Advanced characterization of ferroelectric domains in hafnia-based thin films

Hafnium oxide (HfO₂) has reactivated significant interest since the discovery of its ferroelectric (FE) properties in 2011 [1]. Currently, it is one of the most promising materials for emerging low-power non-volatile memory and logic, essential for in-memory computing (IMC) architectures. The working principle is simple; information is stored by leveraging the characteristic switchable spontaneous polarization of a FE layer. In hafnia, this polarization has been ascribed to the presence the metastable polar orthorhombic (o-, Pca2₁) crystallographic phase, which can be formed via doping, mechanical or thermal stress [2]. Hafnia-based memories are particularly appealing for very high-density mass storage (>10 Tbit/in²) because they retain their ferroelectric properties at very low layer thicknesses (< 10 nm), suitable for 3D integration and ultra-low power operation.

The complex domain structures (regions of uniform polarization) in FE materials can be controlled by tailoring their electrical or mechanical boundary conditions. The size of a domain in a hafnia FE layer is of the order of 10-15 nm, which makes their study challenging, as the resolution of most electron microscopy techniques is limited to several tenths of nm. We propose to use a systematic methodology to examine them.

The first stage involves the use piezoelectric force microscopy (PFM) to locally write and subsequently image these artificially written microscopic ferroelectric domains. The resulting phase contrast images will allow us to study the phenomena of charge injection and polarization switching with varying voltage pulse parameters (amplitude, pulse duration) (Figure 1.a).

In the second stage, the PhD student will use Low-Energy Electron Microscopy (LEEM) and PhotoEmission Electron Microscopy (PEEM) to characterize the surface potential (an inherent property of the material) of the domains in different polarization states and as a function of the writing voltage. This will allow to correlate the modulation of the surface potential with the polarization and/or injected charge from the PFM writing stage. We also expect to elucidate the effects of the presence (absence) of a metallic electrode above the ferroelectric layer in modulating the electrostatic properties and hence the ferroelectric response of capacitors (Figure 1.b).

The third stage of the project comprises the study of the above-mentioned mechanisms in real time during ferroelectric switching (Figure 2.a). Both switching pulse length and voltage amplitude are expected to determine the switching speed (latency) (Figure 2.b). This requires the use of advanced time-resolved spectromicroscopy techniques, often relying on the use of synchrotron photon sources for high intensity and resolution. Time-resolved imaging using photoemission spectroscopy (XPS) with a dedicated detection system will allow full characterization of domain switch dynamics under real operating conditions (see [3]).

Besides, by integrating the obtained core-level spectra in a specific region of interest (ROI) we will be able to correlate the ferroelectric properties and domain switching with the oxygen vacancy point defect concentration deduced from the hafnium reduced fraction (Figure 2.c) (see [4,5]).

Funding acquired, selection based on CV and eventual interview

Les équipes de l’IRAMIS mènent des recherches en physique et chimie, au carrefour des mondes académiques et des missions du CEA, ainsi que des enjeux sociétaux et de l’innovation. Ses recherches portent sur les nouvelles technologies pour l’énergie, les technologies quantiques, les nouveaux matériaux, les systèmes complexes et l’interaction rayonnement-matière.

La physique de la matière condensée est étudiée au SPEC depuis ses aspects les plus fondamentaux jusqu’aux applications dans les cas qui s’y prêtent. Les approches sont extrêmement variées et permettent l’exploration de mondes qui vont de l’échelle nano aux objets macroscopiques. Fort de ses publications dans des revues scientifiques spécialisées, le SPEC est aussi un lieu de transfert technologique et de brevets. Le SPEC est impliqué dans de nombreuses collaborations, aussi bien avec le CEA qu’au niveau national et international.

La recherche au LENSIS se concentre sur l’étude de la structure électronique et chimique des surfaces, interfaces et films d’oxydes fonctionnels. Pour ce faire, nous utilisons un large éventail de techniques d’analyse de surface basées sur la photoémission, telles que XPS, HAXPES, ARPES et PEEM, ainsi que des sondes électroniques telles que LEEM. Les matériaux et les dispositifs que nous étudions sont réalisés en collaboration étroite avec nos partenaires de recherche technologique, notamment, le CEA LATI (Grenoble) et NaMLab (Dresde), ainsi que des partenaires indutriels tels que ST Microelectronics.

Le candidat devrait avoir de très bonnes bases en physique de l'état condensé. Des bonnes notes et classement en Master sont nécessaires.Previous experience/skills on electron spectroscopy and/or image processing is not mandatory but useful.

Solid grounding in condensed matter physics. Good academic marks and high ranking in Masters courses required