This paper reviews a new field of direct femtosecond laser surface nano/microstructuring and its applications. Over the past few years, direct femtosecond laser surface processing has distinguished itself from other conventional laser ablation methods and become one of the best ways to create surface structures at nano- and micro-scales on metals and semiconductors due to its flexibility, simplicity, and controllability in creating various types of nano/microstructures that are suitable for a wide range of applications. Significant advancements were made recently in applying this technique to altering optical properties of metals and semiconductors. As a result, highly absorptive metals and semiconductors were created, dubbed as the “black metals” and “black silicon”. Furthermore, various colors other than black have been created through structural coloring on metals. Direct femtosecond laser processing is also capable of producing novel materials with wetting properties ranging from superhydrophilic to superhydrophobic. In the extreme case, superwicking materials were created that can make liquids run vertically uphill against the gravity over an extended surface area. Though impressive scientific achievements have been made so far, direct femtosecond laser processing is still a young research field and many exciting findings are expected to emerge on its horizon.

Direct femtosecond laser surface nano/microstructuring and its applications

A power conversion efficiency of 22% is achieved in black silicon back-contacted solar cells through passivation of the nanostructured surface by a conformal alumina layer.

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Black silicon solar cells with interdigitated back-contacts achieve 22.1% efficiency

Black silicon (BSi) represents a very active research area in renewable energy materials. The rise of BSi as a focus of study for its fundamental properties and potentially lucrative practical applications is shown by several recent results ranging from solar cells and light-emitting devices to antibacterial coatings and gas-sensors. In this paper, the common BSi fabrication techniques are first reviewed, including electrochemical HF etching, stain etching, metal-assisted chemical etching, reactive ion etching, laser irradiation and the molten salt Fray-Farthing-Chen-Cambridge (FFC-Cambridge) process. The utilization of BSi as an anti-reflection coating in solar cells is then critically examined and appraised, based upon strategies towards higher efficiency renewable solar energy modules. Methods of incorporating BSi in advanced solar cell architectures and the production of ultra-thin and flexible BSi wafers are also surveyed. Particular attention is given to routes leading to passivated BSi surfaces, which are essential for improving the electrical properties of any devices incorporating BSi, with a special focus on atomic layer deposition of Al2O3. Finally, three potential research directions worth exploring for practical solar cell applications are highlighted, namely, encapsulation effects, the development of micro-nano dual-scale BSi, and the incorporation of BSi into thin solar cells. It is intended that this paper will serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in research currently being undertaken for renewable energy applications.

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Black silicon: fabrication methods, properties and solar energy applications

In this study, we create a multifunctional metal surface by producing a hierarchical nano/microstructure with femtosecond laser pulses. The multifunctional surface exhibits combined effects of dramatically enhanced broadband absorption, superhydrophobicity, and self-cleaning. The superhydrophobic effect is demonstrated by a falling water droplet repelled away from a structured surface with 30% of the droplet kinetic energy conserved, while the self-cleaning effect is shown by each water droplet taking away a significant amount of dust particles on the altered surface. The multifunctional surface is useful for light collection and water/dust repelling.

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Multifunctional surfaces produced by femtosecond laser pulses

Lasers provide the ability to accurately deliver large amounts of energy into confined regions of a material in order to achieve a desired response. For opaque materials, this energy is absorbed near the surface, modifying surface chemistry, crystal structure, and/or multiscale morphology without altering the bulk. This chapter covers a brief introduction to the fundamental principles governing laser propagation and absorption as well as the resulting material responses. We then highlight two case studies of improving efficiency in photovoltaic and optoelectronic devices as well as optimizing cell-surface interactions in biological interfaces.

Fundamentals of Laser-Material Interaction and Application to Multiscale Surface Modification

Micro and nano-structuration of silicon by femtosecond laser: Application to silicon photovoltaic cells fabrication

We have prepared absorbing structures for photovoltaic cells with different nano-texturization, obtained by means of a
femtosecond laser, without the use of corrosive gas (i.e. under vacuum). To take in account the 3D structured front
surface, the emitter doping has been realized by using Plasma Immersion Ion Implantation (so-called PULSION). The
results show a photocurrent increase up to 60 % in the laser texturized zones.

Femtosecond laser for black silicon and photovoltaic cells

To achieve the requirements of the 45nm ITRS technology node and beyond, beamline implantation has reached its limit in terms of low energies. Plasma immersion ion implantation (PIII) is thus an alternative doping technique for the formation of ultrashallow junctions for source/drain extension in silicon devices. In this study, the authors present some results obtained on the PIII prototype called PULSION® designed by the IBS French company. In previous works [F. Torregrosa et al., Proceedings of the 14th International Conference on Ion Implant Technology, 2004 (unpublished); Proceedings of the 16th International Conference on Ion Implant Technology, 2006 (unpublished), p. 6], it has been shown that this machine offers the possibility to reach ultralow energy implantations and then to obtain implantation depths of only a few nanometers. One of the main issues is then to highly activate these junctions with a limited diffusion of dopants. Wafers have been implanted by PULSION® with acceleration voltages fr...

Realization of ultrashallow junctions by plasma immersion ion implantation and laser annealing

To achieve the requirements of ITRS nodes < 45nm, beamline implantation is now limited in terms of low energies. Plasma immersion ion implantation (PIII) is thus an alternative doping technique for the formation of ultra shallow junctions for source/drain extension in silicon devices. In this study, we present some results obtained on the PIII prototype designed by the French company IBS: PULSIONreg. In previous work (7,10), Ultra shallow junctions implanted with BF3 at acceleration voltages down to 20V were shown, with implantation depths of only few nanometers. One of the main issues is then to highly activate these junctions without diffusing too much. Different annealing techniques are known to be potential solutions. Among them: SPER, laser, spike, flash and flash + spike are tested on PULSIONreg implanted samples. The effect of PAI and carbon co-implantation is also studied. The best results obtained using these different annealing technics are presented in this paper. A specific attention is given to the impact of PAI parameters on leakage current.

Ultra shallow junctions fabrication by Plasma Immersion Implantation on PULSION® followed by different annealing processes

It has been recently shown (Mazur et al) (1-7) that a simple way to improve the photocurrent of a silicon-based solar cell is to irradiate the silicon surface with a series of femtosecond laser pulses, in the presence of a sulfur containing gas. This improves the formation of micro- spikes on the silicon surface that strongly reduces the reflectivity of the illuminated surface for the incident solar light (Black Silicon). We have prepared photovoltaic structures with different nano-texturization obtained by means of a femtosecond laser, without the use of corrosive gas (under vacuum). To take in account the 3D structured front surface, the emitter doping has been realized by using Plasma Immersion Ion Implantation (so-called PULSION). The results show a photocurrent increase of about 30 % in the laser textured zones.

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Ultra-short pulsed laser for nano-texturization associated to plasma immersion implantation for 3D shallow doping: Application to silicon photovoltaic structures.

V. Vervisch

Papers

Micro and nano-structuration of silicon by femtosecond laser: Application to silicon photovoltaic cells fabrication

Femtosecond laser for black silicon and photovoltaic cells

Realization of ultrashallow junctions by plasma immersion ion implantation and laser annealing

Ultra shallow junctions fabrication by Plasma Immersion Implantation on PULSION® followed by different annealing processes

Ultra-short pulsed laser for nano-texturization associated to plasma immersion implantation for 3D shallow doping: Application to silicon photovoltaic structures.