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Optical hyperdoping: black silicon

We have developed a novel technique that creates highly doped and structured silicon. Focusing a train of femtosecond laser pulses on silicon wafers in the presence of heavy chalcogens (e.g. S, Se, or Te) dopes a thin layer of silicon at the surface to non-equilibrium levels. This optical hyperdoping process creates black silicon, a highly absorbing surface with extended spectral sensitivity. This material offers new opportunities for silicon-based optoelectronic devices. Black silicon is strongly light-absorbing. During hyperdoping, the polished surface of a silicon wafer is transformed from shiny gray to deep black. In addition to near-unity absorption in the visible, black silicon absorbs over 80 percent of below band-gap, infrared light for wavelengths as long as 2500 nm. This can be used to make photodiodes with remarkable responsivity in both the visible and infrared. It is also possible that this extended absorption range can improve the efficiency of silicon solar cells.
Infrared absorption of femtosecond laser doped silicon: effect of dopant types and thermal treatments, at 5th International Workshop on Crystalline Silicon Solar Cells (Boston, MA), Wednesday, November 2, 2011:
talk_1853.pdf
Doping silicon to concentrations above the metal-insulator transition threshold yields a novel material that has potential for photovoltaic applications. By focusing femtosecond laser (fs-laser) pulses on the surface of a silicon wafer in a sulfur hexafluoride (SF6) environment, silicon is doped with 1% atomic sulfur. Similar concentration of heavy chalcogen dopants (Se and Te) is achieved by fs-laser doping with solid-phase dopant precursors. Fs-laser doped Si:chalcogen systems exhibit near-unity, broadband absorption from the visible to the near infrared (< 0.5 eV, deep below the silicon... Read more about Infrared absorption of femtosecond laser doped silicon: effect of dopant types and thermal treatments
Femtosecond laser texturing and doping of metals and semiconductors for solar harvesting, at SPIE Optics and Photonics (San Diego, CA), Thursday, August 16, 2012:
talk_1977.pdf
Shining intense, ultrashort laser pulses on the surface of a crystalline silicon wafer drastically changes the optical, material and electronic properties of the wafer. The resulting textured surface is highly absorbing and looks black to the eye. The properties of this 'black silicon' make it useful for a wide range of commercial devices. In particular, we have been able to fabricate highly-sensitive PIN photodetectors using this material. The sensitivity extends to wavelengths of 1600 nm making them particularly useful for applications in communications and remote sensing.
Black silicon and the quest for intermediate band semiconductors, at Laser-Based Micro and Nano Processing VIII, Photonics West 2014 (San Francisco, CA), Thursday, February 6, 2014:
talk_2198.pdf
Shining intense, ultrashort laser pulses on the surface of a crystalline silicon wafer drastically changes the optical, material and electronic properties of the wafer. The process has two effects: it structures the surface and incorporate dopants into the sample to a concentration highly exceeding the equilibrium solubility limit. This femtosecond laser "hyperdoping technique" enables the fabrication of defect- and bandgap engineered semiconductors, and laser texturing further enhances the optical density through excellent light trapping. Hyperdoped silicon opens the door for novel... Read more about Black silicon and the quest for intermediate band semiconductors
Laser doping and texturing of silicon for advanced optoelectronic devices, at Frontiers in Optics (FiO)/Laser Science (LS) Conference (Rochester, NY), Monday, October 17, 2016:
talk_2641.pdf
Irradiating a semiconductor sample with intense laser pulses in the presence of dopants drastically changes the optical, material, and electronic properties of the sample. The resulting material has applications for photodetectors and, potentially, intermediate-band solar cells.
Fabrication of Micrometer-Sized Conical Field Emitters Using Femtosecond Laser-Assisted Etching of Silicon, at MRS Spring Meeting (San Francisco, CA), Friday, April 20, 2001:
talk_378.pdf
We produce quasi-ordered arrays of sharp, conical microstructures by structuring the surface of a silicon wafer using femtosecond laser-assisted etching. Analysis of the arrays shows high, stable field emission without any further processing. The sharp, micrometer-sized conical structures result from irradiation of a silicon surface with hundreds of femtosecond-laser pulses in an atmosphere of SF6. These conical microstructures have sharp tips with a radius of curvature of about 250 nm and a subtended angle of less than 20°. They are 10–14 µm tall, have tip-to-tip separations of 6–10 µm, and... Read more about Fabrication of Micrometer-Sized Conical Field Emitters Using Femtosecond Laser-Assisted Etching of Silicon
Comparing properties of femtosecond and nanosecond laser-structured silicon, at Materials Research Society Fall Meeting (Boston, MA), Monday, December 2, 2002:
talk_451.pdf
Sharp microcones form on crystalline silicon surfaces upon irradiation with either femtosecond or nanosecond laser pulses in a sulfur hexafluoride environment. While the general shape and aspect ratio of femtosecond and nanosecond laser cones are similar, several features (such as size and position relative to the original surface) suggest that different mechanisms may be involved in the formation of these structures. The microscopic structure and optoelectronic properties of surfaces covered with nanosecond or femtosecond laser cones could therefore differ as well. We compare the optical... Read more about Comparing properties of femtosecond and nanosecond laser-structured silicon
Femtosecond laser doping of silicon, at Photonics West 2007 (San Jose, CA), Wednesday, January 24, 2007:
talk_1291.pdf
Silicon is an abundant, stable, and efficient material for use in photovoltaic devices. However, it is costly to process, and is transparent at wavelengths longer than 1100nm, a spectral region containing 25% of solar energy. The limitations of silicon have spurred significant research into complex heterostructures that capture a greater fraction of sun’s energy. Engineering silicon to extend its effective spectral range, however, might offer a simpler way to increase the efficiency and decrease the cost of silicon-based photovoltaics. We report the creation of a thin, highly absorbing layer... Read more about Femtosecond laser doping of silicon
Optical hyperdoping: Extending silicon's reach, at Jones Seminar, Thayer School of Engineering, Dartmouth University (Hanover, NH), Friday, February 13, 2009:
talk_1444.pdf
Silicon is the world's most widely used semiconductor. As the building block of a photovoltaic cell, silicon offers a combination of stability, efficiency, and manufacturability currently unmatched by any other material. However, as an indirect absorber of light, thick layers of highly-pure, expensive material are required for efficient light absorption and charge collection. Furthermore, silicon does not absorb in the infrared, a spectral region that contains about a quarter of the sun's radiation. In this talk, I will discuss optical hyperdoping, a non-equilibrium laser-doping technique we... Read more about Optical hyperdoping: Extending silicon's reach
Optical Hyperdoping: Transforming Semiconductor Band Structure for Solar Energy Harvesting, at Third-Generation Solar Technologies Multidisciplinary Workshop: Synergistic Chemistry-Materials-Mathematical Sciences Approaches to Addressing Solar Energy Problems (San Francisco, CA), Monday, April 5, 2010:
talk_1613.pdf
Harvesting solar energy at the terawatt scale requires technologies that can be produced inexpensively using Earth-abundant materials. Although technologies built from Earth-abundant materials exist for converting solar energy to electrical energy or chemical energy, none are yet cost-competitive with fossil fuels. Meeting the challenge of harvesting solar energy with Earth-abundant materials such as Si and TiO2 will require transformative approaches to increase efficiency, lower manufacturing cost, and reduce material requirements. While these materials have been widely studied, we bring a... Read more about Optical Hyperdoping: Transforming Semiconductor Band Structure for Solar Energy Harvesting
Applications of femtosecond lasers in materials processing, at Data Storage Institute, NUS (Singapore), Friday, March 30, 2012:
talk_1894.pdf
The intersection of materials research and ultrafast optical science is producing many valuable fundamental scientific results and applications, and the trend is expected to evolve as new and exciting discoveries are made. Femtosecond laser micromachining presents unique capabilities for three-dimensional, material-independent, sub-wavelength processing. At the same time the surface processing of materials permits the creation of novel materials that cannot (yet) be created under other conditions.

