Browsing by Subject "Blue"
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Item 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 VolkanWe 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 AmericaItem Open Access Enhanced memory effect via quantum confinement in 16 nm InN nanoparticles embedded in ZnO charge trapping layer(AIP Publishing, 2014) El-Atab, N.; Cimen, F.; Alkis, S.; Ortac, B.; Alevli, M.; Dietz, N.; Okyay, Ali Kemal; Nayfeh, A.In this work, the fabrication of charge trapping memory cells with laser-synthesized indium-nitride nanoparticles (InN-NPs) embedded in ZnO charge trapping layer is demonstrated. Atomic layer deposited Al2O3 layers are used as tunnel and blocking oxides. The gate contacts are sputtered using a shadow mask which eliminates the need for any lithography steps. High frequency C-Vgate measurements show that a memory effect is observed, due to the charging of the InN-NPs. With a low operating voltage of 4 V, the memory shows a noticeable threshold voltage (Vt) shift of 2 V, which indicates that InN-NPs act as charge trapping centers. Without InN-NPs, the observed memory hysteresis is negligible. At higher programming voltages of 10 V, a memory window of 5 V is achieved and the Vt shift direction indicates that electrons tunnel from channel to charge storage layer. © 2014 AIP Publishing LLCItem Open Access Novel hybrid light emitting diodes with multiple assemblies of nanocrystals to generate and tune white light(2007) Nizamoğlu, SedatToday approximately one third of the world population (about two billion people) in under-developed countries has no access to electricity and relies on unhealthy, costly and low-quality fuel-based lighting for home illumination. In the rest of the world, lighting consumes a large portion (20%) of the total electricity production, which significantly contributes to global warming problem. Also given limited resources, such large energy consumption needs to be reduced for sustainable economic growth. Solid state lighting provides remedy to these problems for the entire globe. Therefore, the advancement of white light emitting diodes (WLEDs) becomes a key point for development of human civilization. To this end, the strong demand for the development of high quality WLEDs around the globe motivates our research work on the investigation of white light generation with high color rendering index. In this thesis, we develop and demonstrate nanocrystal hybridized light emitting diodes with high color rendering index. By the hybridization of multiple layer-by-layer assemblies of CdSe/ZnS core-shell nanocrystals on blue and near ultraviolet (n-UV) InGaN/GaN light emitting diodes, we show white light generation with highly tunable optical properties such as tristimulus color coordinates, correlated color temperature, and color rendering index. Additionally, by using dual hybridization of nanocrystals in combination with conjugated polymers, we obtain white light sources with high color rendering indices exceeding the requirements of the future solid state lighting applications. In this thesis, we present design, growth, fabrication, experimental characterization and theoretical analysis of these hybrid white LEDs.Item 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 VolkanWe 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 AmericaItem Open Access Stable and low ‐ threshold optical gain in CdSe/CdS quantum dots: an all ‐ colloidal frequency up ‐ converted laser(Wiley-VCH Verlag, 2015) Güzeltürk, B.; Keleşemur, Y.; Güngor, K.; Yeltik, A.; Akgül, M. Z.; Wang, Y.; Chen R.; Dang, C.; Sun, H.; Demir, Hilmi VolkanAn all-solution processed and all-colloidal laser is demonstrated using tailored CdSe/CdS core/shell quantum dots, which exhibit highly stable and low-threshold optical gain owing to substantially suppressed non-radiative Auger recombination.Item Open Access White light generation with CdSe/ZnS core-shell nanocrystals and InGaN/GaN light emitting diodes(IEEE, 2006) Nizamoğlu, Sedat; Özel, Sedat; Sarı, Emre; Demir, Hilmi VolkanWe present hybrid white light sources that integrate CdSe/ZnS core-shell nanocrystals on blue InGaN/GaN light emitting diodes (LED). We report on the demonstrations of white light generation using yellow nanocrystals (λPL=580 nm) hybridized on a blue LED (λEL= 440 nm) with tristimulus coordinates of x=0.37 and y=0.25, correlated color temperature of Tc=2692 K, and color rendering index of R a=14.6; cyan and red nanocrystals (λPL=500 nm and 620 nm) on a blue LED (λEL=440 nm) with x=0.37, y=0.28, T c=3246 K, and Ra=19.6; and green, yellow, and red nanocrystals (λPL=540 nm, 580 nm, and 620 nm) on a blue LED (λEL=452 nm) with x=0.30, y=0.28, Tc =7521 K, and Ra=40.9. © 2006 IEEE.