Black silicon

Black silicon is a semiconductor material, a surface modification of silicon with very low reflectivity and correspondingly high absorption of visible (and infrared) light. The modification was discovered in the 1980s as an unwanted side effect of reactive ion etching (RIE).[1][2] Another method for forming a similar structure was developed in Eric Mazur's laboratory at Harvard University (1998).

Contents

Properties

Black silicon is a needle-shaped surface structure where needles are made of single-crystal silicon and have a height above 10 microns and diameter <1 micron.[2] Its main feature is an increased absorption of incident light – the high reflectivity of the silicon, which is usually 20–30% for quasi-normal incidence, is reduced to about 5%. This is due to the formation of a so-called effective medium[3] by the needles. Within this medium, there is no sharp interface, but a continuous change of the refractive index that reduces Fresnel reflection.

Applications

The unusual optical characteristics, combined with the semiconducting properties of silicon make this material interesting for sensor applications. The potential applications include:[4]

Production by RIE

In semiconductor technology, RIE is a standard procedure for producing trenches and holes with a depth of up to several hundred microns and very high aspect ratios. In Bosch process RIE, this is achieved by repeatedly switching between an etching and passivation. With cryogenic RIE, the low temperature and oxygen gas achieve this sidewall passivation by forming SiO2, easily removed from the bottom by directional ions. Both RIE methods can produce black silicon, but the morphology of the resulting structure differs substantially. The switching between etching and passivation of the Bosch process creates undulated sidewalls, which are visible also on the black silicon formed this way.

During etching, however, small debris remain on the substrate; they mask the ion beam and produce structures that are not removed and in the following etching steps and result in tall silicon pillars.[12] The process can be set so that a million needles are formed on an area of one square millimeter.[10]

Production by Mazur's method

Material

In 1999, a group led by Eric Mazur and James Carey at the Harvard University developed a process in which black silicon was produced by irradiating silicon with femtosecond laser pulses.[13] After irradiation in the presence of a gas containing sulfur hexafluoride and other dopants, the surface of silicon develops a self-organized microscopic structure of micrometer-sized cones. The resulting material has many remarkable properties, such as an enhanced absorption that extends to the infrared below the band gap of silicon, including the wavelengths for which unmodified silicon is transparent. This property is caused by sulfur atoms being forced to the silicon surface, creating a structure with a lower band gap and therefore the ability to absorb longer wavelengths.

Similar surface modification can be achieved in vacuum using the same type of laser and laser processing conditions. In this case, the individual silicon micro-cones are lack of sharp tip but more in a penguin-like form. The reflectivity of such a micro-structured surface is very low, 3~14% in the spectral range 350–1150 nm.[14] Such reduction in reflectivity is considered to be contributed by the geometry of these micro-cones, which increases the light internal reflections between themselves and hence the possibility of light absorption by the silicon is increased. The gain in absorption achieved by fs laser texturization is found to be superior to that achieved by using alkaline chemical etch method,[15] which is a standard industrial approach for surface texturization of mono-crystalline silicon wafers in solar cell manufacturing. It is also found that such surface modification is independent to local crystalline orientation. Uniform texturisation effect can be achieved across the whole surface of a multi-crystalline silicon wafer. The very steep angles lower the reflection to near zero and also increase the probability of recombination, the latter is the reason that it thus far has not been used in solar cell manufacturing.

Function

When the material is biased by a small electric voltage, absorbed photons are able to excite dozens of electrons. The sensitivity of black silicon detectors is 100–500 times higher than that of untreated silicon (conventional silicon), in both the visible and infrared spectra.[16][17]

Uses and commercialization

The material has found commercial applications in a number of photodetectors for various imaging and night vision applications. Black silicon is currently being commercialized by SiOnyx, a Massachusetts-based venture-funded startup company which acquired licensing for the process from Harvard in 2006.

Black silicon also has potential application for high-efficiency solar cells, which is being explored by Solasys, an EU Seventh Framework Programme (FP7) funded demonstration project aiming at lowering manufacturing costs while increasing cell efficiency at the same time.

