Graphene sees more light with silicon carbide substrate

Graphene could be utilised in a wider range of highly sensitive optical devices by combining it with a silicon carbide substrate, claim researchers in the US.

Graphene GFET (Purdue University image/Erin Easterling)
Graphene GFET (Purdue University image/Erin Easterling)

Their findings, detailed in a research paper appearing in Nature Nanotechnology, overcome the limitations of graphene-based photodetectors that only have a small area that is sensitive to light, thereby limiting their performance.

By combining graphene with a comparatively much larger silicon carbide substrate, the researchers at Purdue University, the University of Michigan and Pennsylvania State University created graphene field-effect transistors – GFETs – that can be activated by light.

High-performance photodetectors might be useful for applications including high-speed communications and ultra-sensitive cameras for astrophysics, plus sensing applications and wearable electronics. Arrays of the graphene-based transistors might bring high-resolution imaging and displays.

“In most cameras you need lots of pixels,” said Igor Jovanovic, a professor of nuclear engineering and radiological sciences at the University of Michigan. “However, our approach could make possible a very sensitive camera where you have relatively few pixels but still have high resolution.”

“In typical graphene-based photodetectors demonstrated so far, the photoresponse only comes from specific locations near graphene over an area much smaller than the device size,” Jovanovic said. “However, for many optoelectronic device applications, it is desirable to obtain photoresponse and positional sensitivity over a much larger area.”

According to Purdue University, new findings show the device is responsive to light even when the silicon carbide is illuminated at distances far from the graphene. The performance can reportedly be increased by as much as 10 times depending on which part of the material is illuminated. The new phototransistor also is “position-sensitive,” meaning it can determine the location from which the light is coming, which is important for imaging applications and for detectors.

“This is the first time anyone has demonstrated the use of a small piece of graphene on a large wafer of silicon carbide to achieve non-local photodetection, so the light doesn’t have to hit the graphene itself,” said Yong Chen, a Purdue University professor of physics and astronomy and electrical and computer engineering, and director of the Purdue Quantum Center. “Here, the light can be incident on a much larger area, almost a millimetre, which has not been done before.”

A voltage is applied between the backside of the silicon carbide and the graphene, setting up an electric field in the silicon carbide. Incoming light generates photo carriers in the silicon carbide.

“The semiconductor provides the media that interact with light,” Jovanovic said. “When light comes in, part of the device becomes conducting and that changes the electric field acting on graphene.”

This change in the electric field also changes the conductivity of graphene itself, which is detected. The approach is called field-effect photo detection.

The silicon carbide is un-doped, unlike conventional semiconductors in silicon-based transistors. Being un-doped makes the material an insulator unless it is exposed to light, which temporarily causes it to become partially conductive, changing the electric field on the graphene.

“This is a novelty of this work,” Chen said.

The research is related to work to develop new graphene-based sensors designed to detect radiation and was funded with a joint grant from the National Science Foundation and the U.S. Department of Homeland Security and another grant from the Defense Threat Reduction Agency.