Browsing by Author "Ji, Y."

Now showing 1 - 11 of 11

Open Access
Energy-saving quality road lighting with colloidal quantum dot nanophosphors
(Walter de Gruyter GmbH, 2014) Erdem, T.; Kelestemur, Y.; Soran-Erdem, Z.; Ji, Y.; Demir, Hilmi Volkan
Here the first photometric study of road-lighting white light-emitting diodes (WLEDs) integrated with semiconductor colloidal quantum dots (QDs) is reported enabling higher luminance than conventional light sources, specifically in mesopic vision regimes essential to street lighting. Investigating over 100 million designs uncovers that quality road-lighting QD-WLEDs, with a color quality scale and color rendering index ≥85, enables 13-35% higher mesopic luminance than the sources commonly used in street lighting. Furthermore, these QD-WLEDs were shown to be electrically more efficient than conventional sources with power conversion efficiencies ≥16-29%. Considering this fact, an experimental proof-of-concept QD-WLED was demonstrated, which is the first account of QD based color conversion custom designed for street lighting applications. The obtained white LED achieved the targeted mesopic luminance levels in accordance with the road lighting standards of the USA and the UK. These results indicate that road-lighting QD-WLEDs are strongly promising for energy-saving quality road lighting. © 2014 Science Wise Publishing & De Gruyter 2014.
Open Access
A hole accelerator for InGaN/GaN light-emitting diodes
(AIP Publishing, 2014) Zhang, Z. H.; Liu, W.; Tan, S. T.; Ji, Y.; Wang, L.; Zhu, B.; Zhang, Y.; Lu, S.; Zhang, X.; Hasanov, N.; Sun, X. W.; Demir, Hilmi Volkan
The quantum efficiency of InGaN/GaN light-emitting diodes (LEDs) has been significantly limited by the insufficient hole injection, and this is caused by the inefficient p-type doping and the low hole mobility. The low hole mobility makes the holes less energetic, which hinders the hole injection into the multiple quantum wells (MQWs) especially when a p-type AlGaN electron blocking layer (EBL) is adopted. In this work, we report a hole accelerator to accelerate the holes so that the holes can obtain adequate kinetic energy, travel across the p-type EBL, and then enter the MQWs more efficiently and smoothly. In addition to the numerical study, the effectiveness of the hole accelerator is experimentally shown through achieving improved optical output power and reduced efficiency droop for the proposed InGaN/GaN LED. (C) 2014 AIP Publishing LLC.
Open Access
Improved hole distribution in InGaN/GaN light-emitting diodes with graded thickness quantum barriers
(AIP Publishing, 2013) Ju, Z. G.; Liu, W.; Zhang, Z. H.; Tan, S. T.; Ji, Y.; Kyaw, Z. B.; Zhang, X. L.; Lu, S. P.; Zhang, Y. P.; Zhu, B.; Hasanov, N.; Sun, X. W.; Demir, Hilmi Volkan
InGaN/GaN light-emitting diodes (LEDs) with graded-thickness quantum barriers (GTQB) are designed and grown by metal-organic chemical-vapor deposition. The proposed GTQB structure, in which the barrier thickness decreases from the n-GaN to p-GaN side, was found to lead to an improved uniformity in the hole distribution and thus, radiative recombination rates across the active region. Consequently, the efficiency droop was reduced to 28.4% at a current density of 70 A/cm2, which is much smaller than that of the conventional equal-thickness quantum barriers (ETQB) LED, which is 48.3%. Moreover, the light output power was enhanced from 770 mW for the ETQB LEDs to 870 mW for the GTQB LEDs at 70 A/cm2. © 2013 AIP Publishing LLC.
Open Access
Improved InGaN/GaN light-emitting diodes with a p-GaN/n-GaN/p-GaN/n-GaN/p-GaN current-spreading layer
(Optical Society of American (OSA), 2013) Zhang, Z.-H.; Tan, S.T.; Liu W.; Ju, Z.; Zheng, K.; Kyaw, Z.; Ji, Y.; Hasanov, N.; Sun X.W.; Demir, Hilmi Volkan
This work reports both experimental and theoretical studies on the InGaN/GaN light-emitting diodes (LEDs) with optical output power and external quantum efficiency (EQE) levels substantially enhanced by incorporating p-GaN/n-GaN/p-GaN/n-GaN/p-GaN (PNPNP-GaN) current spreading layers in p-GaN. Each thin n-GaN layer sandwiched in the PNPNP-GaN structure is completely depleted due to the built-in electric field in the PNPNP-GaN junctions, and the ionized donors in these n-GaN layers serve as the hole spreaders. As a result, the electrical performance of the proposed device is improved and the optical output power and EQE are enhanced. © 2013 Optical Society of America.
