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Polarizing nuclear spins in quantum dots by injection of a spin-polarized current
Pablo Asshoff, Gunter Wüst, Andreas Merz, Heinz Kalt, and Michael Hetterich
Citation: AIP Conference Proceedings 1399, 681 (2011);
View online: https://doi.org/10.1063/1.3666560
View Table of Contents: http://aip.scitation.org/toc/apc/1399/1
Published by the American Institute of Physics
Polarizing nuclear spins in quantum dots by injection of a
spin-polarized current
Pablo Asshoff, Gunter Wüst, Andreas Merz, Heinz Kalt and Michael Hetterich
Institut für Angewandte Physik, Karlsruhe Institute of Technology (KIT) and DFG Center for Functional
Nanostructures (CFN), 76131 Karlsruhe, Germany
Abstract.
We demonstrate spin-polarization of the nuclei in self-assembled InGaAs quantum dots due to electrical injection of a spinpolarized current. High-resolution optical spectroscopy of the Overhauser shift is used as detection method. The electrically
induced spin-polarization is of comparable magnitude as for spin-injection by photoexcitation. The results imply that efficient
nuclear spin-polarization in quantum dots can be achieved by purely electrical means.
Keywords: Electrical spin-injection, quantum dots, dynamic nuclear polarization, spin-LED
PACS: 72.25.Dc, 85.75.-d, 72.25.Hg, 72.25.Rb, 78.67.Hc
Quantum dots (QDs) are candidates for a spin-based
quantum information processing. The information can
be stored either in the spin state of an electron confined
within the QD, or in nuclear spin states. These states
are coupled to each other by the hyperfine interaction:
the injected spin-polarized electrons align the nuclear
spins via electron-nuclei flip-flop processes. The aligned
nuclear spins produce an effective magnetic field BN ,
which in turn affects the electron spins. The electron
spins experience a total magnetic field of an externally
applied field B0 plus the nuclear magnetic field BN .
The difference of the Zeeman splitting between the spin
subbands originating from the nuclear field results in
the so-called Overhauser shift, which can be resolved by
spectroscopic methods [1, 2].
Here, we investigate if efficient spin-polarization of
the nuclei is possible by electrical injection of spinpolarized electrons. A convenient choice for this is to use
single quantum dots embedded in a spin light emitting
diode (spin-LED) [3], which allows for analyzing the
Overhauser shift from the light emission from the QDs.
We compare the result with an optically induced spinpolarization of the nuclei. These two injection methods
are relevant for approaches involving the use of spin
states [4, 5, 6].
The investigated quantum dots are incorporated in
an all-semiconductor spin-LED (Fig. 1). For electrical
spin injection, electrons are injected through the diluted
magnetic semiconductor ZnMnSe, which acts as a spin
aligner when a magnetic field is applied. The layers were
deposited in two molecular-beam epitaxy (MBE) systems on a GaAs:Zn(001) substrate (p ∼ 1 × 1019 cm−3 ).
In the first system, designed for the growth of III–V materials, on the substrate a ∼ 500 nm layer of GaAs:Be
(p ∼ 1 × 1019 cm−3 ) was grown, followed by 100 nm
FIGURE 1. Cross-sectional schematical view (not to scale)
of the spin-LED. For electrical operation, electrons are injected
from the top In contact pad, traverse the Zn0.95 Mn0.05 Se:Cl
layer and leave it with a defined spin-polarization. The optically
active InGaAs QDs and the wetting layer are sandwiched between thin GaAs layers, which renders a suitable band structure
for the injection process. Unpolarized holes are flowing into
the InGaAs QDs and the WL from the bottom part of the spinLED, leading to radiative recombination. For optical excitation,
electron-hole pairs are generated in the WL, both carrier types
are captured in the QD states, and recombine. The gold mask on
top of the structure, which contains the apertures, is indicated.
