Description du sujet de thèse

Multiscale correlation of structural and ferroelectric properties of ScAlN alloys through advanced STEM and X-Ray techniques under electrical/biasing stimuli
The development of new materials and the control of their properties are fundamental pillars of technological research. In electronics— one CRHEA's field of expertise — new nitride-based semiconductors such as ScAlN are emerging as candidates for next-generation devices (RF filters, non-volatile memory devices) due to their ferroelectric property. This property is related to the non-centrosymmetric crystalline structure of the material in which the polarity is determined by the cation- anion atomic bond direction. In ferroelectric materials, the polarity can be modified when applying an external electric field, enabling information to be stored, erased, and reprogrammed in a non-volatile manner at nanometer range limit. The control of the polarity requires the precise position of the anionic and cationic atomic columns to be characterized. To sustain the development of these new materials, versatile characterization techniques are essential to determine their structural properties and correlate them with their electrical behaviour. Scanning transmission electron microscopy (STEM) characterization technique, in particular through the integrated differential phase contrast (iDPC) technique, provides this capability and allows materials analyses at different scales, from the micrometric down to the nanometric scale. Moreover, when coupled with external electrical-biasing stimuli, one can determine real time microstructure dynamical changes with an atomic resolution. In addition, in situ synchrotron X-ray diffraction studies under electrical field pulses allows studying i) the piezoelectric and ferroelectric response of the thin films and ii) the characteristic time of the switching of the ferroelectric domains. The expected results will provide insight into the internal stresses of the layers induced by the transition of the ferroelectric domains during biasing.
The proposed subject is part of the BE-SAFE collaborative project, which is funded by the French National Research Agency (ANR) and aims to optimize the composition and growth methods of ScAlN-based alloys. The project seeks to provide a ferroelectric material that can be scaled down to nanometric thicknesses, while meeting the requirements for integration onto an III-nitride electronic platform.

Contexte de travail

The research works addressed in the project will be organized in three distinct parts which will clearly contribute to a fruitful exploration of new fields of application in materials:
1) Investigation of ScAlN films with different Sc compositions and thicknesses using classical TEM techniques;
2) An in situ electrical biasing TEM approach, the main part of the thesis, for studying real-time polarization reversal phenomena occurring within ScAlN ferroelectric materials. This characteristic is directly linked to the structural reorganization of the position of the constituent atoms, and observing it requires the use of specific tools such as the STEM-iDPC imaging technique;
3) In situ X-ray diffraction studies performed under synchrotron radiation while electrical pulses are applied. These challenging experiments have already been carried out by the Mechanics of Nano-Objects (MNO) group at the IM2NP laboratory in Marseille on other ferroelectric materials.
All the TEM analyses will be performed using the high-resolution transmission electron microscope Thermo Fischer Spectra 200 which is available at CRHEA. This will enable to observe material microstructural changes up to atomical scale. In addition, we expect that defects like dislocations, and other microstructural features may play a crucial role in polarization behaviour, all of which can be probed with an atomic resolution. For all analyses a focused ion beam (FIB) sample preparation protocol will be used.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

The candidate should have completed an M2 training programme or an engineering course with a solid background in physics, and ideally, in semiconductors. Knowledge of materials science would be highly appreciated. While it is not necessary for the candidate to be already familiar with all these topics and techniques, he/she must be motivated and interested in exploring all these aspects in depth. The PhD student should have good organization skills and team spirit. Experience in programming languages would be advantageous.
The collaborative nature of the project (involving CRHEA and IM2NP) requires the candidate to be available to travel for experimental work carried out at IM2NP in Marseille and at the synchrotron facility (either in Grenoble or in the Paris area but also abroad).