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Polarization-resolved photoluminescence spectroscopy of III-nitride quantum dots
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, results from studies on (In)GaN quantum dots (QDs) are presented, including investigations of the structural, optical and electronic properties. The experimental studies were performed on GaN and InGaN QDs grown by molecular beam epitaxy, taking advantage of the Stranki-Krastanov growth mode for the GaN QD samples and the composition segregation for the InGaN QD samples.

Optical spectroscopy of the (In)GaN QDs was performed with a combination of different experimental techniques, e.g. stationary microphotoluminescence (μPL) and timeresolved μPL. The μPL spectroscopy is suitable for studies of single QDs due to the wellfocused excitation laser spot, and it typically does not require any special sample preparation. The powerful combination of power and polarization dependences was used to distinguish the exciton and the biexciton emissions from other emission lines in the recorded spectra.

The QDs could be observed with random in-plane anisotropy, as determined by the strong linear polarization for single QDs but with different angular orientation from dot to dot. Additionally, these experimental results are in good agreement with the computational results revealing a similar degree of polarization for the exciton and the biexciton emissions. Further, the theory predicts that the discrepancy of the polarization degree is larger between the positive and negative trions in comparison with the exciton and the biexciton. Based on this result, polarization resolved spectroscopy is proposed as a simple tool for the identification of trions and their charge states.

The fine-structure splitting (FSS) and the biexciton binding energy (Ebxx) are essential QD parameters of relevance for the possible generation of quantum entangled photon pairs in a cascade recombination of the biexciton. In general, the Coulomb interaction between the negatively charged electron and the positively charged hole lifts the fourfold degeneracy of the electron and hole pair ground state, forming a set of zero-dimensional exciton states of unequal energies. This Coulomb-induced splitting, referred to as the FSS, results in an electronic fine structure, which is strongly dependent on the symmetry of the exciton wave function. The FSS was in this work resolved and investigated for excitons in InGaN QDs, using polarization-sensitive μPL spectroscopy employed on the cleaved-edge of the samples. As expected, the FSS is found to exhibit identical magnitudes, but with reversed sign for the exciton and the biexciton. For quantum information applications, a vanishing FSS is required, since otherwise the emissions of the polarization-entangled photon pairs in the cascade biexciton recombination will be prohibited.

The biexcitons are found to exhibit both positive and negative binding energies for the investigated QDs. Since a negative binding energy indicates a repulsive Coulomb interaction, such biexcitons (or exciton complexes) cannot exist in structures of higher dimensionality. On the other hand, a biexciton with a negative binding energy can be found in QDs, since the exciton complexes still remain bound due to their three dimensional confinement. Moreover, the biexciton binding energy depends on the dot size, which implies that a careful size control of dots could enable manipulation of the biexciton binding energy. A large Ebxx value enables better and cheaper spectral filtering, in order to purify the single photon emission, while a proposed time reordering scheme relies on zero Ebxx for the generation of entangled photons.

The dynamics of the exciton and the biexciton emissions from InGaN QD were measured by means of time-resolved μPL. The lifetimes of the exciton related emissions are demonstrated to depend on the dot size. Both the exciton and the biexciton emissions reveal mono-exponential decays, with a biexciton lifetime, which is about two times shorter than the exciton lifetime. This implies that the QD is small, with a size comparable to the exciton Bohr radius. The photon generation rates can be manipulated by controlling the QDs size, which in turn can be utilized for generation of single- and entangled-photons on demand, with a potential for applications in e.g. quantum information.

