Abstract
The structural evolution of a Pt thin film following photo-thermal excitation by 1 ps optical laser pulses was studied with a time resolution of 100 ps over a total time period of 1 ms. Laser pulse fluences below 50 mJ/cm2 were insufficient to relax the residual stress state of the as-prepared film even after 10 000 pulses. In this fluence regime, a rapid initial lattice expansion and a decrease in the lattice coherence length due to ultrafast photo-thermal heating were observed. The lattice expansion reached a maximum, and the coherence length reached a minimum, 100–200 ps after excitation before monotonically decaying back to their initial values in about 1 µs. Laser pulse fluences greater than 50 mJ/cm2 produced irreversible stress relaxation within the first 10 optical pulses. In this regime, the lattice expansion was qualitatively similar to that in the low fluence regime, except that the initial structural state was not recovered. The evolution in the coherence length, however, was more complex. Following an initial decrease similar to that observed at low fluence, the coherence length then increased to a broad maximum greater than the initial value, before recovery.
Highlights
Our interest in time-resolved xray diffraction (TRXRD) studies of irreversible processes, in general,13 and the practical importance of gaining a scitation.org/journal/adv better understanding of irreversible thin film stress relaxation, in particular, provided the primary motivation for the work described in this article
The results of a typical TRXRD experiment in the low and high laser pulse energy regimes are shown in Fig. 2 for the pre-annealed samples
The quantity Δεdiff represents the difference between the measured strain for an as-prepared sample at a given time delay and total number of laser pulses [e.g., the solid red circles in Fig. 4(a)] and the average strain of an annealed sample at the same time delay [dashed red line in Fig. 4(a)] after shifting both to a common origin. These results clearly show that laser fluences less than 50 mJ/cm2 do not produce stress relaxation, while larger laser fluences produce increasing stress relaxation
Summary
Scitation.org/journal/adv better understanding of irreversible thin film stress relaxation, in particular, provided the primary motivation for the work described in this article. After 1200 laser pulses at a fluence of 78 mJ/cm2, the measured values of d111 = 0.226 19(2) nm and d111/111 = 0.226 39(5) nm, corresponding to lattice strains of −0.16% and −0.08%, respectively, revealed a significant stress reduction with respect to the bulk
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