"If we don't need to add non-silicon components and can monolithically integrate the material with electronics into a single silicon chip, the receivers become much cheaper," said professor Saif Islam.
According to the team, silicon can be used as a photodetector, but there's a trade off between speed and efficiency. To capture most of the photons, the piece of silicon needs to be thick, and that makes it slow. Make the silicon thinner so it works faster and too many photons get lost.
Instead, circuit designers have used materials such as gallium arsenide and indium phosphide. Gallium arsenide, however, is significantly more expensive and cannot be monolithically integrated with silicon electronics.
The group began by experimenting with ways to increase the efficiency of silicon by adding tiny pillars or columns, then holes to the silicon wafer. After two years of experiments, they settled on a pattern of holes that taper towards the bottom.
The detector uses the tapered holes to divert photons sideways, preserving the speed of thin-layer silicon and the efficiency of a thicker layer.
"We came up with a technology that bends the incoming light laterally through thin silicon," Islam said.
According to the researchers, when entering the holes, the photons are pulled sideways into the silicon. The wafer itself is about two microns thick, but because they move sideways, the photons travel through 30 to 40microns of silicon.
So far, the group has built an experimental photodetector and solar cell. The photodetector is said to convert data from optical to electronics at 20Gbytes per second, with a quantum efficiency of 50%.