See also

References

  1. ^ Jansen, H; Boer, M de; Legtenberg, R; Elwenspoek, M (1995). "The black silicon method: a universal method for determining the parameter setting of a fluorine-based reactive ion etcher in deep silicon trench etching with profile control". Journal of Micromechanics and Microengineering 5 (2): 115. Bibcode 1995JMiMi...5..115J. doi:10.1088/0960-1317/5/2/015. 
  2. ^ a b c Black Silicon as a functional layer of the micro-system technology
  3. ^ C. Tuck Choy (1999). Effective Medium Theory: Principles and Applications. Oxford University Press. ISBN 0198518927. http://books.google.com/?id=SK_Jn3YwAu4C&printsec=frontcover. 
  4. ^ Carsten Meyer: Black Silicon: sensor material of the future? Heise Online. 5th February 2009
  5. ^ Koynov, Svetoslav; Brandt, Martin S.; Stutzmann, Martin (2006). "Black nonreflecting silicon surfaces for solar cells". Applied Physics Letters 88 (20): 203107. Bibcode 2006ApPhL..88t3107K. doi:10.1063/1.2204573. http://www.wsi.tum.de/Portals/0/media/e25/brandt/pdfs/apl_koynov_nonreflecting.pdf. 
  6. ^ Koynov, Svetoslav; Brandt, Martin S.; Stutzmann, Martin (2007). "Black multi-crystalline silicon solar cells". Physica status solidi (RRL) – Rapid Research Letters 1 (2): R53. Bibcode 2007PSSRR...1R..53K. doi:10.1002/pssr.200600064. http://www.wsi.tum.de/Portals/0/media/e25/brandt/pdfs/pss_koynov_black_mukticrystalline.pdf. 
  7. ^ Gail Overton: TECHNOLOGY:-Black-silicon-emits terahertz-radiation-Teraherz Technology: Black silicon emits terahertz radiation. In:Laser Focus World, 2008
  8. ^ Cheng-Hsien Liu: Formation of Silicon nanopores and Nanopillars by a Maskless Deep Reactive Ion Etching Process, 11 Nov. 2008
  9. ^ Zhiyong Xiao et al. (2007). "Formation of Silicon Nanopores and Nanopillars by a Maskless Deep Reactive Ion Etching Process". TRANSDUCERS 2007 - 2007 International Solid-State Sensors, Actuators and Microsystems Conference. pp. 89–92. doi:10.1109/SENSOR.2007.4300078. ISBN 1-4244-0841-5. 
  10. ^ a b Martin Schaefer: Velcro in miniature - "silicon grass "holds together micro-components In:wissenschaft.de. 21st June 2006.
  11. ^ Black Silicon Comes Back - And Cheaper than Ever, 7 September 2010
  12. ^ Mike Stubenrauch, Martin Hoffmann, Siliziumtiefätzen (DRIE), 2006
  13. ^ William J. Cromie arises:Black Silicon, A New Way To Trap Light.In:Harvard Gazette.9th December 1999, accessed on 16 February 2009.
  14. ^ Torres, R., Vervisch, V., Halbwax, M., Sarnet, T., Delaporte, P., Sentis, M., Ferreira, J., Barakel, D., Bastide, S., Torregrosa, F., Etienne, H., and Roux, L., "Femtosecond laser texturization for improvement of photovoltaic cells: Black silicon", Journal of Optoelectronics and Advanced Materials, Volume 12, No. 3, pp. 621-625, 2010.
  15. ^ Sarnet, T., Torres, R., Vervisch, V., Delaporte, P., Sentis, M., Halbwax, M., Ferreira, J., Barakel, D., Pasquielli, M., Martinuzzi, S., Escoubas, L., Torregrosa, F., Etienne, H., and Roux, L., "Black silicon recent improvements for photovaltaic cells", Proceedings of The International Congress on Applications of Lasers & Electro-Optics, 2008.
  16. ^ Wade Roush: SiOnyx Brings "Black Silicon" into the Light; Material Could upend Solar, Imaging Industries. In: Xconomy.10th Dec. 2008
  17. ^ 'Black Silicon' A new type of silicon promises cheaper, more-sensitive light detectors, Technology Review Online. 29th October 2008

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