Big sparks create glowing porous silicon.

A century-old device used to demonstrate sparking has helped shed new light on the mechanism of photoluminescence in porous silicon.

Since first reported almost two years ago, the discovery that light causes acid-etched silicon to glow red has created much debate about the reason for this luminescence. Some researchers attribute the glow to quantum confinement effects in microscopic silicon "wires" left behind after etching: Electrons confined by these wires recombine with positive charges to produce light. Other evidence suggests that etching modifies silicon chemically and that either a silicon compound called siloxene or bonds between hydrogen -- or perhaps oxygen -- and silicon play a key role in luminescence (SN: 5/16/92, p.324).

Now physicist Rolf E. Hummel and graduate student Sung-Sik Chang of the University of Florida in Gainesville have discovered an acid-free way to make silicon porous. Their approach limits the chances that silicon will undergo the chemical changes of etching, Hummel says.

He and Chang place two silicon wafers in a Tesla transformer and generate sparks between the wafers for several hours. "You can make very, very intense, large sparks, like lightning in a thunderstorm," says Hummel.

The porous wafers then emit red light when subjected to a laser, they report in the Oct. 19 APPLIED PHYSICS LETTERS. "The longer you spark, the more porous the silicon and the better the photoluminescence," Hummel notes.

To guard against possible reactions between the wafers and the humid Florida air, they eroded a few silicon wafers in pure, dry nitrogen. "There was no difference" in luminescence between the resulting wafers, Hummel says. "In our case, the siloxene, the impurity, and the hydrogen theories can be ruled out [as the cause of photoluminescence]."

These results, coupled with others reported during the past few months, "cast a lot of doubt on models that attribute luminescence to specific [chemical] species," says Leigh T. Canham of the Defense Research Agency in Malvern, England. He, Hummel, and Chang caution that some contamination could occur as the scientists transfer the very reactive silicon from the nitrogen atmosphere to the instrument that measures light emitted.

But Canham also cites work in which German and Japanese researchers heated porous silicon to high temperatures and then exposed it to oxygen. Their results support his idea that quantum confinement may be responsible for this luminescence, Canham says.