laser beam graphic
A laser beam (yellow) reflects off a 2D material (orange) highlighting a grain boundary defect in the atomic lattice. (Image: MRI/Penn State)

To further shrink electronic devices and to lower energy consumption, the semiconductor industry is interested in using 2D materials, but manufacturers need a quick and accurate method for detecting defects in these materials to determine if the material is suitable for device manufacture. A team of Penn State researchers has developed a technique to quickly and sensitively characterize these defects.

Two-dimensional materials, the most well-known being graphene — a single-atom-thick layer of carbon atoms — are atomically thin. The researchers’ goal is to have a 2D material on a four-inch wafer with at least an acceptable number of defects, and to be able to quickly evaluate it.

Their solution is to use laser light combined with Second Harmonic Generation (SHG), a phenomenon in which the frequency of the light shone on the material reacts at double the original frequency. They add dark field imaging, a technique in which extraneous light is filtered out so that defects shine through. According to the researchers, this is the first instance in which dark field imaging was used, and it provides three times the brightness of the standard bright field imaging method, making it possible to see types of defects previously invisible.

The localization and identification of defects with the commonly used bright field second harmonic generation is limited because of interference effects between different grains of 2D materials. By the use of dark field SHG they remove the interference effects and reveal the grain boundaries and edges of semiconducting 2D materials. Such a novel technique has good spatial resolution and can image large area samples that could be used to monitor the quality of the material produced at industrial scale.

Crystals are made of atoms, and so the defects within crystals — where atoms are misplaced — are also of atomic size. Usually, powerful, expensive and slow experimental probes that do microscopy using beams of electrons are needed to discern such fine details in a material. Here, a fast and accessible optical method pulls out just the signal that originates from the defect itself to rapidly and reliably find out how 2D materials are stitched together out of grains oriented in different ways.

The semiconductor industry wants to have the ability to check for defects on the production line, but 2D materials will likely be used in sensors before they are used in electronics. Because 2D materials are flexible and can be incorporated into very small spaces, they are good candidates for multiple sensors in a smartwatch or smartphone and the myriad of other places where small, flexible sensors are required.