Open Access
Low thermal-mass LEDs: Size effect and limits
(Optical Society of American (OSA), 2014) Lu, S.; Liu W.; Zhang, Z.-H.; Tan, S.T.; Ju, Z.; Ji, Y.; Zhang X.; Zhang, Y.; Zhu, B.; Kyaw, Z.; Hasanov, N.; Sun X.W.; Demir, Hilmi Volkan
In this work, low thermal-mass LEDs (LTM-LEDs) were developed and demonstrated in flip-chip configuration, studying both experimentally and theoretically the enhanced electrical and optical characteristics and the limits. LTM-LED chips in 25 × 25 μm2, 50 × 50 μm2, 100 × 100 μm2 and 200 × 200 μm2 mesa sizes were fabricated and comparatively investigated. Here it was revealed that both the electrical and optical properties are improved by the decreasing chip size due to the reduced thermal mass. With a smaller chip size (from 200 μm to 50 μm), the device generally presents higher current density against the bias and higher power density against the current density. However, the 25 × 25 μm2 device behaves differently, limited by the fabrication margin limit of 10 μm. The underneath mechanisms of these observations are uncovered, and furthermore, based on the device model, it is proven that for a specific flip-chip fabrication process, the ideal size for LTM-LEDs with optimal power density performance can be identified. ©2014 Optical Society of America
Open Access
Nonradiative recombination-Critical in choosing quantum well number for InGaN/GaN light-emitting diodes
(Optical Society of American (OSA), 2015) Zhang, Y.P.; Zhang, Z.-H.; Liu W.; Tan, S.T.; Ju, Z.G.; Zhang X.L.; Ji, Y.; Wang L.C.; Kyaw, Z.; Hasanov, N.; Zhu, B.B.; Lu, S.P.; Sun X.W.; Demir, Hilmi Volkan
In this work, InGaN/GaN light-emitting diodes (LEDs) possessing varied quantum well (QW) numbers were systematically investigated both numerically and experimentally. The numerical computations show that with the increased QW number, a reduced electron leakage can be achieved and hence the efficiency droop can be reduced when a constant Shockley-Read-Hall (SRH) nonradiative recombination lifetime is used for all the samples. However, the experimental results indicate that, though the efficiency droop is suppressed, the LED optical power is first improved and then degraded with the increasing QW number. The analysis of the measured external quantum efficiency (EQE) with the increasing current revealed that an increasingly dominant SRH nonradiative recombination is induced with more epitaxial QWs, which can be related to the defect generation due to the strain relaxation, especially when the effective thickness exceeds the critical thickness. These observations were further supported by the carrier lifetime measurement using a pico-second time-resolved photoluminescence (TRPL) system, which allowed for a revised numerical modeling with the different SRH lifetimes considered. This work provides useful guidelines on choosing the critical QW number when designing LED structures. © 2014 Optical Society of America.
Open Access
On the effect of N-GaN/P-GaN/N-GaN/P-GaN/N-GaN built-in junctions in the n-GaN layer for InGaN/GaN light-emitting diodes
(Optical Society of America, 2014-01-07) Kyaw, Z.; Zhang, Z. H.; Liu, W.; Tan, S. T.; Ju, Z. G.; Zhang, X. L.; Ji, Y.; Hasanov, N.; Zhu, B.; Lu, S.; Zhang, Y.; Sun, X. W.; Demir, Hilmi Volkan
N-GaN/P-GaN/N-GaN/P-GaN/N-GaN (NPNPN-GaN) junctions embedded between the n-GaN region and multiple quantum wells (MQWs) are systematically studied both experimentally and theoretically to increase the performance of InGaN/GaN light emitting diodes (LEDs) in this work. In the proposed architecture, each thin P-GaN layer sandwiched in the NPNPN-GaN structure is completely depleted due to the built-in electric field in the NPNPN-GaN junctions, and the ionized acceptors in these P-GaN layers serve as the energy barriers for electrons from the n-GaN region, resulting in a reduced electron over flow and enhanced the current spreading horizontally in the n-GaN region. These lead to increased optical output power and external quantum efficiency (EQE) from the proposed device. (C) 2014 Optical Society of America
Open Access
On the mechanisms of InGaN electron cooler in InGaN/GaN light-emitting diodes
(Optical Society of America, 2014) Zhang, Z. H.; W. L.; Tan, S. T.; Ju, Z.; Ji, Y.; Kyaw, Z.; Zhang, X.; Hasanov, N.; Zhu, B.; Lu, S.; Zhang, Y.; Sun, X. W.; Demir, Hilmi Volkan
Electron overflow limits the quantum efficiency of InGaN/GaN light-emitting diodes. InGaN electron cooler (EC) can be inserted before growing InGaN/GaN multiple quantum wells (MQWs) to reduce electron overflow. However, detailed mechanisms of how the InGaN EC contributes to the efficiency improvement have remained unclear so far. In this work, we theoretically propose and experimentally demonstrate an electron mean-free-path model, which reveals the InGaN EC reduces the electron mean free path in MQWs, increases the electron capture rate and also reduces the valence band barrier heights of the MQWs, in turn promoting the hole transport into MQWs. (C) 2014 Optical Society of America
Open Access
P-doping-free InGaN/GaN light-emitting diode driven by three-dimensional hole gas
(2013) Zhang, Z.-H.; Tiam Tan, S.; Kyaw, Z.; Liu W.; Ji, Y.; Ju, Z.; Zhang X.; Wei Sun X.; Volkan Demir H.
