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Black Silicon: A Mystery in Infrared
Despite being literally as common as sand, silicon is an invaluable substance. Its ability to absorb visible light and transform it into electrical pulses makes silicon an essential component of the sensors used in digital cameras, solar cells, and other photovoltaic devices.
A recent experiment in processing black silicon—silicon whose surface has been treated to decrease its reflectivity—revealed surprising results. The processing method not only greatly increased the efficiency with which silicon absorbs visible light, it also made it possible for silicon to absorb light in the infrared spectrum. This new trait could have far-reaching consequences in that it could revolutionize the production of a wide range of technologically sophisticated products, including night-vision goggles, heat-seeking missiles, and others.
With the support of the company's internal research program, a MITRE research team decided to pursue the mysteries of black silicon.
A Harvard research team first announced the unexpected properties of black silicon in 1998. Their unorthodox process for creating black silicon involves firing high-powered bursts of laser light at a silicon wafer suspended in a gas of sulfur hexafluoride. The process creates microscopic peaks and valleys on the surface of the silicon. Similar to an anechoic chamber, which is free from echoes and reverberations because its ridged walls suck up sound waves, the black silicon's peaked surface increases the amount of visible light energy it can absorb.
The team claimed that its experiment resulted in an almost 100 percent efficiency in visible light absorption. However, another claim—that their process also resulted in the silicon being able to absorb infrared energy—elicited skepticism from their peers. According to current scientific thinking, infrared light has too long of a wavelength to be affected by the micron-sized peaks of black silicon's textured surface.
"It's generally accepted that silicon detectors aren't going to work in the infrared without doping them with other exotic materials," explains Jody Mandeville, a group leader in MITRE's Department of Defense FFRDC. "The fact that the Harvard team achieved near 100 percent light absorption almost completely across the visible spectrum and way into the infrared made this almost too good to be true."
A Chaotic Process
For Mandeville, the possibility of silicon absorbing infrared energy was too compelling not to pursue. Based at the MITRE site in Colorado Springs, he set out with research partners at the nearby Air Force Academy to recreate the results of the Harvard researchers. His team not only succeeded in achieving basically the same results, but did so with an off-the-shelf laser that was much less powerful than the state-of-the-art laser used in the earlier experiment.
After publishing a paper with the results of his investigation in the journal Applied Physics A in March 2011, Mandeville knew he had only taken the initial step in solving the mystery. He realized that he had to discover not only if the process worked, but also how it worked. Without solving the mystery of silicon's expansion into the infrared, it would be impossible to optimize the manufacturing methods to make new infrared-sensing technologies feasible.
"We're trying to understand the fundamental physics of what's going on," he says. "If you don't understand the physics, it's hard to optimize an engineering process."
Puzzling out all the chemical reactions of Harvard's experiment, however, proved to be an impossible task. "It's such a chaotic process when the laser blasts the surface of the silicon," says Mandeville. "There's etching and evaporating and melting and gases being broken down. It's way too complex to figure it all out. So we had to simplify the process as much as possible and take it one step at a time."
3,000 Degrees Celsius
Mandeville proceeded with his research based on the premise that the laser texturing would not allow black silicon to capture the broad waves of infrared. If it wasn't the texturing, he reasoned, it must have something to do with the embedding of sulfur into the silicon during the texturizing process.
To isolate the effect of the sulfur from the effects of the texturing, the MITRE team evaporated a solution of silicon and sulfur onto the smooth surface of glass. If the sulfur was allowing the silicon to absorb infrared energy, Mandeville anticipated two results from combining smooth silicon with sulfur on glass: it would not show the increase in visible light absorption provided by texturing, and it would show absorption of infrared light. The test, however, failed—very little infrared was absorbed.
Mandeville remained stumped until he realized that the evaporation process he used had resulted in clumps of sulfur atoms embedding onto the silicon. Molecular sulfur has much less ability to absorb infrared energy than do individual atoms of sulfur. He recognized he would have to figure out a way to keep the sulfur from clumping during the evaporation process.
A peek into the chemistry books revealed a Nobel Prize-winning technique from the 1940s for creating single atoms that requires heating molecules to 3,000 degrees Celsius—half the temperature of the sun. Mandeville is currently preparing a new series of experiments using this technique. He is confident that the solution of silicon and atomic sulfur will exhibit infrared-absorbing abilities and solve the black silicon mystery.
"The original crux of this project was skepticism," he says. "We are now obtaining a better understanding of the physics involved, which should enhance our ability to pass along the process to manufacturers."
If Mandeville has in fact solved the mystery, his research may help pave the way for a new era in infrared technology.
—by Christopher Lockheardt
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