627 results on '"Hilmi Volkan Demir"'
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602. Superior warm-white light-emitting diodes integrated with quantum dot nanophosphors for high luminous efficacy and color rendering
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Hilmi Volkan Demir, Xiao Wei Sun, Talha Erdem, and Sedat Nizamoglu
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Brightness ,Correlated color temperature ,Color rendering index ,Color rendering ,Color ,Phosphor ,High luminous efficacy ,law.invention ,Photometry (optics) ,Optics ,law ,Semiconductor quantum dots ,Diode ,Physics ,business.industry ,Color image processing ,Luminous efficacy ,Light emitting diodes ,Nanophosphors ,Luminance ,Quantum dot ,Optoelectronics ,business ,Quantum dot lasers ,Light-emitting diode - Abstract
Conference name: CLEO: Applications and Technology 2011 Date of Conference: 1–6 May 2011 Quantum dot nanophoshor hybridized warm-white LEDs are reported to exhibit high photometric performance of luminous efficacy exceeding 350 lm/Wopt and color rendering index close to 90 at correlated color temperatures
603. Superior warm-White light-emitting diodes integrated with quantum dot nanophosphors for high luminous efficacy and color rendering
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Hilmi Volkan Demir
604. Type-tuning of quasi-type-II CdSe/CdS seeded core/shell nanorods: Type-I vs. type-II
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Hilmi Volkan Demir
605. Electromodulation of photoluminescence from CDSE nanorods film
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Ibrahim Murat Soganci, E. Ustinovich, A. A. Lutich, Mikhail Artemyev, and Hilmi Volkan Demir
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Stark shift ,Materials science ,Photoluminescence ,Electric fields ,business.industry ,Transparent electrode ,Electromodulation ,Optoelectronic devices ,Photoluminescence properties ,Optoelectronics ,Nanorod ,Pl quenching ,Wavelength ranges ,Nanorods ,business ,CdSe nanorod - Abstract
We studied photoluminescence (PL) properties of CdSe nanorods integrated in a thin film sandwiched between transparent electrodes to which an electric field applied. Nearly 20 % of PL quenching accompanied with the weak Stark shift have been observed. This effect is proposed to be used for PL modulation, in particular in the wavelength range beyond the range that traditional optoelectronic devices may cover.
606. Förster-type nonradiative energy transfer directed from colloidal quantum dots to epitaxial quantum wells for light harvesting applications
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Hilmi Volkan Demir
607. Room-temperature, high-efficiency conversion of Mott-Wannier excitons to Frenkel excitons in hybrid semiconductor quantum dot/polymer composites
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Hilmi Volkan Demir
608. Using Metasurfaces to Control Random Light Emission
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Hilmi Volkan Demir
609. CdSe/ZnS core-shell nanocrystal based scintillators for enhanced detection in UV.
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Hilmi Volkan Demir, Ibrahim Murat Soganci, and Evren Mutlugun
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- 2006
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610. InGaN/GaN based LEDs with electroluminescence in violet, blue, and green tuned by epitaxial growth temperature.
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Emre Sari, Sedat Nizamoglu, Tuncay Ozel, Hilmi Volkan Demir, Ayse Inal, Erkin Ulker, Ekmel Ozbay, Yilmaz Dikme, and Micheal Heuken
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- 2006
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611. Single ultrafast diffusive conduction based optoelectronic switch for multi-channel operation.
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Fatih Hakan Koklu, Hilmi Volkan Demir, Yairi, M., Harris, J.S., Jr., and Miller, D.A.B.
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- 2005
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612. Decoupling contact and mirror: an effective way to improve the reflector for flip-chip InGaN/GaN-based light-emitting diodes.
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Binbin Zhu, Wei Liu, Shunpeng Lu, Yiping Zhang, Namig Hasanov, Xueliang Zhang, Yun Ji, Zi-Hui Zhang, Swee Tiam Tan, Hongfei Liu, and Hilmi Volkan Demir
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OPTICAL reflectors ,ELECTROMAGNETIC waves ,LIGHT emitting diodes ,VACUUM tubes ,MATHEMATICAL decoupling - Abstract
In the conventional fabrication process of the widely-adopted Ni/Ag/Ti/Au reflector for InGaN/GaN-based flip-chip light-emitting diodes (LEDs), the contact and the mirror are entangled together with contrary processing conditions which set constraints to the device performance severely. Here we first report the concept and its effectiveness of decoupling the contact formation and the mirror construction. The ohmic contact is first formed by depositing and annealing an extremely thin layer of Ni/Ag on top of p-GaN. The mirror construction is then carried out by depositing thick layer of Ag/Ti/Au without any annealing. Compared with the conventional fabrication method of the reflector, by which the whole stack of Ni/Ag/Ti/Au is deposited and annealed together, the optical output power is improved by more than 70% at 350 mA without compromising the electrical performance. The mechanism of decoupling the contact and the mirror is analyzed with the assistance of contactless sheet resistance measurement and secondary ion mass spectrometry (SIMS) depth profile analysis. [ABSTRACT FROM AUTHOR]
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- 2016
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613. Plasmon-based photopolymerization: near-field probing, advanced photonic nanostructures and nanophotochemistry.
