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Larsson, Arvid
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Larsson, A., Moskalenko, E. & Holtz, P.-O. (2011). Manipulating the Spin Polarization of Excitons in a Single Quantum Dot by Optical Means. Applied Physics Letters, 98(7), 071906
Open this publication in new window or tab >>Manipulating the Spin Polarization of Excitons in a Single Quantum Dot by Optical Means
2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 7, p. 071906-Article in journal (Refereed) Published
Abstract [en]

Circular polarization studies of photoluminescence from the neutral (X0) and the positively charged (X+) exciton are reported for individual InAs/GaAs quantum dots (QDs). High polarization degrees, 60 % for X0 and 73 % for X+, were recorded without any external magnetic field applied. These studies show that that the QD polarization and population dynamics are controllable either by varying the photo-excitation intensity, or by using a second IR laser excitation.

Place, publisher, year, edition, pages
American Institute of Physics, 2011
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-64725 (URN)10.1063/1.3554422 (DOI)000287507200017 ()
Note
Original Publication: Arvid Larsson, Evgenii Moskalenko and Per-Olof Holtz, Manipulating the Spin Polarization of Excitons in a Single Quantum Dot by Optical Means, 2011, Applied Physics Letters, (98), 7, 071906. http://dx.doi.org/10.1063/1.3554422 Copyright: American Institute of Physics http://www.aip.org/ Available from: 2011-02-03 Created: 2011-02-03 Last updated: 2017-12-11Bibliographically approved
Larsson, A. (2011). Optical spectroscopy of InGaAs quantum dots. (Doctoral dissertation). Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>Optical spectroscopy of InGaAs quantum dots
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis deals with optical studies of semiconductor quantum dots (QDs) in the InGaAs material system. It is shown that for self-assembled InAs QDs, the interaction with the surrounding GaAs barrier and the InAs wetting layer (WL) in particular, has a very large impact on their optical properties.

The ability to control the charge state of individual QDs is demonstrated and attributed to a modulation in the carrier transport dynamics in the WL. After photo-excitation of carriers (electrons and holes) in the barrier, they will migrate in the sample and with a certain probability become captured into a QD. During this migration, the carriers can be affected by exerting them to an external magnetic field or by altering the temperature.

An external magnetic field applied perpendicular to the carrier transport direction will lead to a decrease in the carrier drift velocity since their trajectories are bent, and at sufficiently high field strength become circular. In turn, this decreases the probability for the carriers to reach the QD since the probability for the carriers to get trapped in WL localizing potentials increases. An elevated temperature leads to an increased escape rate out of these potentials and again increases the flow of carriers towards the QD. These effects have significantly different strengths for electrons and holes due to the large difference in their respective masses and therefore it constitutes a way to control the supply of charges to the QD.

Another effect of the different capture probabilities for electrons and holes into a QD that is explored is the ability to achieve spin polarization of the neutral exciton (X0). It has been concluded frequently in the literature that X0 cannot maintain its spin without application of an external magnetic field, due to the anisotropic electron – hole exchange interaction (AEI). In our studies, we show that at certain excitation conditions, the AEI can be by-passed since an electron is captured faster than a hole into a QD. The result is that the electron will populate the QD solely for a certain time window, before the hole is captured. During this time window and at polarized excitation, which creates spin polarized carriers, the electron can polarize the QD nuclei. In this way, a nuclear magnetic field is built up with a magnitude as high as ~ 1.5 T. This field will stabilize the X0 spin in a similar manner as an external magnetic field would. The build-up time for this nuclear field was determined to be ~ 10 ms and the polarization degree achieved for X0 is ~ 60 %.

In contrast to the case of X0, the AEI is naturally cancelled for the negatively charged exciton (X-) and the positively charged exciton (X+) complexes. This is due to the fact that the electron (hole) spin is paired off in case of X- (X+).  Accordingly, an even higher polarization degree (~ 73 %) is measured for the positively charged exciton.

