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The irradiation of silicon surfaces with femtosecond laser
pulses in the presence of a halogen containing gas transforms the flat, mirror-like
surface of a silicon wafer into a forest of microscopic spikes. The spiked surface
is strongly light-absorbing:
the surface of silicon, normally gray and shiny, turns
deep black. The spikes are tens of micrometers tall and have tip sizes on the
order of hundreds of nanometers.
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Silicon spikes as viewed under an electron microscope. |
Black silicon surface viewed at high magnification |
Spike formation depends strongly on the characteristics
of the laser pulses: they must be ultrashort and very
intense. Also, the gas in which the silicon is
placed during the irradiation is critical. Depending on the background gas the
shape of microstructures can vary from sharp and tall to rounded and short. The importance of the
surrounding gas suggests that chemical reactions are involved in the formation of the spikes.
The crystal structure of the substrate does not appear to be important, since spikes form on both
Si(100) and Si(111); the interaction of the laser with the surface semms to dictate where the gas and surface
react since structuring only appears where the laser strikes the surface.
We are currently studying the remarkable optical and electronic properties of
this novel form of silicon. The infrared absorption and field emission characteristics
are of particular interest for industrial application.
We are also interested in better understanding the
influence of the light and the electronic excitation of the substrate on the
surface chemistry that gives rise to these structures. Finally, spikes form on some
but not all other semiconductors under similar conditions.
| | PLAINLY SPEAKING |
Silicon is the material of computer chips and solar cells. It is ubiquitous throughout the
high-tech world. Ordinarily, silicon absorbs a moderate amount of visible light, but a substantial
amount of visible light is reflected as well, and infrared and ultraviolet light are transmitted
through silicon or reflected from it with very little absorption.
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Silicon wafer with patch of black silicon (actual size).
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Spiked silicon surfaces, in contrast, absorb nearly all light at
wavelengths ranging from the ultraviolet to the infrared. This suggests it may be very useful
in improving the performance of existing silicon devices, such as detectors and photovoltaics.
Spiked silicon is made by shining a series of very short, very intense laser pulses
at a silicon surface in a chamber filled with a gas such as sulfur hexafluoride or chlorine.
In the presence of the laser light, the gas reacts with the silicon surface
etches away some of it, leaving a pattern of conical spikes
behind.
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Publications on this subject