C.H. Li

Low-resistance spin injection into silicon using graphene tunnel barriers

Spin manipulation in a semiconductor offers a new paradigm for device operation beyond Moores law. Ferromagnetic metals are ideal contacts for spin injection and detection, but the intervening tunnel barrier required to accommodate the large difference in conductivity introduces defects, trapped charge and material interdiffusion, which severely compromise performance. Here, we show that single-layer graphene successfully circumvents the classic issue of conductivity mismatch between a metal and a semiconductor for electrical spin injection and detection, providing a highly uniform, chemically inert and thermally robust tunnel barrier. We demonstrate electrical generation and detection of spin accumulation in silicon above room temperature, and show that the contact resistance-area products are two to three orders of magnitude lower than those achieved with oxide tunnel barriers on silicon substrates with identical doping levels. Our results identify a new route to low resistance-area product spin-polarized contacts, a key requirement for semiconductor spintronic devices that rely on two-terminal magnetoresistance, including spin-based transistors, logic and memory.

Optical detection of spin Hall effect in metals

Optical techniques have been widely used to probe the spin Hall effect in semiconductors. In metals, however, only electrical methods such as nonlocal spin valve transport, ferromagnetic resonance, or spin torque transfer experiments have been successful. These methods require complex processing techniques and measuring setups. We show here that the spin Hall effect can be observed in non-magnetic metals such as Pt and β-W, using a standard bench top magneto-optical Kerr system with very little sample preparation. Applying a square wave current and using Fourier analysis significantly improve our detection level. One can readily determine the angular dependence of the induced polarization on the bias current direction (very difficult to do with voltage detection), the orientation of the spin Hall induced polarization, and the sign of the spin Hall angle. This optical approach is free from the complications of various resistive effects, which can compromise voltage measurements. This opens up the study of spin Hall effect in metals to a variety of spin dynamic and spatial imaging experiments.

Direct comparison of current-induced spin polarization in topological insulator Bi2Se3 and InAs Rashba states.

Three-dimensional topological insulators (TIs) exhibit time-reversal symmetry protected, linearly dispersing Dirac surface states with spin–momentum locking. Band bending at the TI surface may also lead to coexisting trivial two-dimensional electron gas (2DEG) states with parabolic energy dispersion. A bias current is expected to generate spin polarization in both systems, although with different magnitude and sign. Here we compare spin potentiometric measurements of bias current-generated spin polarization in Bi2Se3(111) where Dirac surface states coexist with trivial 2DEG states, and in InAs(001) where only trivial 2DEG states are present. We observe spin polarization arising from spin–momentum locking in both cases, with opposite signs of the measured spin voltage. We present a model based on spin dependent electrochemical potentials to directly derive the sign expected for the Dirac surface states, and show that the dominant contribution to the current-generated spin polarization in the TI is from the Dirac surface states.

A graphene solution to conductivity mismatch: Spin injection from ferromagnetic metal/graphene tunnel contacts into silicon

Spin-injection into silicon from a ferromagnetic metal requires a solution to the conductivity mismatch. Oxide tunnel barriers such as MgO, Al2O3, or SiO2 are typically used to solve this problem, but often include defects and must be several monolayers thick to avoid pinholes. At these thicknesses, the overall tunnel-barrier becomes highly resistive, preventing these junctions to be used in devices based on local magnetoresistance. Besides providing a spin dependent interface resistance, these barriers also prevent metal ions from diffusing into silicon, which would severely compromise device performance. Here, we show that we can lower the junction resistance by 2–3 orders of magnitude when using a single layer of graphene as the tunnel barrier rather than SiO2 or Al2O3. Hanle measurements show that the spin lifetime is independent of the tunnel barrier material (graphene, Al2O3, SiO2), demonstrating that the lifetime measured is not dominated by some characteristics of the tunnel barrier. The graphene provides a highly uniform barrier, with well-controlled thickness and minimal defect and trapped charge density, while successfully circumventing the conductivity mismatch between a ferromagnetic metal and Si and preventing metal ion diffusion from the FM contact.