Oxide-based architectures for electronics and spintronics – Oxitronics

For many decades electronics has been based on the electron charge and built from semiconductors, typically Si and III-Vs. Even though semiconducting nanostructures will certainly be the basis of microelectronics for a few more years, a massive research effort is being made worldwide to find solutions that will ultimately form the basis of information processing/storage devices beyond 2015-2020. On the materials research front, one of the routes currently being investigated focuses on transition metal oxides. Perovskites (ABO3) are especially interesting because they exhibit an exceptionally broad range of functionalities (superconductivity, semiconductivity, ferromagnetism, ferroelectricity, etc) within the same crystalline structure. Furthermore, perovskites present many advantages: (i) as for III-V systems, atomically sharp heterostructures can now be grown ; (ii) they can have extremely high responses to external stimuli (strain, magnetic field, electric field) ; (iii) their interfaces may exhibit exotic properties absent in the bulk, thereby opening a route to a vast terra incognita of novel metamaterials. To explore the potential of a future oxide-based electronics, SrTiO3, a band insulator in its bulk form, is a key material : upon minute n-type doping it can be turned into a metal with a long mean free path and spin-diffusion length, or into a ferroelectric with the application of a few percent of biaxial strain. SrTiO3 is also a model system to study correlated electron physics as upon electron filling the Ti 3d band it turns into a Mott insulator. Furthermore, when combined with other perovskite insulators SrTiO3 is also a good template to create novel interfacial phases. Within this framework, the main goal of the project OXITRONICS is to make use of the many advantages of oxides to fabricate perovskite-based heterostructures in order to inject, manipulate and detect charge and spin currents in three-dimensional (3D) and two-dimensional (2D) electron gases. En route to this goal, strong efforts will be made to obtain a detailed understanding (fabrication conditions, transport properties, atomic and electronic structure) of 3D and 2D electronic gases based on SrTiO3. The first half of the project will be devoted to the physics of engineered 3D electron gases (3DEG) based on SrTiO3 and to the generation of 2D gases (2DEG), either through charge transfer at interfaces between SrTiO3 and LaAlO3, or in ultrahigh-quality electron-doped SrTiO3 thin films. The main effort within this latter task will be focused on the search for definitive indications of a 2D metallic character at SrTiO3/LaAlO3 interfaces. Specific signatures of quantization effects in the growth directions (Shubnikov-de Haas oscillations and quantum Hall effect) will be looked for through magnetotransport experiments at very low temperature and very high magnetic field. The second half of the project will aim at utilizing the 3DEG and 2DEG as channels to transport charge and spin currents. Special care will be taken in the design of these architectures (geometry, injection and detection electrodes). Electric-field control of the charge-based properties of the channel (carrier density, carrier mobility, dimensionality of the gas) will be achieved through different strategies (back-gates, dielectric and ferroelectric gate insulators). Spin-injection and detection will be performed with different types of ferromagnetic contacts (half-metals, spin-filters, etc). Attempts to manipulate the spins in the channel will eventually be carried out through the Hanle effect or via gate effects studied previously. The consortium gathered to meet these objectives consists of four laboratories. A substantial effort will be put on theory in order to orient experimental research. In particular, theoreticians will simulate the influence of parameters like doping, oxygen vacancies, charge-transfer and strain on the properties and the dimensionality of the electron gases. They will also participate in the design and the interpretation of the transport experiments. On the experimental part, growth will be carried out by the coordinating partner, a world-leading group in oxide electronics and spintronics. A strong effort will be put on high-resolution transmission electron microscopy, electron-energy loss spectroscopy, scanning probe microscopy and angle-resolved photoemission spectroscopy, in order to obtain a complete description of the atomic and electronic structure of the 3DEG and 2DEG. Magnetotransport will be another strong component of the project. Measurements at very high field, a key tool to determine the dimensionality and characterize the Fermi surface of the electron gases, will be performed at one of the few available facilities worldwide. Finally, spin-injection devices will be designed, fabricated and characterized by the coordinating partner who has a very strong background on the topic.