The polarization of the emitted single photons can be manipulated by using a polarizer, but to the prize of photon loss and reduced emission intensity. Alternative methods to control the polarization of the emission light are a manipulation of the dot symmetry statically by its shape or dynamically by an externally applied electric field. Predictions based on performed calculations show that in materials with a small spin-orbit split-off energy (ΔSO), like the III-nitride materials, the polarization degree of the emission is more sensitive to dot asymmetry than in materials with a large value for ΔSO, e.g. the III-arsenide materials. Moreover, for an electric field applied in the 1͞10 and the 11͞2 directions of the zinc-blende lens-shaped QDs grown on the (111) plane, the polarization degree of InN QDs is found to be significantly more, by a factor of ~50 times, sensitive to the electric field than for GaN QDs. This work demonstrates that especially the InN based QD, are suitable for manipulation of the polarization by the direct control of the dot symmetry or by externally applied electric fields.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. , 68 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1494
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-87748ISBN: 978-91-7519-715-9 (print)OAI: oai:DiVA.org:liu-87748DiVA: diva2:599904
Public defence
2013-02-07, Plank, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-01-22 Created: 2013-01-22 Last updated: 2015-01-23Bibliographically approved
List of papers
1. Excitons and biexcitons in InGaN quantum dot like localization centers
Open this publication in new window or tab >>Excitons and biexcitons in InGaN quantum dot like localization centers
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Indium segregation in a narrow InGaN single quantum well creates quantum dot (QD) like exciton localization centers. Cross section transmission electron microscopy reveals varying shapes and lateral sizes in the range ~1-5 nm of the QD-like features, while scanning near field optical microscopy demonstrates a highly inhomogeneous spatial distribution of optically active individual localization centers. Microphotoluminescence spectroscopy confirms the spectrally inhomogeneous distribution of localization centers, in which the exciton and the biexciton related emissions from single centers of varying geometry could be identified by means of excitation power dependencies. Interestingly, the biexciton binding energy (Ebxx) was found to vary from center to center, between 3 to -22 meV, in correlation with the exciton emission energy. Negative binding energies justify the three-dimensional quantum confinement, which confirms QD-like properties of the localization centers.! The observed energy correlation is proposed to be understood as variations of the lateral extension of the confinement potential, which would yield smaller values of Ebxx for reduced lateral extension and higher exciton emission energy. The proposed relation between lateral extension and Ebxx is further supported by the exciton and the biexciton recombination lifetimes of a single QD, which suggest a lateral extension of merely ~3 nm for a QD with strongly negative Ebxx = -15.5 meV.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-87746 (URN)
Available from: 2013-01-22 Created: 2013-01-22 Last updated: 2016-06-07
2. Dynamic characteristics of the exciton and the biexciton in a single InGaN quantum dot
Open this publication in new window or tab >>Dynamic characteristics of the exciton and the biexciton in a single InGaN quantum dot
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2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, no 6Article in journal (Refereed) Published
Abstract [en]

The dynamics of the exciton and the biexciton related emission from a single InGaN quantum dot (QD) have been measured by time-resolved microphotoluminescence spectroscopy. An exciton-biexciton pair of the same QD was identified by the combination of power dependence and polarization-resolved spectroscopy. Moreover, the spectral temperature evolution was utilized in order to distinguish the biexciton from a trion. Both the exciton and the biexciton related emission reveal mono-exponential decays corresponding to time constants of similar to 900 and similar to 500 ps, respectively. The obtained lifetime ratio of similar to 1.8 indicates that the QD is small, with a size comparable to the exciton Bohr radius.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-84908 (URN)10.1063/1.4742343 (DOI)000307862400022 ()
Note

Funding Agencies|Thaksin University in Thailand||Swedish Research Council (VR)||Swedish Foundation for Strategic Research (SSF)||Knut and Alice Wallenberg Foundation||

Available from: 2012-10-26 Created: 2012-10-26 Last updated: 2017-12-07
3. Polarization-resolved fine-structure splitting of zero-dimensional InxGa1-xN excitons
Open this publication in new window or tab >>Polarization-resolved fine-structure splitting of zero-dimensional InxGa1-xN excitons
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2011 (English)In: PHYSICAL REVIEW B, ISSN 1098-0121, Vol. 83, no 20, 201307- p.Article in journal (Refereed) Published
Abstract [en]

The fine-structure splitting of quantum confined InxGa1-x Nexcitons is investigated using polarization-sensitive photoluminescence spectroscopy. The majority of the studied emission lines exhibits mutually orthogonal fine-structure components split by 100-340 mu eV, as measured from the cleaved edge of the sample. The exciton and the biexciton reveal identical magnitudes but reversed sign of the energy splitting.

