Browsing by Author "Sari, E."

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Open Access
Blue quantum electroabsorption modulators based on reversed quantum confined Stark effect with blueshift
(AIP Publishing LLC, 2007-01-02) Sari, E.; Nizamoglu, S.; Ozel, T.; Demir, Hilmi Volkan
The authors present the design, growth, fabrication, experimental characterization, and theoretical analysis of blue quantum electroabsorption modulators that incorporate similar to 5 nm thick In0.35Ga0.65N/GaN quantum structures for operation between 420 and 430 nm. Growing on polar c plane on sapphire, they obtain quantum structures with zigzag potential profile due to alternating polarization fields and demonstrate that their optical absorption blueshifts with applied electric field, unlike the redshift of conventional quantum confined Stark effect. In InGaN/GaN quantum structures, they report the largest absorption change of 6000 cm(-1) for 6 V bias swing around 424 nm, holding promise for blue optical clock generation and injection directly into silicon chips.
Open Access
Comparative study of electroabsorption in InGaN/GaN quantum zigzag heterostructures with polarization-induced electric fields
(AIP Publishing LLC, 2008-05-19) Sari, E.; Ozel, T.; Koc, A.; Ju, J. W.; Ahn, H.-K.; Lee, I.-H.; Baek, J.-H.; Demir, Hilmi Volkan
We present a comparative study on InGaN/GaN quantum zigzag structures embedded in p-i-n diode architecture that exhibit blue-shifting electroabsorption in the blue when an electric field is externally applied to compensate for the polarization-induced electric field across the wells. With the polarization breaking their symmetry, the same InGaN/GaN quantum structures redshift their absorption edge when the external field is applied in the same direction as the well polarization. Both computationally and experimentally, we investigate the effects of polarization on electroabsorption by varying compositional content and structural parameters and demonstrate that electroabsorption grows stronger with weaker polarization in these multiple quantum well modulators.
Open Access
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
Open Access
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.
Open Access
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.
Open Access
Green/yellow solid state lighting via radiative and nonradiative energy transfer involving colloidal semiconductor nanocrystals
(IEEE, 2009-08-05) Nizamoglu, S.; Sari, E.; Baek, J. H.; Lee, I. H; Demir, Hilmi Volkan
LEDs made of In(x)Ga(1-x)N and (Al(x)Ga(1-x))(1-y)In(y)P suffer from significantly reduced quantum efficiency and luminous efficiency in the green/yellow spectral ranges. To address these problems, we present the design, growth, fabrication, hybridization, and characterization of proof-of-concept green/yellow hybrid LEDs that utilize radiative and nonradiative [Forster resonance energy transfer (FRET)] energy transfers in their colloidal semiconductor nanocrystals (NCs) integrated on near-UV LEDs. In our first NC-LED, we realize a color-converted LED that incorporate green-emitting CdSe/ZnS core/shell NCs (lambda(PL) = 548 nm) on near-UV InGaN/GaN LEDs (lambda(EL) = 379 nm). In our second NC-LED, we implement a color-converted FRET-enhanced LED. For that, we hybridize a custom-design assembly of cyan-and green-emitting CdSe/ZnS core/shell NCs (lambda(PL) = 490 and 548 nm) on near-UV LEDs. Using a proper mixture of differently sized NCs, we obtain a quantum efficiency enhancement of 9% by recycling trapped excitons via FRET. With FRET-NC-LEDs, we show that it is possible to obtain a luminous efficacy of 425 lm/W(opt) and a luminous efficiency of 94 lm/W, using near-UV LEDs with a 40% external quantum efficiency. Finally, we investigate FRET-converted light-emitting structures that use nonradiative energy transfer directly from epitaxial quantum wells to colloidal NCs. These proof-of-concept demonstrations show that FRET-based NC-LEDs hold promise for efficient solid-state lighting in green/yellow.
Open Access
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.
Open Access
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
Open Access
Spatially Selective Assembly of Quantum Dot Light Emitters in an LED Using Engineered Peptides
(American Chemical Society, 2011-02-23) Demir, Hilmi Volkan; Seker, U. O. S.; Zengin, G.; Mutlugun, E.; Sari, E.; Tamerler, C.; Sarikaya, M.
Semiconductor nanocrystal quantum dots are utilized in numerous applications in nano- and biotechnology. In device applications, where several different material components are involved, quantum dots typically need to be assembled at explicit locations for enhanced functionality. Conventional approaches cannot meet these requirements where assembly of nanocrystals is usually material-nonspecific, thereby limiting the control of their spatial distribution. Here we demonstrate directed self-assembly of quantum dot emitters at material-specific locations in a color-conversion LED containing several material components including a metal, a dielectric, and a semiconductor. We achieve a spatially selective immobilization of quantum dot emitters by using the unique material selectivity characteristics provided by the engineered solid-binding peptides as smart linkers. Peptide-decorated quantum dots exhibited several orders of magnitude higher photoluminescence compared to the control groups, thus, potentially opening up novel ways to advance these photonic platforms in applications ranging from chemical to biodetection.
