Laser & Photonics Reviews

Cover image for Vol. 8 Issue 2

Editor: Katja Paff

Impact Factor: 7.976

ISI Journal Citation Reports © Ranking: 2012: 2/80 (Optics); 8/128 (Physics Applied); 9/68 (Physics Condensed Matter)

Online ISSN: 1863-8899

Associated Title(s): Advanced Optical Materials, Journal of Biophotonics


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March 27, 2014

As black as can be

As black as can beA novel ultra-black broadband absorber concept based on a needle-like silicon nanostructure has been developed: ultra-black silicon (ub-Si) exhibits an absorptance of more than 99.5% between 350 nm and 2000 nm and about 99.8% between 1000 nm and 1250 nm.

Jena (Germany) – From solar thermal energy conversion over optical spectroscopy to photothermal light detection – these and many more applications need highly efficient light absorbers. Materials with broadband absorptance as high as possible are high on the wish list/highly desirable. From that point of view, there are three interesting materials with broadband absorptance of more than 99%: gold-black coatings, large area carbon nanotube arrays (CNT) and the so-called ultra-black nickel-phosphorus (ub-NiP). Although CNT exhibit the highest absorptance of more than 99.9% around 550 nm, the mostly used material is ub-NiP because of its well established, simple fabrication and its absorptance of up to ∼99.8%. Martin Steglich and his colleagues from Friedrich Schiller University Jena (Germany) now add an attractive absorber concept based on needle-like silicon nanostructure called Black Silicon.
The novel absorber material consists of a 1.6 μm deep Black Silicon nanostructure established on a highly doped silicon substrate by inductively coupled plasma reactive ion etching (ICP-RIE) and an additional dielectric coating (Al2O3) prepared by atomic layer deposition. The fabrication procedure is free of lithography and only consists of a dry etching and an oxide deposition step. The etching relies on a randomly distributed formation of silicon oxyfluoride particles (SiOxFy) on the silicon surface. The new method is reliable, well-repeatable, up-scalable and does not require lithography or any sample pre-treatment. Fabrication of the absorbers is consistent with CMOS standards.
The absorber concept with Black Silicon yields an absorptance beyond 99% in a broad wavelength range between 350 nm and 2250 nm. Particularly and in contrast to other publications, the high absorptance in the wavelength region beyond the silicon bandgap (λ > 1100 nm) is accomplished by applying highly doped, degenerate silicon substrates which exhibit a finite coefficient of light absorption in this range. To stay consistent with the term ultra-black NiP, the concept is similarly referred to as ultra-black silicon (ub-Si).
Improved absorbers also incorporate an additional oxide capping layer on the nanostructures and reach an absorptance of A > 99.5% in the range of 350 to 2000 nm and A ∼ 99.8% between 1000 and 1250 nm.
(Text contributed by K. Maedefessel-Herrmann)

See the original publication: M. Steglich, D. Lehr, S. Ratzsch, T. Käsebier, F. Schrempel, E.-B. Kley, and A. Tünnermann, An ultra-black silicon absorber. Laser Photonics Rev., 8:2. L13-L17 (2014);

March 27, 2014
As black as can be