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Xuan Zhou, Olivier Soppera, Jérôme Plain, Safi Jradi, Xiao Wei Sun, Hilmi Volkan Demir, Xuyong Yang, Claire Deeb, Stephen K Gray, Gary P Wiederrecht, and Renaud Bachelot
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PHOTOPOLYMERIZATION ,PLASMONS (Physics) ,NEAR-fields ,NANOPHOTONICS ,PHOTOCHEMISTRY ,NANOSTRUCTURED materials ,CHEMICAL synthesis - Abstract
Hybrid nanomaterials are targeted by a rapidly growing group of nanooptics researchers, due to the promise of optical behavior that is difficult or even impossible to create with nanostructures of homogeneous composition. Examples of important areas of interest include coherent coupling, Fano resonances, optical gain, solar energy conversion, photocatalysis, and nonlinear optical interactions. In addition to the coupling interactions, the strong dependence of optical resonances and damping on the size, shape, and composition of the building blocks provides promise that the coupling interactions of hybrid nanomaterials can be controlled and manipulated for a desired outcome. Great challenges remain in reliably synthesizing and characterizing hybrid nanomaterials for nanooptics. In this review, we describe the synthesis, characterization, and applications of hybrid nanomaterials created through plasmon-induced photopolymerization. The work is placed within the broader context of hybrid nanomaterials involving plasmonic metal nanoparticles and molecular materials placed within the length scale of the evanescent field from the metal surface. We specifically review three important applications of free radical photopolymerization to create hybrid nanoparticles: local field probing, photoinduced synthesis of advanced hybrid nanoparticles, and nanophotochemistry. [ABSTRACT FROM AUTHOR]
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- 2014
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614. Tailoring the electronic structures of zinc oxide based nanowires for optoelectronic applications
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Xin Zhao, Sun Handong, School of Physical and Mathematical Sciences, and Hilmi Volkan Demir
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Science::Physics::Optics and light [DRNTU] ,Materials science ,chemistry ,business.industry ,Nanowire ,chemistry.chemical_element ,Optoelectronics ,Nanotechnology ,Zinc ,business - Abstract
ZnO nanowires (NWs) have attracted tremendous attention in the past 15 years after the first demonstration of optically pumped ultraviolet laser by Yang et al.1, 2 As a wide band gap (3.3 eV, RT) material possessing room-temperature stable excitons (60 meV binding energy), ZnO NWs have been treated as one of the most promising platform for next-generation optoelectronic devices.3, 4 The global lightening market calls for materials with controllable doping, energy band gap, as well as novel device design schemes to achieve high-efficiency devices emitting light with arbitrary mono or mixed colors.5, 6 In order to fulfill these requirements, we have synthesized ZnO NWs with designed electronic structures including 1) the n-/p-type doped MgZnO alloy wires where we tuned the carrier concentration and band gap simultaneously, 2) core/doped-shell NWs to spatially tuned band structure via integrated utilization of two growth methods: hydrothermal synthesis and pulsed laser deposition (PLD). The synthesized ZnO and ZnMgO NWs were characterized mainly by methodologies such as photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS), etc. Self-consistent models were proposed to shine some light on the physical correlations between the carrier density, surface depletion, and luminescence, which could provide instructions on designing novel light emitting devices. Furthermore, taking into account conjugated factors aforementioned, we proposed a novel design scheme of ZnO NW-based white-light-emitting-diodes via Förster resonant energy transfer (FRET) from ZnO defect levels to quantum dots (QDs) and a prototype device was demonstrated for the first time. The core part of the thesis could be divided into three main parts accordingly as follows: Part-I Synthesis of MgZnO NWs with controlled conduction type via Ga and P doping In Chapter 3, Ga and P-doped MgZnO nanostructures were synthesized by PLD to explore the possibility of modulating the band gap and conduction type simultaneously. The motivation of this work lay in the great importance of tuning the band offset ratio as well as extending the emission and transparent region of doped ZnO NWs. The Ga:MgZnO NWs array grew perpendicular to the sapphire substrates and the following XPS and PL confirmed the existence of gallium and magnesium. Under pulsed laser excitation, the doped wires exhibited Fabry-Perot lasing at 386 nm with a quality factor Q exceeding 1000. For the P-doped MgZnO, the product displayed as very dense bundle of NWs with an hierarchical arrangement. The following XPS confirmed the successful incorporation of Mg and high concentration of P, which act as the key to compensate the background carriers and achieve p-type. Photoluminescence further confirmed the band gap tuning and existence of acceptor-related optical transitions. The stable p-type conductivity was proved by the fabricated homojunction n-ZnO thin film/P:ZnMgO NW diode, which shown rectification behavior even after three-month storage in air. Part-II Forming ZnO/Ga:ZnO core-shell structure via doping gallium the surface to modulate the optical properties. ZnO/Ga:ZnO core-shell NWs were fabricated to spatially tailor the electronic band gap via introducing the n+ region near the NWs surface. This Ga-doped shells would act as the hole blocking layer preventing the non-radiative recombination of holes with trapped electrons induced by surface depletion effect. We have proposed an integrated electronic band structure model considering the conjugating factors such as mid-gap defects, surface depletion, and carrier concentration-induced band gap bending to explain the successful suppression of deep-level emissions (DLE). The following XPS measurement and temperature-dependent PL could fit the model very well. In short, this novel scheme and integrated model provided non-trivial physical insights on NW-based light emitting devices design and fabrication. Part-III Excitonic energy recycling from ZnO defect states: towards electrically driven NW-QDs hybrid white light-emitting-diodes. It is widely known that QDs, though be treated as eminent lumophores due to their high quantum yield and color tunability, are hinder in the path towards practical light emitting devices owing to the surface ligand-induced carrier injection difficulty. On another hand, as promising UV emitting material, ZnO NWs suffer from the undesirable visible emission due to the DLE, which result in uncontrollable emission color and waste of exciton energy. To address this problem, we proposed a novel device designing scheme for w-LEDs, which incorporated CdSe QDs with p-GaN/n-ZnO NWs heterostructures. For the first time, QDs were excited by the recombination of carriers captured by deep-level states in electrically injected ZnO NWs. The device exhibited an achromatic emission with chromaticity coordinate (0.327, 0.330) and color temperature 5783 K, equal to the sun. Doctor of Philosophy (SPMS)
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- 2020
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615. InGaN/GaN light-emitting diodes : from modeling to their hybrid applications with novel nanomaterials
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Namig Hasanov, Sun Xiaowei, Hilmi Volkan Demir, and School of Electrical and Electronic Engineering
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Materials science ,law ,business.industry ,Engineering::Electrical and electronic engineering [DRNTU] ,Optoelectronics ,Nanotechnology ,business ,Nanomaterials ,Light-emitting diode ,law.invention - Abstract