Place, publisher, year, edition, pages
American Physical Society, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-68780 (URN)10.1103/PhysRevB.83.201307 (DOI)000290715600002 ()
Note
Original Publication: Supaluck Amloy, Y T Chen, K F Karlsson, K H Chen, H C Hsu, C L Hsiao, L C Chen and Per-Olof Holtz, Polarization-resolved fine-structure splitting of zero-dimensional InxGa1-xN excitons, 2011, PHYSICAL REVIEW B, (83), 20, 201307. http://dx.doi.org/10.1103/PhysRevB.83.201307 Copyright: American Physical Society http://www.aps.org/ Available from: 2011-06-08 Created: 2011-06-07 Last updated: 2015-01-23
4. On the polarized emission from exciton complexes in GaN quantum dots
Open this publication in new window or tab >>On the polarized emission from exciton complexes in GaN quantum dots
2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 100, no 021901Article in journal (Refereed) Published
Abstract [en]

The optical linear polarization properties of exciton complexes in asymmetric Stranski-Krastanov grown GaN quantum dots have been investigated experimentally and theoretically. It is demonstrated that the polarization angle and the polarization degree can be conveniently employed to associate emission lines in the recorded photoluminescence spectra to a specific dot. The experimental results are in agreement with configuration interaction computations, which predict similar polarization degrees for the exciton and the biexciton (within 10%) in typical GaN quantum dots. The theory further predicts that the polarization degree can provide information about the charge state of the dot.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
Keyword
biexcitons, gallium compounds, III-V semiconductors, light polarisation, photoluminescence, semiconductor quantum dots, wide band gap semiconductors
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-73599 (URN)10.1063/1.3675572 (DOI)000299126800013 ()
Note
funding agencies|Thaksin University in Thailand||Swedish Research Council (VR)||Swedish Foundation for Strategic Research (SSF)||Knut and Alice Wallenberg Foundation||Available from: 2012-01-10 Created: 2012-01-10 Last updated: 2017-12-08
5. Size dependent biexciton binding energies in GaN quantum dots
Open this publication in new window or tab >>Size dependent biexciton binding energies in GaN quantum dots
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2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 25, no 251903Article in journal (Refereed) Published
Abstract [en]

Single GaN/Al(Ga)N quantum dots (QDs) have been investigated by means of microphotoluminescence. Emission spectra related to excitons and biexcitons have been identified by excitation power dependence and polarization resolved spectroscopy. All investigated dots exhibit a strong degree of linear polarization (∼90%). The biexciton binding energy scales with the dot size. However, both positive and negative binding energies are found for the studied QDs. These results imply that careful size control of III-Nitride QDs would enable the emission of correlated photons with identical frequencies from the cascade recombination of the biexciton, with potential applications in the area of quantum information processing.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-73545 (URN)10.1063/1.3670040 (DOI)000299031600019 ()
Note

funding agencies|Thaksin University in Thailand||Swedish Research Council (VR)||Swedish Foundation for Strategic Research (SSF)||Knut and Alice Wallenberg Foundation||

Available from: 2012-01-09 Created: 2012-01-09 Last updated: 2017-12-08
6. III-nitride based quantum dots for photon emission with controlled polarization switching
Open this publication in new window or tab >>III-nitride based quantum dots for photon emission with controlled polarization switching
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Computational studies based on 6 band k⋅p!theory are employed on lens-shaped III-nitride quantum dots (QDs) with focus on the polarization properties of the optical interband transitions. The results predict pronounced linear polarization of the ground-state related transitions for asymmetric QDs of a material with small split-off energy. It is demonstrated that a moderate externally applied electric field can be used to induce a linear polarization and to control its direction. InN is found to be the most efficient choice for dynamic polarization switching controlled by an electric field, with potential for polarization control on a photon-by-photon level.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-87747 (URN)
Available from: 2013-01-22 Created: 2013-01-22 Last updated: 2016-06-07Bibliographically approved

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