Nanoholes in Silicon Bolster Photodetector Speed, Efficiently

A photodetector that could move more data at a lower cost has been built by propagating photons through nanoholes in a silicon wafer. The detector uses tapered holes to divert photons sideways, preserving both the speed of thin-layer silicon and the efficiency of a thicker layer.

Photodetectors for optical to electronic signal conversion usually make use of efficient, but expensive materials other than silicon. A new approach uses tapered holes in silicon to trap photons and send them sideways through a silicon wafer, boosting efficiency. The approach allows high-efficiency, high-speed photodetectors that could be cheaper and more efficient for use in data centers. Courtesy of Saif Islam, UC Davis Department of Electrical and Computer Engineering.

Existing high-speed, high-efficiency photodetector devices use materials such as gallium arsenide and indium phosphide. A photodetector made with gallium arsenide is about ten times as efficient as a silicon photodetector at the same scale and wavelength — but it is significantly more expensive and cannot be monolithically integrated with silicon electronics.

A team of electrical engineers from the University of California, Davis and W&WSens Devices, Inc., Los Altos, Calif. experimented with ways to increase the efficiency of silicon by adding tiny pillars, then holes to a silicon wafer. After two years of experiments, they identified a pattern of tapered nanoholes that could move light sideways.

“We came up with a technology that bends the incoming light laterally through thin silicon,” professor Saif Islam said.

The team demonstrated its technology on a silicon wafer about 2 μm thick. The photons moved sideways through the wafer, causing them to actually travel through 30 to 40 μm of silicon. The micro- and nanostructured holes enhanced, by an order of magnitude, the absorption efficiency of the thin intrinsic silicon layer, allowing for a data rate of 20 gigabits per second or higher at a wavelength of 850 nm. The micro- and nanoscale holes exhibited an ultrafast impulse response of 30 picoseconds and a high efficiency of more than 50 percent.

So far, the engineers have built an experimental photodetector and solar cell using the new technology. The team believes that further optimization could improve the efficiency of their device to more than 70 percent; and that the device could potentially work with a wider range of wavelengths than current photodetector technology.

“We're trying to take advantage of silicon for something silicon cannot usually do,” said Islam. “If we don't need to add nonsilicon components and can monolithically integrate with electronics into a single silicon chip, the receivers become much cheaper.”

The growth of data centers to power cloud services has created a demand for devices to move large amounts of data, very fast, over distances of a few yards to hundreds of yards. Such devices could also be used for high-speed home connections, Islam said.

The research was published in Nature Photonics (doi: 10.1038/nphoton.2017.37).

Video models the propagation of photons through a silicon wafer after they enter a tapered nanohole. These patterns of tapered holes could be used as photodetectors, replacing expensive materials such as gallium arsenide in optical-to-electronic connections. Courtesy of Saif Islam, UC Davis Department of Electrical and Computer Engineering.