Black silicon

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Black silicon (or colloquially 'black silicon', English black silicon ) is a surface modification of crystalline silicon . High-energy bombardment with ions or ultra-short laser pulses creates needle-shaped structures on the surface, which greatly reduce the reflection of the substrate. This change was originally observed in the mid-1980s, when it was a negative side effect of reactive ion deep etching (DRIE).

properties

Scanning electron microscope image of black silicon, produced by reactive ion deep etching (ASE process)

Black silicon is a needle-shaped surface structure (with a length> 10 µm and a diameter <1 µm on monocrystalline silicon), which is why the structure is also known as “ silicon grass ” or “ RIE grass ”.

An essential feature is an increased absorption of incident light. Similar to the moth-eye effect (mechanism of certain anti-reflective layers ), the structures significantly reduce the high degree of reflection of the silicon (depending on the wavelength, 20 to 30% for quasi-perpendicular incidence) (to approx. 5%). The reason for this is the formation of a so-called effective medium through the microstructure. It causes a constant transition of the refractive index of the effective medium, so that there is no sharp optical interface at which the light (according to the Fresnel formulas ) can be reflected. Instead, the light is "gently" guided into the material and hardly reflected, which makes the object appear black.

application

The unusual optical properties of the semiconductor also make the material interesting for sensor applications. As of February 2009, the applications are in development. Areas of application are:

  • Image sensors with increased sensitivity
  • Residual light amplifier
  • Thermal imaging cameras
  • Photovoltaics with increased efficiency through increased absorption. Increased long-term stability is to be expected due to the enlarged surface. In January 2012, researchers succeeded in improving the efficiency and simplifying the manufacture of solar cells by means of extremely short laser pulse treatment of black-silicon solar cells .
  • Mechanical contacts and interfaces.

Research is being carried out into further applications in the terahertz , nano-pore and fastening areas.

Production with reactive ion deep etching

Scanning electron microscope image of a single "needle" of black silicon, produced by reactive ion deep etching (ASE process)

In semiconductor technology, reactive ion deep etching is a standard process for the production of trenches and holes (with a depth of up to a few 100 micrometers, sometimes with very high aspect ratios). This is achieved by repeatedly switching between an etching and a passivation step.

When etching, however, small deposits of passivation can remain on the floor and “mask” it. When the process is shifted to passivation, structures to be formed arise that are not removed in the subsequent etching steps. This creates vertical surfaces on which a polymer layer can be deposited. This way, long silicon columns can remain, covered by the deposits from above and the polymer from the sides. The process can be set so that millions of needles can form on a square millimeter.

Manufactured using the Mazur method

In 1999 a group at Harvard University (led by Eric Mazur and James Carey ) developed a process in which black silicon is produced by bombardment with extremely high-energy pulsed femtosecond lasers . The spatial structure is changed by the laser bombardment, and a needle-shaped surface (approx. 300 nm long) is created, which is relatively uniform and easily reproducible.

In the presence of sulfur hexafluoride , a significantly higher amount of sulfur can be incorporated into the silicon during laser irradiation ( doping ), which reduces the band gap and thus changes the electrical and optical properties of the material. Due to the smaller band gap, low-energy light (up to the infrared range) is also sufficient to excite electrons in the conduction band (see photo effect ). By additionally applying a small bias - DC voltage which can sensitivity , and thus the generated current can be increased larger by a factor 100. The reason is that a photon releases many electrons here .

literature

  • Xiaogang Liu, Paul R. Coxon, Marius Peters, Bram Hoex, Jacqueline M. Cole, Derek J. Fray: Black silicon: fabrication methods, properties and solar energy applications . In: Energy Environ. Sci. tape 7 , no. 10 , August 4, 2014, p. 3223-3263 , doi : 10.1039 / C4EE01152J .

Web links

Individual evidence

  1. ↑ H. Jansen, MJ de Boer, R. Legtenberg, MC Elwenspoek: 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 . In: J. Micromech. Microeng . tape 5 , 1995, p. 115-120 , doi : 10.1088 / 0960-1317 / 5/2/015 .
  2. ↑ a b c 'Black Silicon' as a functional layer in microsystem technology. TU Ilmenau, February 2, 2007, accessed on August 7, 2016.
  3. ^ Tuck C. Choy: Effective Medium Theory: Principles and Applications . Oxford University Press, 1999, ISBN 0-19-851892-7 .
  4. ↑ Carsten Meyer: Black Silicon: Sensor Material of the Future? On: Heise Online. February 5, 2009, accessed February 16, 2009.
  5. ↑ Svetoslav Koynov, Martin S. Brandt, Martin Stutzmann: Black nonreflecting silicon surfaces for solar cells. In: Applied Physics Letters. 88, 2006, p. 203107, doi: 10.1063 / 1.2204573 ( wsi.tum.de ( Memento from July 24, 2011 in the Internet Archive ) PDF)
  6. ↑ Svetoslav Koynov, Martin S. Brandt, Martin Stutzmann: Black multi-crystalline silicon solar cells. In: Physica status solidi-rapid research letters 1, No. 2, 2007, pp. R53 – R55, doi: 10.1002 / pssr.200600064 ( wsi.tum.de ( Memento from July 24, 2011 in the Internet Archive ) PDF).
  7. ↑ Solar cells on black silicon - efficiency doubled through new technology ( Memento from July 14, 2014 in the Internet Archive ).
  8. ^ Gail Overton: Terahertz Technology: Black silicon emits terahertz radiation . In: Laser Focus World 2008, accessed February 20, 2009.
  9. ↑ Zhiyong Xiao, Chunhua Feng, PCH Chan, I-Ming Hsing: Formation of Silicon Nanopores and Nanopillars by a Maskless Deep Reactive Ion Etching Process . In: International Solid-State Sensors, Actuators and Microsystems Conference, 2007 . 2007, p. 89-92 , doi : 10.1109 / SENSOR.2007.4300078 .
  10. ↑ a b Martin Schäfer: Velcro fastener in mini format - "silicon grass" holds micro-components together. In: Wissenschaft.de. June 21, 2006, accessed September 8, 2019 .
  11. ↑ Mike Stubenrauch, Martin Hoffmann, Ilona Hirt: Silicon Deep Etching (DRIE)  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. . @1@ 2Template: Toter Link / www.zlw-ima.rwth-aachen.de  [PDF] 2006, p. 31 (presentation).
  12. ^ William J. Cromie: Black Silicon, A New Way To Trap Light ( Memento January 13, 2010 in the Internet Archive ). In: Harvard Gazette. December 9, 1999, accessed February 16, 2009.
  13. ↑ Wade Roush: Xconomy: SiOnyx Brings "Black Silicon" into the Light; Material Could Upend Solar, Imaging Industries . On: Xconomy. Dec 10, 2008, accessed on Feb 16, 2009 (explanation of how it works).
  14. ↑ Prachi Patel-Predd: 'Black Silicon' A new type of silicon promises cheaper, more-sensitive light detectors. . On: Technology Review Online. October 29, 2008, accessed February 16, 2009.