In a different study, pyramidal QD structures were employed. In contrast to fabrication of self-assembled QDs, the position of QDs can be controlled in these samples as they are grown in inverted pyramids that are etched into a substrate. After sample processing, the result is free-standing AlGaAs pyramids with InGaAs QDs inside. Due to the pyramidal shape of these structures, the light extraction is considerably enhanced which opens up possibilities to study processes un-resolvable in self-assembled QDs. This has allowed studies of Auger-like shake-up processes of holes in single QDs. Normally, after radiative recombination of X+, the QD is populated with a ground state hole. However, at recombination, a fraction of the energy can be transferred to the hole so that it afterwards occupies an excited state instead. This process is detected experimentally as a red-shifted luminescence satellite peak with an intensity on the order of ~ 1/1000 of the main X+ peak intensity. The identification of the satellite peak is based on its intensity correlation with the X+ peak, photoluminescence excitation measurements and on magnetic field measurements.

Abstract [sv]

Arbetet som presenteras i denna avhandling rör studier av kvantprickars optiska egenskaper. En kvantprick är en halvledarkristall som endast är några tiotals nanometer stor. Den ligger oftast inbäddad inuti en större kristall av ett annat halvledarmaterial och pga. den begränsade storleken får en kvantprick mycket speciella egenskaper. Bland annat så kommer elektronerna i en kvantprick endast att kunna anta vissa diskreta energinivåer liknande situationen för elektronerna i en atom. Följaktligen kallas kvantprickar ofta för artificiella atomer.

För halvledarmaterial gäller det generellt att det inte endast är fria elektroner i ledningsbandet, som kan leda ström utan även tomma elektrontillstånd i valens­bandet, vilka uppträder som positivt laddade partiklar, kan leda ström. Dessa kallas kort och gott för hål. I en kvantprick har hålen såsom elektronerna helt diskreta energinivåer.

Precis som är fallet i en atom, så kommer elektroniska övergångar mellan olika energi­nivåer i en kvantprick att resultera i att ljus emitteras. Energin (dvs. våglängden alt. färgen) för detta ljus bestäms av hur energinivåerna i kvant­pricken ligger, för elektronerna och hålen, och genom att analysera ljuset kan man således studera kvantprickens egenskaper.

Studierna i den här avhandlingen visar att växelverkan mellan en kvantprick och den omgivande kristallen, som den ligger inbäddad i, har stor inverkan på kvantprickens optiska egenskaper. T.ex. visas att man kan kontrollera antalet elektroner, som kommer att finnas i kvantpricken genom att modifiera hur elektronerna kan röra sig i omgivningen. Dessa rörelser modifieras här genom att variera temperaturen och genom att lägga på ett magnetiskt fält.

Ett magnetiskt fält, vinkelrätt mot en elektrons rörelse, kommer att böja av dess bana och dess chans att nå fram till kvantpricken kan således minskas. Elektronen kan då istället fastna i andra potentialgropar i kvantprickens närhet. Genom att öka temperaturen, vilket ger elektronerna större energi, kan deras chans att nå fram till kvantpricken å andra sidan öka.

En annan effekt, som studerats, är möjligheten att kontrollera spinnet hos elektronerna i en kvantprick. Även i dessa studier visar det sig att växelverkan med omgivningen spelar stor roll och kan användas till att kontrollera elektronens spin.

Mekanismen som föreslås är att om elektronerna hinner före hålen till kvantpricken, så hinner de överföra sitt spin till atomkärnorna i kvantpricken. På detta sätt kan man få atomkärnornas spin polariserat, vilket resulterar i ett inbyggt magnetfält, i storleksordningen 1.5 Tesla, som i sin tur hjälper till att upprätthålla en hög grad av spinpolarisering även hos elektronerna. För att få elektronerna att hinna först, måste deras rörelser i omgivningen kontrolleras.