In last two decades, InGaN/GaN light-emitting diodes have been one of the main focus of research thanks to their low power consumption, high efficiency, long lifetime, high color purity and color quality, narrow luminescence, possibility to tune the emission wavelength from near ultraviolet to green by increasing the In content, and several other promising properties. In early stages of the development of InGaN-based LEDs, growing high quality epitaxial films on a suitable substrate was the main issue. This issue was effectively bypassed by growing the main device layers on the lattice-matched buffer layer. Another big issue was the difficulty to achieve high p-type conductivity in GaN; the problem was solved by high temperature annealing and low energy electron beam irradiation methods. In order to achieve a well operating InGaN/GaN light-emitting diode, both optical and electrical properties should be in the desired level. The main performance measure of these devices is the external quantum efficiency. To increase the external quantum efficiency, the issues with the carrier injection, radiative recombination, light-extraction, ohmic contacts, and several other factors that limit the overall performance of the device need to be properly addressed. Although InGaN-based light emitting diodes have been strongly developed, the performance of the devices still needs to be increased by novel methods. Moreover, the devices need to be properly characterized in both wafer-level and chip-level states to deeply investigate the drawbacks and advantages of each element during the growth and fabrication. Furthermore, the applications of light emitting diodes needs to be extensively investigated which can be realized through making hybrid systems and combining the advantages of these light emitting devises and novel materials. Although, there are some studies on this kind of hybrid applications, there is still a long way to go to make use of InGaN/GaN light emitting diode structures in pumping novel materials and devices. In this thesis, we systematically investigate the design, growth, wafer-level characterization, device fabrication, and device-level characterization of InGaN/GaN light-emitting diode structures grown on polar sapphire substrates. We demonstrate optimized growth process and wafer-level characterization of high crystal quality InGaN/GaN device. The fabrication and device-level characterization of improved conventional, flip-chip and vertical light-emitting diodes are demonstrated following the growth of the epitaxial layers. Next, we investigate the advantages and drawbacks of Ni/Ag/Ni/Au and Ni/Ag/Ti/Au ohmic reflectors and compare the devices fabricated with both types of the contact-mirrors. The proposed device outperforms the reference device in terms of electrical (lower forward voltage) and optical properties (higher reflectance). Moreover, the critical role of the incorporation of sputtered TiW was studied and it was concluded that the TiW-incorporated light-emitting diodes exhibit higher light extraction, higher optical power, and higher external quantum efficiency compared with those without TiW. The enhancement in the device performance was mainly attributed to the robustness of the device against high temperature annealing. Electroluminescence measurements further confirmed that TiW-incorporated light emitting diodes possess better heat management. We also investigated the critical role of InGaN epitaxial thin layer on the electrical performance of the device. By testing the layers with several thicknesses and In compositions, we came to the conclusion that a 2 nm thick InGaN layer with high In composition can enhance the current-voltage characteristics of the device by creating a 2-dimensional hole gas in the interface. The generation of holes in the interface is attributed to band bending induced by the piezoelectric polarization owing to the lattice mismatch. We investigated the effect of grading the InGaN quantum wells along the growth directions and compared the carrier distribution, radiative recombination rate, optical power, and external quantum efficiency with those of the conventional structure device. Moreover, we performed extensive study on the thickness-dependent performance enhancement of the quantum wells and concluded that the device with 6.5 nm thick graded quantum wells outperforms the conventional device with 2.5 nm thick quantum wells. Furthermore, studies also showed that having only three graded quantum wells with 6.5 nm thickness in the active region is more effective than having eight non-graded 2.5 nm thick quantum wells. Quantum confined stark effect is a well-known phenomenon which strongly reduces the performance of the devices owing to the separation of charge carriers due to the piezoelectric polarization induced-band bending. This bending in both conduction and valence bands strongly affects the carrier transport. The bending reduces effective barrier height of AlGaN electron blocking layer for electrons and increases barrier height for holes. Bearing in mind the low mobility and concentration of holes and the leakage of electrons, we introduced two additional InGaN quantum wells in the electron blocking layer to recycle the leaked electrons. The radiative recombination was significantly increased which was consistent with optical power and external quantum efficiency results. Moreover, we showed that, by having only six quantum wells in the active region and two quantum wells within the electron blocking layer can still significantly increase the performance of the conventional device with eight quantum wells. The synthesis of novel nanomaterials and their hybridization with optoelectronic materials and devices have recently become one the main research areas which combines the electrical injection properties of the one and the optical, structural, and geometrical properties of another material. In that manner, we chemically synthesized novel 2D CdSe nanoplatelets, and performed optical and morphological characterization. Moreover, we increased the photoluminescence of CdSe solid films with the incorporation of localized surface plasmons. The photoluminescence enhancement was attributed to the electric field enhancement and increased number of radiative channels in the presence of metallic nanoparticles, which was also confirmed with theoretical studies and time-resolved photoluminescence spectroscopy experiments. We believe this method will be useful in devices incorporating CdSe nanoplatelets as main active materials. Moreover, we investigated the critical role of CdSe nanoplatelets in the performance of color-converted InGaN/GaN light-emitting diodes as an exciton donor for the color converter CdSe/ZnS nanocrystal quantum dots. The hybrid device fabricated with the CdSe nanoplatelets outperformed the one without the nanoplatelets in terms of power conversion efficiency. The enhancement was ascribed to the exciton migration from donor CdSe nanoplatelets to acceptor CdSe/ZnS quantum dots both of which were pumped with InGaN/GaN light-emitting diode. Nonradiative Forster-type excitonic energy transfer between these donor-acceptor pairs was further confirmed with time-resolved photoluminescence spectroscopy and photoluminescence excitation measurements. Furthermore, we investigated Forster resonance energy transfer to CdSe nanoplatelets from InGaN quantum wells of bulk and nanopillar structures. Optical characterization revealed that the internal quantum efficiency and light extraction efficiency of nanopillar device structure were higher than those of the as-grown structure owing to the increased surface to volume ratio and the strain-relaxation. Next, the excitonic energy transfer between InGaN/GaN nanopillars and chemically synthesized CdSe nanoplatelets were monitored with time-resolved photoluminescence decay measurements. Resonance energy transfer from bulk quantum well capped with 3 nm GaN cap layer was also investigated in a similar manner following short characterization of bulk quantum well epitaxial structure. The energy transfer efficiency of the bulk quantum well system was higher than that of the nanopillar structure. The enhanced exciton migration was attributed to the reduced separation between the quantum wells and the nanoplatelets in the bulk quantum well structure compared with the nanopillar structure. Stacking of nanoplatelets is believed to strongly reduce the chance of nanoplatelets to be in the close proximity of the quantum wells in the InGaN/GaN nanopillar arrays. In summary, the thesis includes the epitaxial growth, device fabrication, wafer-level and device-level characterization, studies of novel device designs, and hybrid applications of InGaN/GaN light-emitting diodes structures with novel CdSe nanoplatelets. Effective methods to increase the electrical and optical performance of the device were discussed in detail and the performance of the devices was compared with the conventional structures. The thesis work has provided important insights for design, growth, fabrication, characterization, and applications of high performance InGaN/GaN light-emitting diodes and heterostructures. Doctor of Philosophy (EEE)
