Spin Transport in Two-Dimensional Material Heterostructures
Doctoral thesis, 2015
Spintronics is considered as an alternative for information processing beyond the charge based technology. The spintronic device performance depend on the spin relaxation mechanisms in the channel material. Si and graphene are interesting for their long spin coherence lengths and ideal for spin transport channels. Additionally, the interest in newly discovered two-dimensional semiconductors (2D SC), topological insulators (TI) and hexagonal boron nitride (h-BN) increases due to their strong spin-orbit coupling, existence of spin polarized surface states and insulating band structure, respectively. Despite the recent advances in spintronics, most of these new materials are not explored and the spin physics is not fully understood yet.
In this thesis, we create large spin polarizations up to 34% in the Si bulk using ozone oxidized SiO2 as an ideal tunnel barrier to and study the influence of its Schottky barrier (SB) on the spin injection at room temperature. In graphene, we investigate the effect of ferromagnetic (FM) tunnel contacts and channel length dependence on the spin signal achieving spin transports over distances of 16 µm and spin lifetimes of 1.2 ns in CVD graphene. Using the 2D insulator h-BN as an alternative barrier material in magnetic tunnel junctions and on Si reveals a good tunnel spin polarization, whereas h-BN on graphene significantly increases the spin lifetimes and results in spin polarizations up to 65%. For the 2D SCs MoS2 and black phosphorous we demonstrate a significant reduction of their interface SB by using FM tunnel contacts, which circumvent the conductivity mismatch problem required for magnetoresistance measurements. Finally, we measure the spin-momentum locking in the surface states of the topological insulator Bi2Se3 using FM tunnel contacts up to room temperature.
These excellent spintronic properties of the individual materials and their heterostructures promise novel devices with custom-designed spin properties.
Spin polarized tunnelling