Solvents Polish Porous Silicon’s Efficiency

Daniel C. McCarthy

With regard to porous silicon, efficiency applies both to the material's light-emissive qualities and to its cost-performance ratio. But until recently, stability has not been among the material's strong points. Recent research performed by several European collaborators shows that the introduction of solvents into the pores of optical waveguides made from porous silicon can dramatically increase waveguide transmissive properties. The discovery, however, resurrects the old issue of how to balance this increased efficiency with intrinsic instability of the materials.

Results of the work, which appeared last October in IEEE Photonics Technology Letters, were published by researchers from the UK including the Defence Evaluation and Research Agency in Malvern, the University of Nottingham in Nottingham and British Telecom Laboratories in Martlesham Heath; and from the Institute for Thin Film and Ion Technology in Julich, Germany.

"We weren't looking for this -- it was really a bonus," said Armando Loni, a project manager at the Defence Evaluation and Research Agency. "We had been looking at using the porous nature of the waveguides to detect various chemical vapors such as acetone, IPA and methane."

What Loni and his colleagues found was that the introduction of solvents onto the waveguide surface reduced scattering losses -- in some cases, dropping them to zero. The most dramatic results came from the solvent isopropan-2-ol (IPA), a common cleaning agent for semiconductor materials. Loni reported that transmission of the wavelengths 633 and 1320 nm through a self-aligned waveguide structure typically had been 34 dB before the introduction of IPA. Placing IPA into the pores increased transmission by 100 percent within measurement error.

Steve Kershaw, a research scientist at British Telecom Labs, was careful to point out that the potential applications remain speculative, largely because the volatile nature of the solvents makes them too unstable to provide repeatable results over a long period.

Loni pointed out, however, that it may be possible to contain a solvent within the porous network by capping the waveguide with a surface layer to sustain the low loss over a long period. He added, "You can broaden the context in terms of impregnating porous silicon with other materials, either to render it optically or electrically active or to render it sensitive to specific things. The ideas might also apply to other porous materials like alumina."