines high-resolution microscopy with ultrafast lasers.

The technique, described in the journal Photonics of nature, enables researchers to spot mismatched atoms in semiconductors with unprecedented precision. Semiconductor physics labels these atoms “defects,” which sounds negative, but they’re usually added to materials on purpose and are critically important to the performance of semiconductors in today’s and tomorrow’s devices.

“This is particularly important for components with nanoscale structures,” said Tyler Cocker, the Jerry Cowen Endowed Chair in Experimental Physics and leader of the new study.

This includes things like computer chips, which routinely use semiconductors with nanoscale characteristics. And researchers are working to take nanoscale architecture to an extreme by engineering materials that are a single atom thick.

“These nanoscopic materials are the future of semiconductors,” said Cocker, who also directs the Ultrafast Terahertz Nanoscopy Laboratory in MSU’s Department of Physics and Astronomy. “When you have nanoscale electronics, it’s really important to make sure the electrons can move the way you want them to.”

Defects play a big role in that electron movement, so scientists like Cocker are keen to learn exactly where they are and how they behave. Cocker’s peers have been excited to learn that his team’s new technique will allow them to easily obtain that information.

“One of my colleagues said to me, ‘I hope you went out and celebrated,'” Cocker said.

Vedran Jelic, who led the project as a postdoctoral researcher in Cocker’s group and is now with the National Research Council of Canada, is the first author of the new report. The research team also included PhD students Stefanie Adams, Eve Ammerman and Mohamed Hassan, as well as undergraduate researcher Kaedon Cleland-Host.

Cocker added that the technique is simple to implement with the right equipment, and his team is already applying it to atomically thin materials such as graphene nanoribbons.

“We have a number of projects open where we’re using the technique with more materials and more exotic materials,” Cocker said. “We’re basically folding it into everything we do and using it as a standard technique.”

A light touch (almost).

There are already tools, notably scanning tunneling microscopes, or STMs, that can help scientists spot single-atom defects.

Unlike the microscopes many people would recognize from high school science classes, STMs do not use lenses and lamps to magnify objects. Instead, STMs scan the surface of a sample using a sharp atomic tip, almost like a stylus on a recorder.

But the STM tip doesn’t touch the surface of the sample, it just gets close enough so that electrons can bounce, or tunnel, between the tip and the sample.

STMs record how many electrons bounce and from where, along with other information, to provide atomic-scale information about samples (hence why Cocker’s lab refers to this as nanoscopy instead of microscopy).

But STM data alone is not always sufficient to clearly resolve defects within a sample, especially in gallium arsenide, an important semiconductor material found in radar systems, high-efficiency solar cells and modern telecommunications equipment.

For their latest publication, Cocker and his team focused on samples of gallium arsenide that had been deliberately injected with silicon defect atoms to tune how electrons move through the semiconductor.

“The silicon atom basically looks like a deep hole for electrons,” Cocker said.

Although theorists have studied this type of defect for decades, experimentalists have not been able to directly detect these single atoms, until now. Cocker and his team’s new technique still uses an STM, but the researchers also shine laser pulses right on top of the STM.

These pulses are made up of light waves with terahertz frequencies, meaning they move up and down a trillion times a second. Recently, theorists have shown that this is the same frequency that silicon atom defects must move back and forth within a sample of gallium arsenide.

By combining STM and terahertz light, the MSU team created a probe that has unparalleled sensitivity to defects.

When the STM tip reached a silicon defect on the gallium arsenide surface, a sudden and intense signal appeared in the team’s measurement data. When the researchers moved the tip one atom away from the defect, the signal disappeared.

“Here was this bug that people have been chasing for over forty years, and we could see it ringing like a bell,” Cocker said.

“At first it was hard to believe because it was so distinct,” he continued. “We had to measure it in every way to be sure that this was true.”

Once they were convinced that the signal was real, however, it was easy to explain thanks to years of theoretical work devoted to the subject.

“When you discover something like this, it’s really rewarding when there’s already decades of theoretical research that fully characterizes it,” said Jelic, who, along with Cocker, is also a corresponding author on the new paper.

Although Cocker’s lab is at the forefront of this field, there are groups around the world currently combining STMs and terahertz light. There are also a number of other materials that can benefit from this technique for applications beyond defect detection.

Now that his team has shared its approach with the community, Cocker is excited to see what other discoveries await.