Most of the semiconductor industry’s effort goes toward making silicon as pure as possible. But nearly ten years ago Harvard graduate students in physics professor Eric Mazur's lab accidentally discovered the opposite approach worked better: finding that it can actually be more useful be to create silicon devices with impurities, defects, and unconventional structures.
“We are messing up perfectly good silicon,” says James Carey, one of those students, and now co-founder of Harvard spin-off company SiOnyx “But in the end, the properties speak for themselves.” As a result, says Carey “We have seen a 100 to 500 times increase in sensitivity to light compared to conventional silicon detectors.”
Scientists already try to improve silicon crystal efficiency by doping them with atoms of various elements.
But what the grad students found was that if, in addition, they hit 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 turned out to be much better at absorbing photons and releasing electrons, up to 500 times better.
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.
This leaves the wafer festooned with tiny cones, which the photons were apparently bouncing between; greatly increasing their chances of being absorbed. Once the silicon has absorbed all wavelengths of light (even infra-red) it turns black.
Because this "black silicon" is just normal silicon that has been roughed up by their femtosecond laser pulses and chemical treatment, this 500-fold improvement could simply be integrated into current production lines fairly easily. “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.
You might wonder why this is news now, 10 years after their discovery.
Till very recently, Harvard actually took a dim view of commercializing student discoveries made on school time by its tuition-paying students. The school’s technology transfer office simply “wasn’t very excited” about their discovery of "black silicon" back then, according to Carey.
However, three years ago, in a shakeup of technology licensing policy, they hired the university licensing veteran Isaac Kohlberg, who saw the potential of roughed up silicon and began to iron out the licensing deal that has now made SiOnyx possible.
For Matter Network