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- 2020
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616. High power gallium nitride light-emitting diodes for efficient solid state lighting and displays
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Yun Ji, Sun Xiaowei, Hilmi Volkan Demir, School of Electrical and Electronic Engineering, and LUMINOUS! Centre of Excellence for Semiconductor Lighting and Displays
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chemistry.chemical_compound ,Solid-state lighting ,Materials science ,chemistry ,law ,business.industry ,Optoelectronics ,Gallium nitride ,business ,Engineering::Electrical and electronic engineering::Microelectronics [DRNTU] ,Power (physics) ,Light-emitting diode ,law.invention - Abstract
Nitride based light-emitting diodes (LEDs) are considered to be the next generation lighting source, owing to their superior advantages including high brightness, high energy conversion efficiency, long life span, compact size, fast response, and low maintenance cost. Tremendous work has been devoted to improving the performance of InGaN/GaN multiple quantum well LEDs in the past decades. The GaN based white LED has currently achieved reasonably high efficacy and displays a huge potential of replacing the incandescent and fluorescent lamps in the current market. However, the performance of InGaN/GaN LED light sources is still limited by some technical issues. One of the challenges is to reduce the efficiency droop, the phenomenon of reduction of external quantum efficiency at high current levels, such that the device could maintain high luminous efficacy for high brightness lighting applications. The root cause of the efficiency droop is still under debate, although there have been more convincing reports lately. Also, the lighting source should provide high chromaticity quality comparable to the conventional lighting sources to generate comfortable visual perception. Tremendous research work has been contributed to the improvement of InGaN/GaN LED lighting quality. This thesis research work focuses on the design, growth and fabrication of InGaN/GaN LED structures for enhanced efficiency of light emission. InGaN/GaN multiple quantum wells LED wafers with high crystal quality and high uniformity were grown using metal-organic chemical vapor deposition technique. The fabrication processes of LED chips were developed, and characterization methods for both wafer-level and chip-level electrical and optical performance were employed. Based on these standard growth and fabrication procedures, new device epi-structures have been designed for the performance improvement of InGaN/GaN LEDs. Excess electron overflow and hole injection shortage into the quantum wells cause the non-uniform carrier distribution and carrier crowding within the MQWs. Hence, all the carriers are involved in the radiative recombination process to generate photons, suppressing the optical output power as well as the energy conversion efficiency of LED devices. In this thesis, to enhance the hole transport depth, a partially p-type doped quantum barriers structure has been proposed. In order to investigate the carrier transport behavior within the active region, the quantum wells are intentionally grown at different temperatures, to incorporate different indium content. Through examining the emission intensity at different wavelengths, it is found that, in conventional LED structures, the holes could only be injected into shallow quantum wells close to the p-GaN layer. By inserting p-type doped GaN layer into the quantum barriers close to the p-GaN, the potential energy barriers for holes are reduced. Hence, the holes are able to reach deeper quantum wells close to the n-GaN side, as indicated by the increased light emission from deeper quantum wells. As a benefit, the hole distribution is more uniform within the MQWs, and more QWs are involved in the light emission. The conventional LED employs a p-type doped AlGaN layer as the electron blocking layer (EBL) to confine electrons within the MQWs region and prevent them from overflowing into the p-GaN layer. However, the p-EBL, inserted between the MQWs and the p-GaN layer, also suppresses the hole injection from the p-GaN to the MQWs, since the large band gap AlGaN layer creates an energy barrier for both electrons and holes. In contrast, when an n-type AlGaN EBL is adopted instead, the EBL blocks excess electrons before they enter the MQWs region. Hence, the electron crowding is avoided. Also, since the n-EBL is not on the path of hole transport into the active region, the amount of holes injected into the QWs is not suppressed. Simulation results suggest that the n-EBL structure results in more uniform electron and hole distributions, a higher hole concentration, and a higher radiative recombination rate in each individual QW, which agrees with the experimentally measured electroluminescence emission intensity and optical power output. The study on the influence of polarization fields on the device efficiency has also been carried out. So far, most research on the improvement of GaN LED performance focus on the structures grown on (0001) c-plane sapphire substrates. Due to the strong polarization field caused by the spontaneous and piezoelectric polarizations, the electron and hole wave functions are spatially separated, hence reducing the radiative recombination rates within the QWs. LED structures grown on nonpolar and semipolar planes have been proposed to eliminate this problem. However, it is still unclear how the polarities of different growth planes affect the carrier recombination dynamics and device performance. In this thesis work, the MQW LED structures grown on (0001) polar and (11-22) semipolar planes are investigated for the comparative investigation of carrier dynamics, in collaboration with the University of California, Santa Barbara (UCSB). The underlying physics behind the performance difference is revealed through photoluminescence and electroabsorption measurements and energy band analysis. Finally, a GaN based white light emitting source with tunable optical parameters is demonstrated for the extended application of GaN LED as lighting and display sources. The blue light emitting GaN LED chip has been designed to yield a current-dependent dual-wavelength emission centered at 425 and 460 nm. Nano-sized phosphor particles with photoluminescence emission peaks at 560 and 625 nm are coated on the LED chip to convert the blue light into green and red color light. As the operating current of the device varies, the blue emission intensity of the two peaks from the GaN LED chip changes, leading to intensity change of the green and red light. Hence, the color temperature and color rendering index could be adjusted accordingly. The proposed device structure provides a new method for generating high quality tunable white light source for indoor lighting and display applications. In summary, this dissertation includes the epitaxial growth, device fabrication, and theoretical studies of InGaN/GaN LEDs. Novel epitaxial structures have been proposed and realized in LED devices for performance enhancement, and the physics behind the performance improvement has been revealed for each proposed structure. The thesis work has provided insights important for design, growth and fabrication of high performance GaN LED based solid state lighting and display sources. DOCTOR OF PHILOSOPHY (EEE)
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- 2019
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617. Application of metal oxide microstructures in organic photovoltaics
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Amoolya Nirmal, Hilmi Volkan Demir, and School of Electrical and Electronic Engineering
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Metal ,chemistry.chemical_compound ,Materials science ,Organic solar cell ,chemistry ,Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics [DRNTU] ,visual_art ,visual_art.visual_art_medium ,Oxide ,Nanotechnology ,Microstructure - Abstract