In this talk we will discuss how shining intense, ultrashort laser pulses on the surface of...

Read more about Applications of femtosecond lasers in materials processing
Black silicon: A new light absorber for photovoltaic applications, at APS Centennial Meeting 1999 (Atlanta, GA), Tuesday, March 23, 1999:
talk_265.pdf
We demonstrate a new technique for texturing silicon surfaces using femtosecond laser pulses. Sharp micron-sized spikes are created by repeatedly irradiating a silicon surface with femtosecond laser pulses in the presence of SF6. The spikes are highly light-absorbing and enhance the light absorption in silicon close to 100 (increase in photocurrent of more than 60) over flat silicon. Spiked silicon is of potential use as a highly efficient light-absorber for solar cells and photodetectors.
Ultrafast laser microtexturing of silicon for optoelectronic devices, at Photonics West: Commercial and Biomedical Applications of Ultrafast Lasers (San Jose, CA), Thursday, January 24, 2002:
talk_406.pdf
Arrays of sharp, conical microstructures are obtained by texturing the surface of a silicon wafer using femtosecond laser-assisted chemical etching. The one step, maskless texturing process drastically changes the optical and electronic properties of the original silicon wafer. These properties make the textured silicon viable for use in a wide range of commercial devices. Near-unity absorption of light, from visible to infrared wavelengths, offer opportunities for use in optically active devices such as solar cells and detectors. Significant enhancement of below-band-gap photocurrent... Read more about Ultrafast laser microtexturing of silicon for optoelectronic devices
Femtosecond laser-assisted microstructuring of silicon surfaces for novel detector, sensing, and display technologies, at Physics Colloquium, University of Massachusetts Lowell (Lowell, MA), Wednesday, March 10, 2004:
talk_506.pdf
Irridiating silicon surfaces with trains of ultrashort laser pulses in the presence of a sulfur containing gas drastically changes the structure and properties of silicon. The normally smooth and highly reflective surface develops a forest of sharp microscopic spikes. The microstructured surface is highly absorbing even at wavelengths beyond the bandgap of silicon and has many interesting novel applications.

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