I en ytterligare studie undersöktes den process där en elektronisk övergång i kvantpricken inte enbart resulterar i emission av ljus, utan även i att en annan partikel tar över en del av energin och blir exciterad. Dessa processer avspeglas i att en del av det ljus som emitteras har lägre energi. Detta ljus är också mycket svagt, ca 1000 ggr lägre intensitet, och möjligheten att kunna mäta detta är helt beroende på hur ljusstarka kvantprickarna är. De prover som använts i denna studie består av pyramidstrukturer, ca 7.5 mikrometer stora, med kvantprickar inuti. Denna geometri ger ca 1000 ggr bättre ljusutbyte jämfört med traditionella strukturer, vilket möjliggjort studien.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2011. p. 48
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1363
Keywords
quantum dot, semiconductor, optical properties, spin, transport, wetting layer, polarization, kvantprick, halvledare, optiska egenskaper, spin
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Materials Engineering Condensed Matter Physics Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-64707 (URN)978-91-7393-238-7 (ISBN)
Public defence
2011-02-25, Planck, Hus E, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2011-02-03 Created: 2011-02-02 Last updated: 2011-02-03Bibliographically approved
Moskalenko, E., Larsson, A. & Holtz, P.-O. (2010). Spin polarization of neutral excitons in quantum dots: the role of the carrier collection area. Nanotechnology, 21(34)
Open this publication in new window or tab >>Spin polarization of neutral excitons in quantum dots: the role of the carrier collection area
2010 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, NANOTECHNOLOGY, Vol. 21, no 34Article in journal (Refereed) Published
Abstract [en]

A high degree (approximate to 55%) of circular polarization has been observed for the neutral exciton in InAs/GaAs quantum dots (QDs). The possibility to record non- zero polarization of the neutral exciton is explained in terms of different capture times of the light electron compared with the heavier holes into the QDs from the wetting layer. This interpretation is supported by the progressive reduction of the polarization degree with increasing QD density, and also with increasing temperature.

Place, publisher, year, edition, pages
Institute of Physics, 2010
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-58644 (URN)10.1088/0957-4484/21/34/345401 (DOI)000280632500009 ()
Note
Original Publication: Evgenii Moskalenko, Arvid Larsson and Per-Olof Holtz, Spin polarization of neutral excitons in quantum dots: the role of the carrier collection area, 2010, NANOTECHNOLOGY, (21), 34, 345501. http://dx.doi.org/10.1088/0957-4484/21/34/345401 Copyright: Institute of Physics; 1999 http://www.iop.org/ Available from: 2010-08-22 Created: 2010-08-20 Last updated: 2017-12-12Bibliographically approved
Larsson, L. A., Larsson, M., Moskalenko, E. S. & Holtz, P.-O. (2010). Temperature and Magnetic Field Effects on the Transport Controlled Charge State of a Single Quantum Dot. NANOSCALE RESEARCH LETTERS, 5(7), 1150-1155
Open this publication in new window or tab >>Temperature and Magnetic Field Effects on the Transport Controlled Charge State of a Single Quantum Dot
2010 (English)In: NANOSCALE RESEARCH LETTERS, ISSN 1931-7573, Vol. 5, no 7, p. 1150-1155Article in journal (Refereed) Published
Abstract [en]

Individual InAs/GaAs quantum dots are studied by micro-photoluminescence. By varying the strength of an applied external magnetic field and/or the temperature, it is demonstrated that the charge state of a single quantum dot can be tuned. This tuning effect is shown to be related to the in-plane electron and hole transport, prior to capture into the quantum dot, since the photo-excited carriers are primarily generated in the barrier.