With the emerging issues of climate change and depleting fossil fuel energy, solar cells are gaining widespread interest in both research labs and in the industry. Most of today's commercially available solar cells however are based on inorganic materials, mainly silicon. Organic PhotoVoltaics (OPVs), though lagging behind its inorganic counterpart in efficiency, has the potential for low cost, ease of fabrication and compatibility with flexible substrates to its advantage. In this thesis, the application of microstructures on the zinc oxide (ZnO) which form the electron selective inter-layers in inverted OPVs to enhance performance was investigated. Porous zinc oxide (ZnO) was demonstrated to serve as a microstructured electron selective layer enhancing light scattering in inverted organic photovoltaics. The use of porous ZnO structure led to a marked improvement in device performance when compared to non-porous ZnO, with 35% increase in current density and 30% increase in efficiency. The use of porous structure was extended to doped zinc oxide electron selective layers, namely indium-doped and aluminium dopes zinc oxide layers. The enhanced performance of OPV employing porous structure was observed for these devices also by virtue of the efficient light scattering property of the porous layer. Thus porous microstructure on metal oxide proved to be a portable efficient method of power conversion efficiency enhancement. Finally, photonic crystal was employed on the zinc oxide electron selective layer of inverted OPVs. By optimizing various process parameters, efficiency enhancement was observed for OPV devices with photonic crystal ZnO layer compared to planar ZnO electron selective layer. The highly ordered periodic structures of the photonic crystals provided effective light trapping which resulted in increased absorption in the active layer and subsequent current density improvement. DOCTOR OF PHILOSOPHY (EEE)
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- 2019
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618. Efficient quantum dot light emitting diodes for solid state lighting and displays
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Xuyong Yang, Sun Xiaowei, School of Electrical and Electronic Engineering, and Hilmi Volkan Demir
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Solid-state lighting ,Materials science ,Engineering::Materials::Nanostructured materials [DRNTU] ,Quantum dot ,business.industry ,law ,Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics [DRNTU] ,Optoelectronics ,business ,Engineering::Electrical and electronic engineering::Nanoelectronics [DRNTU] ,Engineering::Electrical and electronic engineering::Semiconductors [DRNTU] ,law.invention ,Light-emitting diode - Abstract
Quantum dot-based light-emitting diodes (QLEDs) with advantages in colour quality, stability and cost-effectiveness are emerging as a candidate for single-material, full colour light sources. The focus of this study is to solve some challenging problems that hinder the ultimate research transformation and commercialization of the QLED technology. Our light out-coupling strategy enhanced device efficiency up to 9.34%, the record high value for green QLEDs. The substitution of organic charge transport layers with metal oxide nanoparticles significantly improved the operating lifetime of QLEDs by a factor of 20. Our development of the narrowest-linewidth, high-quality InP-based QDs with wavelength tunability across full visible spectrum solved the toxicity problem of QDs effectively. The use of Kapton tape and metal electrodes in top-emitting LED architecture made a significant breakthrough in flexible QLED performance with the best efficiency of 4.03%. Our study reported in this thesis contributed to the fast development of the QLEDs. DOCTOR OF PHILOSOPHY (EEE)
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- 2019
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619. Colloidal quantum dot lasing for lighting and displays
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Yuan Gao, Hilmi Volkan Demir, School of Physical and Mathematical Sciences, LUMINOUS! Centre of Excellence for Semiconductor Lighting and Displays, and Sun Handong
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Science::Physics::Optics and light [DRNTU] ,Distributed feedback laser ,Materials science ,Active laser medium ,business.industry ,Physics::Optics ,Backlight ,law.invention ,Optics ,Engineering::Materials::Nanostructured materials [DRNTU] ,law ,Quantum dot ,Optical cavity ,Optoelectronics ,Whispering-gallery wave ,business ,Lasing threshold ,Light-emitting diode - Abstract
Semiconductor light-emitting diodes (LEDs) enable artificial lighting with an unprecedented level of efficiency. However, “efficiency droop” occurs in LEDs under high power injection density, practically limiting the feasible efficiency levels at high output powers. To address this problem, the concept of laser lighting has been proposed. Also, the current liquid crystal displays (LCDs) suffer the problems of low energy efficiency and small colour gamut, which can be addressed by employing the polarized white backlighting and saturated primary colours. Lasers with colloidal quantum dots (CQDs) as a gain medium can provide solutions to these limitations of current lighting and display technologies. Thus, the target of my Ph.D. thesis work is to develop and demonstrate low threshold colloidal quantum dot lasing with high linear polarization. In Chapter 2 of this thesis, unique properties of CQDs, including size dependent bandgap and discrete energy levels, which result from quantum confinement effect, are discussed. Here by adjusting the size, structure and chemical composition, CQDs that emit at various targeted wavelengths were synthesized. The resulting optical and structural characterizations are also presented. In Chapter 3, a brief review of the optical gain from CQDs is given, and the means that can be adopted to abate Auger recombination are discussed. In experimental part, CQD lasing of red, green, and blue CQDs was demonstrated. Moreover, a FRET-assisted indirect pumping scheme for CQD green lasing with standard pumping source was developed. In Chapter 4 and Chapter 5, highly polarized lasing from CQDs was demonstrated by utilization of the optical cavity effect and adoption of the polarized gain medium, respectively. The CQD DFB laser with mechanically flexible substrate was shown and analysed in Chapter 4. However, for a cylindrical optical cavity of a large diameter, which has low selectivity of TE and TM mode, the polarized gain medium that was fabricated by aligned nanorods was employed for realizing highly polarized Whispering Gallery mode lasing. These results indicate that highly polarized CQD lasing can find important uses in future lighting and displays. DOCTOR OF PHILOSOPHY (SPMS)
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- 2019
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620. Design and growth of high-power gallium nitride light-emitting diodes
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Zi-Hui Zhang, Hilmi Volkan Demir, School of Electrical and Electronic Engineering, and Sun Xiaowei
- Subjects
Materials science ,business.industry ,Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics [DRNTU] ,Gallium nitride ,Engineering::Electrical and electronic engineering::Semiconductors [DRNTU] ,Engineering::Electrical and electronic engineering::Microelectronics [DRNTU] ,law.invention ,Power (physics) ,chemistry.chemical_compound ,chemistry ,law ,Optoelectronics ,business ,Light-emitting diode - Abstract
