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This work will investigate the crystallization of various Ge-rich GST alloy layers with other alloys elements and compositions provided par STMicroelectronics. For each defined stack, the nucleation and growth of crystalline phases, the phase transition as well as segregation and diffusion phenomena, including the role of the interfaces, will be followed. The physical properties (structural, optical and electrical) will be characterized in situ (i.e. during thermal annealing) using a panel of several original techniques, already used at IM2NP laboratory: combined in situ X-ray diffraction (XRD), X-Ray reflectivity (XRR) and sheet resistance measurements (Rs) as well as spectroscopic ellipsometry (SE). The main work focusses on in situ XRD during thermal annealing to understand the crystallization mechanisms. Such in situ experiments will be conducted using both lab and synchrotron sources.

Different types of experiments will be performed:

• Crystallization temperature and phase identification: in situ XRD will be performed during ramp annealing to measure the crystallization temperature Tx at a fixed ramp rate. The temperature range studied will extend up to the degradation of the thin films. The crystalline structures formed will be tracked as a function of annealing temperature.
• Crystallization kinetics: in situ XRD will be performed during isothermal annealing at several temperatures below Tx. The incubation time before the onset of crystallization will be measured for all crystalline phases of that form. The incubation time of the first crystallized phase will be linked to the annealing temperature, enabling the extraction of the activation energy for crystallization. The crystallization sequence and kinetics of each newly formed crystalline phase will be tracked. The XRD results from both ramp and isothermal annealing will be quantitatively analyzed using Rietveld refinements performed on the full XRD spectrum to identify the crystalline structures, average grain sizes, lattice parameters, and crystallized fractions, etc.). Selection Samples for TEM: For selected temperatures, in situ annealing will be stopped at different annealing durations, notably just before and after the formation of each new crystalline phase. The samples will be kept for the following characterization using scanning transmission electron microscopy (STEM). Such experiments will be performed at CEMES laboratory and by other partners in the frame of the same project. This work will be performed in strong partnership with STMicroelectronics.

Phase Change Random Access Memories (PCRAM) are among the most mature non-volatile emerging memories (NVM): they allow data storage at high programing speed with enhanced endurance compared to today Flash technology , as recently demonstrated by the INTEL OPTANETM. PCRAM are based on the ultrafast (<10 ns) and reversible transition between the amorphous and crystalline states of phase change materials (PCMs) initially used for optical data storage. Data are stored thanks to the high resistivity contrast between these two structural phases of PCMs, the crystalline phase having a low resistance state, and the amorphous one a high resistance state. Both amorphous and crystalline states of PCMs exhibits also very different optical properties, and are stable enough to be used as 0 and 1 states in non-volatile memories. The most promising PCM for NVM is the chalcogenide Ge-rich GexSbyTez (Ge-rich GST) used by STMicroelectronics for embedded automotive applications, requiring high stability under temperature variations . However, critical issues occurring at nanoscale needs further material studies concerning the role of interfaces (nitride, dielectrics, etc..), the alloy composition (phase separation, precipitation, segregation, etc..), the effect of impurities and the impact of such issues on the crystallization kinetics (nucleation and growth). This work aims to closely study the optimized chalcogenides (typically Ge-rich GST with various added alloys element) integrated in STMicrolectronics memory cells during the crystallization process and post-annealing treatments.

The ideal candidate holds a PhD degree in Material or Engineering Sciences, has a solid background in physics, several years of research experience, good communication skills and is proficient in written and spoken English. Knowledge of XRD, deposition processes, PCM and programming skills will be highly appreciated.