Browsing by Subject "LEDs"

Now showing 1 - 6 of 6

Open Access
Common white light sources
(Springer, 2019-01) Erdem, Talha; Demir, Hilmi Volkan; Erdem, Talha; Demir, Hilmi Volkan
In this Chapter we describe the features of common light sources. We first present the spectral features of the sun and discuss its colorimetric properties. Next, we summarize the properties of traditional light sources including incandescent lamps, fluorescent lamps, and high-pressure sodium lamps. Subsequently, we discuss the white light-emitting diodes of various types.
Open Access
Conclusions and future outlook
(Springer Singapore, 2022-10-28) Erdem, Onur; Demir, Hilmi Volkan
Open Access
Electric field dependent optoelectronic nature of InGaN/GaN quantum structures and devices
(2012) Sarı, Emre
In the past two decades we have been witnessing the emergence and rapid development of III-Nitride based optoelectronic devices including InGaN/GaN light-emitting diodes (LEDs) and laser diodes with operation wavelengths ranging from green-blue to near-UV. These InGaN/GaN devices are now being widely used in applications important for lighting, displays, and data storage, collectively exceeding a total market size of 10 billion USD. Although InGaN/GaN has been studied and exploited very extensively to date, its field dependent nature is mostly unknown and is surprisingly prone to quite unexpected behavior due to its intrinsic polarization property. In this thesis, we report our systematic study on the electric field dependent characteristics of InGaN/GaN quantum structures and devices including modulators and LEDs. Here we present our comparative study of electroabsorption in polar c-plane InGaN/GaN multiple quantum wells (MQWs) with different built-in polarization induced electrostatic fields. Analyzing modulator structures with varying structural MQW parameters, we find that electroabsorption grows stronger with decreasing built-in electrostatic field strength inside the well layer, as predicted by our theoretical model and verified by our experimental results. To further explore the field dependent optoelectronic nature of c-plane grown InGaN/GaN quantum structures, we investigate radiative carrier dynamics, which is of critical importance for LEDs. Our time and spectrum resolved photoluminescence measurements and numerical analyses indicate that the carrier lifetimes, the radiative recombination lifetimes, and the quantum efficiencies all decrease with increasing field. We also study the physics of electroabsorption and carrier dynamics in InGaN/GaN quantum heterostructures grown intentionally on nonpolar a-plane of the wurtzite crystal structure, which are free of the polarization-induced electrostatic fields. We compare these results with the conventional c-plane grown polar structures. In the polar case, we observe blue-shifting absorption profile and decreasing carrier lifetimes with increasing electric field. In the nonpolar case, however, we observe completely the opposite: a red-shifting absorption profile and increasing carrier lifetimes. We explain these observations in the context of basic physical principles including Fermi‟s golden rule and quantum-confined Stark effect. Also, we present electroabsorption 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 electroabsorption performance. While keeping the ELOG window widths constant, compared to photoluminescence behavior, however, electroabsorption surprisingly exhibits the largest performance variation, making the electroabsorption the most sensitive to the mask stripe widths. This thesis work provides significant insight and important information for the optoelectronics of InGaN/GaN quantum structures and devices to better understand their field dependent nature.
Open Access
Electroluminescence efficiency enhancement in quantum dot light-emitting diodes by embedding a silver nanoisland layer
(Wiley-VCH Verlag, 2015) Yang, X.; Hernandez-Martinez, P. L.; Dang C.; Mutlugün, E.; Zhang, K.; Demir, Hilmi Volkan; Sun X. W.
A colloidal quantum dot light-emitting diode (QLED) is reported with substantially enhanced electroluminescence by embedding a thin layer of Ag nanoislands into hole transport layer. The maximum external quantum efficiency (EQE) of 7.1% achieved in the present work is the highest efficiency value reported for green-emitting QLEDs with a similar structure, which corresponds to 46% enhancement compared with the reference device. The relevant mechanisms enabling the EQE enhancement are associated with the near-field enhancement via an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Ag nano-islands, which are found to lead to good agreement between the simulation results and the experimental data, providing us with a useful insight important for plasmonic QLEDs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Introduction
(Springer, Singapore, 2019) Erdem, T.; Demir, Hilmi Volkan
Here we briefly emphasize the importance of lighting for our daily lives as well as its role in energy consumption. We very briefly introduce the problems that need to be addressed and finally summarize the contents of this brief.
Open Access
Near-UV InGaN/GaN-based dual-operation quantum optoelectronic devices
(SPIE, 2007) Özel, Tuncay; Sarı, Emra; Nizamoğlu, Sedat; Demir, Hilmi Volkan
We present a novel dual-operation InGaN/GaN based quantum optoelectronic device (QOD) that operates as a quantum electroabsorption modulator in reverse bias and as a light emitter in forward bias in the spectral range of near-ultraviolet (UV). Here we report the design, epitaxial growth, fabrication, and characterization of such QODs that incorporate ∼2-3 nm thick InGaN/GaN quantum structures for operation between 380 nm and 400 nm. In reverse bias, our QODs show an optical absorption coefficient change of ∼14000 cm -1 with a reverse bias of 9 V (corresponding to ∼40 cm -1 absorption coefficient change for 1 V/μm field swing) at 385 nm, reported for the first time for InGaN/GaN quantum structures in the near-UV range. In forward bias, though, our QODs exhibit optical electroluminescence spectrum centered around 383 nm with a full width at half maximum of 20 nm and photoluminescence spectrum centered around 370 nm with a full width at half maximum of 12 nm. This dual operation makes such quantum optoelectronic devices find a wide range of optoelectronics applications both as an electroabsorption modulator and a light emitting diode (LED).