ACTA

Ultrafast Pulsed Laser Annealing of Pd_100-xSi_x Thin Films

O. Liubchenko^a, I. Jacyna^a, T.J. Albert^b, J. Antonowicz^c, M. Chojnacki^a, A.C. Dippel^d, P. Dzięgielewski^c, O. Gutowski^d, D. Klinger^a, Z. Kostera^c, R. Minikayev^a, K. Sokolowski-Tinten^b, W. Zajkowska-Pietrzak^a, R. Sobierajski^a
^aInstitute of Physics Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, Poland
^bFaculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany
^cFaculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland
^dDeutsches Elektronen Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany

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Excitation of materials by ultrashort laser pulses is characterized initially by a strong non-equilibrium between the electronic and lattice degrees of freedom. This is followed by a rapid transfer of the electronic excess energy to the lattice, which leads to heating within a few picoseconds. In thin films deposited on a substrate, the subsequent nanosecond-scale quenching is driven by rapid heat diffusion into the substrate. This non-equilibrium heating-cooling cycle with ultrahigh heating and cooling rates enables access to metastable states that are difficult to reach with conventional techniques. In this work, we report on a comprehensive characterization of structural changes driven by ultrashort pulsed laser annealing in thin Pd_100-xSi_x films (x=0,3,5,10,17%). The combined use of microscopy and X-ray diffraction for post-mortem analysis provides new insights into the composition-dependent crystallization behavior. By analyzing optical images from a series of laser irradiations at varying pulse energies (fluence scan), a two-dimensional fluence map was reconstructed for an arbitrary pulse energy. This enabled a direct correlation between deposited energy density, surface morphology, and structural changes derived from the micro-X-ray diffraction measurements analyzed using the Rietveld method. Above a fluence threshold of 25 mJ/cm², all samples exhibit significant structural modifications - changes of the surface morphology, an increase of the lattice parameters, reduction of strain, grain growth, and reduced amorphous contributions. These observations are consistent with a scenario in which the sample undergoes ultrafast melting, followed by rapid cooling and recrystallization, thereby relaxing towards a lower energy state. Notably, only the face-centered cubic phase of crystalline Pd was observed across all samples and irradiation conditions, in contrast to the equilibrium phase diagram including the Pd-Si compounds. This confirms that ultrafast laser treatment produces metastable states, previously unreported in Pd-Si alloys.

DOI:10.12693/APhysPolA.148.303
topics: Pd-Si alloys, X-ray diffraction (XRD), ultrashort pulsed laser annealing