FOR RELEASE: Tuesday March 23, 9:48 am.
Claudia Wu, Tsing-Hua
Her, Eric Mazur
(617) 495-9616
Harvard University
Cambridge, MA 02138, USA
To contact the authors during the meeting: (404) 659-6500 (Wu), (404) 659-1400 (Mazur)
Popular Version of Paper IC07.10
Tuesday, March 23, 9:48 a.m.
APS Centennial Meeting, Atlanta
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| After irradiating a square patch of silicon with a series of very short laser pulses the silicon surface turns black. |
When a material is hit by laser pulses of less than a trillionth of a second duration, the material can undergo drastic transformations. Because the energy of such ultrashort laser pulses is confined to a very short time interval, the intensity of the laser pulse can be extremely high, easily matching the intensity one would obtain by focusing all the sunlight that hits the earth onto an area the size of a fingernail. These ultrashort, high-intensity pulses can induce new physical and chemical phenomena. We will show how such short laser pulses can be used to transform the flat, mirror-like surface of a silicon wafer into a forest of microscopic spikes. The silicon spikes are strongly light-absorbing: the surface of silicon, which is normally gray and shiny, turns deep black.
Spike formation
After irradiating a flat silicon surface in a halogen-containing
gas (for instance, chlorine or sulfur hexafluoride) with ultrashort
laser pulses, we find that an array of microscopic spikes is formed
at the surface. The spikes form spontaneously and are fairly regular
in size and distribution. They are only several tens of micrometers
tall (less than the diameter of a human hair) and very sharp.
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Silicon spikes as viewed under an electron microscope. |
Black silicon surface viewed at high magnification |
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The formation of the spikes depends strongly on the characteristics of the laser pulses. It is essential that the pulses be ultrashort and very intense. With longer or weaker pulses, there is no formation of spikes. We also find that the halogen gas in which the silicon is placed during the irradiation is critical. We do not find spikes in vacuum, nitrogen, or helium. This dependence on the surrounding gas suggests that chemical reactions are involved in the formation of the spikes.
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Multiple reflections enhance light absorption. |
Black silicon
The spiky silicon is an excellent light trap. Multiple reflections at the exterior and interior of the surface lead to reduced reflection and thus enhanced absorption at the air-silicon interface (the same principle underlies anechoic, egg-carton shaped acoustic surfaces). In fact, the silicon spikes absorb almost all visible light that falls on them, which is why it looks black.
Silicon is not only the material used to manufacture computer chips, but also a key material for electro-optic devices such as solar cells and photodetectors, which convert light into electric energy. However, ordinary silicon is highly reflective -- for some colors of visible light it can reflect almost 50% of the incident light, leaving less to be absorbed and converted into electric energy. Black silicon could thus have important applications as a light absorber in solar cells and photodetectors. We are in the process of characterizing the electrical properties of black silicon to determine its applicability in future solar cells or photodetectors.
The microscopic spikes on black silicon may also find applications in display technology and as micro needles for drug delivery through the application of medicated skin patches. Having just begun to explore these silicon spikes and their applications, we look forward to many exciting new developments in the near future.
Additional information can be found on the web site of the principal investigator.
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