UCLA

The continuous scaling of Si transistor feature size has driven the advancement of semiconductor technology in the past decades; however, such aggressive scaling is approaching the ultimate physical limit soon. Novel materials and devices are in urgent need to resolve a number of critical challenges. In particular, spintronic devices have been proposed and extensively studied by using the spin of electrons as another degree of freedom in devices for information processing, which enables advanced electronic devices that could potentially outperform Si devices with lower power dissipation and faster switching.

In this work, the carrier and spin transport in Ge nanowires will be presented. Atomic-scale thermal annealing was established as a convenient approach to make high-quality nanoscale source/drain contacts in high-performance Ge nanowire transistors. Electrical spin injection and detection in both p- and n-type Ge nanowires were demonstrated using ferromagnetic Mn5Ge3 Schottky contacts and Fe/MgO tunnel junctions, respectively. The measured spin lifetime and spin diffusion length in Ge nanowires were much larger than those reported for bulk Ge, suggesting that the spin relaxation was significantly suppressed in nanowires.

Furthermore, we studied the spin transport in topological insulators, in which the spin-momentum locking of helical surface states was preserved by the strong spin-orbit interaction and time-reversal symmetry. We demonstrated the electrical detection of the spin-polarized surface states conduction in topological insulator (Bi0.53Sb0.47)2Te3 using a Co/Al2O3 ferromagnetic tunnel contact. Voltage (resistance) hysteresis was observed when sweeping the magnetic field, and the two resistance states were reversible by changing the electric current direction. Our results showed a direct evidence of the charge current-induced spin polarization in the topological surface states. With the understanding of spin injection and detection, it might open up great opportunities to explore novel spintronic devices based on topological insulators and Ge nanowires.