New use found for exotic material

A novel use has been found for black silicon, an exotic material discovered accidentally in a Harvard research lab three years ago.

In 1999, when researchers used laser pulses to etch the surface of silicon, the most common substance used in electronic devices, they created a material that efficiently traps light. Called black silicon, it holds amazing potential for efficiently converting sunlight to electricity, for communicating by light, and for monitoring the environment for evidence of pollution and global warming (see Dec. 9, 1999, Gazette).

Now, further experiments reveal that, when placed in a strong electric field, black silicon also emits electrons with surprising efficiency. Since many electronic devices depend on beams of electrons to operate, the new discovery is sparking intense interest over its possible commercial applications. Such applications include ultra-thin television screens, ultra-sensitive pollution monitors, and even satellite maneuvering systems.

“We’re still trying to figure out why black silicon emits so well,” says James Carey, a graduate student in Harvard’s Division of Engineering and Applied Sciences. “Even in such a preliminary stage, it is generating a lot of excitement because it has a number of wonderful possible applications. Anything that emits electrons has countless uses, from cell phones to supercomputers.”

Carey received an award for his work on black silicon at the International Vacuum and Microelectronics Conference in August. The award honors young graduate students who do outstanding research. Carey, 24, received a medal, plaque, a good bottle of champagne, and a bad tie. He drank the champagne, proudly displays the medal and plaque, and sometimes wears the tie “for fun.”

Unexpected, beautiful, and practical

Black silicon was serendipitously discovered by a group led by Eric Mazur, Harvard College Professor and Gordon McKay Professor of Applied Physics. Mazur’s team was investigating what kinds of new chemistry might occur when lasers shine on different materials. One day, the experimenters put a chip of gray silicon into a vacuum chamber, added some gas, and bombarded it with ultra-short laser pulses, just to see what would happen.

The silicon turned black, and its surface became etched into a dazzling forest of minute, needlelike spires. “Apart from making something beautiful and unexpected, we came up with something that is tantalizingly practical,” Mazur says.

Light shone on the surface of black silicon bounces back and forth between the spikes in such a way that most of it never comes back out. Anything that absorbs light that well should make an excellent solar cell for converting sunlight into electricity. Mazur and his colleagues are pursing this possibility with a Norwegian company called Scanwafer, the world’s largest maker of solar cells.

The spiky silicon also absorbs infrared radiation (heat) in a way that would make it an excellent detector of pollutants, toxic chemicals, and human bodies. “You could equip sensors or robots with such devices for searching bombed-out buildings, investigating chemical spills, or exploring other planets,” Carey notes.

Mazur assigned Carey to see if the silicon spikes would produce electrons when stimulated with an electric field, much the way cathode ray tubes in television sets emit electrons when heated. With the help of postdoctoral fellow Catherine Crouch, graduate student Mike Sheehy, and Rebecca Younkin, who has since graduated, Carey began a series of experiments that turned out to be frustrating at first, then surprisingly successful.

The procedure involved applying a voltage to the thin silicon tips, which are only about 500 atoms across. It was hypothesized that such an electric field would provide enough energy for electrons to “jump” off atoms on the needle tips and create an electron beam. That process is known as field emission.

At first, short circuits plagued the experimenters. “I spent months trying to get rid of those short circuits,” Carey recalls. “Then I realized that the problem was not a problem at all; it was due to electrons being emitted at lower voltages than I thought possible. In other words, black silicon turns out to be a much more efficient emitter than we ever expected.”

Getting a better rear view

The award given to Carey and the attention his report has received attest to the interest generated by people who want to turn black silicon emitters into useful products. Silicon is cheap and such emitters are easy to make.

“Many field emitters now on the market require a large number of manufacturing steps, together with difficult-to-maintain ‘clean rooms’ and expensive fabrication machines,” Carey points out. “And they are not as efficient as black silicon.”

Take television sets, for example. The electron beam that makes pictures on the screen is produced by applying heat to a bulky cathode ray tube. Generating the same beams with a black silicon device promises to be cheaper and much more energy efficient. In addition, the screen could be less than an inch thick.

Such thinness opens the way to other types of display devices, from 40-inch flat screens to small displays mounted on gambling machines or the dashboards of cars. One proposed application would replace side and rear view mirrors on cars and trucks. Instead, sensors on the rear bumper would feed information to displays on the dashboard.

The National Aeronautics and Space Administration is looking into the possibility of employing black silicon field emitters to steer small satellites. Beams of electrons would provide the thrust to turn and otherwise maneuver the spacecraft. Such propulsion would be significantly more efficient than burning liquid fuels.

If an electron beam is made intense enough, its high energy might even be used as a ray-like weapon.

“Electron beams are used in electronic devices from household appliances to sophisticated medical instruments, so future applications and products are limitless,” Carey says enthusiastically.

In the next breath, however, he admits that these field emitters are still in the laboratory stage. Carey and his colleagues still don’t know why they work as well as they do. So far, the best gas for generating the necessary silicon surfaces turns out to be sulfur hexafluoride. When laser pulses strike the silicon in its presence, atoms of sulfur are deposited in the outer layers of the ultra-sharp spikes.

“We believe sulfur raises the number of electrons available for emission from the tips of the silicon,” Carey says. “But we don’t know for sure. We’re also analyzing the chemical and structural makeup of the surface of black silicon to see if more surprises, good or bad, await us.”

At least there will be no more short circuits or ugly ties.