Mixing periodic topographies and structural patterns on silicon surfaces mediated by ultrafast photoexcited charge carriers

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Mixing periodic topographies and structural patterns on silicon surfaces mediated by ultrafast photoexcited charge carriers

Jean-Philippe Colombier, Anton Rudenko, Elena Silaeva, Hao Zhang, Xxx Sedao, Emile Bévillon, Stéphanie Reynaud, Claire Maurice, Florent Pigeon, Florence Garrelie, and Razvan Stoian

Phys. Rev. Research 2, 043080 – Published 15 October 2020

Abstract

Ultrafast laser irradiation of silicon can significantly modify charge densities and optical indices, impacting the formation and the development of nanoscale-arranged periodic structures. Photoexcitation degree as well as thermodynamic, hydrodynamic, and structural aspects are reported for crossed orientation of laser-induced periodic surface structures generated on single-crystal silicon after multiple-pulse femtosecond laser irradiation. The periodic topography and microstructure generated by light scattering on surface nanoroughness were characterized to gain insights into the regime of photoexcitation, subsequent thermodynamic conditions, and inhomogeneous energy deposition related to periodic nanostructure formation and growth. A generated free-carrier density around $(3 \pm 2) \times 10^{21} {cm}^{- 3}$ is estimated from time-resolved ellipsometry and supported by time-dependent density-functional theory calculations. A finite-difference time-domain solution of the far-field interference of the surface scattered light and the refracted laser wave confirms the periodically crossed energy deposition for this excitation degree. The interference process does not necessarily involve surface plasmon polaritons, and quasicylindrical evanescent waves are identified as plausible scattered waves requiring less restrictive conditions of photoexcitation. Ab initio calculations are also performed to evaluate the transient state of silicon under strong electron-phonon nonequilibrium upon fs laser irradiation. For the reached excitation degree, an electron temperature up to 8000 K is deduced, supporting local amorphization processes observed as a result of high mechanical stress and quenching rates. Ab initio combined with electromagnetic calculations agree well with the results of topography and structural characterization.

Received 12 June 2020
Revised 15 September 2020
Accepted 22 September 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.043080

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Carrier generation & recombination Density functional theory Light-matter interaction Ultrafast phenomena

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Authors & Affiliations

Jean-Philippe Colombier ^1,*, Anton Rudenko ¹, Elena Silaeva ¹, Hao Zhang¹, Xxx Sedao ¹, Emile Bévillon¹, Stéphanie Reynaud¹, Claire Maurice², Florent Pigeon¹, Florence Garrelie¹, and Razvan Stoian¹

¹Univ Lyon, UJM-St-Etienne, CNRS, Institute of Optics Graduate School, Laboratoire Hubert Curien UMR 5516, F-42023 Saint-Etienne, France
²Ecole Nationale Supérieure des Mines de Saint-Etienne, Laboratoire Georges Friedel, CNRS, UMR5307, 42023 St-Etienne, France

^*Email address: jean.philippe.colombier@univ-st-etienne.fr

Article Text

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Vol. 2, Iss. 4 — October - December 2020

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Images

Figure 1
A site on silicon irradiated by 13 laser pulses of 60 fs at $F_{peak} = 0.27 J / {cm}^{2}$ . (a) SEM image reveals typical features including LSFL, HSFL, ablation crater, and flat zone with contrast. The laser polarization is indicated by the arrow in the upper-right corner. A cross-section lamella (indicated by dashed line AB) is extracted and shown in (e). (b) EBSD map ( $IPF + BS$ ) shows crystalline state and laser-induced lattice damage. The lower-right corner inset is cube representation of the crystal orientation. (c) AFM topography of the laser impact. Segments AB and CD are indicative of 2D profiles extraction along LSFL and HSFL. The 2D profiles are shown as insets. (d) 2D-Fourier transform of the SEM image with axis normalized by the free-space wave number $k_{0} = 2 π / λ$ . The dashed circles represent $| k | = Re (\tilde{n}) k_{0}$ and $| k | = k_{0}$ . (e) STEM images of the cross section illustrate the entire cross section at low magnification and reveal distribution of amorphous silicon. The selected area (indicated by the dashed rectangle) is further magnified and shown below in (f). The carbon and platinum are protective layers deposited before FIB sectioning.
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Figure 2
Theoretical reflectivity of silicon for the pump and probes incident angles ( $0^{\circ}, 65 . 8^{\circ}, 27 . 1^{\circ}$ , respectively, as a function of electron density. Insets: Time-resolved $p$ reflectivity changes at two probe angles on silicon ( $27 . 1^{\circ}$ and $65 . 8^{\circ}$ ) for two fluences, corresponding to two excitation degrees.
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Figure 3
Evolution of excited electron density in silicon illuminated with a laser pulse of $τ_{f w h m} = 60$ fs and fluence of 0.25 J/ ${cm}^{2}$ (black solid line) and 0.15 J/ ${cm}^{^{2}}$ (blue solid line). Normalized intensity profile of the laser pulse is shown by a red dashed line.
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Figure 4
(a) Density of states of silicon, with a band gap artificially increased to reach the 1.17 eV experimental value. $N_{e}$ corresponds to the magnified proportion of holes and electrons created during the heating process. (b) Electron density $N_{e}$ and heat capacity $C_{e}$ as a function of the electron temperature. (c) Primitive cell of silicon, with the three basal planes (100), (010), and (001) showing the difference between 8000 K and 0 K electron density spatial distribution. Blue and red gradients indicate areas of respectively decreasing and increasing electron densities.
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Figure 5
(a)–(d) Free-carrier-density evolution below a rough silicon surface irradiated by an ultrashort laser pulse with a pulse duration of 60 fs and a fluence $F = 0.3 J / {cm}^{2}$ . Roughness is introduced by a random distribution of nanocavities with a characteristic length $2 l = 20$ nm and a concentration $C = 1 %$ . At the upper right corner of the figure, the energy absorption by a single nanocavity on the surface with homogeneous free-carrier density $N_{e}$ normalized to $N_{0} = 10^{21} {cm}^{- 3}$ is shown. At the lower left corner, the time corresponding to the current free-carrier density snapshot is indicated on the incident Gaussian pulse. (e) Electric field contributions including quasicylindrical wave (CW, blue), surface plasmon polariton (SPP, red), and total contributions (total, black) scattered by a single surface nanocavity in transverse plane for different inhomogeneous electron densities of the silicon surface. Note that SPP contributions are considered only for the highest carrier density. For nonplasmonic cases, only the total contribution is shown in the absence of surface plasmon wave.
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Figure 6
A site on silicon irradiated by 30 laser pulses of 60 fs at 0.22 J/ ${cm}^{2}$ . Three areas are identified with a threshold marked by a circle. The larger zone encompasses the annealing process area, the intermediate zone covers the transient melting area, and the central zone undergoes significant capillary gradients. Scanning electron microscopy (SEM) image reveals HSFL with $Λ_{H S F L} = 400$ nm at the center of the impact and LSFL with $Λ_{L S F L} = 730$ nm, referred to as LSFL I in the melting area and LSFL II in the annular annealing area. The laser polarization is indicated by the arrow in the upper-left corner.
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