Graphene 'Phototransistor' Promising for Optical Technologies

Posted Apr 19, 2017 by

A team of researchers from Purdue University, University of Michigan and Pennsylvania State University have claimed to solve a problem hindering development of highly sensitive optical devices made of a material called graphene, an advance that could bring applications from imaging and displays to sensors and high-speed communications. Graphene is an extremely thin layer of carbon that is promising for optoelectronics, and researchers are trying to develop graphene-based photodetectors, devices that are critical for many technologies. However, typical photodetectors made of graphene have only a small area that is sensitive to light, limiting their performance.

The researchers have solved the problem by combining graphene with a comparatively much larger silicon carbide substrate, creating graphene field-effect transistors, or GFETs, which can be activated by light. High-performance photodetectors might be useful for applications including high-speed communications and ultra-sensitive cameras for astrophysics, as well as sensing applications and wearable electronics. Arrays of the graphene-based transistors might bring high-resolution imaging and displays.

The new findings which were detailed in a research paper appearing in the journal Nature Nanotechnology states that, 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. However, for many optoelectronic device applications, it is desirable to obtain photoresponse and positional sensitivity over a much larger area. The device is claimed to be responsive to light even when the silicon carbide is illuminated at distances far from the graphene. The performance can 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. Here, the light can be incident on a much larger area, almost a millimeter, which has not been done before. A voltage is applied between the back side 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. 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.

According to the researchers, the paper is about a sensor to detect photons, but the principles are the same for other types of radiation. They are using the sensitive graphene transistor to detect the changed electric field caused by photons, light in this case, interacting with a silicon carbide substrate. Light detectors can be used in devices called scintillators, which are used to detect radiation. Ionizing radiation creates brief flashes of light, which in scintillators are detected by devices called photo multiplier tubes, a roughly century-old technology. The researchers also explained their findings with a computational model. The transistors were fabricated at the Birck Nanotechnology Center in Purdue's Discovery Park.