Structural and optical property tailoring of black silicon with fs-laser pulses
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Structural and optical property tailoring of black silicon with fs-laser pulses S. Kontermann1, A. L. Baumann1, T. Gimpel2, K.M. Guenther2, A. Ruibys1, Ulrike Willer2, and W. Schade1,2 1 Fraunhofer Heinrich Hertz Institute, EnergieCampus, Am Stollen 19B, 38640 Goslar, Germany 2 Clausthal University of Technology, Institute of Energy Research and Physical Technologies and EFZN, EnergieCampus, Am Stollen 19A, 38640 Goslar, Germany ABSTRACT Irradiating a planar silicon surface with femtosecond laser pulses under a sulfuric atmosphere creates first a structured surface featuring cones of up to 20 microns in height, and second a 0.1 – 1 m thick layer of multi-crystalline silicon on theses cones containing up to 1 at.% sulfur acting as n-type dopant. Further, the sulfur establishes energy states within the band gap of silicon allowing for the absorption of infrared (IR) light with energies below the band gap energy of silicon. This black silicon process is distinguished by the fact that only one single laser process is required to tailor three material characteristics in on step: the surface structure, the doping and the light absorption. In this work we study structural and optical material characteristics of black silicon. For the first time this work presents properties of black silicon processed with shaped femtosecond laser pulses. Finally, black silicon substrate is used as substrate for manufacturing a black silicon solar cell with a femtosecond laser pulse formed sulfur emitter. For such a black silicon solar cell we achieved a record efficiency of 4.5 % . INTRODUCTION The interaction of laser light with silicon can result in melting, ablation or structuring depending on the laser fluence [1]. When a silicon surface is irradiated with some hundred femtosecond laser pulses of a fluence in the range of 10 kJ/m 2 , cones on the micro to nanometer scale are formed at the surface [2]. This material is called black silicon because of its black appearance. Light is perfectly trapped between the cones when it hits such a surface. Carrying out the laser processing under a sulfur containing atmosphere leads to below band gap absorption due to multiple photon absorption [3]. The absorption characteristics of this black silicon depend on the laser fluence, the number of laser pulses per spot [4], and the parameters of the processing atmosphere [5]. Post processing annealing can significantly alter the absorption spectrum [4]. EXPERIMENTAL As substrates we use Si-(111) and Si (100) Floatzone and Czochralski boron doped ptype silicon with a base resistivity between 1 cm base 6 cm . For laser processing we irradiate the substrate with 1-500 fs laser pulse with a pulse energy of Epulse 100 μJ corresponding to a fluence of F 20 kJ/m 2 for an irradiated laser spot diameter of D 80 m . The laser system is a commercially available Mantis seed laser and a Spitfire amplifier from Spectra Physics with a repetition rate of 10 kHz at a wavelength of 800 nm. The processing chamber is mounted on a x-y table for scanning a
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