Silicon is a wonderfully cooperative element. It takes relatively little energy to promote the electrons in a silicon crystal from their usual, docile orbits around the atomic nuclei into wild, free circulation. That’s what makes silicon a semiconductor—valuable for electronic switching devices such as transistors, sensing devices such as the CCDs in cameras and X-ray machines, and energy-generating devices such as photovoltaic cells.
But silicon would be more wondrous if it were even more responsive—if an incoming photon needed less energy to knock loose an electron, for example, or if a single photon could kick loose many electrons. In pursuit of this vision, chemists, physicists, and engineers have spent decades trying out various ways of modifying silicon crystals—for example, by doping them with atoms of arsenic or other elements that put more free electrons into the mix.
Almost ten years ago, graduate students in the laboratory of physics professor Eric Mazur at Harvard University stumbled across a new way of making silicon more responsive: they found that if they blasted the surface of a silicon wafer with an incredibly brief pulse of laser energy in the presence of gaseous sulfur and other dopants, the resulting material—which they called “black silicon”—was much better at absorbing photons and releasing electrons. And this week, after nearly three years in hyper-stealth mode, a spinoff company with an exclusive license from Harvard to commercialize the process has begun talking with reporters.
Executives for the company, called SiOnyx, believe that its technology will help semiconductor manufacturers build far more sensitive detectors and far more efficient photovoltaic cells, using essentially the same silicon-based processes they currently depend on—thereby revolutionizing areas such as medical imaging, digital photography, and solar energy generation.
The venture-funded startup has emerged with a bang, securing exclusive coverage by New York Times technology writer John Markoff in today’s edition. But SiOnyx CEO Stephen Saylor and principal scientist James Carey, a PhD graduate of Mazur’s lab, also showed me around their Beverly, MA, facility last week, on the condition that this post would appear after Markoff’s story.
“You’ve never been able to detect light the way this stuff detects light,” says Saylor, referring to black silicon’s remarkable sensitivity to incoming photons, especially photons at infrared energies, which pass through normal silicon as if it were transparent. That property could make it an ideal, and inexpensive, replacement for less-sensitive detectors in devices as varied as X-ray and CRT machines, surveillance satellites, night-vision goggles, and consumer digital cameras. “It means that you solve a clear and obvious pain point for a very large number of customers,” Saylor says.
And because black silicon is just silicon that’s been roughed up a bit by femtosecond laser pulses and chemical treatment, SiOnyx’s technology could theoretically be integrated into existing semiconductor fabrication lines without much disruption. “You can do everything we’re talking about without extraordinary, Herculean effort, and you can do it in a way that fits with high-volume manufacturing flows,” says Carey.
SiOnyx was incorporated in 2005, secured the Harvard license in early 2006, and obtained $11 million in venture financing from Harris & Harris, Polaris Venture Partners, and RedShift Ventures in 2007. The company is going public with its story because “we have enough momentum now both with strategic partners and with the technology that it makes sense at this point to share a little more about what we are up to,” say Saylor.
Harvard, for its part, is holding up SiOnyx as one early result of the ongoing overhaul of the university’s technology licensing efforts. The school gained a reputation early in this decade as being unresponsive, even hostile, toward faculty and students who wished to commercialize discoveries made in the university’s labs, especially in areas outside of biotechnology and drug development. For years after the discovery of black silicon in Mazur’s lab, the school’s technology transfer office “wasn’t very excited” about the work, according to Carey.
But in 2005 the university brought in university licensing veteran Isaac Kohlberg to rebuild its technology transfer operation from scratch. Saylor and Carey say it was Kohlberg and his staff who finally understood black silicon’s potential and ironed out the licensing deal that made SiOnyx possible.
“The exciting steps being taken to develop [black silicon] for commercial application serve as even more evidence of the entrepreneurial energy that continues to gel and accelerate at Harvard,” Kohlberg says in a press release set to be issued tomorrow by SiOnyx and Harvard’s Office of Technology Development.
Bob Metcalfe, a general partner at Polaris Ventures who sits on SiOnyx’s board, thinks Kohlberg is right: “Harvard seems to be getting its act together in patent licensing,” he says.
Exactly what makes black silicon such an effective absorber of photons is a question that even Mazur and Carey couldn’t answer at first. The material is one of many offshoots of work going on in Mazur’s lab in the late 1990s using femtosecond lasers—devices that can emit an intense pulse of light lasting only a millionth of a billionth of a second. Mazur lab researchers found that zapping a silicon wafer with such pulses in the presence of sulfur hexafluoride gas—an experiment initially carried out on a whim—left the wafer festooned with tiny cones. Silicon roughened in this way soaks up almost all of the light that strikes it in visible wavelengths, appearing black—hence the name.
“It took several years for us to begin thinking properly about what we had,” says Carey. “The original thought was that the surface roughening process was what created the advantage.” The researchers hypothesized that photons were bouncing from cone to cone—and that the more times they bounced, the higher the likelihood that they’d be absorbed, thus dislodging electrons. But then Carey and his coworkers realized that black silicon was also absorbing infrared light, “which you can’t explain just by
