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Wednesday, October 29, 2008

Black Silicon

Light trap: Treating silicon with short, intense femtosecond laser pulses in the presence of sulfur creates tiny cones on its surface. The rough, sulfur-infused surface is an excellent light trap, capturing nearly all of the sun’s light, including the parts of the spectrum that pass through normal silicon.
Credit: SiOnyx

Silicon's ability to absorb light and produce electric current has made it the material of choice for light sensors and solar cells. Yet about half of the light from the sun--red light and most of the infrared--passes right through silicon.

SiOnyx, a startup based in Beverly, MA, is making a new type of silicon material, dubbed black silicon, which captures nearly all of the sun's light. "It is basically a sponge for light, both visible and infrared," says CEO Stephen Saylor. The material uses the light more effectively, generating hundreds of times more current than conventional silicon. The company, which has licensed technology developed at Harvard University, also claims that the material makes it possible to use less silicon for light sensors, making the devices cheaper, smaller, and lighter.

Saylor says that the highly sensitive light detectors made from black silicon would have many advantages. In medical x-ray imaging, he says, "if you have a very high-sensitivity detector, you could lower the radiation dose of x-rays to get that image." Because the detectors pick up extremely low light signals, they could be used for in vitro imaging, night-vision goggles, and light sensors in digital cameras. Low-light applications currently use more exotic and expensive gallium arsenide.

The material could also be used to make infrared detectors, a new application for silicon. Infrared detectors, used in fiber-optic telecommunications, astronomy, and security systems, are made of gallium arsenide and other materials that are difficult and expensive to process in addition to containing toxic chemicals such as lead and mercury. "Black silicon extends the technology that we know extremely well and makes it usable in a region of spectrum where it wasn't useful before," says Eric Mazur, a professor of applied physics at Harvard, who discovered the material in his lab. "I really believe it's a new class of materials, just as semiconductors were a new class of materials 60 years ago." Mazur cofounded SiOnyx in 2006 with his then graduate student James Carey, now the company's chief science officer.

The company makes the material by putting conventional silicon in a chamber full of sulfur hexafluoride gas and bombarding it with short, intense pulses from a femtosecond laser. This roughens the surface by creating millions of tiny cones on it. The rough layer is about 300 nanometers thick and infused with sulfur atoms.

This thin surface layer does all the light capturing. Conventional silicon devices use 0.5-millimeter-thick silicon. Black-silicon devices would use hundreds of times less silicon, which would cut costs, Saylor points out. The thin devices would also be easier to incorporate into an integrated circuit.

Researchers at SiOnyx and Harvard are still investigating why black silicon produces much more current than does normal silicon when exposed to the same light. The theory is that this happens because of a mechanism called photoconductive gain. In regular silicon, each photon will knock loose only one electron to contribute to electric current. But in the new material, each photon sends multiple electrons cruising through the circuit, boosting the current 200 to 300 times. "We believe this is really the first time photonic gain has been seen in silicon," Saylor says.

The material's potential for photovoltaic solar cells remains to be seen. In a light detector, an external voltage is applied to the silicon. When a photon hits the material, it knocks loose an electron. The voltage sweeps the electron out into an external electric circuit to produce current. But photovoltaic materials have to create a voltage in response to light. It is not clear if black silicon can be coaxed into doing that efficiently, says MIT mechanical-engineering professor Tonio Buonassisi.

Buonassisi is now exploring the material for photovoltaic applications. He and his group are trying to understand the atomic structure of the material so that they can harness it to make a solar cell. The material's high absorbance makes it a promising candidate. "This is a very interesting material, and it certainly is intriguing for solar cells . . . although a lot of the mysteries have yet to be unraveled," Buonassisi says.

SiOnyx, meanwhile, is developing a black-silicon fabrication process. Saylor says that the company wants to develop a scalable way to make uniform black-silicon wafers. Then it plans to license the manufacturing method to companies that make silicon light detectors and solar cells.