Open Access
Violet to deep-ultraviolet InGaN∕GaN and GaN∕AlGaN quantum structures for UV electroabsorption modulators
(AIP Publishing LLC, 2007) Ozel, T.; Sari, E.; Nizamoglu, S.; Demir, Hilmi Volkan
In this paper, we present four GaN based polar quantum structures grown on c-plane embedded in p-i-n diode architecture as a part of high-speed electroabsorption modulators for use in optical communication (free-space non-line-of-sight optical links in the ultraviolet) UV: the first modulator incorporates 4 – 6 nm thick GaN/AlGaN quantum structures for operation in the deep-UV spectral region and the other three incorporate 2 – 3 nm thick InGaN/GaN quantum structures tuned for operation in violet to near-UV spectral region. Here, we report on the design, epitaxial growth, fabrication, and characterization of these quantum electroabsorption modulators. In reverse bias, these devices exhibit a strong electroabsorption (optical absorption coefficient change in the range of 5500– 13 000 cm−1 with electric field swings of 40– 75 V/um at their specific operating wavelengths. In this work, we show that these quantum electroabsorption structures hold great promise for future applications in ultraviolet optoelectronics technology such as external modulation and data coding in secure non-line-of-sight communication systems.
Open Access
White light generation by resonant nonradiative energy transfer from epitaxial InGaN/GaN quantum wells to colloidal CdSe/ZnS core/shell quantum dots
(Institute of Physics Publishing Ltd., 2008) Nizamoglu, S.; Sari, E.; Baek, J.-H.; Lee, I.-H.; Demir, Hilmi Volkan
We propose and demonstrate white-light-generating nonradiative energy transfer (ET) from epitaxial quantum wells (QWs) to colloidal quantum dots (QDs) in their close proximity. This proof-of-concept hybrid color-converting system consists of chemically synthesized red-emitting CdSe/ZnS core/shell heteronanocrystals intimately integrated on epitaxially grown cyanemitting InGaN/GaN QWs. The white light is generated by the collective luminescence of QWs and QDs, for which the dot emission is further increased by 63% with nonradiative ET, setting the operating point in the white region of CIE chromaticity diagram. Using cyan emission at 490 nm from the QWs and red emission at 650 nm from the nanocrystal (NC) luminophors, we obtain warm white light generation with a correlated color temperature of Tc = 3135 K and tristimulus coordinates of (x, y) = (0.42, 0.39) in the white region. By analyzing the time-resolved radiative decay of these NC emitters in our hybrid system with a 16 ps time resolution, the luminescence kinetics reveals a fast ET with a rate of (2 ns)-1 using a multiexponential fit with χ2 = 1.0171.
Open Access
White light generation tuned by dual hybridization of nanocrystals and conjugated polymers
(Institute of Physics Publishing Ltd., 2007) Demir, Hilmi Volkan; Nizamoglu, S.; Ozel, T.; Mutlugun, E.; Huyal I.O.; Sari, E.; Holder, E.; Tian, N.
Dual hybridization of highly fluorescent conjugated polymers and highly luminescent nanocrystals (NCs) is developed and demonstrated in multiple combinations for controlled white light generation with high color rendering index (CRI) (> 80) for the first time. The generated white light is tuned using layer-by-layer assembly of CdSe/ZnS core-shell NCs closely packed on polyfluorene, hybridized on near-UV emitting nitride-based light emitting diodes (LEDs). The design, synthesis, growth, fabrication and characterization of these hybrid inorganic-organic white LEDs are presented. The following experimental realizations are reported: (i) layer-by-layer hybridization of yellow NCs (λPL = 580nm) and blue polyfluorene (λPL = 439 nm) with tristimulus coordinates of (x, y) = (0.31, 0.27), correlated color temperature of Tc = 6962 K and CRI of R a = 53.4; (ii) layer-by-layer assembly of yellow and green NCs (λPL = 580 and 540 nm) and blue polyfluorene (λPL = 439 nm) with (x, y) = (0.23, 0.30), Tc = 14395 K and Ra = 65.7; and (iii) layer-by-layer deposition of yellow, green and red NCs (λPL = 580, 540 and 620 nm) and blue polyfluorene (λPL = 439 nm) with (x, y) = (0.38, 0.39), T c = 4052 K and Ra = 83.0. The CRI is demonstrated to be well controlled and significantly improved by increasing multi-chromaticity of the NC and polymer emitters.
Open Access
White light generation using CdSe/ZnS core–shell nanocrystals hybridized with InGaN/GaN light emitting diodes
(Institute of Physics Publishing Ltd., 2007) Nizamoglu, S.; Ozel, T.; Sari, E.; Demir, Hilmi Volkan
We introduce white light generation using CdSe/ZnS core-shell nanocrystals of single, dual, triple and quadruple combinations hybridized with InGaN/GaN LEDs. Such hybridization of different nanocrystal combinations provides the ability to conveniently adjust white light parameters including the tristimulus coordinates (x,y), correlated colour temperature (Tc) and colour rending index (Ra). We present the design, growth, fabrication and characterization of our white hybrid nanocrystal-LEDs that incorporate combinations of (1) yellow nanocrystals (λPL = 580 nm) on a blue LED (λEL = 440 nm) with (x,y) = (0.37,0.25), T c = 2692 K and Ra = 14.69; (2) cyan and red nanocrystals (λPL = 500 and 620 nm) on a blue LED (λEL = 440 nm) with (x,y) = (0.37,0.28), Tc = 3246 K and Ra = 19.65; (3) green, yellow and red nanocrystals (λPL = 540, 580 and 620 nm) on a blue LED (λEL = 452 nm) with (x,y) = (0.30,0.28), Tc = 7521 K and Ra = 40.95; and (4) cyan, green, yellow and red nanocrystals (λPL = 500, 540, 580 and 620 nm) on a blue LED (λEL = 452 nm) with (x,y) = (0.24,0.33), Tc = 11 171 K and Ra = 71.07. These hybrid white light sources hold promise for future lighting and display applications with their highly adjustable properties.