09 Décembre – Thesis defense - Paloma Martinez
11 h Amphi G - Building A29 / University of Bordeaux (Talence campus)
Ultrafast dynamics in chalcogenide phase-change transition materials and glasses.
Chalcogenide are materials made of Te, S or Se alloyed with an electropositive element such as Ge, In or Ag. Key-components in memories with the development of the 3D X-point cross-bar structure by Intel, they are now prospective candidates for future artificial intelligence synaptic devices. Their outstanding optical and electrical properties that make them well-suited for data storage applications.
First, they demonstrate a high optical and electrical contrast between their crystalline and amorphous state : in Germanium Tellurium, a prototypical chalcogenide used in memory devices, the reflectivity of the crystalline state is the double of the amorphous state and several orders of magnitude separate the resistivities of these two phases. Second, the threshold switching phenomenon, similar to a transistor effect, allows a very fast transition, reversible, which can be as fast as hundreds of picoseconds upon electronic excitation of the material using a light or an electrical pulse. This threshold switching occurs in two ways, which define two sub-categories of chalcogenides. In the first case, a permanent (non-volatile) transition is seen between an amorphous and a crystalline phase. This characterizes the so-called 'Phase-Change Memory' (PCM) materials. In the second case, a non-permanent transition is triggered between an initial highly resistive amorphous phase and another amorphous phase of low resistivity, defining the 'Ovonic Threshold Switching' (OTS) materials.
As both processes are triggered by electronic excitation, the interaction of chalcogenide PCM with ultra-short light pulse has attracted significant attention in the hunt for a possible non-thermal transition that would overcome the actual thermal "speed limits" in memory devices.
Thin films of amorphous GeTe, a well-known PCM material, and three selenide chalcogenide glasses (Ge-Sb-Se-N alloys) candidates for OTS applications, were pumped at 800 nm by 30-fs laser pulses in order to trigger out-of-equilibrium states. Frequency-domain interferometry (FDI), a pump-probe technique that allows for a sub-picosecond time resolution and nanometric longitudinal resolution was used to investigate the resulting ultrafast dynamics. This method not only retrieves the evolution of the dielectric properties, but also the surface nature and dynamics. Complementary out-of-equilibrium ab initio molecular dynamics (AIMD) simulations were conducted to get a deeper understanding of the experimental results.
Experiment and simulations reveal that photo-excited GeTe undergoes an amorphous - amorphous transition, with some subtleties on a closer look. We measure a non-thermal transition occuring within 300 fs, leading to an out-of-equilibrium state that is structurally speaking very similar to the liquid phase. On the picosecond timescale, the electron-phonon coupling drives a thermal transition for high irradiation intensity leading to the development of a liquid phase and a shrinkage of the film. This whole dynamics mainly affects the Ge atoms, switching from tetrahedral local sites to octahedral sites.
Chalcogenide glasses respond differently to the laser impingement. First, we see a higher dependency of the dielectric functions to the laser intensity. Second, though a liquid state is also appearing due to thermal effect, this liquid state is found to be very different from the early-times out-of-equilibrium states. However, it is interesting to note that in both GeTe and chalcogenide glasses, the local environment of the Ge atoms tends to octahedral sites upon excitation.
These experimental and theoretical results shed a new light on the optically highly excited states in chalcogenide materials involved in both important processes in non-equilibrium conditions : phase-change materials transitions in memory device and ovonic threshold switching phenomenon induced by static field.