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Chalcogenide Phase-Change Materials (PCMs), mainly GeSbTe-based alloys, have already been widely used for optical data storage in DVD-RAM or CD-RW. Thanks to their unique reversible and very fast amorphous to crystalline phase transition which is characterized by an uncommon huge change in optical and electrical properties, PCMs are now extensively studied aiming at developing innovative emerging non-volatile memories such as phase change random access memory (PCRAM) or storage class memories (SCM) in order to replace current dominant Flash memory technology [1]. The interaction of PCMs with a fs light pulse has attracted significant attention due to fundamental interest since the possible non-thermal amorphous↔crystal phase transition could be used as a process to drive the phase change above the thermal “speed limits” [2]. Our experiments address the investigation of ultra-fast phenomena of fundamentals laser-material interaction.
Frequency domain interferometry (FDI) [3] is a pump-probe experiment that gives access to the variation of the refractive index of a material. A pump pulse (25 fs, 800 nm, 1kHz) is used to trigger a phase transition. The probe beam is made of two pulses (120 fs, 532 nm) delayed by 9 ps in our case which are focused on the pump/sample interaction point. The first probe pulse impinges the surface of the sample before the pump pulse, and is thus reflected on the unperturbed material, while the second one that arrives after the pump pulse, is reflected on the pump-heated material. Both pulses are then sent in a spectrometer where they interfere in the frequency domain. The intensity variation and phase shifts in the interference pattern (right image in the fig. 1) can be used to retrieve variations of the optical constant of the heated material. The interference pattern is simultaneously measured for the S ans P polarization independently.
The samples are amorphous GeSbTe-based thin film deposited by magnetron sputtering in a 200 mm industrial deposition tool at in the LETI clean-rooms. A 10 nm thick SiN capping layer of hwas been coated deposited on top of the GST films in order to prevent surface oxidation.
We will present the results obtained on prototypical PCMs thin films, i.e. Ge2Sb2Te5 and GeTe. Experiments have been conducted in the fluence range (from 17 to 31 mJ/cm2 ) allowing us to trigger the amorphous to crystal phase transition. Dynamics on the sub-ps time scale shows a very rapid switch mainly attributed to the real part of the refractive index. The polarisation resolved FDI permits to foster information on the behaviour of the surface. A clear phase shift is attributed to a contraction, in the nm range, and the sub-ps time scale. The results presented will be discussed and compared to on-going ab-initio simulations.
[1] P. Noé et al., “Phase Change Materials for Non-Volatile Memory devices: From Technological Challenges to Materials Science Issues”, Topical Review in Semicond. Sci. Technol., to be published (2017).
[2] D. Loke et al. “Breaking the Speed Limits of Phase-Change Memory” Science 336, 1566 (2012)
[3] J.P. Geindre et al., “Frequency-domain interferometer for measuring the phase and amplitude of a femtosecond pulse probing a laser-produced plasma” Optics Letters 19, 1997 (1994).
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