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'Black silicon'

The need for highly efficient photovoltaic solar cells has been clear for quite a while.  A bit less obvious to the broad scientific community is the need for highly efficient light sensors, for example in night vision and thermal imaging applications.

As with many important discoveries, a class of materials called "black silicon" was discovered by accident, in this case as a byproduct of microfabrication technology.  These materials are called "black" because they have unusually broad and high absorption spectra, and so appear black both to the naked eye and to instruments that can reach into infrared.

In the late 1990s, methods were developed to study and commercialize these potentially very useful materials.  A well-established method, tricky and expensive, involves exposing a silicon substrate to femtosecond laser radiation in the presence of sulfur dopants.  The sulfur doping is thought to lower the band gap of silicon to allow absorption of long-wavelength IR photons.  A surface texturization also results from the laser irradiation, yielding microscale spikes or pillars that are thought to produce many total internal reflection paths at a variety of scales, resulting in much greater absorption of light.

How much greater?  The photoelectric effect in black silicon is currently several hundred times more sensitive than crystalline silicon alone.

It is not uncommon to wait ten years or more for a laboratory result to become efficient and reliable enough to reach into the marketplace.  A new method using nanosecond laser pulses has yielded similar results, but involving much less expensive and temperamental laser apparatus.  This last development may drive commercialization efforts into self-sustaining growth.

Another interesting application may emerge from black silicon: when excited, it can emit terahertz (THz) radiation.  The transparency and opacity of many materials to THz waves suggest applications in biomedical imaging, package inspection, scientific spectroscopy, and airport security.

The author's affiliation with The MITRE Corporation is provided for identification purposes only, and is not intended to convey or imply MITRE's concurrence with, or support for, the positions, opinions or viewpoints expressed by the author.

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