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Optical spectroscopy of InGaAs quantum dots
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
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. , 48 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1363
Keyword [en]
quantum dot, semiconductor, optical properties, spin, transport, wetting layer, polarization
Keyword [sv]
kvantprick, halvledare, optiska egenskaper, spin
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Materials Engineering Condensed Matter Physics Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-64707ISBN: 978-91-7393-238-7 (print)OAI: oai:DiVA.org:liu-64707DiVA: diva2:394191
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
List of papers
1. Effective tuning of the charge state of a single InAs/GaAs quantum dot by an external magnetic field
Open this publication in new window or tab >>Effective tuning of the charge state of a single InAs/GaAs quantum dot by an external magnetic field
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2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 7, 075306- p.Article in journal (Refereed) Published
Abstract [en]

A microphotoluminescence study of single InAs/GaAs quantum dots (QDs) in the presence of an applied external magnetic field is presented. Attention is focused on the redistribution between the spectral lines of a single QD observed at increasing magnetic field parallel to the growth direction (Faraday geometry). The redistribution effect is explained by considering the electron drift velocity in the QD plane that affects the probability for capture into the QD. In contrast, no redistribution is observed when applying the magnetic field perpendicular to the growth direction (Voigt geometry).

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16333 (URN)10.1103/PhysRevB.78.075306 (DOI)
Note

Original Publication: Evgenii Moskalenko, Andréas Larsson, Mats Larsson, Per-Olof Holtz, W. V. Schoenfeld and P. M. Petroff, Effective tuning of the charge state of a single InAs/GaAs quantum dot by an external magnetic field, 2008, Physical Review B Condensed Matter, (78), 075306. http://dx.doi.org/10.1103/PhysRevB.78.075306 Copyright: American Physical Society http://www.aps.org/

Available from: 2009-01-15 Created: 2009-01-15 Last updated: 2013-05-07Bibliographically approved
2. Comparative Magneto-Photoluminescence Study of Ensembles and of Individual InAs Quantum Dots
Open this publication in new window or tab >>Comparative Magneto-Photoluminescence Study of Ensembles and of Individual InAs Quantum Dots
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2009 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 9, no 1, 353-359 p.Article 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: 2011-02-03Bibliographically approved
3. Temperature and Magnetic Field Effects on the Transport Controlled Charge State of a Single Quantum Dot
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, 1150-1155 p.Article 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
Keyword
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
4. Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field
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, 193413- p.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.

Keyword
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: 2011-02-03Bibliographically approved
5.
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6. Manipulating the Spin Polarization of Excitons in a Single Quantum Dot by Optical Means
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, 071906- p.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: 2011-03-30Bibliographically approved
7. Hole Shake-Up in Individual InGaAs Quantum Dots
Open this publication in new window or tab >>Hole Shake-Up in Individual InGaAs Quantum Dots
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We report on a spectroscopic study of hole shake-up processes in single InGaAs quantum dots. By studying dots with very high luminescence efficiency these processes are unveiled and further tested in an applied magnetic field. The mechanism is attributed to shake-up of a hole from the S ground state to the D excited state. The experimental results are confirmed by configuration interaction calculations that also reveal a dependence of the shake-up intensity on the relative extension of electron and hole wave functions.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-64726 (URN)
Available from: 2011-02-03 Created: 2011-02-03 Last updated: 2016-06-07Bibliographically approved

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