PhD subjects

The rise of room-temperature applications - nanoscale magnetometry, thermometry, single photon emission, solid-state implementation of qubits - of the negatively charged nitrogen-vacancy NV- center in diamond has motivated a renewed interest in the search, with theoretical methods, of other point defects - in diamond in another material- with a desired property for quantum application, e.g. a bright photoluminescence adna long coherence time of the spin ground state.

However, the fact that the local atomic structure of the defect ground-state or of the excited states is hardly accessible with direct experimental techniques prevents a direct understanding of the thermodynamics stability of defect charge states in the bulk, and of the expected quantum property. This makes the on-demand control of the defect charge state challenging, a problem even more complex near to the surface, because band bending induces a surface modification of the charge state and surface states of ubiquitous defects may be present.

In this Ph.D. work, theoretical methods will be used to predict the defect charge states and explore the effect of the proximity of the surface on the thermodynamic stability and on the spin structure. The objective is threefold: To apply the theoretical framework developed at LSI and predict the defect charge states in bulk; To study changes in the charge state brought by the proximity of the surface; To extend the Hubbard model used to compute the excited states and to account for the electron-lattice interaction in order to compute the zero-phonon line also for the excited states that cannot be predicted by the DFT only. Materials under considerations are carbides -diamond and silicon carbide- and borides - elemental boron and boron compounds. The theoretical method will rely on the Hubbard model developed at LSI in collaboration with IMPMC, and density functional theory (DFT) calculations.

The major argument for promoting the development of spin electronics is the low power dissipation. The aim of the thesis is to determine and optimize the power efficiency of these devices. We focus the study on the power dissipated by two kinds of devices. On the one hand, the devices allowing the reversal of the magnetization of a magnetic layer by a transverse spin current, namely the Spin-Orbit Torque effect (SOT), and on the other hand the devices based on topological materials.

In this context, the definition of useful power - or efficiency - is an open problem. Indeed, the thermodynamics of this type of non-equilibrium system involves cross-effects between the degrees of freedom of the electric charge carriers, those of the spin of these carriers, as well as those of the magnetization of the adjacent layer.

We have developed a variational method in order to establish the stationary state of a Hall bar and the power dissipated in a load circuit. Preliminary measurements have recently validated the prediction in the case of the anomalous Hall effect. The project aims to generalize the study to SOT and topological materials.

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).

The international community is engaged in developing the policy to reduce greenhouse gases (GHGs) emission, in particular carbon dioxide (CO2), in order to reduce the risks associated to the global warming. Consequently, it is very important to find low-cost processes to dissociate and then capture carbon dioxide (CO2), as well as to develop low power, high performance sensors suitable to monitor GHGs reductions.A common and existing method for sensing the concentration of gases is achieved by using semiconducting metal oxides surfaces (MOS) like SnO2, ZnO, and TiO2. Moreover, one route to achieve CO2 dissociation is plasma assisted catalytic decomposition. However, surface defects, and in particular oxygen vacancies and charged trapped therein, play an important role in the (photo)reactivity of MOS. The way optical properties of surfaces are modified by such defects is not completely understood, nor is the additional effect of the presence of the gas. In some models, the importance of charge transfer is also emphasized.

In this Ph.D. work, theoretical methods will be used to model the surface with defects and predict the optical properties. The objective is threefold: To apply the theoretical frameworks developed at LSI for the study of defects to predict the defect charge states in bulk; To study the effect of the surface on the defect stability; to study bulk and surface optical properties, and find out spectroscopic fingerprints of the molecular absorption and dissociation near to the surface. Materials/gas under considerations are oxides like titanium oxide, eventually deposited on a layer on gold, and carbon dioxide. The theoretical method will be the time dependent density functional perturbation theory method (TDDFPT) developed at LSI in collaboration with SISSA, Trieste (Italy).

Ref.: I. Timrov, N. Vast, R. Gebauer, S. Baroni, Computer Physics Communications 196, 460 (2015).

Scientific and technological development of the Terahertz (THz) frequency range, a domain of the electromagnetic spectrum that is located in between microwaves and infrared photonics, is timely and subjected to an intense research activity recently. We have recently developed TeraHertz cavities using a plasmonic mechanism based on the surface plasmons of a doped semiconductor. We have demonstrated the remarkable property of these resonators of being able to confine TeraHertz photons in record volumes of the order of 10-7 times smaller than the diffraction limit. This plasmonic mechanism also allows the functionalization and tunability of the cavities thanks to external parameters such as the electric or magnetic field or the temperature, among others. The aim of this thesis will be to develop THz plasmonic cavities and in particular to study their nonlinear behaviour when subjected to intense THz pulses. The aim will be to TeraHertz frequency conversion based on this nonlinearity.

Today, in the context of climate change and the search for frugal numerical technologies, there is an urgent need to develop a portfolio of thermoelectric materials offering thermal stability, especially for the temperature range 300-400 K, where a large amount of heat is wasted into the environment. Compared to bulk materials, low-dimensional materials, such as nanowires and thin films, offer interesting possibilities for improvement of their thermoelectric properties. In this theoretical project, we aim to describe the coupled dynamics of hot electrons and phonons in low dimensional materials via an approach based on Density Functional Theory and on the solution of coupled Boltzmann transport equations for electrons and phonons. The focus of the project will be to describe main effects of reduced dimensionality and the role of interface and substrate on thermoelectric transport properties in 1D and 2D dimensional materials. The choice of materials is motivated by the potential applicability in the field of next generation energy harvesting, as well as by the ongoing collaborations with experimentalists. Recently, GEEPS researchers have demonstrated that 2D Bi2O2Se allows to achieve a thermoelectric power which is 6-fold larger and closer to room temperature operation than that measured recently by another team. This preliminary result is very encouraging and, at the same time, raises fundamental questions on the physical reasons which led to such outstanding power factor. This is what our theoretical project aims to elucidate.

N-doped oxides and/or oxinitrides constitute a booming class of compounds with a broad spectrum of useable properties and in particular for novel technologies of carbon-free energy production and multifunctional sensors. In this research field the search for new materials is particularly desirable because of unsatisfactory properties of current materials. The insertion of nitrogen in the crystal lattice of an oxide semiconductor allows in principle to modulate its electronic structure and transport properties enabling new functionalities. The production of corresponding single crystalline thin films is highly challenging. In this thesis work, single crystalline oxynitride heterostructures will be grown by atomic plasma-assisted molecular beam epitaxy. The heterostructure will combine two N doped layers: a N doped BaTiO3 will provide ferroelectricity and a heavily doped ferrimagnetic ferrite whose magnetic properties can be modulated using N doping to obtain new artificial multiferroic materials better suited to applications. The resulting structures will be investigated with respect to their ferroelectric and magnetic characteristics as well as their magnetoelectric coupling, as a function of the N doping. These observations will be correlated with a detailed understanding of crystalline and electronic structures.

The student will acquire skills in ultra-high vacuum techniques, molecular beam epitaxy, ferroelectric and magnetic characterizations as well as in state-of-the-art synchrotron radiation techniques.

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.