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Spin Properties in InAs/GaAs Quantum Dot based Nanostructures
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
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.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1426
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-75097ISBN: 978-91-7519-965-8 (print)OAI: oai:DiVA.org:liu-75097DiVA: diva2:503751
Public defence
2012-04-02, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2012-02-16 Created: 2012-02-16 Last updated: 2017-03-27Bibliographically approved
List of papers
1. Spin injection in lateral InAs quantum dot structures by optical orientation spectroscopy
Open this publication in new window or tab >>Spin injection in lateral InAs quantum dot structures by optical orientation spectroscopy
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2009 (English)In: Nanotechnology, ISSN 0957-4484, Vol. 20, no 37, 375401- p.Article in journal (Refereed) Published
Abstract [en]

Optical spin injection is studied in novel laterally-arranged self-assembled InAs/GaAs quantum dot structures, by using optical orientation measurements in combination with tunable laser spectroscopy. It is shown that spins of uncorrelated free carriers are better conserved during the spin injection than the spins of correlated electrons and holes in an exciton. This is attributed to efficient spin relaxation promoted by the electron–hole exchange interaction of the excitons. Our finding suggests that separate carrier injection, such as that employed in electrical spin injection devices, can be advantageous for spin conserving injection. It is also found that the spin injection efficiency decreases for free carriers with high momentum, due to the acceleration of spin relaxation processes.

Keyword
Quantum dots, Photoluminescence, Exciton
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-20249 (URN)10.1088/0957-4484/20/37/375401 (DOI)
Note
Original Publication: Jan Beyer, Irina A Buyanova, Suwaree Suraprapapich, Charles Tu and Weimin Chen, Spin injection in lateral InAs quantum dot structures by optical orientation spectroscopy, 2009, Nanotechnology, (20), 37, 375401. http://dx.doi.org/10.1088/0957-4484/20/37/375401 Copyright: Institute of Physics http://www.iop.org/ Available from: 2009-08-31 Created: 2009-08-31 Last updated: 2017-03-27
2. Efficiency of spin injection in novel InAs quantum dotstructures: exciton vs. free carrier injection
Open this publication in new window or tab >>Efficiency of spin injection in novel InAs quantum dotstructures: exciton vs. free carrier injection
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2010 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Unambiguous experimental evidence for a significant difference in efficiency of excitonic vs. free carrier spin injection is provided in novel laterally arranged self-assembled InAs/GaAs quantum dot structures, from optical orientation and tunable laser spectroscopy. A lower efficiency of exciton spin injection as compared to free carrier spin injection from wetting layers into QDs results in a distinct feature in luminescence polarization of the QDs as a function of excitation photon energy. It is shown that this difference is not related to carrier density and state-filling effects arising from the difference in optical absorption efficiency between the excitons and free carriers. Rather, it is a genuine property for exciton spin injection that suffers stronger spin relaxation due to Coulomb exchange interaction.

Place, publisher, year, edition, pages
IOP, 2010
Series
Journal of Physics: Conference Series, ISSN 1742-6596 ; 245
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-60486 (URN)10.1088/1742-6596/245/1/012044 (DOI)
Conference
Quantum Dots 2010, 26–30 April 2010, Nottingham, UK
Available from: 2010-10-14 Created: 2010-10-14 Last updated: 2017-03-27
3. Strong suppression of spin generation at a Fano resonance in a semiconductor nanostructure
Open this publication in new window or tab >>Strong suppression of spin generation at a Fano resonance in a semiconductor nanostructure
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2012 (English)Manuscript (preprint) (Other academic)
Abstract [en]

We observe remarkable, complete suppression of spin generation under optical excitation in a thin InAs/GaAs wetting layer close to the light-hole excitonic resonance, leading to zero electron spin polarization as monitored by adjacent InAs quantum dots. The suppression is attributed to efficient spin relaxation/scattering at the Fano resonance between the light-hole exciton states and the heavy-hole continuum of the wetting layer. The complete suppression is found to remain effective up to temperatures exceeding 100 K.