Place, publisher, year, edition, pages
Springer Science Business Media, 2010
Keywords
Quantum dot; Wetting layer; Magnetic field; Temperature dependence; Charge state
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-58223 (URN)10.1007/s11671-010-9618-x (DOI)000279674400012 ()
Note
The original publication is available at www.springerlink.com: L. Arvid Larsson, Mats Larsson, E. S. Moskalenko and Per-Olof Holtz, Temperature and Magnetic Field Effects on the Transport Controlled Charge State of a Single Quantum Dot, 2010, NANOSCALE RESEARCH LETTERS, (5), 7, 1150-1155. http://dx.doi.org/10.1007/s11671-010-9618-x Copyright: Springer Science Business Media http://www.springerlink.com/ Available from: 2010-08-10 Created: 2010-08-09 Last updated: 2011-02-03Bibliographically approved
Moskalenko, E., Larsson, A., Larsson, M., Holtz, P.-O., Schoenfeld, W. V. & Petroff, P. M. (2009). Comparative Magneto-Photoluminescence Study of Ensembles and of Individual InAs Quantum Dots. Nano letters (Print), 9(1), 353-359
Open this publication in new window or tab >>Comparative Magneto-Photoluminescence Study of Ensembles and of Individual InAs Quantum Dots
Show others...
2009 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 9, no 1, p. 353-359Article in journal (Refereed) Published
Abstract [en]

We report on magneto-photoluminescence studies of InAs/GaAs quantum dots (QDs) of considerably different densities, from dense ensembles down to individual dots. It is found that a magnetic field applied in Faraday geometry decreases the photoluminescence (PL) intensity of OD ensembles, which is not accompanied by the corresponding increase of PL signal of the wetting layer on which ON are grown. The model suggested to explain these data assumes considerably different strengths of suppression of electron and hole fluxes by a magnetic field. This idea has been successfully checked in experiments on individual ON, where the PL spectra allow to directly monitor the charge state of a OD and, hence, to conclude about relative magnitudes of electron and hole fluxes toward the QD. Comparative studies of different individual QDs have revealed that the internal electric field in the sample (which was altered in the experiments in a controllable way) together with an external magnetic field will determine the charge state and emission intensity of the QDs.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16615 (URN)10.1021/nl803148q (DOI)
Available from: 2009-02-07 Created: 2009-02-06 Last updated: 2017-12-14Bibliographically approved
Larsson, A., Moskalenko, E., Larsson, M. & Holtz, P.-O. (2009). Effective tuning of the charge state of a single InAs/GaAs quantum dot by means of external fields. In: IEEE Nano 2009 in Genua.
Open this publication in new window or tab >>Effective tuning of the charge state of a single InAs/GaAs quantum dot by means of external fields
2009 (English)In: IEEE Nano 2009 in Genua, 2009Conference paper, Published paper (Refereed)
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-50824 (URN)
Available from: 2009-10-14 Created: 2009-10-14 Last updated: 2009-10-17
Larsson, A., Dufåker, D., Karlsson, F. & Holtz, P.-O. (2009). Functionalisation of ZnO Nanoparticles Using Organic Acids. In: Nordic Semiconductor Meeting, June 2009 . Reykjavik
Open this publication in new window or tab >>Functionalisation of ZnO Nanoparticles Using Organic Acids
2009 (English)In: Nordic Semiconductor Meeting, June 2009 , Reykjavik, 2009Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Reykjavik: , 2009
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-50822 (URN)
Available from: 2009-10-14 Created: 2009-10-14 Last updated: 2015-01-23
Larsson, A., Larsson, M., Holtz, P.-O. & Moskalenko, E. (2009). Magnetic field enabled charge state control of single InAs/GaAs quantum dots. In: 2009 9th IEEE Conference on Nanotechnology, IEEE NANO 2009 (pp. 510-512).
Open this publication in new window or tab >>Magnetic field enabled charge state control of single InAs/GaAs quantum dots
2009 (English)In: 2009 9th IEEE Conference on Nanotechnology, IEEE NANO 2009, 2009, p. 510-512Conference paper, Published paper (Refereed)
Abstract [en]

Micro-photoluminescence in the presence of an external magnetic field is employed to study individual InAs/GaAs quantum dots (QDs). By varying the strength of the applied magnetic field, the charge state of the QD is tuned from a double negatively charged exciton to a neutral exciton. This effect is shown to be related to carrier transport in the QD-plane prior to capture into the QD. The temperature dependence of the tuning effect is discussed. © 2009 IEEE NANO Organizers.

