Satoshi Iba

Spin Accumulation and Spin Lifetime in p-Type Germanium at Room Temperature

The electrical creation and detection of spin accumulation in p-type Ge were successfully demonstrated at room temperature by spin-polarized tunneling in epitaxial Fe/MgO contacts on Ge with a hole concentration of 8×1018 cm-3. In Hanle measurements, the spin accumulation produces a spin signal of about 40 µV per mA of tunnel current. The extracted spin lifetime of holes is 13 ps, which is much longer than the momentum relaxation time. The corresponding spin-diffusion length is 80 nm, suggesting that communication of spin information in p-type Ge is possible over the typical channel length of a field-effect transistor.

Growth condition dependence of photoluminescence polarization in (100) GaAs/AlGaAs quantum wells at room temperature

We conducted systematic measurements on the carrier lifetime (τc), spin relaxation time (τs), and circular polarization of photoluminescence (Pcirc) in (100) GaAs/AlGaAs multiple quantum wells grown by molecular beam epitaxy (MBE). The τc values are strongly affected by MBE growth conditions (0.4–9 ns), whereas the τs are almost constant at about 0.13 ns. The result suggests that spin detection efficiency [τs/(τc + τs)], which is expected to be proportional to a steady-state Pcirc, is largely dependent on growth condition. We confirmed that the Pcirc has similar dependence on growth condition to those of τs/(τc + τs) values. The study thus indicates that choosing the appropriate growth condition of the QW is indispensable for obtaining a high Pcirc from a spin-polarized light-emitting diode (spin-LED).

Fabrication of Ge-based light-emitting diodes with a ferromagnetic metal/insulator tunnel contact

We fabricated light-emitting diodes (LEDs) having an n+-Ge/p+-Si structure with ferromagnetic metal/insulator tunnel contact. Clear electroluminescence (EL) with peak wavelength around 1600 nm from the direct band gap of the Ge layer was successfully observed at room temperature, which indicates that tunnel contacts allow efficient direct-gap EL from the LED. Our result is an important step toward developing Ge-based spin-polarized LEDs.

Systematic study of surface morphology, photoluminescence efficiency, and spin-detection sensitivity in (110)-oriented GaAs/AlGaAs quantum wells

We prepared (110)-oriented GaAs/AlGaAs multiple-quantum-well (MQW) samples by molecular beam epitaxy (MBE) under different growth conditions, and conducted systematic measurements of surface morphology, photoluminescence (PL), and spin-detection sensitivity at room temperature. Excellent surface flatness and high PL intensity were observed for the samples grown at temperatures ≥450 °C and As4/Ga flux ratios ≥40. It was found that the PL intensity of (110) MQWs was higher than that of the conventional (100) MQWs grown using the same MBE system. At the same time, we confirmed that the spin-detection sensitivity of (110) MQWs we obtained was an order of magnitude higher than that of the (100) MQWs. These results suggest that the newly developed (110) MQWs indeed have greater advantages than the conventional (100) MQWs for use in emerging spin-optical devices such as spin-controlled lasers.