UV Light Used to Characterize Devices, Improve Performance

Researchers from the Nagoya Institute of Technology (NITech) used UV light to test the performance of miniaturized semiconductors for next-generation electronics. Specifically, the team determined the interface properties of a graphene-gallium nitride (GaN) heterojunction device by characterizing the device under UV illumination. According to the team, an understanding of GaN heterojunction device interfaces and how to improve them is necessary for better device performance.

The researchers demonstrated the effect of UV illumination on electrical hysteresis — a phenomenon in which electrons become trapped at the interface, leading to a behavioral shift in the device — in a graphene-GaN vertical heterojunction device. When they shined UV light on the device, they found photo-excited electrons (excitons) trapped at the interface between the graphene and the GaN and interfering with the transfer of information. The GaN contained surface-level defects and other imperfections that allowed excitons to become trapped at the interface.

Determining the graphene-GaN heterojunction interface under ultraviolet illumination. The study shows the fabrication process of a vertical Schottky junction with monolayer graphene on free-standing GaN. Courtesy of Golap Kalita, Ph.D., Nagoya Institute of Technology, Japan.

When UV light was shined on the heterojunction, the excitons populated at the interface and remained trapped, creating a large hysteresis window. A fabricated graphene-GaN device that was free from electrical hysteresis when not illuminated showed the appearance of hysteresis when illuminated with UV light. The residual impurities of graphene and the surface defects of the GaN acted as trap sites for the photocarriers, leading to interface-dependent photoresponsivity.

When the researchers applied a more refined layer of graphene to the GaN, they did not observe any hysteresis without light illumination, suggesting that refining the graphene layer could lead to a cleaner match at the interface. But due to inherent defects in the GaN, illumination with UV light still instigated frenetic behavior in the excitons.

The ability to evaluate the purity of the interface could be valuable in the development of high-performance devices, the researchers said. “This study will open up new possibilities to characterize other heterojunction interfaces by an ultraviolet light illumination process,” professor Golap Kalita said. “Ultimately, our goal is to understand the interface of various two- and three-dimensional heterostructures to develop novel optoelectronic devices with graphene.”

The research was published in Applied Physics Letters (https://doi.org/10.1063/1.5084190).