Field: Physics/Astrophysics
Session Topic:
Spintronics
Speaker:
Yuzo Ohno, Tohoku University
Title: Spin Injection and Spin Control in Semiconductors
Semiconductor spin-electronics (spintronics), where both charge and spin degrees of
freedom are utilized, is currently of great interest. This is because it is expected to enable
integration of magnetic functionalities and semiconductor circuitry as well as quantum
information technology based on coherent nature of spins in semiconductors. For
developing practical semiconductor "spintronics" devices, injection of spin-polarized
electrical current into non-magnetic semiconductors is one of key elements. For a number
of spin-based device schemes, electrical spin injection through high quality semiconductor
heterojunctions in the absence of magnetic field is preferable. Ferromagnetic
semiconductor that can be epitaxially grown on high-quality nonmagnetic
heterostructures is a candidate to induce spin polarized current in nonmagnetic
semiconductors. (Ga,Mn)As, which exhibits a ferromagnetic phase at relatively high
temperatures (Curie temperature TC~160 K), is a natural choice as it can be combined
with existing devices made of III-V compounds.
In order to demonstrate spin injection into semiconductors, we fabricated light
emitting diodes (LEDs) which consist of pn junctions of 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: Spin-polarization 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 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. The known ferromagnetic semiconductors compatible with high
quality heterostructures are, however, all p-type. This is believed to be due to the small
exchange interaction between magnetic spins and conduction band electron spins.
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
non-magnetic semiconductor via interband tunneling. 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.
It is also of great importance to understand the spin dephasing mechanism in
semiconductor quantum structures. The spin relaxation time of electron spin must be
sufficiently long to process information stored in the form of the polarization of spins. The
lack of inversion symmetry of zinc-blende structure, like GaAs, results in spin splitting of
the conduction band via spin-orbit coupling, which is the driving force for the spin
relaxation. For a two-dimensional electron systems confined in a QW potential, the spin
relaxation due to spin-orbit interaction has been predicted to have strong dependence on
the growth axis, although in most experiments QWs formed on a (100) plane were
investigated. We thus 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.
Compared to the conduction electron spins, nuclear spins in semiconductors have
several orders of magnitude longer lifetimes and are thus favorable candidates for storing
quantum information. Local manipulation of nuclear spin can be achieved by controlling
hyperfine interaction with electron spins. Our time-resolved measurements of electron
spin precession provided unambiguous signatures of all optical nuclear magnetic
resonance, in which electron spins excited by repetitive optical pulses not only polarize
but also tip the nuclear spins. This enables spatially selective manipulation of nuclear
spin through confinement of the tipped field. Furthermore, we demonstrated the
gate-control of dynamic nuclear polarization in an n-type GaAs/AlGaAs QW very
recently, using the sensitivity of hyperfine interaction on the metal-insulator phase
transition.
We have investigated and demonstrated the electrical spin injection and local control
of electron and nuclear spins in semiconductor quantum wells. Further developments in
fabrication of magnetic/non-magnetic semiconductor nanostructures and manipulation of
electron and nuclear spins will pave the way to future spintronics devices.