The impact of intrinsic and of extrinsic defects on the dynamic Ron and off-state leakage current of lateral GaN power devices

The intentional doping of lateral GaN power high electron mobility transistors (HEMTs) with carbon (C) impurities is a common technique to reduce buffer conductivity and increase breakdown voltage. However, this comes at the cost of increased intrinsic defects together with degraded dynamic on-resistance (Ron) and current-collapse effects.
The aim of this project is compare the performance of HEMTs devices containing different quantities of extrinsic defects (such as C atoms) and intrinsic defects (such as dislocations), as a function of growths conditions to guide toward optimized buffer structure with good dynamic Ron and low vertical leakage simultaneously.

Electrical polarisation mapping in ferroelectric devices at the nanoscale

Ferroelectric materials, with their high dielectric constant and spontaneous polarisation, are the subject of intense research in microelectronics. Polarisation is an essential parameter for these materials while its characterization remains mainly limited to the macroscopic scale by conventional electrical methods. To deepen the understanding of these materials, particularly in thin layers, and built new devices, local measurements are essential. This thesis project aims to develop a new methodology to directly map polarisation in devices at nanoscale. By combining the expertise of SPEC in thin film growth and of C2N in nanostructuration and electric measurements, we will elaborate and design a particular geometry of nanostructures allowing the use of operando electronic holography (collaboration with CEMES-CNRS, ANR POLARYS) to quantitatively map the local electrical potential in nanodevices upon application of a voltage.

High-throughput experimentation applied to battery materials

High throughput screening, which has been used for many years in the pharmaceutical field, is emerging as an effective method for accelerating materials discovery and as a new tool for elucidating composition-structure-functional property relationships. It is based on the rapid combinatorial synthesis of a large number of samples of different compositions, combined with rapid and automated physico-chemical characterisation using a variety of techniques. It is usefully complemented by appropriate data processing.
Such a methodology, adapted to lithium battery materials, has recently been developed at CEA Tech. It is based, on the one hand, on the combinatorial synthesis of materials synthesised in the form of thin films by magnetron cathode co-sputtering and, on the other hand, on the mapping of the thickness (profilometry), elemental composition (EDS, LIBS), structure (µ-DRX, Raman) and electr(ochim)ical properties of libraries of materials (~100) deposited on a wafer. In the first phase, the main tools were established through the study of Li(Si,P)ON amorphous solid electrolytes for solid state batteries.
The aim of this thesis is to further develop the method so as to enable the study of new classes of battery materials: crystalline electrolytes or glass-ceramics for Li or Na, oxide, sulphides or metal alloys electrode materials. In particular, this will involve taking advantage of our new equipment for mapping physical-chemical properties (X-ray µ-diffraction, Laser-Induced Breakdown Spectroscopy) and establishing a methodology for manufacturing and characterising libraries of thin-film all-solid-state batteries. This tool will be used to establish correlations between process parameters, composition, structure, and electrochemical properties of systems of interest. Part of this work may also involve data processing and programming the characterisation tools.
This work will be carried out in collaboration with researchers from the ICMCB and the CENBG

Modeling and Optimization of 2D Material-Based Field-Effect Transistors: From Multi-Physics Simulations to Atomic-Scale Insights

Field-effect transistors employing 2D materials are emerging as promising candidates due to their superior mobility and atomic thinness. Nonetheless, this technology faces multiple challenges, including minimizing contact resistances, controlling variability, and optimizing short-channel transistors (< 10 nm). At CEA-Leti, a concerted experimental and computational effort is underway to address these issues and propel the development of 2D material-based technologies.

This doctoral research project is situated within this context, aiming to harness multi-physics simulations to evaluate and enhance the performance of 2D material-based FETs by exploring the interplay between technological parameters and device performance. The flexibility in choosing materials and geometric configurations opens the door to pioneering research directions. A pivotal aspect of this work will involve coupling Technology Computer-Aided Design (TCAD) simulations with ab initio methods to achieve a comprehensive understanding of the devices' structural and electronic behaviors at the atomic level.

