A material dubbed black silicon has shown great promise for making cheaper, more sensitive light detectors and imaging devices, while potentially taking advantage of established silicon manufacturing methods. But one of black silicon’s key characteristics–a forest of microscopic cones that form on its surface and give the material its black color–may not be as important as it first appeared to be. The Harvard University researchers who first discovered black silicon are now studying a modified form of the material that has no cones but exhibits the same unique optoelectronic properties. Because of its faint coloring, the new stuff is nicknamed pink silicon, although it can barely be distinguished from a regular silicon wafer.
Black silicon was discovered accidentally by a team led by Harvard physics professor Eric Mazur. The group created the material by throwing together a gaseous sulfur compound and a silicon wafer in a vacuum, then blasting the silicon with a femtosecond laser to restructure it on the nanoscale.
Black silicon can absorb light over a wider spectrum than can normal silicon–from low-frequency visible light through near- and short-wave-infrared wavelengths that would normally pass right through regular silicon. Another property, called photoconductive gain, gives black silicon much greater sensitivity to light. These properties have identified black silicon as a way to make smaller, cheaper, and lighter silicon-based light detectors and to replace more expensive materials used in infrared detectors found in fiber-optic links, security systems, and elsewhere.
But the cones that cover the surface of black silicon, which are created during the high-intensity, short-pulse femtosecond laser restructuring of silicon, can cause problems–for example, by foiling bulk fabrication attempts. James Carey, who studied the properties of black silicon while a grad student in Mazur’s lab at Harvard and cofounded the company SiOnyx, based in Beverly, MA, to commercialize the material, explains that it’s harder to mass-produce wafers with tall cones in a foundry process. This is because a foundry uses thin-film deposition steps, so contact with cones can interfere with smooth processing through the plant. There’s also a risk that the cones could break off. “New materials are hard to introduce to a foundry process with no problems,” Carey says. The cones also interfere with attempts to study the material’s electronic properties in greater detail, because they increase the material’s surface area by one to two orders of magnitude, making it scatter electrons more readily.
Mazur’s lab has now added a new twist to the black-silicon production process, taking advantage of the absorption and high-gain properties of black silicon but keeping the material completely flat. That could help overcome fabrication challenges and allow for more detailed study of the material.
The breakthrough came when the researchers saw “flatter” black silicon cones after changing the parameters of their laser during experiments. It turned out that there were actually two things going on in their original laser production process: ablation, which was responsible for creating the surface cones, and rapid melting combined with resolidification, which trapped doped sulfur atoms in a nanocrystalline structure of the silicon. Both of those processes occurred together originally, but Mazur’s lab has now separated them. “Once we posed the question, it was pretty straightforward to test it,” says Mark Winkler, the grad student who first noticed the strange effects. “It just took changing the way we thought, from taking it as a given that we make spikes to asking how much control we have over the material that we’re making.”
Laser ablation requires a higher energy than melting, so the researchers tuned the intensity of their laser to hit below the ablation threshold but above the melting threshold. Now the laser breaks down the material and lets it recrystallize with 1 to 2 percent sulfur atoms trapped inside–highly doped for a semiconductor–but the surface remains smooth and flat. While the cones no longer form–causing the surface to look slightly pink, instead of black–the material still absorbs straight through to the infrared spectrum.
Without the cones, Mazur says, “we’re finally doing measurements that were impossible” previously, including measuring carrier densities, electron mobility, and other electronic properties. Mazur adds that it’s also easier to study the chemical composition of the substrate and “get a nice profile” of the material below the surface. The big question still remaining, says Winkler, is why black silicon absorbs light in the infrared spectrum in the first place.
Meanwhile, SiOnyx is busy turning black silicon’s potential into commercial devices. While the company’s process doesn’t use completely flat silicon, the SiOnyx researchers cut down the cone height from microns to about 200 nanometers, Carey says, to help the fabrication process. SiOnyx recently completed its first successful foundry run using the shorter cones, thus demonstrating their capability for bulk manufacturing. The company hopes to have commercial photo detectors ready to go this year.
Richard Myers, of Radiation Monitoring Devices, a research and commercial development company that has done some research with black silicon, says that the advantage of the material is that it expands silicon’s functionality. “It comes down to low cost and existing processing technology,” Myers says. The silicon electronics infrastructure is cheap and “in place,” so the new material–whether black or pink–is useful as another way that people are trying to push the limits of silicon.
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