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] Other methods for forming a similar structure include electrochemical etching, stain etching, metal-assisted chemical etching, and laser treatment (which is developed in Eric Mazur's laboratory at Harvard University ), and FFC Cambridge process (an electrochemical reduction process).[3]

Properties

Scanning electron micrograph of black silicon, produced by RIE (ASE process)
SEM micrograph of black silicon formed by cryogenic RIE. Notice the smooth, sloped surfaces, unlike the undulated sidewalls obtained with the Bosch process RIE.

Black silicon is a needle-shaped surface structure where needles are made of single-crystal silicon and have a height above 10 µm and diameter less than 1 µm.[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[4] by the needles. Within this medium, there is no sharp interface, but a continuous change of the refractive index that reduces Fresnel reflection. When the depth of the graded layer is roughly equal to the wavelength of light in silicon (about one-quarter the wavelength in vacuum) the reflection is reduced to 5%; deeper grades produce even blacker silicon.[5] For low reflectivity, the nanoscale features producing the index graded layer must be smaller than the wavelength of the incident light to avoid scattering.[5]

Applications

The unusual optical characteristics, combined with the semiconducting properties of silicon make this material interesting for sensor applications. Potential applications include:[6]

Production

Reactive-ion etching

Scanning electron micrograph of a single "needle" of black silicon, produced by RIE (ASE process)

In semiconductor technology, reactive-ion etching (RIE) is a standard procedure for producing trenches and holes with a depth of up to several hundred micrometres 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 SiO
2, 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.[16] The process can be set so that a million needles are formed on an area of one square millimeter.[12]

Mazur's method

In 1999, a Harvard University group developed a process in which black silicon was produced by irradiating silicon with femtosecond laser pulses.[17] 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 absorption that extends to the infrared range, below the band gap of silicon, including wavelengths for which ordinary silicon is transparent. sulfur atoms are forced to the silicon surface, creating a structure with a lower band gap and therefore the ability to absorb longer wavelengths.

Black silicon made without special gas ambient - laboratory LP3-CNRS

Similar surface modification can be achieved in vacuum using the same type of laser and laser processing conditions. In this case, the individual silicon cones lack sharp tips (see image). The reflectivity of such a micro-structured surface is very low, 3-14% in the spectral range 350–1150 nm.[18] Such reduction in reflectivity is contributed by the cone geometry, which increases the light internal reflections between them. Hence, the possibility of light absorption is increased. The gain in absorption achieved by fs laser texturization was superior to that achieved by using an alkaline chemical etch method,[19] which is a standard industrial approach for surface texturing of mono-crystalline silicon wafers in solar cell manufacturing. Such surface modification is independent of local crystalline orientation. A uniform texturing effect can be achieved across the surface of a multi-crystalline silicon wafer. The very steep angles lower the reflection to near zero and also increase the probability of recombination, keeping it from use in solar cells.

Nanopores

When a mix of copper nitrate, phosphorous acid, hydrogen fluoride and water are applied to a silicon wafer, the phosphorous acid reduction reduces the copper ions to copper nanoparticles. The nanoparticles attract electrons from the wafer’s surface, oxidizing it and allowing the hydrogen fluoride to burn inverted pyramid-shaped nanopores into the silicon. The process produced pores as small as 590 nm that let through more than 99% of light.[20]

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.[21][22]

A group at the National Renewable Energy Laboratory reported black silicon solar cells with 18.2% efficiency.[14] This black silicon anti-reflective surface was formed by a metal-assisted etch process using nano particles of silver. In May 2015, researchers from Finland's Aalto University, working with researchers from Universitat Politècnica de Catalunya announced they had created black silicon solar cells with 22.1% efficiency[23][24] by applying a thin passivating film on the nanostructures by Atomic Layer Deposition, and by integrating all metal contacts on the back side of the cell.

A team led by Elena Ivanova at Swinburne University of Technology in Melbourne discovered in 2012[25] that cicada wings were potent killers of Pseudomonas aeruginosa, an opportunist germ that also infects humans and is becoming resistant to antibiotics. The effect came from regularly-spaced "nanopillars" on which bacteria were sliced to shreds as they settled on the surface.

Both cicada wings and black silicon were put through their paces in a lab, and both were bactericidal. Smooth to human touch, the surfaces destroyed Gram-negative and Gram-positive bacteria, as well as bacterial spores.

The three targeted bacterial species P. aeruginosa, Staphylococcus aureus and Bacillus subtilis, a wide-ranging soil germ that is a cousin of anthrax.

The killing rate was 450,000 bacteria per square centimetre per minute over the first three hours of exposure or 810 times the minimum dose needed to infect a person with S. aureus, and 77,400 times that of P. aeruginosa.

