Hayato Koike

Room-temperature operation of Si spin MOSFET with high on/off spin signal ratio

We experimentally demonstrate a Si spin metal-oxide-semiconductor field-effect transistor (MOSFET) that exhibits a high on/off ratio of source-drain current and spin signals at room temperature. The spin channel is non-degenerate n-type Si, and an effective application of gate voltage in the back-gated structure allows the spin MOSFET operation. This achievement can pave the way to practical use of the Si spin MOSFET.

Local magnetoresistance in Fe/MgO/Si lateral spin valve at room temperature

Room temperature local magnetoresistance in two-terminal scheme is reported. By employing 1.6 nm-thick MgO tunnel barrier, spin injection efficiency is increased, resulting in large non-local magnetoresistance. The magnitude of the non-local magnetoresistance is estimated to be 0.0057 Ω at room temperature. As a result, a clear rectangle signal is observed in local magnetoresistance measurement even at room temperature. We also investigate the origin of local magnetoresistance by measuring the spin accumulation voltage of each contact separately.

Observation of large spin accumulation voltages in nondegenerate Si spin devices due to spin drift effect: Experiments and theory

A large spin-accumulation voltage of more than 1.5 mV at 1 mA, i.e., a magnetoresistance of 1.5 {\Omega}, was measured by means of the local three-terminal magnetoresistance in nondegenerate Si-based lateral spin valves (LSVs) at room temperature. This is the largest spin-accumulation voltage measured in semiconductor-based LSVs. The modified spin drift-diffusion model, which successfully accounts for the spin drift effect, explains the large spin-accumulation voltage and significant bias-current-polarity dependence. The model also shows that the spin drift effect enhances the spin-dependent magnetoresistance in the electric two terminal scheme. This finding provides a useful guiding principle for spin metal-oxide semiconductor field-effect transistor (MOSFET) operations.

Spin drift in highly doped n-type Si

A quantitative estimation of spin drift velocity in highly doped n-type silicon (Si) at 8 K is presented in this letter. A local two-terminal Hanle measurement enables the detection of a modulation of spin signals from the Si as a function of an external electric field, and this modulation is analyzed by using a spin drift-diffusion equation and an analytical solution of the Hanle-type spin precession. The analyses reveal that the spin drift velocity is linearly proportional to the electric field. The contribution of the spin drift effect to the spin signals is crosschecked by introducing a modified nonlocal four-terminal method.

Investigation of spin scattering mechanism in silicon channels of Fe/MgO/Si lateral spin valves

The temperature evolution of spin relaxation time, τsf, in degenerate silicon (Si)-based lateral spin valves is investigated by means of the Hanle effect measurements. τsf at 300 K is estimated to be 1.68 ± 0.03 ns and monotonically increased with decreasing temperature down to 100 K. Below 100 K, in contrast, it shows almost a constant value of ca. 5 ns. The temperature dependence of the conductivity of the Si channel shows a similar behavior to that of the τsf, i.e., monotonically increasing with decreasing temperature down to 100 K and a weak temperature dependence below 100 K. The temperature evolution of conductivity reveals that electron scattering due to magnetic impurities is negligible. A comparison between τsf and momentum scattering time reveals that the dominant spin scattering mechanism in the Si is the Elliott-Yafet mechanism, and the ratio of the momentum scattering time to the τsf attributed to nonmagnetic impurities is approximately 3.77 × 10−6, which is more than two orders of magnitude ...

Electron paramagnetic resonance study of MgO thin-film grown on silicon

The paramagnetic defects in the magnesium-oxide (MgO) thin-film grown on a silicon (Si) substrate (MgO|Si) by a molecular beam epitaxy method are reported. These thin films have been used as an MgO tunnel barrier material for the spin-injection to Si from Fe. According to the crystallinity evaluation, the MgO film possessed a (001) oriented structure. We investigated the paramagnetic defects and impurities of several MgO|Si samples by electron paramagnetic resonance (EPR). The EPR spectra showed a strong isotropic signal with a g-value of 2.002 and six weak satellite peaks which were assigned to oxygen vacancies of highly oriented MgO(001) (F+ center) coupled with 25Mg nuclei (I = 5/2). The spin area density of MgO (thickness = 1.6 nm), which can be used as a tunnel barrier for spin injection, was determined to be 7 × 1013 cm−2. This value corresponds to 1% of O atoms replaced with vacancies. The identification of the paramagnetic centers and the quantitative estimation of their concentration in MgO films...