i-GaAs, the InGaAs QDs / wetting layer (WL) (see [7]
for details) and a 25 nm thick i-GaAs spacer. Plan-view
transmission electron microscopy revealed a QD sheet
density of ∼ 5 × 1010 cm−2 . The heterostructure was
then transferred to the second MBE facility, designed for
the growth of II–VI materials, where 750 nm of the spin
aligner material Zn0.95 Mn0.05 Se:Cl (n ∼ 1018 cm−3 ) followed by a 200 nm layer of ZnSe:Cl (n = 5 × 1018 cm−3 )
were deposited. The latter layer improves the ohmic contact to the subsequently evaporated In contact pad. Then,
gold apertures were fabricated with electron beam lithog-
Physics of Semiconductors
AIP Conf. Proc. 1399, 681-682 (2011); doi: 10.1063/1.3666560
© 2011 American Institute of Physics 978-0-7354-1002-2/$30.00
681
FIGURE 2. CPD of the emitted light from a specific QD
plotted against the Overhauser shift (B = 6 T, T = 5 K), error bars are obtained from the fits. Blue and red data points
correspond to experiments with optically generated and electrically injected charge carriers, respectively. The dashed line
is deduced from the PL experiments. Since for photoexcitation
with linearly polarized light the polarization is very sensitive
to small misalignments of the optical components, the CPD
slightly deviates from zero for this configuration.
raphy to access the luminescence of single QDs. Finally, optical lithography was employed to define squareshaped spin-LEDs, one of which with a surface area of
(400 μ m)2 is investigated here.
The sample was placed in Faraday geometry in a
magneto-optical cryostat at a temperature of T = 5 K and
with a field B0 = 6 T. To investigate a single QD, one of
the gold apertures on top of the spin-LED was placed
in the focus of a 60× microscope objective mounted inside the cryostat with piezoelectric actuators, providing
an optical path from the sample surface to the outside
of the cryostat and vice versa, used for detection and
laser excitation, respectively. The emission of the light
was guided by a fiber to a 0.85 m double monochromator (Horiba, Spex 1402) with two 1200 grooves/mm gratings. The dispersed light was then detected by a CCD
(Andor, iDus DU420A-BR-DD). By inserting a quarterwave plate and a linear polarizer in the optical path, we
could additionally determine the circular polarization degree (CPD) of the emitted light, which corresponds to
the spin-polarization inside the quantum dot [8], and is
defined as (Iσ + − Iσ − )/(Iσ + + Iσ − ), with Iσ +(−) denoting
the intensity of σ +(−) -polarized light.
In a first experiment, we determined the Overhauser
shift and the CPD from the spectra when the quantum dots were electrically excited. We applied a voltage
across the structure and generated a spin-polarized current, leaving the quantum dots filled with spin-polarized
electrons [3]. When resident electrons recombine with
holes flowing into the device from the p-doped side of
the spin-LED, new electrons can populate the quantum
dot, and transfer spin-polarization to the nuclei, leading to a quasi-equilibrium. A constant current I = 8 mA
682
(U ∼ 4 V) was flowing through the device, resulting in
a high current density, with the Overhauser shift being
saturated.
For comparison, in a second experiment we generated
a spin-population in the QDs by CW photoexcitation,
and measured the Overhauser shift and CPD. As for the
electroluminescence measurements, high enough excitation intensities were used to reach a regime where the
Overhauser shift is saturated. For the photoexcitation, we
changed the helicity of the impinging light in order to obtain Overhauser shifts for varying average electron spinpolarizations.
In Fig. 2 the magnitude of the Overhauser shift in relation to the spin-injection efficiency (corresponding to the
CPD) is shown for a single QD. Results obtained from
the spectra for the cases of electrical excitation and photoexcitation are plotted. Data obtained from photoexcitation are shown as blue data points. A linear approximation [2] is indicated by the dashed line in the graph.
As can be seen, a similar relationship between CPD and
Overhauser shift as for the photoexcitation occurs. Indeed, the Overhauser shift is of comparable magnitude
as deduced from the photoexcitation experiments. This
implies that electrical excitation leads to efficient spinpolarization of the nuclei which corresponds to the case
of photoexcitation.
This work has been performed within project A2 of
the DFG Research Center for Functional Nanostructures
(CFN). It has been further funded by a grant from the
Ministry of Science, Research and the Arts of BadenWürttemberg (Az.: 7713.14-300). P.A. acknowledges financial support from the Karlsruhe School of Optics and
Photonics (KSOP) and the Karlsruhe House of Young
Scientists (KHYS).
REFERENCES
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