Scanning tunneling microscopy and terahertz laser technology Researchers develop a new method for precise semiconductor analysis

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To incorporate increasingly intelligent and powerful electronics into ever smaller devices, the development of tools and methods for precise analysis of materials is essential. Researchers combine atomic-scale imaging with extremely short laser pulses for this purpose.

Physicists at Michigan State University are combining high-resolution microscopy with ultrafast lasers for semiconductor analysis. The method described in the journal 'Nature Photonics' enables researchers to detect mismatched atoms in semiconductors with high precision. These mismatched atoms are commonly referred to as "defects." This sounds negative, but it is meant differently. The defects are usually intentionally added to the materials and are crucial for the performance of semiconductors.

"This is particularly important for components with nanoscale structures," says Tyler Cocker, the head of the study detailing the method; for example, computer chips that use semiconductors with such structures. Furthermore, researchers are pushing nano-architectures to the limit by developing materials that are only one atom thick.

Controlling electrons

"These nanoscopic materials are the future of semiconductors," says Cocker. "When you have electronics at the nanoscale, it's really important to ensure that the electrons move as you want them to." Defects play a significant role in this movement, which is why scientists want to know exactly where they are located and how they behave. Cocker's team's new method allows this information to be obtained in a straightforward way because it is easy to implement with the right equipment. The team has already applied it to atomically thin materials such as graphene nanoribbons. "We have a number of open projects where we are applying the method to further and more exotic materials," it is explained.

An almost touch

There are already instruments, primarily the scanning tunneling microscope (STM), that allow scientists to detect single atom defects. STMs do not use lenses and light bulbs to magnify objects but instead scan the surface of a sample with an extremely fine tip. However, the STM tip does not touch the surface of the sample; it only comes close enough for electrons to tunnel between the tip and the sample.

STMs record how many and from where electrons tunnel to provide insights at the atomic level about the samples along with other information. However, this data alone is not always sufficient to clearly resolve defects in a sample, especially in gallium arsenide, an important semiconductor material used in radar systems, high-efficiency solar cells, and modern telecommunications devices.

For their latest publication, Cocker and his team focused on gallium arsenide samples that were intentionally doped with silicon defect atoms to influence the movement of electrons through the semiconductor. "The silicon atom basically looks like a deep pothole to the electrons," says Cocker. Although theorists have been studying this type of defect for decades, these individual atoms have not yet been directly detected in experiments.

Microscope and laser pulses

The method by Cocker and his team uses an STM with laser pulses shining on its tip. These pulses consist of light waves with terahertz frequencies, meaning they oscillate one trillion times per second. Through this coupling, the team created a probe with unprecedented sensitivity to defects. When the tip comes close to a silicon defect on the surface of the gallium arsenide, an intense signal appears in the measurement data.

After his team shared the method with the community, Cocker is eager to see what other discoveries will emerge. Meanwhile, many groups are combining STMs and terahertz light. At the same time, there are numerous other materials that could benefit from this technique for applications beyond defect detection. (sb)

Link: Single atoms show their true color