Browsing by Author "Lee, I. H."

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    Dislocation density dependent electroabsorption in epitaxial lateral overgrown InGaN/GaN quantum structures
    (Optical Society of America, 2013-01-14) Sari, E.; Jang, L. W.; Baek, J. H.; Lee, I. H.; Sun, X. W.; Demir, Hilmi Volkan
    We study electroabsorption (EA) behavior of InGaN/GaN quantum structures grown using epitaxial lateral overgrowth (ELOG) in correlation with their dislocation density levels and in comparison to steady state and time-resolved photoluminescence measurements. The results reveal that ELOG structures with decreasing mask stripe widths exhibit stronger EA performance, with a maximum EA enhancement factor of 4.8 compared to the reference without ELOG. The analyses show that the EA performance follows similar trends with decreasing dislocation density as the essential parameters of the photoluminescence spectra (peak position, width and intensity) together with the photoluminescence lifetimes. While keeping the growth window widths constant, compared to photoluminescence behavior, however, EA surprisingly exhibits the largest performance variation, making EA the most sensitive to the mask stripe widths. (C) 2013 Optical Society of America
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    Efficient nonradiative energy transfer from InGaN/GaN nanopillars to CdSe/ZnS core/shell nanocrystals
    (AIP Publishing, 2011-04-20) Nizamoglu, S.; Guzelturk, B.; Jeon, D. W.; Lee, I. H.; Demir, Hilmi Volkan
    In this study, we propose and demonstrate efficient electron-hole pair injection from InGaN/GaN multiple quantum well nanopillars (MQW-NPs) to CdSe/ZnS core/shell nanocrystal quantum dots (NQDs) via Forster-type nonradiative energy transfer. For that we hybridize blue-emitting MQW-NPs with red-emitting NQDs and the resultant exciton transfer reaches a maximum rate of (0.192 ns)(-1) and a maximum efficiency of 83.0%. By varying the effective bandgap of core/shell NQDs, we conveniently control and tune the excitonic energy transfer rate for these NQD integrated hybrids, and our measured and computed exciton transfer rates are found to be in good agreement for all hybrid cases.
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    Electric field dependent radiative decay kinetics of polar InGaN/GaN quantum heterostructures at low fields
    (American Institute of Physics, 2009-05-29) Sari, E.; Nizamoglu, S.; Lee, I. H.; Baek, J. H.; Demir, Hilmi Volkan
    Electric field dependent photoluminescence decay kinetics and its radiative component are studied in polar InGaN/GaN quantum heterostructures at low fields. Under externally applied electric field lower than polarization fields, spectrally and time resolved photoluminescence measurements are taken to retrieve internal quantum efficiencies and carrier lifetimes as a function of the applied field. Subsequently, relative behavior of radiative recombination lifetimes is obtained in response to the applied field. In these characterizations of polar InGaN/GaN structures, we observe that both the carrier lifetime and the radiative recombination lifetime decrease with increasing external electric field, with the radiative component exhibiting weaker field dependence.
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    FRET-LEDs involving colloidal quantum dot nanophosphors
    (Webcom Communications, 2010) Nizamoğlu, S.; Sari, E.; Baek, J. H.; Lee, I. H.; Sun, X. W.; Demir, Hilmi Volkan
    Semiconductor nanocrystal quantum dots (NQD) with their narrow and tuneable emission are promising candidates to serve as color convertors integrated on light-emitting diodes (LEDs). The use of nonradiative energy transfer, also known as Förster-type resonance energy transfer (FRET), in such NQD nanophosphors provides additional benefits for color-conversion in solid state lighting. In this paper we discuss these NQD-integrated FRET-LEDs for lighting applications.
