CC BY
8LM-O-12
Silicon surface amorphization and re-crystallization via single
femtosecond laser pulses
C. Florian1'2 *. D. Fischer1, K. Freiberg3, M. Duwe4, M. Sahre1, S. Schneider4, A. Hertwig1, J. Krüger1, M. Rettenmayr3, U. Beck1, A. Undisz5, J. Bonse1^
1- Bundesanstalt für Materialforschung und -prüfung (BAM), Unter den Eichen 87, D-12205 Berlin, Germany 2- Princeton University, 70 Prospect Avenue, NJ-08540, Princeton, USA
3- Friedrich-Schiller-Universität Jena, D-07743 Jena, Germany 4- Accurion GmbH, Stresemannstraße 30, D-37079 Göttingen, Germany 5- Technische Universität Chemnitz, Erfenschlager Straße 73, D-09125 Chemnitz, Germany
camilo.florian@princeton. edu 1joern. bonse@bam. de
Silicon is the material responsible for most of the technological developments during the past century, making it one of the most studied materials along different disciplines. However, there are still unturned stones regarding its superficial re-solidification after femtosecond laser-induced local melting. In this presentation, we report irradiation experiments with single femtosecond pulses (790 nm, 30 fs) with a spatially Gaussian distribution on two different types of silicon with orientations <111> and <100>. The surface modifications were studied in detail via different techniques, including optical microscopy, atomic force microscopy, spectroscopic imaging ellipsometry, energy dispersive X-ray spectroscopy and high-resolution transmission electron microscopy. We quantitatively estimate the resulting radial amorphous layer depth profiles with maximum thicknesses around some tenths of nanometers for fluences in between the melting and ablation thresholds [1].
Fig. 1. General representation of a single fs laser pulse impinging a sample of crystalline silicon (c-Si) from the top. Data from the surface modifications gathered via AFM (among other techniques) allows the detection of different annular disks. Depending on the local laser fluence, different superficial modifications are produced as indicated by the colored semi-disks (bottom). A cross-sectional TEM characterization is included as an inset (right) to illustrate the thickness of the amorphous layer (a-Si) formed. Reproduced from [1]. (CC-BY 4.0 License).
In particular, spectroscopic imaging ellipsometry (SIE) allowed fast data acquisition using multiple wavelengths to provide experimental measurements for calculating the nanometric radial amorphous layer thickness profiles with micrometric lateral resolution based on a thin-film layer model. SIE proved to be capable of detecting and measuring nanometric structural and chemical modifications (oxidation) on the studied laser spots. The accuracy of the SIE-based calculations is verified experimentally by characterizing an in-depth material lamella via high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (STEM-EDX). Foi completeness, we present a mathematical modelling for the melt layer thickness considering different optical absorption processes including one photon absorption, two photon absorption and free-carrier absorption, highlighting the relevance of the latter one in the femtosecond laser-induced melting of silicon.
[1] C. Florian, D. Fischer, K. Freiberg, M. Duwe, M. Sahre, S. Schneider, A. Hertwig, J. Krüger, M. Rettenmayr, U. Beck, A. Undisz, J Bonse, Single Femtosecond Laser-Pulse-Induced Superficial Amorphization and Re-Crystallization of Silicon, Materials, 14, 1651 (2021) https://doi .org/10.3390/ma14071651
