Black silicon: A circuit like this SiOnyx test array could eventually be used in imaging devices such as an infrared sensor or a cell-phone camera.
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.