Complimentary metal-oxide semiconductors (CMOS) have served as the backbone of the electronics industry for over four decades. However, the last decade has been marked by increasing concerns that CMOS will not be able to continue to meet the demands of Moore’s Law in which the number of transistors in a dense integrated circuit doubles approximately every two years. If CMOS is going to continue to be a force in electronics, it will become necessary to integrate CMOS with other semiconductor materials other than silicon.
It appears that research out of The Institute of Photonic Sciences in Barcelona (ICFO) and supported by The Graphene Flagship has found a way to integrate graphene into a CMOS integrated circuit.
In research described in the journal Nature Photonics, the ICFO researchers combined the graphene-CMOS device with quantum dots to create an array of photodetectors.
While the photodetector arrays could enable digital cameras capable of seeing UV, visible and infrared light simultaneously, the technology could have a wide range of applications, including microelectronics to low-power photonics.
“The development of this monolithic CMOS-based image sensor represents a milestone for low-cost, high-resolution broadband and hyperspectral imaging systems" said, Frank Koppens, a professor at ICFO in a press release.
Koppens, who The Graphene Council interviewed back in 2015, believes that "in general, graphene-CMOS technology will enable a vast amount of applications, that range from safety, security, low cost pocket and smartphone cameras, fire control systems, passive night vision and night surveillance cameras, automotive sensor systems, medical imaging applications, food and pharmaceutical inspection to environmental monitoring, to name a few."
The researchers were able to integrate the graphene and quantum dots into a CMOS chip by first depositing the graphene on the CMOS chip. Then this graphene layer is patterned to define the pixel shape. Finally a layer of quantum dots is added.
“No complex material processing or growth processes were required to achieve this graphene-quantum dot CMOS image sensor,” said Stijn Goossens, another researcher from ICFO in Barcelona. “It proved easy and cheap to fabricate at room temperature and under ambient conditions, which signifies a considerable decrease in production costs. Even more, because of its properties, it can be easily integrated on flexible substrates as well as CMOS-type integrated circuits."
The graphene-enabled CMOS chip achieves its photoresponse through something called the photogating effect, which starts as the quantum dot layer absorbs light and transfers it as photo-generated holes or electrons to the graphene. These holes or electrons move through the material because of a bias voltage applied between two pixel contacts. The photo signal triggers a change in the conductivity of the graphene and it is this change that is sensed. Because graphene has such high conductivity, a small change can be quickly detected giving the device extraordinary sensitivity.
Andrea Ferrari, science and Technology offficer of the Graphene Flagship added: "The integration of graphene with CMOS technology is a cornerstone for the future implementation of graphene in consumer electronics. This work is a key first step, clearly demonstrating the feasibility of this approach.”