Gallium Nitride (GaN) based materials have emerged as the leading technology for a wide range of optoelectronic applications in the visible range. In particular, blue emitting lasers and light emitting diodes (LEDs) have led to the development of widely used “white LEDs” and “Blu-Ray” technologies. A next step, which represents one of the most important challenges for nitride LEDs, is to replicate in the ultra-violet (UV) range, i.e. below 360 nm, the performances obtained in the blue. Indeed, AlxGa1-xN materials, which allow covering an emission spectrum between 360 nm and 200 nm, are well adapted to become the next technology in the UV, in replacement of the mercury-vapour lamps, which are hindered by environmental issues (toxicity, hazardous waste) and technological limitations (large size equipments, low efficiency and lifetime, etc.). However, the internal quantum efficiency (IQE) of UV LEDs – which is given by the product of the radiative recombination efficiency (RE) and the injection efficiency (IE) - is rapidly decreasing when going towards shorter wavelengths. Therefore, the general objective of NANOGANUV project is to develop alternative solutions to the current technologies and designs, adopted by most R&D laboratories, by focusing on two major key elements of AlxGa1-xN LED structures: - 1) the active region (on which depends the RE); - 2) the p-type region (on which depends the IE). In order to address these locks, different approaches will be developed. Presently, RE and EI are limited in AlxGa1-xN materials by high defect densities and an impediment to the doping, respectively. Regarding the doping issue, which is due to the dopant ionization energy increase with Al content, the main lock concerns the case of p-type doping. Three paths will be followed to improve the IQE: - 1) the use of quantum dots (QDs), grown by Molecular Beam Epitaxy (MBE, the most mature epitaxial technique for QD fabrication). Indeed, owing to the spatial confinement of carriers in 3 dimensions (D) instead of the 1D spatial confinement in the case of quantum wells, QDs strongly improve the RE by reducing the influence of defects; - 2) the optimization of Mg doping in AlxGa1-xN layers by MBE, for which p-type GaN doped layers have been demonstrated with the largest hole concentrations (close to 10^19 cm^-3). In order to reach the level of optimized doping conditions and associated electrical characteristics, the fabrication of a high-stability dopant evaporation cell dedicated to Mg will also be developed; - 3) the use of AlN bulk substrates and the development of a specific high-temperature growth furnace to improve the structural quality of AlxGa1-xN. These two approaches will allow determining the potential of MBE to reach high-quality AlxGa1-xN layers. The overall ambition is to develop a novel route towards the fabrication of efficient UV LEDs, by using alternative scientific and technical solutions at both the nanoscale level, i.e. involving QD modelling, fabrication and quantum engineering, and the micro/macroscopic level i.e. investigating optical and transport properties, to identify, design and assemble the building blocks for the fabrication of QD-based UV sources. The final targets are to design and fabricate UV LEDs operating in the 260 – 360 nm spectral region. This large UV range (from UV-A to UV-C regions) should allow addressing a wide range of applications, from UV curing and counterfeit analysis to medical phototherapy, water and air purification. At the end of the project, devices presenting the best performances will be further processed in LED packages with the aim of performing a series of tests on experimental workbenches by companies specialized in LED testing for UV applications (which will be defined by the specific UV region covered by the LED prototypes).

The project aims at growing germanene, the germanium equivalent of graphene, and study the physics of Dirac fermions in this two-dimensional (2D) material. Indeed, germanene departs from conventional 2D electrons systems and graphene by a buckled atomic structure and a significant spin orbit coupling. It should thus form a rich playground for fundamental studies in low-dimensional physics. Based on the expertise recently gained with the growth of germanene on Al(111) by partners of this project, we want to explore the growth of van der Waals heterostructures, consisting of germanene and 2D layered materials, that allow to minimize the interaction between germanene and these supporting materials. For that purpose, our consortium will rely on state of the art in depth characterization tools at the nanoscale: synchrotron radiation, scanning probe microscopy at low temperature with multiple tips and time-resolved spectroscopy capability. Our analysis based on versatile multi-physical characterization will be compared with calculations performed in the framework of the density functional theory, highlighting the impact of the atomic arrangement on the band structure of germanene and how the nature of the substrate might perturb the structural and electronic properties of this remarkable sheet of Ge atoms. Relevant to this project will be the measurement of the Dirac cone hallmark, the band gap, the carrier mobility and the charge transfer from the underlying layer. Also, we will strive to demonstrate the existence of the quantum spin Hall effect, that is expected due to the substantial spin-orbit coupling in germanene. Of particular interest is the study of defects and lattice deformations, that opens the door to topological transitions, like the Kekulé distortion, causing the attachment of mass to Dirac Fermions. Because of the anticipated poor resistance of germanene to ambient conditions, what would severely limit a deeper characterization and prevent its use in spin/opto-electronic applications, efforts will also be devoted to encapsulate germanene. We want to achieve the growth of germanene on Al(111) ultra-thin films on silicon, followed by the removal of the Si parent substrate and the oxidation of the Al layer, and, to protect the top face of germanene with 2D layered materials transferred in ultra-high vacuum. These schemes will take place along with innovations in instrumentations, in particular Raman spectroscopy in ultra-high vacuum that is the tool of choice for fingerprinting 2D materials. French companies that are involved in the Equipex and Labex investment awards of two of the partners will benefit from transfers of know-how in advanced instrumentations. Progress in the field of the synthesis of germanene, in the understanding of the physics of this material and in the design of dedicated tools will be key to turn germanene into practical technologies at the end of the project.