The project benefits from access to state-of-the-art computational resources and software (Sentaurus, VASP, GPAW, etc.), supported by CEA-Leti's expertise in simulation methodologies and close collaboration with experimental teams. This doctoral endeavor offers a unique opportunity to develop a wide-ranging skill set in electronic device simulation, contributing to the scientific community through presentations at leading international conferences and publications in esteemed journals.

HPC Parallel Integrodifferential Solver for Dislocation Dynamics

Context : Understanding the behavior of metals at high deformation rate [4] (between 104 and 108 s-1) is a huge scientific and technologic challenge. This irreversible (plastic) deformation is caused by linear defects in the crystal lattice : these are called dislocations, which interact via a long-range elastic field and contacts.
Nowadays, the behavior of metals at high deformation rate can only be studied experimentally by laser shocks. Thus, simulation is of paramount importance. Two approaches can be used : molecular dynamics and elastodynamics simulations. This thesis follows the second approache, based on our recent works [1, 2], thanks to which the first complete numerical simulations of the Peierls-Nabarro Equation (PND) [5] was performed. The latter equation describes phenomena at the scale of the dislocation.
PND is a nonlinear integrodifferential equation, with two main difficulties : the non-locality in time and space of the involved operators. We simulated it thanks to an efficient numerical strategy [1] based on [6]. Nevertheless, the current implementation is limited to one CPU –thus forbidding thorough investigations on large-scale systems and on long-term behaviors.

Thesis subject : There are two main objectives :
- Numerics. Based on the algorithmic method of [1], implement a HPC solver (High Performance Computing) for the PND equation, parallel in time and space, with distributed memory.
- Physics. Using the solver developped, investigate crucial points regarding the phenomenology of dislocations in dynamic regime. For exploiting the numerical results, advanced data-processing techniques will be employed, potentially enhanced by resorting to AI techniques.
Depending on the time remaining, the solver might be employed for investigating dynamic fractures [3].

Candidate profile : The proposed subject is multidisciplinary, between scientific computing, mechanics, and data-processing. The candidate shall have a solid background in scientific computing applied to Partial Differential Equations. Mastering C++ with OpenMP and MPI is recommended. Moreover, interest and knowledge in physics –especially continuum mechanics- will be a plus.
The PhD will take place at the CEA/DES/IRESNE/DEC in Cadarache (France), with regular journeys to Paris, for collaboration with CEA/DAM and CEA/DRF.

[1] Pellegrini, Josien, Shock-driven motion and self-organization of dislocations in the dynamical Peierls model, submitted.
[2] Josien, Etude mathématique et numérique de quelques modèles multi-échelles issus de la mécanique des matériaux. Thèse. (2018).
[3] Geubelle, Rice. J. of the Mech. and Phys. of Sol., 43(11), 1791-1824. (1995).
[4] Remington et coll., Metall. Mat. Trans. A 35, 2587 (2004).
[5] Pellegrini, Phys. Rev. B, 81, 2, 024101, (2010).
[6] Lubich & Schädle. SIAM J. on Sci. Comp. 24(1), 161-182. (2002).

Semiconductor perovskites for the future of medical radiography: experimental analysis of doping and link to electrooptical performance

X-rays is the most widely used medical imaging modality for the detection of pathologies, the monitoring of their evolution and during certain surgical procedures.
The objective of this thesis is to study a new semiconductor material based on perovskites for direct X-ray detection. Their use in the form of photoconductive devices in matrix imagers should make it possible to improve the spatial resolution of images and increase the signal, and thus to treat patients better. Prototype X-ray imagers manufactured at the CEA provide better spatial resolution than current systems, but the detector material still needs to be improved.
To this end, the doctoral student, a physicist and experimenter, will study the link between the structural properties of CsPbBr3 layers and the transport properties of charge carriers in these layers. He will then analyse the effect of intrinsic and extrinsic doping of the layers on the dark current, photocurrent and electrical stability of the devices. The results of this thesis will provide a detailed understanding of the mechanisms responsible for the performance of CsPbBr3-based X-ray imagers.

