Silicon and Germanium Spintronics – SiGeSPIN

The aim of this SiGeSPIN project is to develop silicon and germanium spintronics at room temperature. We focus on silicon and germanium since they are the basic materials used in nowadays microelectronics.
Germanium appears as a key material in spintronics since its strong spin-orbit coupling makes spin manipulation by an electric field possible. In contrast, silicon with a negligible spin-orbit coupling is more suitable to carry spin currents over long distances and may be used in the “spin circuitry” or quantum computing. Thus the fundamental study of spin injection in both silicon and germanium (and more generally their alloys) is a key step in the way to the development of semiconductor spintronics.
The first objective is to efficiently inject spin polarized electrons into these materials. For this purpose, a large effort will be devoted to material and surface science to understand in detail the microscopic mechanisms involved in the electrical injection of spins from a ferromagnet into Si or Ge channels. This represents a real scientific challenge since recent reports have shown that spin transport is fully determined by the presence of interface states, impurities, interface roughness, and how the electronic bands line up at the interface between the ferromagnet and the semiconductor. Only a perfect fundamental knowledge and control of each of these parameters will allow us to achieve efficient spin injection in Si and Ge and propose new spin transport models. In this part, different growth techniques including molecular beam epitaxy will be explored and both ferromagnetic metals and semiconductors like Mn doped Ge will be tested as spin injectors. Interface parameters will be deduced from electrical measurements, electron paramagnetic resonance and x-ray photoemission spectroscopy. They will be supported by ab-initio calculations. Spin injection, detection and manipulation measurements will be performed on lateral spin valves fabricated by optical and electron beam lithography. The combination of both three-terminal and four-terminal measurements will allow us to discriminate between spin accumulation into interface localized states and spin injection in the semiconductor conduction band. A constant feedback between growth and characterization will allow us to find the best spin injector in Si and Ge channel at room temperature.
The second objective of this project is to manipulate the spin signal using both electric fields and geometrical effects. First, by using SOI (resp. GOI) wafers we will apply a back gate voltage to the Si (resp. Ge) channel in order to modify the band profile within the semiconductor channel and induce spin precession through the Rashba spin-orbit interaction. Then we expect to enhance the spin signal by reducing the channel thickness and lateral dimensions below the spin diffusion length.
Finally we will explore one-dimensional spin transport in Si, Ge and SiGe nanowires. By using the one-dimensional confinement of carriers in nanowires, we expect much longer spin diffusion lengths and larger spin signals. They will be grown using both the vapour-liquid-solid growth method from Chemical Vapor Deposition (bottom-up approach) and the e-beam lithography (top-down approach). In this project, we will address in particular the major issue of making good tunnel contacts to the nanowires in order to ensure efficient spin injection. For this purpose, an original method of “flat contacting” by shadow implantation of dopants in semi-insulating Si and Ge films will be developed during the project.