ESPADON

Semiconductor quantum dots are promising building blocks for the development of future spintronics and photonics devices. The spin of a confined carrier can be used as a quantum bit, the recombination of electron-hole pairs provides triggered sources of single photons, confined carriers induce a ferromagnetic interaction between magnetic atoms and form a nano-sized magnetic object. Most experimental studies so far have been performed on self-assembled dots in spite of strong limitations in terms of optical selection rules and magnetic anisotropy: these limitations are intrinsic, since they their growth requires the existence of a strong lattice mismatch, so that only a small number of material associations can be exploited, and both their lens-shape and the built-in strain distribution conspire to make the ground hole state a heavy hole state. A more flexible system exists based on tailored quantum dot inserted in nanowires. ESPADON proposes to grow and study (combining experiment and modeling) novel structures based on semiconductor nanowires containing quantum dots with tailored anisotropic properties, controlled by their aspect ratio and elastic strain. The project proposes an in depth evaluation of the novel opportunities these systems offer for the optical manipulation of single spin qubits and for the control of the spin anisotropy of magnetic dots. More precisely, it is based on - A systematic study of strain configurations in dots in nanowires with various aspect ratio and material combinations (III-V and II-VI), and of their effect on electronic properties. The nanowire configuration is particularly flexible for the association of different materials, and for the realization of dots hosting light holes as well as heavy holes, which remains a challenge in other types of dots. It is also particularly flexible to associate several dots in the same nanowire and to combine different mechanisms (magneto-optics, electric field manipulation, transport, etc.) which up to now have been optimized in different configurations (exciton quantum dots, gate quantum dots, photon emission) and appear essential for different steps of the quantum information continuum. - The association of different approaches - magnetic characterization and selection-rules studies – which provides complementary information on the electronic states, and a strong support by modeling with complementary skills developed on the different systems. A collective effort will be mobilized towards an overall description and identification of the opportunities in photonics and spintronics. - A strong interaction between the teams growing and studying III-V and II-VI’s, two classes of materials which appear to complement each other not only by broadening the range of parameters they offer, but also by complementary stages of achievements and knowledge: (1) narrow lines in III-Vs at the state of the art of quantum dots, partly compensated in II-VIs by a stronger Coulomb interaction allowing to achieve single photon emission at room temperature, and (2) carrier induced ferromagnetism observed in both systems with a higher Curie temperature in GaAs but formidable difficulties to push the system to the single nano-object limit. For instance, elaborate the corresponding III-V nanowires remains a challenge while it has been already demonstrated with II-VIs.