Quantum optomechanics and electromechanics is a fast growing field with promising applications in quantum information. Recently non-classical mechanical states have been realized. However, full control of a quantum mechanical mode, which is necessary for successful quantum information processing based on electromechanical systems, has yet to be demonstrated. This project aims to develop a novel quantum electromechanical device capable of obtaining full quantum control of a macroscopic mechanical resonator by integrating a phononic microcavity with a superconducting transmon qubit. We expect to achieve a very strong qubit-phonon coupling coefficient that will allow the realization of any quantum unitary operation on the phononic mode. This will have important applications in quantum information and quantum sensing. In addition this project opens the route to test the relevance of the quantum mechanics to the macroscopic world.

RF signals are everywhere in today’s connected society. Surface Acoustic Wave (SAW) filters are widely used to distinguish between signals at different frequencies. Unfortunately, the performance of SAW filters drops above 5 GHz. Thin films of epitaxial LiNbO3 on sapphire host “guided” AW. These modes comply with the demand for higher frequency and higher efficiency. Unfortunately, despite the success of AW technology and its frequency progresses, two limitations remain inherent to AWs, specifically: (i) The absence of tunability once the geometry and material are defined. If several frequency bands are needed in an application, this requires several AW devices. (ii) The absence of non-reciprocity of acoustical wave propagation. In AW devices, the energy flows as easily in the forward and in the backward directions. The input of any AW device cannot be isolated from the influence of its output, as would be desirable for information processing purposes. Spin waves (SWs) are the eigenexcitations of the magnetization. They display rich linear and nonlinear physics and they offer the same miniaturization capability as acoustical waves. The dispersion relation of SWs can be engineered by material and geometry, and later adjusted finely by magnetic fields or spin-torque effects; Upon proper design, SWs can be strongly non-reciprocal, i.e. they propagate differently in opposite directions. The central hypothesis of our research is that coupling AWs with SWs is a route to overcome the intrinsic limitations plaguing acoustic wave technology: by researching at the interface between material science, magnetism, acoustics and microwave engineering, the objective of MAXSAW is to use specific features of spin-waves (SW) –tunability and non-reciprocity– to add new capabilities to state-of-the-art LiNbO3 AW-based filters. We will harness the ability to engineer the dispersion laws of both SW and guided AW to achieve tangential nesting of the propagation characteristics of AW and SW, i.e. match their frequency, wavevector, and group velocities. This last (novel) point, supplemented by the high confinement of the acoustical energy near the interface with the coupled spin-wave medium, ensures that even if the SW-AW coupling (i.e. the magneto-elasticity) is weak, truly magneto-elastic resonance with strong hybridized character can be harnessed and confer non-reciprocity and tunability to the wave propagating medium as well as to dedicated transducers. The end goal of MAXSAW is to demonstrate new rf components with unprecedented attributes: this includes adjustable delay lines, compact broadband isolators, and frequency-tunable filters all potentially perfectly adapted for 5G standards, that may offer valorization opportunities for us. To achieve its goals, MAXSAW comprises 4 technical work packages: WP1 defines the propagation medium for the hybrid waves by enhancing the magneto-elastic cooperativity. WP2 is devoted to the making of the propagation medium, including the acoustical materials growth and the customization of the magnetic materials. WP3 develops augmented transducers matched to the propagating medium to best benefit from the medium developments. Finally, the WP4 is the demonstration of novel rf devices that harness hybrid AW-SWs in the strong coupling regime. To demonstrate its objectives, the consortium shall build upon the expertise in state-of-the-art acoustic wave devices (FEMTO-ST, team of Pr. Bartasyte), spin-wave dynamics (C2N, team of Thibaut Devolder, coordinator), their mutual coupling (INSP, team of Laura Thevenard) and optimized non-reciprocal magnetic materials (CEA-SPEC, team of Grégoire de Loubens), complemented by the support of Frec|n|sys as industrial subcontractor.

The DYNAMELT project proposes an experimental and numerical study of the melting dynamics in multiphase, eutectic and peritectic alloys (multiphase melting). It aims at a first comprehensive investigation of pattern formation and microstructure selection during melting and melting/solidification (M/S) processes involving two solid phases. By combining experimental and numerical methods, it takes advantage of the winning methodological strategy in solidification science. Fundamental and engineering-oriented aspects of interdisciplinary interest for nonlinear physics and materials science will be addressed. Main objectives are 1-to unveil elementary mechanisms during early stages of multiphase melting, 2-to analyse coupled-melting patterns in peritectic and eutectic alloys, and, on this new basis, 3- to cast light on the memory effects during M/S cycles. Despite an apparent similarity, asymmetries exist between melting and solidification concerning nucleation and migration of interfaces, as well as solute redistribution. One striking example is the markedly low chemical diffusivity in solid phases that causes the freezing of microsegregation patterns. These inhomogeneous microstructures affect the partial melting dynamics and M/S cycles, and thus change materials properties on a large scale. The project addresses multiphase melting by controlled experiments employing temperature gradients, thin films as well as bulk samples with tailored microstructures, along with phase-field numerical simulations considering moving boundary conditions and diffusive couplings of interface motion on the relevant scaling lengths. Focus will be made on in situ diagnostics to investigate hitherto unresolved dynamic processes occurring during melting of multiphase alloys. The work program is based on a well-established expertise of the French-German consortium in the science of solidification and melting, with synergic interaction between experiments and modeling.