Ultrafast Pulsed Laser Annealing of Pd100-xSix Thin Films

Abstract

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.