Understanding Dopant Incorporation Mechanisms in Heavily-Doped Semiconductors using combinations of high resolution (S)TEM techniques

Context: There is a need for very highly-doped semiconductor specimens for the continued development of Si/Ge CMOS devices and for doping III-V materials where the ionisation energy leads to a low concentration of carriers. In order to provide these highly doped specimens, new growth and implantion methods are required. These need to be better understood and characterised with nm-scale resolution.
Proposed Subject: We will combine various (scanning) transmission electron microscope (S)TEM techniques, such as electron holography, precession electron diffraction, spectroscopy and high-resolution imaging on the same specimen. Advanced data processing techniques will be developed in order to combine the different maps to provide information about the total dopant concentration, the quantity of dopants on substitutional sites, and the active dopant concentrations. This work will provide methodology to assess the effectiveness of the different processes that are used for doping in advanced CMOS research. This includes FD-SOI 10 nm and below, raised and embedded source and drains, Si:P, SiGe:B, Low Temperature / Coolcube.

Multi-level functionality in ferroelectric, hafnia-based thin films for edge logic and memory

The numerical transition to a more attractive, agile and sustainable economy relies on research on future digital technologies.

Thanks to its non-volatility, CMOS compatibility, scaling and 3D integration potential, emerging memory and logic technology based on ferroelectric hafnia represents a revolution in terms of possible applications. For example, with respect to Flash, resistive or phase change memories, ferroelectric memories are intrinsically low power by several orders of magnitude.

The device at the heart of the project is the FeFET-2. It consists of a ferroelectric capacitor (FeCAP) wired to the gate of a standard CMOS transistor. These devices have excellent endurance, retention and power rating together with the plasticity required for neuromorphic applications in artificial intelligence.

The thesis will use advanced characterization techniques, in particular photoemission spectroscopy and microscopy to establish the links between material properties and the electrical performance of the FeCAPs.

Operando experiments as a function of number of cycles, pulse amplitude and duration will allow exploring correlations between the kinetics of the material properties and the electrical response of the devices.

The thesis work will be carried out in close collaboration with NaMLab (Dresden) and the CEA LETI (Grenoble).

Contribution of metal-semiconductor interfaces to the operation of the latest generation of infrared photodiodes

This thesis concerns the field of cooled infrared detectors used for astrophysical applications. In this field, the DPFT/SMTP (Infrared Laboratory) of CEA-LETI-MINATEC works closely with Lynred, a world leader in the production of high-performance infrared focal planes. In this context, the infrared laboratory is developing new generations of infrared detectors to meet the needs of future products.
One of the current development axes concerns the quality of the p-type semiconductor metal interface. These developments are driven by the increase in the operating temperature of the detectors, as well as by the very strong performance requirements for space applications.
The challenge of this thesis is to contribute to a better understanding of the chemical species present at the interface of interest as a function of different surface treatment types and to link them to the electrical properties of the contact made.
The candidate will join the infrared laboratory, which includes the entire detector production process. He/she will produce these samples using the technological means available in the LETI clean room, in collaboration with experts in the field. He/she will also have access to the necessary characterization tools (SIMS, XPS, AFM…) available on the nano-characterization platform (PFNC) or in the CEA clean room. Finally, he/she will be involved in the electro-optical characterization of the material, in collaboration with the Cooled Infrared Imaging Laboratory (LIR), which specializes in fine material characterization.

Stabilisation of Perovskite photovoltaic devices by passivation with Metal-Organic Frameworks type materials

MOFs are a type of porous organic-inorganic hybrid material with interesting properties in terms of the passivation of defects in the perovskite and its stability, particularly versus light. For example:
- Direct effect of MOF components as passivation agents: Metal ions and organic ligands can passivate defects at the MOF/PK interface.
- Downconversion of incident radiation: Certain metals (such as europium) or ligands (with aromatic groups) can absorb high-energy radiation (typically violet/near-UV), then re-emit this energy in the form of lower-energy radiation or transmit it directly in a non-radiative manner to the perovskite by Förster resonance (or FRET). This protects the perovskite from high-energy photons, and therefore a priori improves light stability, with little energy loss.
The thesis work will focus on
- integrating MOFs into the perovskite layer, either as a surface treatment or as a mixture of suspensions
- Materials studies (in particular advanced studies using XPS and UPS)
- Favrication of single-junction devices and then tandem devices with silicon sub-cells
- Study of lifetime under illumination (continuous, cycling) with associated characterisations (electrical measurements, photoluminescence, etc.).

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