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Spectroscopy of semiconductor quantum dots
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Quantum dots in the Si/Ge and InAs/GaAs materials systems are examined by means of photoluminescence. The spectroscopic study of Si/Ge quantum dots has demonstrated two different radiative recombination channels in the type II band alignment: The spatially direct transition inside the dot and the spatially indirect transition across the dot interface. Increased sample temperature results in a gradual transfer from the spatially indirect to the spatially direct recombination due to higher oscillator strength combined with the increased electron population inside the dot. In contrast to the spatially direct transition, the spatially indirect transition is shown to be sensitive to the carrier density due to the band bending at the Si/Ge interface. Due to an increased Si/Ge intermixing and hence reduced strain in the Si barrier, a reduction of the conduction band offset at increased growth temperatures is observed utilizing the different recombination channels as probes. The optical properties as derived from photoluminescence are correlated with the structural properties obtained by atomic force microscopy. Furthermore, by applying an electric field across the Si/Ge quantum dot structure, a reversed quantum confined Stark effect is demonstrated for the spatially indirect transition. By switching between the two different field directions, unique information on the growth related asymmetric strain profile derived at the through self-assembly of the quantum dots can be gained since corresponding information can not be obtained for type I systems.

The studies of the InAs/GaAs quantum dots show that external electric and magnetic fields alter the in-plane carrier transport to the dots. The results obtained from the micro-photoluminescence exciton spectra of a single dot demonstrate a redistribution of the excitonic lines when a lateral electric field is applied. This fact exhibits an effective charge reconfiguration of the dot from a purely negative charge state to a neutral state, demonstrating that the number of electrons and holes are controlled by the electric field. The model proposed to explain the charge redistribution is based on an effective hole localization at the potential fluctuations of the wetting layer at low temperatures and low fields. Furthermore, it is demonstrated that the quantum dot photoluminescence signal is considerably increased (up to a factor of 4) depending on the magnitude of the external electric field. The experimental results also show that the internal field is altered by an additional infrared illumination of the sample. An applied magnetic field perpendicular to the quantum dot layer at low temperatures is found to enhance the carrier localization in the wetting layer and accordingly reduce the quantum dot photoluminescence intensity. At higher temperatures (>100K), an enhanced photoluminescence intensity is instead observed due to increased capture, localization, and recombination rate of the carriers in the quantum dots.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2005. , 65 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 976
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-31204Local ID: 16952ISBN: 91-85457-48-5 (print)OAI: oai:DiVA.org:liu-31204DiVA: diva2:252027
Public defence
2005-11-18, Hörsal Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2012-12-11
List of papers
1. Spatially direct and indirect transitions observed for Si/Ge quantum dots
Open this publication in new window or tab >>Spatially direct and indirect transitions observed for Si/Ge quantum dots
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2003 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 82, no 26, 4785-4787 p.Article in journal (Refereed) Published
Abstract [en]

The optical properties of Ge quantum dots embedded in Si were investigated by means of photoluminescence, with temperature and excitation power density as variable parameters. Two different types of recombination processes related to the Ge quantum dots were observed. A transfer from the spatially indirect to the spatially direct recombination in the type-II band lineup was observed with increasing temperature. A blueshift of the spatially indirect Ge quantum-dot-emission energy with increasing excitation power is ascribed to band bending at the type-II Si/Ge interface for high carrier densities. Comparative studies were performed on uncapped Ge dot structures.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13794 (URN)10.1063/1.1587259 (DOI)
Available from: 2006-02-27 Created: 2006-02-27 Last updated: 2017-12-13
2. Reversed quantum-confined Stark effect and an asymmetric band alignment observed for type-II Si∕Ge quantum dots
Open this publication in new window or tab >>Reversed quantum-confined Stark effect and an asymmetric band alignment observed for type-II Si∕Ge quantum dots
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2005 (English)In: Physical Review B, ISSN 1098-0121, Vol. 71, no 11, 113301- p.Article in journal (Refereed) Published
Abstract [en]

We report on the quantum-confined Stark effect for spatially indirect transitions in Stranski-Krastanov grown type-II Si∕Ge quantum dots. A linear blueshift of the spatially indirect transition is observed at increasing electric field in contrast to the commonly observed redshift for type-I transitions. A shift of the emission-peak position and different quenching rates of the photoluminescence for p-i-n and n-i-p diodes at increased electric field and temperature indicate a deeper notch potential for electrons above the dot than below due to a strain-induced asymmetry in the band alignment.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16331 (URN)10.1103/PhysRevB.71.113301 (DOI)
Note

Original Publication: Mats Larsson, Per-Olof Holtz, Anders Elfving, Göran Hansson and Wei-Xin Ni, Reversed quantum-confined Stark effect and an asymmetric band alignment observed for type-II Si∕Ge quantum dots, 2005, Physical Review B, (71), 113301. http://dx.doi.org/10.1103/PhysRevB.71.113301 Copyright: American Physical Society http://www.aps.org/

