Spin Transport in Ge Nanowires for Diluted Magnetic Semiconductor-Based Nonvolatile Transpinor

Spintronic devices, in particular spin field-effect transistors (spinFETs), have been studied for decades as a promising candidate to replace Si CMOS devices. Several variants of spinFETs have been proposed to use the spin of electrons as another degree of freedom for information processing. However, all the previous proposals still use charge current, which generates power dissipation. To eliminate the charge current, we propose a novel spin-based transistor (transpinor) based on diluted magnetic semiconductor (DMS) with electric field-controlled paramagnetism-to-ferromagnetism phase transition. One promising prototype is to integrate a Mn_xGe_1-x DMS nanowire, whose ferromagnetism can be effectively controlled by a gate electrode, with Mn₅Ge₃ferromagnetic contacts for spin injection (Figure 1). Another variant of the ferromagnetic contacts could be epitaxial Fe/MgO tunnel junctions. In this transpinor device, the magnetic moments are transferred from the source to the channel, and then to the drain through spin injection and exchange interaction. As the gate electrode manipulates the ferromagnetism of the DMS channel, it controls the communication between the source and drain.

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 λ_sf = 480 nm and a spin flip lifetime of τ_sf = 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 λ_sf = 2.57 μm and the spin lifetime of τ_sf = 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 Mn_xGe_1-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 Mn_xGe_1-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.

Acknowledgement: This work was in part supported by Western Institution of Nanoelectronics (WIN) and National Science Foundation through grant ECCS 1308358.