Spin Transport in Ge Nanowires for Diluted Magnetic Semiconductor-Based Nonvolatile Transpinor
In order to realize the proposed transpinor device, we first studied the spin transport in Ge nanowires and demonstrated the electrical spin injection and detection using both ferromagnetic Mn5Ge3 Schottky contacts and Fe/MgO tunnel junctions (Figure 2). The Mn5Ge3 contacts were fabricated by germanidation through rapid thermal annealing. This convenient approach produced single-crystalline Mn5Ge3 contacts with a controlled channel length and atomically clean interfaces to the Ge nanowire, which helped alleviate Fermi level pinning at the interface. Subsequently, electrical spin injection and detection was successfully demonstrated in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor. A spin diffusion length of λsf = 480 nm and a spin flip lifetime of τsf = 244 ps are revealed in p-type Ge nanowires at T = 10 K, which were much larger than those reported for bulk p-type Ge. Meanwhile, spin injection into Ge nanowires with epitaxial Fe/MgO tunneling contacts was performed in nonlocal spin valve measurements. The spin diffusion length of λsf = 2.57 μm and the spin lifetime of τsf = 7.2 ns were obtained in n-type Ge nanowires at T = 40 K, which are again much larger than those in bulk n-type Ge. The significant enhancement in the spin lifetime and diffusion length in nanowires suggested that the spin relaxation was suppressed in nanowires compared with bulk materials.
Furthermore, we successfully demonstrated the pattern-assisted growth of MnxGe1-x DMS nanowires (Figure 3), which serve as the transpinor channel. The growth was performed using molecular beam epitaxy (MBE) chamber with SiO2 nanotrenches as the mask. The ferromagnetic MnxGe1-xnanowires showed a Curie temperature above 400 K without any precipitate. The ferromagnetism can be further modulated by a gate voltage at 100 K in a metal-oxide-semiconductor capacitor structure, demonstrating electric field control of ferromagnetism. This result, along with the electrical spin injection into Ge nanowires, lays the foundation to build the transpinor device as a possible realization of all-spin logic devices with built-in memory for low-power applications.