Available from: 2009-01-15 Created: 2009-01-15 Last updated: 2012-12-11Bibliographically approved
3. Band alignment studies in Si/Ge quantum dots based on optical and structural investigations
Open this publication in new window or tab >>Band alignment studies in Si/Ge quantum dots based on optical and structural investigations
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The present work is a photoluminescence study of Si-embedded Stranski-Krastanov Ge quantum dots. The value of the conduction band offset is a result of the magnitude of the tensile strain in the Si surrounding the compressive strained Ge dot. Due to the increased Si/Ge intermixing and reduced strain in the Si barrier, a reduction of the conduction band offset is observed at increased growth temperatures. The optical properties as derived from photoluminescence spectroscopy are correlated with structural properties obtained as a function of the growth temperature. High growth temperatures result in large Ge dots with low density due to the pronounced surface diffusion and Si/Ge intermixing. As confirmed by photoluminescence, the band gap of the Ge dots increases with increased growth temperature due to the higher degree of Si/Ge intermixing. The band alignment is of type-II in these structures, but the occurrence of both spatially indirect and spatially direct transitions are confirmed in temperature dependent photoluminescence measurements with varied excitation power conditions. An increasing temperature results in a gradual transition from the spatially indirect to the spatially direct recombination in the type-II band lineup, due to higher oscillator strength for the spatially direct transition combined with a higher population factor at higher temperatures.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-86206 (URN)
Available from: 2012-12-11 Created: 2012-12-11 Last updated: 2012-12-11
4. Magnetic field effects on optical and transport properties in InAs/GaAs quantum dots
Open this publication in new window or tab >>Magnetic field effects on optical and transport properties in InAs/GaAs quantum dots
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2006 (English)In: Physical Review B, ISSN 1098-0121, Vol. 74, no 24Article in journal (Refereed) Published
Abstract [en]

A photoluminescence study of self-assembled InAs/GaAs quantum dots under the influence of magnetic fields perpendicular and parallel to the dot layer is presented. At low temperatures, the magnetic field perpendicular to the dot layer alters the in-plane transport properties due to localization of carriers in wetting layer (WL) potential fluctuations. Decreased transport in the WL results in a reduced capture into the quantum dots and consequently a weakened dot-related emission. The effect of the magnetic field exhibits a considerable dot density dependence, which confirms the correlation to the in-plane transport properties. An interesting effect is observed at temperatures above approximately 100  K, for which magnetic fields, both perpendicular and parallel to the dot layer, induced an increment of the quantum dot photoluminescence. This effect is ascribed to the magnetic confinement of the exciton wave function, which increases the probability for carrier capture and localization in the dot, but affects also the radiative recombination with a reduced radiative lifetime in the dots under magnetic compression.

Keyword
indium compounds, gallium arsenide, III-V semiconductors, semiconductor quantum dots, photoluminescence, magneto-optical effects, self-assembly, localised states, excitons, wave functions, radiative lifetimes, galvanomagnetic effects
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16330 (URN)10.1103/PhysRevB.74.245312 (DOI)
Note
Original Publication: Mats Larsson, Evgenii Moskalenko, Andréas Larsson, Per-Olof Holtz, C. Verdozzi, C.-O. Almbladh, W. V. Schoenfeld and P. M. Petroff, Magnetic field effects on optical and transport properties in InAs/GaAs quantum dots, 2006, Physical Review B, (74), 245312. http://dx.doi.org/10.1103/PhysRevB.74.245312 Copyright: American Physical Society http://www.aps.org/ Available from: 2009-01-15 Created: 2009-01-15 Last updated: 2012-12-11Bibliographically approved
5. Single InAs/GaAs quantum dot spectroscopy in a lateral electric field
Open this publication in new window or tab >>Single InAs/GaAs quantum dot spectroscopy in a lateral electric field
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We report on the comprehensive study of InAs/GaAs single quantum dots subjected to a lateral external electric field by means of micro-photoluminescence (μ-PL) technique. The results obtained on the exciton in the μ-PL spectra of a single dot demonstrate a considerable PL intensity enhancement (up to a factor of 4) of the dot as well as a redistribution of the excitonic lines when an electric field is applied. The latter fact exhibits an effective charge reconfiguration of the dot from a purely negatively charged to a neutral state. The model proposed to explain the charge redistribution is based on an effective hole localization at the potential fluctuations of the wetting layer at low temperature and bias.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-86208 (URN)
Available from: 2012-12-11 Created: 2012-12-11 Last updated: 2012-12-11
6. The effect of the external lateral electric field on the luminescence intensity of InAs/GaAs quantum dots
Open this publication in new window or tab >>The effect of the external lateral electric field on the luminescence intensity of InAs/GaAs quantum dots
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2007 (English)In: Physics of the solid state, ISSN 1063-7834, E-ISSN 1090-6460, Vol. 49, no 10, 1995-1998 p.Article in journal (Refereed) Published
Abstract [en]

We report on low-temperature microphotoluminescence (μ-PL) measurements of InAs/GaAs quantum dots (QDs) exposed to a lateral external electric field. It is demonstrated that the QDs’ PL signal could be increased severalfold by altering the external and/or the internal electric field, which could be changed by an additional infrared laser. A model which accounts for a substantially faster lateral transport of the photoexcited carriers achieved in an external electric field is employed to explain the observed effects. The results obtained suggest that the lateral electric fields play a major role for the dot luminescence intensity measured in our experiment—a finding which could be used to tailor the properties of QD-based optoelectronic applications.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-39862 (URN)10.1134/S1063783407100307 (DOI)51544 (Local ID)51544 (Archive number)51544 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-12-13

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