In this dissertation, the InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been studied from multiple aspects including improvement of material quality, suppression of quantum confined Stark effect (QCSE), promotion of carrier transport, enhancement of current spreading, reduction of electron overflow as well as increase in hole concentration of p-GaN through the generation of three-dimensional hole gas by dopant-free methods, with the aim to improve the optical performance of the devices. The InGaN/GaN LED epitaxial wafers are grown by metal-organic chemical vapor deposition (MOCVD) system on c-plane sapphire substrates. As it is well-known that the crystalline quality is crucial for high-efficiency InGaN/GaN LEDs, we have discussed and demonstrated the epitaxial films with optimized crystalline quality in this thesis. Besides, InGaN/GaN LEDs grown along polar-orientations (i.e., c+/c- orientations) suffer from the QCSE in the MQWs, which significantly reduces the spatial overlap of the electron-hole wave functions,and thus decreases the radiative recombination rates in the device active region. We have designed and demonstrated that the QCSE can be effectively suppressed by Si step-doping the quantum barriers,which enhances the optical output power and external quantum efficiency (EQE). In addition, by further Mg doping the Si step-doped quantum barriers with a proper Mg doping level and doing position, we have obtained the InGaN/GaN LEDs with PN-type quantum barriers. With this quantum barrier architecture, on one hand, the QCSE has been effectively screened, and on the other hand, the hole transport has also been promoted significantly.Hence, an even more enhancement in the optical output power and EQE has been obtained. The current crowding in the InGaN/GaN LEDs due to the low p-type conductivity is another challenge for achieving high-efficiency LEDs.In this dissertation, we have proposed a solution for current spreading enhancement by embedding a thin weakly doped n-GaN layer into the p-GaN layer.Such that, the p-GaN/n-GaN/p-GaN (PNP-GaN) current spreading layer is formed. The advantage of this design is to achieve the current spreading layer directly in the MOCVD growth, which saves the post-growth treatment in the fabrication process for modulating the current distribution. Besides incorporating a resistive layer (i.e., weakly doped n-GaN) in the p-GaN layer to improve the current spreading, another way for a better current spreading is to increase the electrical conductivity of the top contact layer, i.e., p+-GaN layer for the conventional InGaN/GaN LEDs. Therefore, we have grown the heavily doped n+-GaN layer on top of the p+-GaN layer to form a tunnel junction as the top contact layer. It is proved that the p+-GaN/n+-GaN tunnel junction has significantly improved the current spreading and thus the device performance. The additional voltage drop on the p+-GaN/n+-GaN tunnel junction can be relieved by inserting an InGaN layer between the p+-GaN and n+-GaN, i.e., a polarization tunnel junction which can also further improve the optical performance of the devices. We have also suggested InGaN/GaN LED devices with the electron overflow reduction through incorporating n-type InGaN layer and n-type AlGaN layer into the n-GaN layer, respectively, accompanied with which a mean-free-path model has been proposed and demonstrated to explain the mechanism of the electron overflow reduction through decreasing the electron mean free path. Lastly, as it has been mentioned previously, the low p-type conductivity in the InGaN/GaN LEDs has substantially limited the LED performance. The Mg dopants in the p-type GaN layers are only ionized by less than 1% at room temperature due to the large binding energy of Mg acceptors.In this dissertation, we have increased the hole concentration by inducing the three-dimensional (3D) hole gas without additional Mg doping. Our experiment shows that the generation of the 3D hole gas has nothing to do with any external dopants. More importantly, the 3D hole gas can be injected into the MQWs for the radiative recombination. Hence, the 3D hole gas can be another hole source besides the one donated by the ionized Mg dopants. DOCTOR OF PHILOSOPHY (EEE)
- Published
- 2019
- Full Text
- View/download PDF
621. Efficiency optimization of organic light-emitting diodes
- Author
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Shuwei Liu, Hilmi Volkan Demir, Sun Xiaowei, and School of Electrical and Electronic Engineering
- Subjects
Materials science ,business.industry ,Engineering::Electrical and electronic engineering [DRNTU] ,OLED ,Electrical engineering ,Optoelectronics ,business - Abstract
Interest in Organic light-emitting diode (OLEO) technology was extensive lately, because of the unique advantages compared to its inorganic counterpart and other technologies. With extensive research of more than two decades, OLEO has advanced from pure laboratory research and stepped into the commercial world, however, due to current limitation in efficiency, yield and cost, OLEO technology has still a long way to go to be a dominant force in display and lighting market. DOCTOR OF PHILOSOPHY (EEE)
- Published
- 2019
- Full Text
- View/download PDF
622. Growth and fabrication of InGan/GaN light-emitting diodes from planar to microwall structures : epitaxial and device designs, modelling and characterization
- Author
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Zhu, Binbin, Hilmi Volkan Demir, and School of Physical and Mathematical Sciences
- Subjects
Science::Physics [DRNTU] - Abstract
The thesis has studied the InGaN/GaN-based LEDs, from growth to fabrication and from planar structure to micro-wall structure, to alleviate their efficiency droop and improve their performance. For the epi-wafers, extraordinary performance is observed in tandem LEDs, which is attributed to reduced forward voltage and more uniform carrier distribution, while reduced forward voltage and improved electrical thermal stability are observed in LEDs with Mg doping in the barriers, which are due to reduced depletion length and increased acceptor concentration. For the fabricated devices, the reflective contact is studied, in which the decoupled contact is introduced and InGaxNyOz interfacial layer is designed, while both methods help realize ohmic contact and high reflectivity. In addition, GaN micro-walls are prepared by selective area growth method and micro-wall LEDs are realized. This thesis has performed systematic study on LEDs, and improved performance is realized in three aspects, which are growth, fabrication and epi-wafer structures. Doctor of Philosophy (SPMS)
- Published
- 2017
623. Wavelength tuning of the soft-approached whispering gallery mode microlasers for display and sensing
- Author
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Yang, Shancheng, Hilmi Volkan Demir, Sun Handong, School of Physical and Mathematical Sciences, and Photonics Research Centre
- Subjects
Science::Physics [DRNTU] - Abstract
Whispering gallery mode (WGM) microcavities and microlasers have attracted enormous research attentions in recent years due to the high quality factors, small mode volumes, enhanced light-matter interactions, and abundant applications in various fields such as optoelectronics, biological and chemical sensing, high-quality lasers, nonlinear studies, etc. Although the mature but sophisticated top-down and bottom-up approaches for semiconductor processing can provide WGM microcavities and microlasers with high quality and ultra-compact integration, the intrinsic rigid nature of the materials hinders the development of flexible applications like displayand sensing, which require the tuning of the wavelengths within the cavity. As the confined resonances are ultra-sensitive to the gain medium, the refractive index and the cavity size, doping-flexible, elastic and cost-efficient soft-approached WGM microlasers are competitive in wavelength tuning and promising in application broadening. However, though the reported soft candidates based on polymer materials have shown outstanding optical performances and improved flexibility, the lack of controllable fabrication techniques dramatically decreases the practical values of such microlasers. Therefore, engineering soft-approached WGM microlasers with controllable approaches and enhanced flexibilities in wavelength tuning for practical applications remains challenging. In this thesis, three works that emphasize the challenges are introduced. By employing a commercially available microplotter, the proposed fabrication processes for the hemispheres and microfibers are wellx controlled. In addition, wavelength tuning for new applications in display and sensing are demonstrated by manipulating the gain medium and the cavity geometry of the two configurations, respectively. Furthermore, a novel floating quasi-disk microlaser, which is bi-directionally and reconfigurably tunable, is proposed and exploited as a sensitive sensor for water-soluble chemicals in microfluidics. The proposed works not only broaden the applications for the tunable microlasers but are also significant in the spread of the soft-approached WGM microcavities for industrialization and commercialization. Doctor of Philosophy (SPMS)
- Published
- 2017
624. Growth and fabrication of low thermal-mass gallium nitride light-emitting diodes
- Author