Here, GaN/AlxGa1-xN heterostructures with a graded AlN composition, completely lacking external p-doping, are designed and grown using metal-organic-chemical-vapour deposition (MOCVD) system to realize three-dimensional hole gas (3DHG). The existence of the 3DHG is confirmed by capacitance-voltage measurements. Based on this design, a p-doping-free InGaN/GaN light-emitting diode (LED) driven by the 3DHG is proposed and grown using MOCVD. The electroluminescence, which is attributed to the radiative recombination of injected electrons and holes in InGaN/GaN quantum wells, is observed from the fabricated p-doping-free devices. These results suggest that the 3DHG can be an alternative hole source for InGaN/GaN LEDs besides common Mg dopants. © 2013 AIP Publishing LLC.
Open Access
A PN-type quantum barrier for InGaN/GaN light emitting diodes
(Optical Society of American (OSA), 2013) Zhang, Z.-H.; Tan, S.T.; Ji, Y.; Liu W.; Ju, Z.; Kyaw, Z.; Sun X.W.; Demir, Hilmi Volkan
In this work, InGaN/GaN light-emitting diodes (LEDs) with PN-type quantum barriers are comparatively studied both theoretically and experimentally. A strong enhancement in the optical output power is obtained from the proposed device. The improved performance is attributed to the screening of the quantum confined Stark effect (QCSE) in the quantum wells and improved hole transport across the active region. In addition, the enhanced overall radiative recombination rates in the multiple quantum wells and increased effective energy barrier height in the conduction band has substantially suppressed the electron leakage from the active region. Furthermore, the electrical conductivity in the proposed devices is improved. The numerical and experimental results are in excellent agreement and indicate that the PN-type quantum barriers hold great promise for high-performance InGaN/GaN LEDs. © 2013 Optical Society of America.
Open Access
Room-temperature larger-scale highly ordered nanorod imprints of ZnO film
(Optical Society of American (OSA), 2013) Kyaw, Z.; Wang J.; Dev, K.; Tiam Tan, S.; Ju, Z.; Zhang, Z.-H.; Ji, Y.; Hasanov, N.; Liu W.; Sun X.W.; Demir, Hilmi Volkan
Room-temperature large-scale highly ordered nanorod-patterned ZnO films directly integrated on III-nitride light-emitting diodes (LEDs) are proposed and demonstrated via low-cost modified nanoimprinting, avoiding a high-temperature process. with a 600 nm pitch on top of a critical 200 nm thick Imprinting ZnO nanorods of 200 nm in diameter and 200 nm in height continuous ZnO wetting layer, the light output power of the resulting integrated ZnO-nanorod-film/semi- transparent metal/GaN/InGaN LED shows a two-fold enhancement (100% light extraction efficiency improvement) at the injection current of 150 mA, in comparison with the conventional LED without the imprint film. The increased optical output is well explained by the enhanced light scattering and outcoupling of the ZnOrod structures along with the wetting film, as verified by the numerical simulations. The wetting layer is found to be essential for better impedance matching. The current-voltage characteristics and electroluminescence measurements confirm that there is no noticeable change in the electrical or spectral properties of the final LEDs after ZnO-nanorod film integration. These results suggest that the low-cost high-quality large-scale ZnOnanorod imprints hold great promise for superior LED light extraction. ©2013 Optical Society of America.