The macroscopic properties of solids are largely related to the geometric structure of the atoms within the solid, which is referred to as a phase. External energy input (by applying pressure, increasing temperature, providing light or electrical energy) can induce a transition from one phase to another, thereby altering the properties of solids. This is a phase transition. Our work focuses on studying phase changes on femtosecond time scales using various sources of ultrafast light: optical lasers, high-order harmonic generation, or X-ray free-electron lasers. We are particularly interested in chalcogenide materials (alloys containing at least one of the elements S, Se, or Te). These materials underpin some of the most innovative concepts in microelectronics: from neuromorphic components to spin-orbitronics. Their operation involves non-equilibrium physics induced by electronic excitation resulting from the absorption of a fs laser pulse, both in the optical [1] and X-ray [2] regimes.
The family of PCM (phase change materials), and more specifically GeSbTe alloys, provides an example of our work. These materials have been widely used in DVD data storage technology. Binary digital data storage (0 or 1) relies on the contrast in optical or electrical properties between the crystalline and amorphous phases. In the case of the simple alloy GeTe, the crystalline form is metallic and the amorphous phase is a semiconductor. A lot of work have been dedicated to understand the phase transition starting form the cristalline phase, but the amorphous phase has been less investigated. Starting from the samples made at the CEA-LETI the effect of a fs laser pulse has been measured using frequency domain interferometry [3]. This technique allows us to retrieve the refractive index of the excited material. Subsequently, ab-initio simulations are employed to model the excited state, enabling the computation of the refractive index for a specific combination of electronic (Te) and ionic (Ti) temperatures. These computed values are subsequently juxtaposed with experimental data. Femtosecond laser irradiation of amorphous GeTe thin film induces a transition towards a distinct amorphous GeTe phase. At the sub-ps timescale, we observe a transient excitation with a structure akin to the liquid phase. Over a longer duration, we witness material contraction of up to 4 nm within 9 ps for fluences exceeding 26 mJ cm−1. At these higher fluences, a liquid phase emerges, whereas at lower fluences, we anticipate a solid–solid transition, as evidenced by the maintained steepness of the interface.
We are employing various techniques to investigate phase transitions and specific processes in materials
– in carbon based material induces by x-ray fs pulses with time-resolved x-ray diffraction [2]
– in metal using time resolved photoemission spectroscopy [4]
– generation of random structures in chalcogenides materials [5]
contact : jerome.gaudin[at]u-bordeaux.fr
Collaborations:
– P. Noé, J. B. Dory, CEA LETI, Grenoble
– J.-Y. Raty, Université de Liège
– A. Levy, Institut des Nano-Sciences de Paris (INSP)-Sorbonne Université
– F. Dorchies, CELIA groupe PETRUX
– Ph. Heinmann, Linac Coherent Light Source (LCLS), Stanford
[1] Noé et al. »Toward ultimate nonvolatile resistive memories: The mechanism behind ovonic threshold switching
revealed. » Science Advance 6 : eaay2830 (2020)
[2] Heimann et al., « Non-thermal structural transformation of diamond driven by x-rays. » Struct. Dyn. 10, 054502 (2023)
[3] Martinez et al. « Sub-Picosecond Non-Equilibrium States in the Amorphous Phase of GeTe Phase-Change Material Thin
Films. » Adv. Mater. 33, 2102721 (2021)
[4] Lévy et al., » Mitigating space charge in time-resolved photoelectron spectroscopy to study laser-heated copper dynamics in the high fluence regime », Phys. Rev. B, accepted (2024)
[5] Martinez et al., « Laser Generation of Sub-Micrometer Wrinkles in a Chalcogenide Glass Film as Physical Unclonable Function. » Adv. Mater. 2020, 32, 2003032″