Building a Better Detector with Black Silicon

Hank Hogan

Exploiting an accidental discovery, researchers at the University of Texas at Austin, at Harvard University in Cambridge, Mass., and at the University of Virginia in Charlottesville have developed a silicon photodetector that has a high responsivity from 600 to 1100 nm. Built of black, or microstructured, silicon, the detector’s photoresponse extends to 1.6 μm, well beyond the 1.07 μm provided by standard silicon.

Photodetectors based on black silicon — formed by irradiating silicon with femtosecond laser pulses while exposing it to a sulfur-containing gas — exhibit a spectral response out to 1.6 μm, beyond the 1.07 μm of standard silicon. The photodetectors could be used for night vision or for environmental sensing. Courtesy of Eric Mazur, Harvard University.

The investigators constructed the black silicon from standard semiconductor silicon by irradiating it with a femtosecond laser while exposing the material to sulfur hexafluoride. The laser energy prods the normally very stable SF₆ molecule into reacting with the silicon.

The flat, mirrorlike surface of the silicon transforms into a black material covered by micron-scale spikes that taper from a base that is microns across to a tip a few hundred nanometers wide. Harvard professor of physics and applied physics Eric Mazur, in whose lab the microstructured silicon was initially fabricated, recalled that the discovery was both accidental and serendipitous.

“We were totally surprised to see the blackness of the resulting surfaces and the structure under the electron microscope,” he said.

The exact dimensions of the microstructures are a function of the gas used and of the laser setup. In building the photodetector, the scientists fired 100-fs pulses from a Ti:sapphire laser at an n-doped silicon wafer at a 1-kHz repetition rate, achieving an average fluence of 4 kJ/m² at the surface.

They moved the silicon relative to the beam at such a speed that any given spot received ~200 laser shots. They annealed the silicon, creating a forest of microstructures ~2 to 3 μm high and spaced 2 to 3 μm apart. Finally, they used the black silicon to create devices measuring 50 to 500 μm in diameter, with contacts to both front and back to allow device operation and characterization.

Using a tungsten-halogen lamp filtered through a tunable monochromator and semiconductor lasers, they measured the response versus wavelength. The photodetector exhibited high responsivity in the visible, with some response out to 1.6 μm. The researchers attributed the infrared photoresponsivity to the ambient gas, noting that the sulfur implanted in the surface by the manufacturing process was important.

Mazur said that future research will involve better understanding of the physics of the photoelectric properties and a better grasp of the structure formation. However, the group has been concentrating on developing devices and applications. He also noted that the unique spectral response of the material made likely such uses as night vision and environmental sensing applications.

Because the microstructures form only where the laser strikes the silicon, the material potentially could be incorporated into a standard semiconductor chip.

“Process integration is a topic that we are actively investigating, with the goal of determining how to best fit into current imaging device manufacturing processes,” said team member James E. Carey.

Applied Physics Letters, Vol. 89, 2006, 033506.