Physicists are developing a method to detect single-atom defects in semiconductors

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

Physicists at Michigan State University have taken a long-awaited step forward in this field with an approach that combines high-resolution microscopy with ultrafast lasers.

The technique described in the magazine Photonics of nature, allows researchers to detect misfit atoms in semiconductors with unparalleled precision. Semiconductor physics refers to these atoms as “defects,” which sounds negative, but they’re usually added to materials on purpose and are critical to the performance of semiconductors in today’s—and tomorrow’s—devices.

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

This includes things like computer chips that routinely use nanoscale semiconductors. And researchers are working to take nanoscale architecture to the extreme with engineering materials that are one atom thick.

“These nanoscopic materials are the future of semiconductors,” said Cocker, who also leads 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 this electron movement, so scientists like Cocker want to find out exactly where they are and how they behave. Cocker’s peers were excited to discover that his team’s new technique would allow them to easily obtain this information.

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

Vedran Jelic, who led the project as a postdoctoral researcher in Cocker’s group and is now at the National Research Council 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 can be easily implemented with the right equipment, and his team is already applying it to atomically thin materials such as graphene nanorods.

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

A light (almost) touch

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

Unlike the microscopes that many people would be familiar with from high school science class, STMs do not use lenses and light bulbs to magnify objects. Rather, STMs scan the surface of the sample with an atomically sharp tip, almost like the stylus on a record player.

But the STM tip does not touch the surface of the sample, it just gets close enough for electrons to jump or tunnel between the tip and the sample.

STMs record how many electrons jump and where they jump from, along with other information to provide information about patterns at the atomic scale (which is why Cocker’s lab refers to it as nanoscopy instead of microscopy).

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

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

“A silicon atom basically looks like a deep pothole for electrons,” Cocker said.

Although theorists have been studying this type of defect for decades, experimentalists have been unable to directly detect these individual atoms until now. Cocker and his team’s new technique still uses an STM, but the researchers also shine laser pulses directly at the tip of the STM.

These pulses consist of light waves with terahertz frequencies, meaning they oscillate up and down a trillion times per second. Recently, theorists showed that this is the same frequency at which silicon atom defects should oscillate back and forth inside a sample of gallium arsenide.

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

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

“Here was this bug that people had been hunting for over forty years, and we saw it ringing like a bell,” Cocker said.

“It was hard to believe at first because it’s so different,” he continued. “We had to measure it in every way to be sure it was real.

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

“When you discover something like this, it’s really helpful when there’s already decades of theoretical research that thoroughly characterizes it,” said Jelic, who, along with Cocker, is also a correspondent for the new paper.

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

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