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Lu, Shunpeng, Hilmi Volkan Demir, School of Electrical and Electronic Engineering, and Microelectronics Centre
- Subjects
Engineering::Electrical and electronic engineering [DRNTU] - Abstract
Over the past two decades, the technology of InGaN/GaN-based light-emitting diodes (LEDs) has made tremendous progress. The optical performance has been extensively studied to get higher luminous efficacy within a desired cost structure. Today, the efficacy of LEDs has already surpassed that of the incandescent and fluorescent luminaires. In addition, LED lighting has many advantages such as high brightness, long lifetime, high reliability, small size, high efficiency, and low power consumption. Owing to these benefits, LEDs have been widely used in a myriad of applications. Nevertheless, there is still room for improvement of the optical performance for unique applications. For applications on mobile display backlighting, micro-displays, medical devices, visible light communication, and so on, LEDs with larger chip sizes are not effective in achieving higher power densities, higher light extraction efficiency, and faster pulsed operation even when operating at higher current densities. To fulfill the requirements of these different applications, smaller sized low thermal-mass LEDs (LTM-LEDs) are proposed in this thesis. LTM-LED has less thermal mass and better thermal conductivity which enables a lower junction temperature to be maintained. As a result of the reduced thermal mass and better thermal conductivity, LTM-LEDs can sustain much higher current density, higher power density, and faster response speed. In this dissertation, LTM-LEDs in various sizes and geometries were demonstrated and studied based on our standard flip-chip LED fabrication technique to identify the optimal power density performance. Furthermore, partitioned growth LTM-LEDs in different sizes are also grown by the metal-organic chemical-vapor deposition (MOCVD) system for enhanced optical output power performance. Heat is a critical factor for the efficiency droop. Moreover, the distance between the n-contact and the LED mesa, which acts as a conductive path, has a substantial influence on current spreading and output power of LEDs, especially for the smaller sized LEDs. Hence, to increase the output power density of LTM-LEDs, it is important to reduce the thermal mass and decrease the distance between the n-contact and the LED mesa. One of the effective ways is to make LTM-LEDs smaller. Experimental results show that output power density is improved by the decreasing chip size. Our model suggests that smaller size has better uniformity on current density and also generates less heat. As is well known, high light extraction efficiency (LEE) and better current spreading are important criteria for LEDs operating in the higher power density regime. Different shapes always show different characteristics on LEE and current spreading. To increase the power density of LTM-LEDs, triangle-, circle-, and square-shaped LTM-LEDs with the same mesa area are designed and realized in the flip-chip configuration. It was revealed that the circle-shaped LTM-LEDs show the lowest electrical and optical properties, while the triangle-shaped LTM-LEDs deliver the highest power density versus current density. However, our numerical simulations demonstrated that the LEE of triangle-shaped LTM-LEDs is only 0.06% higher than the lowest circle-shaped LTM-LEDs due to only 800 nm depth of the sidewalls. On the other hand, our model and simulation results show that the lower resistance at the mesa edge and the shorter n-GaN current paths, which not only reduce the self-heating but also contribute to higher average radiative recombination rate, account for the superior performance of triangular LTM-LEDs. The quantum-confined Stark effect (QCSE) induced by the lattice mismatch between gallium nitride and the sapphire substrate significantly hinders the optical performance of LEDs. To reduce the QCSE effect, LTM-LEDs with different sizes were grown on patterned c-plane sapphire substrate with the MOCVD technique, i.e., partitioned growth, and the size effect on the optical properties and the indium concentration for the quantum wells is studied experimentally. It is revealed that the optical properties can be improved by decreasing the chip size which subsequently reduces the in-plane compressive stress. With the decreasing chip size (from 1,000 µm to 100 µm), the device performance is enhanced. However, the 50 × 50 µm2 device shows a decreasing of output power which is attributed to more defects induced by the higher indium incorporation in the quantum wells. The underlying mechanisms of these observations are discussed based on different methods of characterization, and furthermore, it is proven that for a specific partitioned growth process, the ideal size for LTM-LEDs with the optimal power performance is identified. In summary, both growth and fabrication techniques are used to study the LTM-LEDs to improve their optical performance. Optimized shape and size for LTM-LEDs with the highest optical performance are found based on certain fabrication conditions, respectively. The optimal size for partitioned growth LTM-LEDs with the highest optical power is found. This provides a good rule of thumb on how to choose the size and geometry to obtain LTM-LEDs with an optimal output power performance. Doctor of Philosophy (EEE)
- Published
- 2017
625. Surface plasmon mediated nonradiative energy transfer and enhanced radiative emission in stratified planar nanostructures
- Author
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Golmakaniyoon, Sepideh, Hilmi Volkan Demir, Sun Xiaowei, and School of Electrical and Electronic Engineering
- Subjects
Engineering::Electrical and electronic engineering [DRNTU] - Abstract
Planar plasmonic nanostructures have gained considerable attention due to their crucial role in the theoretical comprehension of surface enhanced fluorescent along with their wide applications in nonradiative energy transfer (NRET), plasmonic wave guided mode, Raman scattering spectroscopy, color filters, light emitting and light harvesting devices. With the availability of large density of states at the metallic surface, the radiative and nonradiative decay channels of an electric dipole in a vicinity of metal would be dramatically modified. However, the radiative enhancement cannot be realized for any desired emissive dipole as the existing plasmonic resonance frequency is limited to the well-known plasmonic materials. Despite the fact that recent studies in metamaterial structures demonstrate a promising approach of tuning Purcell factor across the emission wavelength, the structures still suffer from an inefficient radiative emission. Moreover, in the case of nonradiative energy transfer, the conventionally sandwiched donor-metal film-acceptor configurations lack the desired efficiency and suffer poor photoemission due to the high energy loss. In this dissertation, we propose and demonstrate the nonradiative energy transfer mechanism between the donor and the acceptor through multi-layered metallic nanostructures – stratified configuration, in which an efficient energy transfer can be realized. This novel approach in NRET uniquely provides us with the ability to overcome the drawback of high energy absorption losses in a thick metal film by inserting a non-absorbing dielectric layer between two thin metal films. Moreover, a strong plasmon-plasmon near-field coupling through the dielectric spacer layer is proven to profoundly extend the effective energy transfer distance/efficiency. The proposed architecture has been demonstrated through theoretical modeling and experiment. A full theoretical model of an oscillating dipole behavior in front of the planar plasmonic nanostructure is given to calculate the radiative and nonradiative decay rates, radiative emission enhancement, electric field enhancement factor and nonradiative energy transfer. We present the results for the two most applicable plasmonic materials, silver and gold, and develop the model from the conventional metallic nanostructure to the stratified metal-dielectric-metal configuration, in which a remarkably enhanced energy transfer efficiency is obtained as a result of stronger surface plasmon coupling at the metal-dielectric boundaries as evidenced by the enhanced electric field