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Hanle effect and electron spin polarization in InAs/GaAs quantum dots up to room temperature
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, USA .
Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California, USA .
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2012 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 23, no 13, 135705- p.Article in journal (Refereed) Published
Abstract [en]

Hanle effect in InAs/GaAs quantum dots (QDs) is studied under optical orientation as a function of temperature over the range of 150-300 K, with the aim to understand the physical mechanism responsible for the observed sharp increase of electron spin polarization with increasing temperature. The deduced spin lifetime Ts of positive trions in the QDs is found to be independent of temperature, and is also insensitive to excitation energy and density. It is argued that the measured Ts is mainly determined by the longitudinal spin flip time (T1) and the spin dephasing time (T2 *) of the studied QD ensemble, of which both are temperatureindependent over the studied temperature range and the latter makes a larger contribution. The observed sharply rising of the QD spin polarization degree with increasing temperature, on the other hand, is shown to be induced by an increase in spin injection efficiency from the barrier/wetting layer and also by a moderate increase in spin detection efficiency of the QD.

Place, publisher, year, edition, pages
IOP Publishing , 2012. Vol. 23, no 13, 135705- p.
National Category
Natural Sciences Condensed Matter Physics
URN: urn:nbn:se:liu:diva-75096DOI: 10.1088/0957-4484/23/13/135705ISI: 000301663900015OAI: diva2:503748
funding agencies|Swedish Research Council| 621-2011-4254 |Available from: 2012-02-16 Created: 2012-02-16 Last updated: 2013-10-02Bibliographically approved
In thesis
1. Spin Properties in InAs/GaAs Quantum Dot based Nanostructures
Open this publication in new window or tab >>Spin Properties in InAs/GaAs Quantum Dot based Nanostructures
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Semiconductor quantum dots (QDs) are a promising building block of future spin-functional devices for applications in spintronics and quantum information processing. Essential to the realization of such devices is our ability to create a desired spin orientation of charge carriers (electrons and holes), typically via injection of spin polarized carriers from other parts of the QD structures. In this thesis, the optical orientation technique has been used to characterize spin generation, relaxation and detection in self-assembled single and multi-QD structures in the InAs/GaAs system prepared by modern molecular beam epitaxy technique.

Optical generation of spin-oriented carriers in the wetting layer (WL) and GaAs barrier was carried  out via circularly polarized excitation of uncorrelated electron-hole pairs from band-to-band transitions or via resonant excitation of correlated electron-hole pairs, i.e. excitons. It was shown that the generation and injection of uncorrelated electron-hole pairs is advantageous for spin-preserving injection into the QDs. The lower spin injection efficiency of excitons was attributed to an enhanced spin relaxation caused by the mutual electron-hole Coulomb exchange interaction. This correlation affects the spin injection efficiency up to elevated temperatures of around 150 K.

Optical orientation at the energy of the WL light-hole (lh) exciton (XL) is accompanied by simultaneous excitation from the heavy-hole (hh) valence band at high ~k-vectors. Quantum interference of the two excitation pathways in the spectral vicinity of the XL energy resulted in occurrence of an asymmetric absorption peak, a Fano resonance. Complete quenching of spin generation efficiency at the resonance was observed and attributed to enhanced spin scattering between the hh and lh valence bands in conjunction with the Coulomb exchange interaction in the XL. This mechanism remains effective up to temperatures exceeding 100 K.

In longitudinal magnetic fields up to 2 T, the spin detection efficiency in the QD ensemble was observed to increase by a factor of up to 2.5 in the investigated structures. This is due to the suppression of two spin depolarization mechanisms of the QD electron: the hyperfine interaction with the randomly oriented nuclear spins and the anisotropic exchange interaction with the hole. At higher magnetic fields, when these spin depolarization processes are quenched, only anisotropic QD structures (such as double QDs, aligned along a specific crystallographic axis) still exhibit a rather strong field dependence of the QD electron spin polarization under non-resonant excitation. Here, an increased spin relaxation in the spin injector, i.e. the WL or GaAs barrier, is suggested to lead to more efficient thermalization of the spins to the lower Zeeman-split spin state before capture to the QD.

Finally, the influence of elevated temperatures on the spin properties of the QD structures was studied. The temperature dependence of dynamic nuclear polarization (DNP) of the host lattice atoms in the QDs and its effect on the QD electron spin relaxation and dephasing were investigated for temperatures up to 85 K. An increase in DNP efficiency with temperature was found, accompanied by a decrease in the extent of spin dephasing. Both effects are attributed to an accelerating electron spin relaxation, suggested to be due to phonon-assisted electronnuclear spin flip-flops driven by the hyperfine interaction. At even higher temperatures, reaching up to room temperature, a surprising, sharp rise in the QD polarization degree has been found. Experiments in a transverse magnetic field showed a rather constant QD spin lifetime, which could be governed by the spin dephasing time T*2. The observed rising in QD spin polarization degree could be likely attributed to a combined effect of shortening of trion lifetime and increasing spin injection efficiency from the WL. The latter may be caused by thermal activation of non-radiative carrier relaxation channels.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. 48 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1426
National Category
Natural Sciences
urn:nbn:se:liu:diva-75097 (URN)978-91-7519-965-8 (ISBN)
Public defence
2012-04-02, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Available from: 2012-02-16 Created: 2012-02-16 Last updated: 2013-10-02Bibliographically approved

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