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    Morphology-dependent energy transfer of polyfluorene nanoparticles decorating InGaN/GaN quantum-well nanopillars
    (American Chemical Society, 2013) Erdem, T.; Ibrahimova, V.; Jeon, D. W.; Lee, I. H.; Tuncel, D.; Demir, Hilmi Volkan
    Conjugated polymer nanoparticles (CPNs), prepared in aqueous dispersion from poly[(9,9-bis{3- bromopropyl}fluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1,3}-thiodiazole)] (PFBT-Br), are incorporated into a nanopillar architecture of InGaN/GaN multiple quantum wells (MQWs) to demonstrate a new organic/inorganic class of nanostructured excitonic model system. This hybrid system enables intimate integration for strong exciton−exciton interactions through nonradiative energy transfer (NRET) between the integrated CPNs and MQW pillars. The NRET of these excitonic systems is systematically investigated at varied temperatures. In these hybrids, InGaN/GaN MQWs serve as the donor of the NRET pair, while immobilized PFBT-Br polymer serves as the acceptor. To understand morphology-dependent NRET, PFBT-Br CPNs coating InGaN/GaN MQWs are made to defold into polymer chains by in situ treatment with a good solvent (THF). The experimental results indicate that NRET is significantly stronger in the case of CPNs compared with their defolded polymer chains. At room temperature, while the NRET efficiency of open polymer chains−nanopillar system is only 10%, PFBT-Br CPNs exhibit a substantially higher NRET efficiency of 33% (preserving the total number of polymer molecules). The NRET efficiency of the nanoparticle systems is observed to be 25% at 250 K, 22% at 200 K, 19% at 150 K, and 15% at 100 K. On the other hand, the defolded polymer chains exhibit significantly lower NRET efficiencies of 17% at 250 K, 16% at 200 K, 11% at 150 K, and 5% at 100 K. This work may potentially open up new opportunities for the hybrid organic/inorganic systems where strong excitonic interactions are desired for light generation, light harvesting, and sensing applications.
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    Nonradiative resonance energy transfer directed from colloidal CdSe/ZnS quantum dots to epitaxial InGaN/GaN quantum wells for solar cells
    (Wiley, 2010-06-04) Nizamoglu, S.; Sari, E.; Baek, J. H.; Lee, I. H.; Demir, Hilmi Volkan
    We report on Förster-type nonradiative resonance energy transfer (NRET) directed from colloidal quantum dots (QDs) to epitaxial quantum wells (QWs) with an efficiency of 69.6% at a rate of 1.527 ns-1 for potential application in III-nitride based photovoltaics. This hybrid exciton generation-collection system consists of chemically-synthesized cyan CdSe/ZnS core/shell QDs (λPL = 490 nm) intimately integrated on epitaxially-grown green InGaN/GaN QWs (λPL = 512 nm). To demonstrate directional NRET from donor QDs to acceptor QWs, we simultaneously show the decreased photoluminescence decay lifetime of dots and increased lifetime of wells in the hybrid dipole-dipole coupled system.
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    Opposite carrier dynamics and optical absorption characteristics under external electric field in nonpolar vs. polar InGaN/GaN based quantum heterostructures
    (Optical Society of America, 2011) Sari, E.; Nizamoglu, S.; Choi, J H.; Lee, S J.; Baik, K H.; Lee, I. H.; Baek, J. H.; Hwang, S M.; Demir, Hilmi Volkan
    We report on the electric field dependent carrier dynamics and optical absorption in nonpolar a-plane GaN-based quantum heterostructures grown on r-plane sapphire, which are surprisingly observed to be opposite to those polar ones of the same materials system and similar structure grown on c-plane. Confirmed by their time-resolved photoluminescence measurements and numerical analyses, we show that carrier lifetimes increase with increasing external electric field in nonpolar InGaN/GaN heterostructure epitaxy, whereas exactly the opposite occurs for the polar epitaxy. Moreover, we observe blue-shifting absorption spectra with increasing external electric field as a result of reversed quantum confined Stark effect in these polar structures, while we observe red-shifting absorption spectra with increasing external electric field because of standard quantum confined Stark effect in the nonpolar structures. We explain these opposite behaviors of external electric field dependence with the changing overlap of electron and hole wavefunctions in the context of Fermi's golden rule. (C) 2011 Optical Society of America