enhancement factor. Moreover, we present the theoretical and experimental demonstration of engineering surface plasmon mode to tune the maximum radiative decay rate frequency of an emitting dipole in a vicinity of a stratified metal-dielectric-metal nanostructure. This effect arises from an efficient surface plasmon coupling at the metal-dielectric interfaces that leads to the highest electric field enhancement at the dipole position in an optimized cascaded nanostructure. The design principle allows modifying the structure to obtain a maximum transmission efficiency at any desired frequency. The current approach uniquely provides the ability to get the highest plasmonic mode outcoupling to the far field emission at the interfaces by adjusting variable parameter including dielectric layer thickness and refractive index. Thanks to the effective cascaded plasmonic modes coupling across the metal-dielectric interfaces, the proposed design uniquely illustrate the ability to optimize the plasmonic nanostructure for noticeable radiative transmission and emission enhancement. In summary, this dissertation studies nonradiative energy transfer and radiative emission in the stratified plasmonic metal-dielectric nanostructures both numerically and experimentally. Our findings show that multilayer plasmonic configurations enhance the nonradiative energy transfer because of the strong surface plasmon near-field coupling created at the interfaces. Furthermore, we demonstrate the leading role of the decay rate improvement over the negligible contribution of excitation enhancement in the proposed stratified nanostructures. Exploiting this effect in light emitting devices, biosensors, and surface-enhance Raman spectroscopy may lead to structures with high radiated intensity at any desired emission frequency. Doctor of Philosophy (EEE)
- Published
- 2017
626. Colloidal quantum dot light-emitting diode architectures for high performance
- Author
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Kheng Swee Leck, Hilmi Volkan Demir, School of Electrical and Electronic Engineering, and Sun Xiaowei
- Subjects
Materials science ,business.industry ,Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics [DRNTU] ,Nanotechnology ,Engineering::Materials::Photonics and optoelectronics materials [DRNTU] ,Engineering::Electrical and electronic engineering::Semiconductors [DRNTU] ,law.invention ,Colloid ,Quantum dot ,law ,Optoelectronics ,Engineering::Nanotechnology [DRNTU] ,business ,Engineering::Electrical and electronic engineering::Nanoelectronics [DRNTU] ,Light-emitting diode - Abstract
Recently scientific research and development on colloidal quantum dot light-emitting diodes (QD-LEDs) have attracted considerable interest thanks to their advantages over conventional epitaxial-based light-emitting diodes, which require expensive deposition tools and high temperature growth on a substrate template, limiting the choices of materials for epitaxial growth. Colloidal quantum dots (QDs) are chemically synthesized tiny nanocrystals that can be easily dispersed in organic/aqueous solvents and integrated into organic/polymer light-emitting diode architectures using cost-effective solution-based fabrication techniques. In addition, the emission spectra of QD-LEDs can be conveniently adjusted (e.g., tuned from blue to red) by simply changing the size of their QDs. This shows a huge potential for future display and lighting applications. Therefore, there is a strong motivation for understanding the underlying device physics and improving the device performance of the QD-LEDs. This thesis work systematically studies the influence of device architecture on the performance of QD-LEDs using various means including optimizing device charge injection and balance and using efficient indium-free electrodes. In this thesis, white QD-LEDs were demonstrated using a specially designed loosely-packed QD layer and device architecture. In the device, 2,2’,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) electron transport layer (ETL) and poly(N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl)-benzidine) (poly-TPD) hole transport layer (HTL) were made partially in contact to each other. As a result, a certain amount of the injected electrons and holes recombined at the interface, giving rise to red exciplex emission. Combining the red exciplex emission, the green emission from the QDs, and the blue emission from poly-TPD layer, a set of white QD-LEDs with high colour rendering index (CRI) of 89 and correlated color temperature (CCT) of 4098 K were successfully demonstrated. Second, an additional HTL was employed between the QD layer and the TPBi ETL to improve the device performance. Adding HTL at the electron side limits excess electrons to be injected into the QD layer, resulting in better charge balance. Here an improvement in the device performance of over five-folds was obtained with the proposed device architecture in comparison to the reference device in which the QD layer is deposited between the HTL and the ETL. The significant improvement results from the balanced hole and electron injection into the QDs emissive layer. Exciton distribution analysis of the studied device showed that nonradiative energy transfer from the organic transport layer to the QDs is minimum and the improvement mainly stems from the optimized charge balance. Third, the effects of different electron injection materials on the operating voltage were investigated. Notable performance improvement was observed from the devices using Cs2CO3 electron injection material. A 35% increase in the EQE, a 19% reduction in the operating voltage, and a 24% improvement in the power conversion efficiency were achieved compared with the reference device using LiF as the electron injection layer (EIL). Device exciton distribution study showed that Cs2CO3 promoted exciton formation in the QD layer and reduced exciton leakage to the organic layer. The increase of exciton recombination in the QD layer thus enabled better device performance. In addition, in the thesis, we used gallium-doped zinc oxide (GZO) to replace tin-doped indium oxide (ITO) used as the electrode in QD-LEDs due to the increasingly expensive raw materials of ITO. The lowest sheet resistance and resistivity of the radio-frequency (RF) sputtered GZO films are 12.28 Ω/□ and 9.48×10-4 Ω·cm, respectively, comparable to those of commercial grade ITO. These resulting QD-LEDs using the GZO electrodes exhibit a similar level of performance as the devices with ITO as the electrode, indicating that the GZO electrode prepared by the RF sputtering process can be a promising ITO replacement for low-cost QD-LEDs. QD-LEDs have made great progress in terms of device performance over the years and there is room for further improvement. With all these findings and demonstrations in this thesis, we conclude that QD-LEDs hold great promise for potential applications in lighting and displays DOCTOR OF PHILOSOPHY (EEE)
- Published
- 2016
627. Multiexciton dynamics in highly excited colloidal quantum dots for laser applications
- Author
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Sushant, Shendre, Hadijah Bte Rahmat, School of Electrical and Electronic Engineering, and Hilmi Volkan Demir
- Subjects
Engineering::Electrical and electronic engineering [DRNTU] - Abstract
Colloidal quantum dots (CQDs) are a novel class of materials which provide an avenue to explore the quantum confinement effects in semiconductors. Due to their high degree of size controlled spectral tunability, they are being widely pursued as the next generation materials for display and solid state lighting applications. Single material CQDs covering the full visible spectrum merely by size control offer a unique solution for the current laser technology. The most desired full color single material lasers would possibly be enabled by this novel material with the most cost effective approach. This MS thesis work aims to study the properties of CQDs along with their excitonic dynamics towards applications in lasers. Here the study of quantum confinement effects and the optical gain in CQDs is carried out. Also, a detailed simulation study of lifetime decay characteristics of excitonic states is performed. The results indicate that CQD lasers hold great promise for our future research work in versatile lasing platforms. Master of Science
- Published
- 2014
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