roughening 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.”
