Optical Properties of In(Ga)As/GaAs Quantum Dots for Optoelectronic Devices 117
As shown above, in bulk GaAs around the Γ -point the heavy and light holes are mixed and
this leads to very rapid spin relaxation of holes, so in the majority of cases it is the electron spin
that is exploited. The dominant spin relaxation mechanism in undoped bulk III–V semiconduc-
tors, especially at low temperature, is the D’yakanov–Perel mechanism [271] . This arises because
in III–V semiconductors there is no centre of symmetry in the unit cell (so-called bulk inversion
asymmetry), which leads to a lifting of the degeneracy of the conduction band spin states, equiv-
alent to applying a small magnetic fi eld. If the direction of motion of the electron differs from the
direction of this effective magnetic fi eld, precession of the electron will lead to spin relaxation.
However, in QDs, since there is 3D confi nement of the electron, the D’yakonov–Perel mechanism
is expected to be suppressed so long spin relaxation times are predicted. Indeed, in early stud-
ies long spin relaxation times of excitons in QDs, exceeding the exciton radiative lifetime, were
reported [272, 273] . These long spin relaxation times and the lifting of the heavy hole/light hole
degeneracy makes QDs attractive for the active region of spin-LEDs.
The fi rst QD-based spin-LEDs were demonstrated by Chye et al . [274] , using structures incor-
porating GaMnAs spin aligner layers [275, 276] . Since GaMnAs is natively p-doped, injection of
spin polarized holes is straightforward; to inject spin polarized electrons a reverse-biased Zener
diode junction formed at a GaMnAs:( n -doped)GaAs interface was used, in which the spin polar-
ised electrons in the valence band tunnel across the junction into the n -GaAs conduction band.
The circular polarization of light emitted from the QDs was comparable to that emitted from QWs
in similar structures.
For magnetic semiconductor layers such as GaMnAs only low-temperature operation is possi-
ble since the Curie temperature of these materials is low (up to 110 K for GaMnAs). For opera-
tion closer to room temperature an alternative is needed, and this is commonly achieved by using
ferromagnetic contacts such as Fe. The maximum spin injection from an Fe contact is around
45%; however, early attempts at spin injection into semiconductors from Fe contacts produced
little or no detected circular polarization of emitted light. This is because ohmic contacts are not
suitable for effi cient spin injection since the conductance mismatch between the layers results in
signifi cant spin dephasing due to carrier scattering at the interface [277] . This may be overcome
using a tunnel junction, for example with a Schottky barrier in reverse bias [278, 279] .
Two examples of InAs/GaAs QD spin-LEDs with Fe contacts have been reported recently [280,
281] , using similar spin-LED structures but different measurement techniques. For successful meas-
urement of circular polarization of emission due to spin polarization of carriers, the geometry of
the system must be correct: the validity of the selection rules for optical transitions in QWs and QDs
depends on the direction of absorption/emission such that normal to the QW or QD plane the rules
outlined above are reliable. However, for in-plane emission the selection rules are ill-defi ned and
optical polarization is not detected [282, 283] , thus the carriers must have a component of spin in
the direction normal to the QD plane (the growth direction). The thin Fe contact layer has its mag-
netization aligned along its plane, so for normal injection conditions the injected carrier spin will be
in the plane and undetectable. To overcome this, measurements are either made using the Faraday
geometry [280] or using the oblique Hanle effect [281] . For the Faraday geometry, a large mag-
netic fi eld (several tesla) is applied in the growth direction in order to align the magnetization of the
Fe contact out of the plane. Injected spins will then be orientated in the growth direction and the
selection rules will be valid. Li et al. [280] observed a noticeable increase in the intensity of positive
circularly polarized light ( s
) with respect to negative circularly polarized light ( s
) emitted from
the LED for an applied fi eld of 3 T (enough to saturate the Fe magnetization in the growth direc-
tion), with a circular polarization (defi ned as P
circ
[ I ( σ
) I ( σ
)]/[ I ( σ
) I ( σ
)]) of around 5%.
Interestingly, this circular polarization remained roughly constant throughout the temperature
range 80–300 K, in contrast to reports for QW spin-LEDs, which show a reduction in circular polar-
ization with increased temperature [284] .
The present authors have demonstrated electrical spin injection into an InAs/GaAs QD spin-
LED using the oblique Hanle effect [281] . For the Hanle effect measurements, a small magnetic
fi eld ( 100 mT) is applied at an angle θ 45 ° to the growth direction. The magnetic fi eld is not
suffi cient to rotate the Fe magnetization out of the plane, so the injected spin is in the direction
of the plane. However, once the spin is injected into the semiconductor, it will precess around the
direction of the applied fi eld, with a Larmor frequency Ω g
*
μ
B
B / , where g
*
is the effective Landé
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