Graphene-based photodetectors could be used to convert optical signals to electrical signals in computer chips, reducing their power consumption.
Such a graphene based device has been described and tested by researchers from the Massachusetts Institute of Technology (MIT), Columbia University and IBM’s T J Watson Research Centre in a study published in the latest issue of Nature Photonics.
Currently, electricity is used to move data within and between computer chips. A graphene-based photodetector would allow using light to perform this function instead, which would, in turn, enable decreasing power consumption and heat generation – two most pressing problems stemming from the growing demands on the computational capacity of the chips.
Optoelectronic devices made of graphene could be much simpler in design than those made from other materials.
“Another advantage, besides the possibility of making device fabrication simpler, is that the high mobility and ultrahigh carrier-saturation velocity of electrons in graphene makes for very fast detectors and modulators,” said Dirk Englund, the Jamieson Career Development Assistant Professor of Electrical Engineering and Computer Science at MIT, who participated in the new research.
Graphene is also responsive to a wider range of light frequencies than the materials typically used in photodetectors. Graphene-based optoelectronic chips could therefore use a conceivably broader-band optical signal, enabling them to move data more efficiently. “A two-micron photon just flies straight through a germanium photodetector,” Englund said, “but it is absorbed and leads to measurable current — as we actually show in the paper — in graphene.”
However, to make the theory a reality, a method for efficient deposition of graphene-layers has to be developed.
So far, the biggest drawback of graphene-based photodetectors has been their low responsivity. A sheet of graphene – consisting of one layer of carbon atoms - would convert only about 2 per cent of the light passing through it into an electrical current. That’s actually quite high for a material only an atom thick, but it’s still too low to be useful.
To increase the responsivity of graphene, electric current has been used previously – a method that increases the noise of the detector’s readings.
The MIT-led team has therefore come up with an innovative design. They made the light enter the detector through a silicon channel — a so called waveguide — etched into the surface of a chip. The layer of graphene is deposited on top of and perpendicular to the waveguide. On the either side of the graphene layer is a gold electrode. But the electrodes’ placement is asymmetrical: One of them is closer to the waveguide than the other.
“There’s a mismatch between the energy of electrons in the metal contact and in graphene,” Englund said, “and this creates an electric field near the electrode.” When electrons are kicked up by photons in the waveguide, the electric field pulls them to the electrode, creating a current.
In experiments, the researchers found their detector would generate 16 milliamps of current for each watt of incoming light using the 20GHz frequency — already competitive with germanium. With the application of a slight bias, induced by the electric current, the detector could get up to 100 milliamps per watt, a responsivity equal to that of germanium.
A similar research, also published in the recent issue of Nature Photonics, was conducted by a team from the Vienna University of Technology, providing comparable results.
“We did not know that we were doing the same thing," said Thomas Mueller, an assistant professor at the Vienna University of Technology’s Photonics Institute, who led the Austrian research group. “But I’m very happy that two papers are coming out in the same journal on the same topic, which shows that it’s an important thing,”