Keywords
Faraday; Magnetic field; Photoluminescence; Quantum dot; Transport; Voigt
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-57052 (URN)
Available from: 2010-06-11 Created: 2010-06-09 Last updated: 2011-01-27
Khranovskyy, V., Tsiaoussis, I., Larsson, A., Holtz, P.-O. & Yakimova, R. (2009). Nanointegration of ZnO with Si and SiC. PHYSICA B-CONDENSED MATTER, 404(22), 4359-4363
Open this publication in new window or tab >>Nanointegration of ZnO with Si and SiC
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2009 (English)In: PHYSICA B-CONDENSED MATTER, ISSN 0921-4526, Vol. 404, no 22, p. 4359-4363Article in journal (Refereed) Published
Abstract [en]

The study is dedicated to some aspects of the controlled heteroepitaxial growth of nanoscaled ZnO structures and an investigation of their general and dimension mediated properties. ZnO nanostructures were synthesized by optimized MOCVD process via two growth approaches: (i) catalyst free self-organized growth of ZnO on Si substrates and (ii) ZnO heteroepitaxy on p-type hexagonal 4H-SiC substrates. The SiC substrate was prepared by sublimation epitaxy and served as a template for the ZnO epitaxial growth. The epitaxial growth of n-ZnO on p-SiC resulted in a regular matrix of well-faceted hexagonally shaped ZnO single crystals. The achievement of ZnO integration with Si encompasses controlled growth of vertically oriented nanosized ZnO pillars. The grown structures were characterized by transmission electron microscopy and microphotoluminescence. Low concentration of native defects due to a stoichiometry balance, advanced optical emission, (excitonic type near-band-edge emission and negligible defect related luminescence) and continuous interfaces (epitaxial relationship ZnO[0 0 0 1]/ SiC[0 0 0 1]) are evidenced. The ZnO nanopillars were further probed as field emitters: the grown structures exhibits advanced field emission properties, which are explained in term of dimensionality and spatial uniformity of the nanopillars. The present results contribute to understanding and resolving growth and device related issues of ZnO as a functional nanostructured material.

Keywords
Crystal growth, Heteroepitaxy, ZnO, Nanopillars, p-n junction
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-52771 (URN)10.1016/j.physb.2009.09.032 (DOI)
Available from: 2010-01-12 Created: 2010-01-12 Last updated: 2011-01-27
Moskalenko, E., Larsson, A. & Holtz, P.-O. (2009). Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field. Physical Review B. Condensed Matter and Materials Physics, 80(19), 193413
Open this publication in new window or tab >>Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field
2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 19, p. 193413-Article in journal (Refereed) Published
Abstract [en]

A high degree of spin polarization for the neutral exciton in individual quantum dots, at zero external magnetic field, is monitored. While a high polarization degree is commonly observed for the charged exciton, a negligible polarization has been predicted for the neutral exciton. The exceptionally high polarization (andgt;60%) observed here is explained in terms of a dynamical nuclear polarization field, stabilizing the electron spin. Such polarization of the quantum dot nuclei, in case of the neutral exciton, is possible due to unequal capture time of electrons and holes.

Keywords
electron spin polarisation, excitons, III-V semiconductors, indium compounds, semiconductor quantum dots
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-52822 (URN)10.1103/PhysRevB.80.193413 (DOI)
Note
Original Publication: Evgenii Moskalenko, L A Larsson and Per-Olof Holtz, Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field, 2009, PHYSICAL REVIEW B, (80), 19, 193413. http://dx.doi.org/10.1103/PhysRevB.80.193413 Copyright: American Physical Society http://www.aps.org/ Available from: 2010-01-12 Created: 2010-01-12 Last updated: 2017-12-12Bibliographically approved
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