Keyword
Quantum Wells, Excitons, Spin Relaxation, Fano resonance
National Category
Natural Sciences Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-74674 (URN)
Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2017-03-27Bibliographically approved
4. Effects of a longitudinal magnetic field on spin injection and detection in InAs/GaAs quantum dot structures
Open this publication in new window or tab >>Effects of a longitudinal magnetic field on spin injection and detection in InAs/GaAs quantum dot structures
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2012 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 14, 145304- p.Article in journal (Refereed) Published
Abstract [en]

Effects of a longitudinal magnetic field on optical spin injection and detection in InAs/GaAs quantum dot (QD) structures are investigated by optical orientation spectroscopy. An increase in optical and spin polarization of the QDs is observed with increasing magnetic field in the range of 0-2 T, and is attributed to suppression of exciton spin depolarization within the QDs that is promoted by hyperfine interaction and anisotropic electron-hole exchange interaction. This leads to a corresponding enhancement in spin detection efficiency of the QDs by a factor of up to 2.5. At higher magnetic fields when these spin depolarization processes are quenched, electron spin polarization in anisotropic QD structures (such as double QDs that are preferably aligned along a specific crystallographic axis) still exhibits rather strong field dependence under non-resonant excitation. In contrast, such field dependence is practically absent in more "isotropic" QD structures (e.g. single QDs). We attribute the observed effect to stronger electron spin relaxation in the spin injectors (i.e. wetting layer and GaAs barriers) of the lower-symmetry QD structures, which also explains the lower spin injection efficiency observed in these structures.

National Category
Natural Sciences Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-74672 (URN)10.1088/0953-8984/24/14/145304 (DOI)000302120500007 ()
Note
funding agencies|Swedish Research Council||Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2017-12-08Bibliographically approved
5. Temperature dependence of dynamic nuclear polarization and its effect on electron spin relaxation and dephasing in InAs/GaAs quantum dots
Open this publication in new window or tab >>Temperature dependence of dynamic nuclear polarization and its effect on electron spin relaxation and dephasing in InAs/GaAs quantum dots
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2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 100, no 14, 143105- p.Article in journal (Refereed) Published
Abstract [en]

Electron spin dephasing and relaxation due to hyperfine interaction with nuclear spins is studied in an InAs/GaAs quantum dot ensemble as a function of temperature up to 85 K, in an applied longitudinal magnetic field. The extent of hyperfineinduced dephasing is found to decrease, whereas dynamic nuclear polarization increases with increasing temperature. We attribute both effects to an accelerating electron spin relaxation through phonon-assisted electron-nuclear spin flip-flops driven by hyperfine interactions, which could become the dominating contribution to electron spin depolarization at high temperatures.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
National Category
Natural Sciences Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-75095 (URN)10.1063/1.3701273 (DOI)000302567800060 ()
Note
funding agencies|Swedish Research Council| 621-2011-4254 |Available from: 2012-02-16 Created: 2012-02-16 Last updated: 2017-12-07Bibliographically approved
6. Strong room-temperature optical and spin polarization in InAs/GaAs quantum dot structures
Open this publication in new window or tab >>Strong room-temperature optical and spin polarization in InAs/GaAs quantum dot structures
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2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 20, 203110- p.Article in journal (Refereed) Published
Abstract [en]

Room-temperature optical and spin polarization up to 35% is reported in InAs/GaAs quantum dots in zero magnetic field under optical spin injection using continuous-wave optical orientation spectroscopy. The observed strong spin polarization is suggested to be facilitated by a shortened trion lifetime, which constrains electron spin relaxation. Our finding provides experimental demonstration of the highly anticipated capability of semiconductor quantum dots as highly polarized spin/light sources and efficient spin detectors, with efficiency greater than 35% in the studied quantum dots.

Place, publisher, year, edition, pages
American Institute of Physics, 2011
National Category
Condensed Matter Physics Telecommunications
Identifiers
urn:nbn:se:liu:diva-68961 (URN)10.1063/1.3592572 (DOI)000290812100058 ()
Note
Original Publication: Jan Beyer, Irina A Buyanova, S. Suraprapapich, C. W. Tu and Weimin Chen, Strong room-temperature optical and spin polarization in InAs/GaAs quantum dot structures, 2011, Applied Physics Letters, (98), 20, 203110. http://dx.doi.org/10.1063/1.3592572 Copyright: American Institute of Physics http://www.aip.org/ Available from: 2011-06-13 Created: 2011-06-13 Last updated: 2017-12-11
7. Hanle effect and electron spin polarization in InAs/GaAs quantum dots up to room temperature
Open this publication in new window or tab >>Hanle effect and electron spin polarization in InAs/GaAs quantum dots up to room temperature
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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
National Category
Natural Sciences Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-75096 (URN)10.1088/0957-4484/23/13/135705 (DOI)000301663900015 ()
Note
funding agencies|Swedish Research Council| 621-2011-4254 |Available from: 2012-02-16 Created: 2012-02-16 Last updated: 2017-12-07Bibliographically approved

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