SiOnyx Brings “Black Silicon” into the Light; Material Could Upend Solar, Imaging Industries
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 byroughening it.” It takes photons of a certain energy to bump electrons in silicon’s outermost layer of electrons, called the “valence band,” into the so-called “conduction band,” where they’re free to circulate between atoms—and infrared photons just don’t have enough. So by all rights, these photons should have been passing right through without interacting with the material, just as if it were frosted glass.
“That was the real discovery point,” says Carey. The genesis of SiOnyx, he explains, came when the Mazur lab dug into the changes caused by the femtosecond laser pulses at the atomic level. And as it turned out, he says, “the cones weren’t really paramount at all”—although they certainly look cool (electron micrographs of the cone forests, like the one below, still appear alongside almost any discussion of black silicon).
What’s really going on—though this is where Carey and Saylor start to get cagey, since it gets at the proprietary heart of SiOnyx’s technology—is that the laser pulses force unusually large numbers of dopant atoms into a thin layer of silicon on the surface of the cones. “The laser allows you to put in a million times more sulfur than you would normally get in if you just combined and heated them,” says Carey. “In that millionth of a billionth of a second you get structural arrangements frozen at the atomic level.”
With its new structure, the “band gap” in this thin silicon layer—the difference in energy between the valence band and the conduction band—is smaller. That means less energy is required to knock electrons into the conduction band, which explains why infrared photons can do the job. Another fringe benefit: applying a small voltage to black silicon (engineers call this “bias”) creates conditions in which a single incoming photon can knock loose dozens of electrons. So, not only is the material responsive to wavelengths that silicon-based devices simply couldn’t detect in the past—it also produces a much stronger signal in response to a weak stimulus. Black silicon is between 100 and 500 times more sensitive to light than untreated silicon, the company says.
These properties mean that SiOnyx is in a position to pioneer new types of solar cells that could capture the sun’s energy across a broader spectrum, achieving greater efficiency than today’s photovoltaic cells.
“Harnessing nuclear fusion energy arriving from Sol—solar energy at 1366 Watts per square meter—is the most promising technology for meeting accelerating world needs for cheap and clean energy,” says Polaris’s Metcalfe. Black silicon “promises to dramatically increase the photo-response (Amps per Watt) of silicon, and not just in the visible spectrum, but also in the infrared, where silicon currently misses half of Sol’s energy. Delivering on that promise is very exciting.”
But that’s the “long shot” application for the material, Metcalfe acknowledges. Closer in is the possibility of major sensitivity improvements in imaging applications such as night vision, surveillance, digital cameras, and medical imaging. Saylor says that the company has negotiated strategic partnerships with two “industry leaders,” and though he won’t name names, he says one of them is active in the medical imaging area.
The attraction of black silicon in medical imaging is obvious: If you could build a more sensitive detector for a CT or mammography machine, you could expose patients to a lower dose of X-rays. (Black silicon, of course, can’t detect X-rays directly; modern digital X-ray machines include a component called a scintillator that emits visible light when struck by X-rays, and that light is what’s recorded by a sensor.) “If we can do something that allows women to get risk-free mammograms twice a year or reduce the number of chest-X-ray equivalents that you get from a CT scan, or address other pain points, we will have an immediate path to market,” says Saylor.
While SiOnyx is telling some of its story, it’s keeping big pieces of it under wraps. Asked how many employees the company has, Saylor says it’s more than 10 and fewer than 50. (Significantly fewer, from what I could see around SiOnyx’s offices—a space in the former United Shoe Machinery factory in Beverly, far outside of Boston, that the company picked because the previous tenant had installed a clean room.) The company won’t build semiconductors or even semiconductor fabrication equipment, but will instead work with as-yet-unnamed partners to develop specifications for machines that can treat isolated areas of silicon wafers to create black silicon.
SiOnyx engineers were using an automated testing device to examine sections of such a wafer when I visited. “We are a process engineering company, not a product engineering company,” says Saylor. “Our job is to make a transferable process that conforms to [our partners’] manufacturing flow. We are doing a tremendous amount of development around what are the optimal conditions for making this black silicon—how do you do it uniformly, how do you make it massively scalable, and how do you transfer it to a foundry.”
Metcalfe says the biggest challenges before SiOnyx right now are “to move the black silicon process from labs to fabs, from experimental facilities/processes at Harvard to production facilities/processes at SiOnyx” and “to navigate through black silicon’s many opportunities to the right go-to-market products.”
Saylor says he hopes the company won’t have to raise any more venture capital to do that. “The first strategic relationships are going to be with very well-aligned industry leaders, so those will lead to development relationships and eventually product-revenue relationships,” he says. The company will be “careful with cash” until it can grow to the point that it “becomes interesting to someone outside the venture investing community,” he says.
There’s an interesting irony to SiOnyx’s business: a large chunk of the semiconductor industry’s effort over the past 50 years has gone toward making silicon as pure as possible. But now SiOnyx and other companies are showing how useful—and perhaps profitable—it can be to craft silicon devices with impurities, defects, and unconventional structures.
“We are messing up perfectly good silicon,” Carey admits. “But in the end, the properties speak for themselves.”