Oct
14
What Is That Black Silicon Stuff?
October 14, 2008 | 6 Comments
You’re aware of silicon wafers made into computer chips and solar cells by very high technological skills with huge investments. There has been an enormous payoff in new products with these things appearing in an ever-growing list of products. Those chips are made by etching circuits into silicon’s shiny surface.
Black Silicon Patch on Silicon Chip
Black Silicon is another kind of chip that is different in that the shiny reflective surface is marred or etched in such a way that light coming in can not be reflected back out – so being called black – as all light is absorbed with little or none coming back out.
Black Silicon By Laser Gas
The news yesterday out of Harvard University about black silicon comes from an etching technique by laser light through a gas and the announcement of a license to a spinoff company that will develop full process engineering. More on that in a moment . . .
The Harvard laser technique irradiates a silicon surface with femtosecond laser pulses through a sulfur containing gas. The effect erodes away the silicon surface making a forest of nanosized silicon tipped spikes micrometers tall on the surface. The laser pulses need to be “ultrashort” and very intense. The gas mix is also critical, as different mixes yield short rounded spikes to the desired sharp and tall ones.
When a silicon surface is reformed it becomes strongly light absorbing. Silicon wafers are usually gray and shiny, and when treated the surface become black which covers the visible light spectrum and the infrared wavelengths as long as 2500 nm. One Harvard page suggests photodiodes with remarkable responsivity for the visible and infrared can be made with the laser/gas process.
Black Silicon Single Spot View
The result is a silicon surface that throws off a greatly increased numbers of electrons. Current silicon cells are coated to resist the escape of reflected light making black silicon a highly desired improvement. But the Harvard effort is not the only player.
The Technical University of Munich Germany is working with a wet process. This method is about scattering nanosized gold (with other catalysts in research) grains as catalysts that are activated by liquids such as hydrogen peroxide and hydrofluoric acid, which will “drill” into the surface. Once the liquids and gold are cleared away, the surface shows 50 to 100 nanometer deep pits.
Plasma is also being looked at, but the techniques so far are difficult to work over large areas and can easily damage the chip making progress slow if not abandoned. Now back to the Harvard news . . .
The laser/gas etching process has nearly ten years of research effort invested. The project is led by Eric Mazur who this week, with about three years of being out of sight or in super stealth mode, has organized the license to a spinoff company called SiOnyx. This suggests that the technology might be ready for commercial scale investing. SiOnyx is reported to be saying the technology will yield more sensitive detectors like in digital cameras and far more efficient photovoltaic solar cells.
The announcement has managed to be scooped by the New York Times’ John Markoff with others allowed to report after NYT publishes their story.
Wade Roush at Xconomy Boston has interviewed the SiOnyx executives CEO Stephen Saylor and Principle Scientist James Carey a PhD graduate of Mazur’s lab for his article. SiOnyx was incorporated in 2005, the license deal done in 2006 with venture capital in hand. Roush explains from the interview what is happening quite well:
“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 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.
What’s really going on—though this is where Carey and Saylor start to get cagey, since it gets at the proprietary heart of SiOnys’x technology—is that the laser pulses force unusually large numbers of (the sulfur) 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, something on the order of between 100 and 500 times more sensitive to light than untreated silicon, the company says.
That leads to a part of the coverage from back in 2001 published by the Harvard Gazette before the silence fell into place. Back then Mazur had Carey see if the silicon spikes would produce electrons when stimulated with an electric field, much the way cathode ray tubes in television sets emit electrons when heated. The experiments turned out to be frustrating at first, then surprisingly successful. The procedure involved applying a voltage to the thin silicon tips. At first, short circuits plagued the experimenters. “I spent months trying to get rid of those short circuits,” Carey recalls. “Then I realized that the problem was not a problem at all; it was due to electrons being emitted at lower voltages than I thought possible. In other words, black silicon turns out to be a much more efficient emitter than we ever expected.”
As you can imagine the commercial prospects are applications both in using incoming light and emitting electrons to be just huge. But it’s way way early:
One of SiOnyx Directors is Bob Metcalfe, a general partner at Polaris Ventures, who cautions that this technology is just now transitioning from laboratory to commercial fabrication. In the early stages the smallest highest value uses are what will get to market first. I’m sure they are aware of the 19,000 square miles of roof space for photovoltaic installations, As Metcalfe points out, current photovoltaic misses the whole of the infrared radiation so if the fabrication can get to scale at low enough prices the market is just huge. Metcalfe also notes that photovoltaic is the “long shot” application or further out in time due to costs and scale one would acknowledge, leaving the sensitivity devices and emitters closer to market.
At the end we can see this breakthrough has come quite a long way over nearly ten years. Soon to make the jump from laboratory to pilot fabrication and hopefully on to vast commercial scale, we are seeing the earliest results. The basics here predate the rise in oil and other energy costs. Americans and Germans are both heading into new territory using silicon for known tasks using new manufacturing to make much better products.
Ten years. That might be a lesson; these kinds of things go pretty slow for consumers looking for better, faster, cheaper and most of all – more efficient. But Black Silicon is here now, and you can be sure more great ideas are coming too.
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