Associate Professor, Research Institute of Electrical Communication, Tohoku University
In order to demonstrate spin injection into semiconductors, we fabricated light emitting diodes (LEDs) which consist of pn junctions of a p-type ferromagnetic semiconductor (Ga, Mn) As and non-magnetic n-GaAs with a strained (In, Ga) As quantum well (QW) inserted as an active region. Here, p-type (Ga, Mn) As is used as a spin polarizer: Spinpolarization of the injected holes is determined directly from the electroluminescence (EL) polarization emitted after the recombination with unpolarized electrons according to the optical selection rule. EL is collected from the cleaved facet to minimize magneto-optical effects due to the nearby (Ga, Mn) As, and its polarization was investigated with a variable magnetic field applied parallel to the easy axis of the (Ga, Mn) As layer, i.e., in Faraday configuration. We observed that the EL polarization below TC draws a clear hysteresis loop: the remanence EL polarization at zero magnetic field is about 1% at T = 6 K. It follows the magnetization of (Ga, Mn) As, which is independently measured by a Superconducting Quantum Interference Device (SQUID) type of magnetometer. The presence of hysteretic EL polarization indicates that the hole spins can be injected and transported in non-magnetic GaAs.
Injection of spin polarized electrons, not holes, is more preferable from the application point of view as electrons usually exhibit longer spin lifetime due to small spin-orbit coupling. Because of the lack of n-type ferromagnetic semiconductor, we employed a spin Esaki diode and demonstrated electrical electron spin injection from the valence band of a p-type ferromagnetic semiconductor (Ga, Mn) As into the conduction band of a nonmagnetic semiconductor via interbond tunnelling. Clear hysteresis loop with±6.5% remanence is observed in the magnetic field dependence of EL polarization from an integrated p-(Ga,Mn)As/n-GaAs/(In,Ga)As/p-GaAs LED.
We also investigated electron spin dynamics in GaAs/AlGaAs QWs grown on (110) oriented substrates with n-doping, in which the spin relaxation mechanism predominant for conventional (100) QWs is shown to be substantially suppressed. It was demonstrated that the spin relaxation time is found to reach nanosecond order in wide temperature range by optical time-resolved measurements. This has enabled us to investigate nuclear spin dynamics by optically detected nuclear magnetic resonance.
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