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Black silicon moves out of the laboratory

09 Dec 2008

A new photonic material could have significant advantages for detection, imaging and power-generation applications. Tim Hayes asks developers at SiOnyx to explain more.

Black silicon, originally invented in a Harvard lab entirely by accident, demonstrates a broader spectral response than conventional silicon and requires a much smaller electrical bias to act as an avalanche photodiode. It combines these advantages in a very thin-layer material, producing a potentially disruptive technology for a range of applications.

"This is a photonics platform that is going to be important in detection, imaging and eventually power generation," said Stephen Saylor of SiOnyx, the US company that has licensed Harvard's black silicon patents. "It is a brand new material with new properties, and it is compatible with the existing infrastructure of silicon processing."

The SiOnyx process exposes the silicon target to high-intensity femtosecond laser pulses in the presence of sulphur hexafluoride. The process changes the chemistry of the silicon surface and also transforms it from a smooth shiny wafer into a nano-structured surface absorbing almost all of the visible light striking it.

"The surface becomes covered with thousands of very ordered microspikes," said Jim Carey of SiOnyx. "We now know that the material's photonic behaviour results from a combination of the morphology and the changes to the material chemistry. The microspikes are dramatic, but not critical."

The black silicon can respond to light from 400 to 2500 nm, covering wavelengths that silicon-based devices could not previously detect. In addition, the bias voltage needed to create a cascade of electrons from an incoming photon can be much lower than the voltage previously needed in a silicon material.

Low bias
The first area to benefit is set to be photodetectors. "In the past, photomultiplier tubes, avalanche photodiodes and intensifier tubes have used brute-force mechanisms to detect very low signal levels," said Saylor. "We can make diodes with a better response than the best avalanche photodiodes on the market, but at a bias of just 2 or 3 V."

The measured optical responsivity of 100 A/W at 950 nm demonstrates a 100-fold increase in sensitivity over traditional detection methodologies. This equates to an external quantum efficiency of 10,000%. This performance is achieved at a mere 3 V of operational bias, enabling direct integration with hybrid and digital circuitry. The extension of silicon's spectral sensitivity to 1550 nm could have significant implications for laser-sensing applications at the 1064, 1330 and 1550 nm nodes.

"Photomultiplier tubes, avalanche photodiodes and the like all use brute-force mechanisms to try to detect very low signal levels," said Saylor. "This has been very limiting in terms of how you can apply them. Detectors in positron emission tomography, for example, are very expensive and have a large footprint because of the biases that they need to run at. A responsive detector operating at low bias would improve the resolution, cost and reliability of these systems."

Using a more sensitive detector could also allow a reduction in the X-ray dosage needed in radiological examinations. "This would be a tremendous potential benefit, just in socio-human terms," noted Saylor.

Consumer cameras that are able to take better photographs in low light could be another benefit, allowing the use of cheaper and simplified lens technology.

Broad response
Black silicon's broad spectral response could also be exploited in photovoltaic cells. "For the first time we have a low-cost process allowing the generation of electron-hole pairs from the infrared wavelengths that normally pass straight through silicon," Saylor explained.

SiOnyx's shallow-junction technique creates an ultra-thin absorbing layer capable of absorbing light energy across the electromagnetic spectrum in just 300 nm of device thickness. Shallow-junction photonics promotes the integration of detectors with other circuitry and holds the promise of ultra-thin-film solar cells with a fraction of the required silicon bulk material.

SiOnyx believes that its discovery will be disruptive in photonics technology for the long term. "It takes time to incorporate a brand new photonics platform into existing infrastructure, but we think that the technology is surprisingly resilient," said Saylor. "As with any new photonics material, in 20 or 30 years we will still be discovering new ways to improve it."

Having licensed the patents from Harvard, SiOnyx is seeking partners to adopt the technology in real-world applications. "We are more of a process development company than a product development company," commented Saylor. "Our main interest is to work with very well capitalized strategics that find this technology enabling in their product lines. For us, that's a great way for a small company to make rapid progress, by aligning with market leaders."

• This article originally appeared in the December 2008 issue of Optics & Laser Europe magazine.

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