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): 115–120. Bibcode:1995JMiMi...5..115J. doi:10.1088/0960-1317/5/2/015.
  2. 1 2 3 Black Silicon as a functional layer of the micro-system technology
  3. 1 2 Liu, Xiaogang; Coxon, Paul; Peters, Marius; Hoex, Bram; Cole, Jacqueline; Fray, Derek (2014). "Black silicon: fabrication methods, properties and solar energy applications". Energy & Environmental Science 7 (10): 3223–3263. doi:10.1039/C4EE01152J.
  4. ↑ C. Tuck Choy (1999). Effective Medium Theory: Principles and Applications. Oxford University Press. ISBN 0-19-851892-7.
  5. 1 2 H.M. Branz, V.E. Yost, S. Ward, B. To, K. Jones, and P. Stradins, "Nanostructured black silicon and the optical reflectance of graded-density surfaces," Appl. Phys. Lett., vol 94, pp. 231121-3, 2009
  6. ↑ Carsten Meyer: "Black Silicon: sensor material of the future?" Heise Online. 5 February 2009
  7. ↑ Koynov, Svetoslav; Brandt, Martin S.; Stutzmann, Martin (2006). "Black nonreflecting silicon surfaces for solar cells" (PDF). Applied Physics Letters 88 (20): 203107. Bibcode:2006ApPhL..88t3107K. doi:10.1063/1.2204573.
  8. ↑ Koynov, Svetoslav; Brandt, Martin S.; Stutzmann, Martin (2007). "Black multi-crystalline silicon solar cells" (PDF). Physica status solidi (RRL) – Rapid Research Letters 1 (2): R53. Bibcode:2007PSSRR...1R..53K. doi:10.1002/pssr.200600064.
  9. ↑ Gail Overton: TECHNOLOGY:-Black-silicon-emits terahertz-radiation-Teraherz Technology: Black silicon emits terahertz radiation. In:Laser Focus World, 2008
  10. ↑ Cheng-Hsien Liu: Formation of Silicon nanopores and Nanopillars by a Maskless Deep Reactive Ion Etching Process, 11 Nov. 2008
  11. ↑ Zhiyong Xiao; et al. (2007). "TRANSDUCERS 2007 - 2007 International Solid-State Sensors, Actuators and Microsystems Conference—Formation of Silicon Nanopores and Nanopillars by a Maskless Deep Reactive Ion Etching Process": 89–92. doi:10.1109/SENSOR.2007.4300078. ISBN 1-4244-0841-5.
  12. 1 2 Martin Schaefer: Velcro in miniature - "silicon grass holds together micro-components" In: wissenschaft.de. 21 June 2006.
  13. ↑ Black Silicon Comes Back - And Cheaper than Ever, 7 September 2010
  14. 1 2 J. Oh, H.-C. Yuan, and H.M. Branz, "Carrier recombination mechanisms in high surface area nanostructured solar cells by study of 18.2%-efficient black silicon solar cells," Nature Nanotechology, vol. 7, pp. 743-8, 2012
  15. ↑ "Black silicon slices and dices bacteria". Gizmag.com. Retrieved 2013-11-29.
  16. ↑ Mike Stubenrauch, Martin Hoffmann, Siliziumtiefätzen (DRIE), 2006
  17. ↑ William J. Cromie arises:Black Silicon, A New Way To Trap Light.In:Harvard Gazette.9 December 1999, accessed on 16 February 2009.
  18. ↑ 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.
  19. ↑ 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.
  20. ↑ Williams, Mike (2014-06-18). "One step to solar cell efficiency". Rdmag.com. Retrieved 2014-06-22.
  21. ↑ Wade Roush: "SiOnyx Brings "Black Silicon" into the Light; Material Could upend Solar, Imaging Industries". In: Xconomy. 10 December 2008
  22. ↑ 'Black Silicon' A new type of silicon promises cheaper, more-sensitive light detectors, Technology Review Online. 29 October 2008
  23. ↑ http://www.sciencedaily.com/releases/2015/05/150518121440.htm
  24. ↑ http://dx.doi.org/10.1038/nnano.2015.89
  25. ↑ Elena P. Ivanova, Jafar Hasan, Hayden K. Web1, Vi Khanh Truon1, Gregory S. Watson, Jolanta A. Watson, Vladimir A. Baulin, Sergey Pogodin, James Y. Wang, Mark J. Tobi6, Christian Löbbe, Russell J. Crawford (20 August 2012). "Natural Bactericidal Surfaces: Mechanical Rupture of Pseudomonas aeruginosa Cells by Cicada Wings". Small 8 (17): 2489–2494. doi:10.1002/smll.201200528. PMID 22674670.

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