Browsing by Author "Ju, Z. G."

Now showing 1 - 7 of 7

Open Access
Advantages of the Blue InGaN/GaN Light-Emitting Diodes with an AlGaN/GaN/AlGaN Quantum Well Structured Electron Blocking Layer
(American Chemical Society, 2014-03-21) Ju, Z. G.; Liu W.; Zhang Z.-H.; Tan S.T.; Ji Y.; Kyaw, Z.; 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 p-(AlGaN/GaN/AlGaN) quantum well structured electron blocking layer (QWEBL) are designed and grown by a metal− organic chemical-vapor deposition (MOCVD) system. The proposed QWEBL LED structure, in which a p-GaN QW layer is inserted in the p-AlGaN electron blocking layer, not only leads to an improved hole injection but also reduces the electron leakage, thus enhancing the radiative recombination rates across the active region. Consequently, the light output power was enhanced by 10% for the QWEBL LED at a current density of 35 A/cm2. The efficiency droop of the optimized device was reduced to 16%. This is much smaller than that of the conventional p-AlGaN electron blocking layer LED, which is 31%.
Open Access
Enhanced hole transport in InGaN/GaN multiple quantum well light-emitting diodes with a p-type doped quantum barrier
(Optical Society of America, 2013) Ji Y.; Zhang, Z. -H.; Tan S.T.; Ju, Z. G.; Kyaw, Z.; Hasanov N.; Liu W.; Sun X. W.; Demir, Hilmi Volkan
We study hole transport behavior of InGaN/GaN light-emitting diodes with the dual wavelength emission method. It is found that at low injection levels, light emission is mainly from quantum wells near p-GaN, indicating that hole transport depth is limited in the active region. Emission from deeper wells only occurs under high current injection. However, with Mg-doped quantum barriers, holes penetrate deeper within the active region even under low injection, increasing the radiative recombination. Moreover, the improved hole transport leads to reduced forward voltage and enhanced light generation. This is also verified by numerical analysis of hole distribution and energy band structure. © 2013 Optical Society of America.
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
Influence of n-type versus p-type AlGaN electron-blocking layer on InGaN/GaN multiple quantum wells light-emitting diodes
(AIP Publishing, 2013-08-01) Ji Y.; Zhang Z.-H.; Kyaw, Z.; Tan S.T.; Ju, Z. G.; Zhang, X. L.; Liu W.; Sun, X. W.; Demir, Hilmi Volkan
The effect of n-AlGaN versus p-AlGaN electron-blocking layers (EBLs) on the performance of InGaN/GaN light-emitting diodes is studied in this work. Experimental results suggest that the n-type EBL leads to higher optical output power and external quantum efficiency, compared to the devices with p-AlGaN EBL, which is commonly used today. Numerical simulations on the carrier distribution and energy band diagram reveal that the n-AlGaN EBL is more efficient in preventing electron overflow, while not blocking the hole injection into the active region, hence leading to higher radiative recombination rate within the multiple quantum wells active region. © 2013 AIP Publishing LLC.
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 origin of the redshift in the emission wavelength of InGaN/GaN blue light emitting diodes grown with a higher temperature interlayer
(AIP Publishing, 2012-03-20) Ju, Z. G.; Tan S.T.; Zhang Z.-H.; Ji Y.; Kyaw, Z. B.; Dikme, Y.; Sun, X. W.; Demir, Hilmi Volkan
A redshift of the peak emission wavelength was observed in the blue light emitting diodes of InGaN/GaN grown with a higher temperature interlayer that was sandwiched between the low-temperature buffer layer and high-temperature unintentionally doped GaN layer. The effect of interlayer growth temperature on the emission wavelength was probed and studied by optical, structural, and electrical properties. Numerical studies on the effect of indium composition and quantum confinement Stark effect were also carried out to verify the experimental data. The results suggest that the redshift of the peak emission wavelength is originated from the enhanced indium incorporation, which results from the reduced strain during the growth of quantum wells.
Open Access
Simultaneous enhancement of electron overflow reduction and hole injection promotion by tailoring the last quantum barrier in InGaN/GaN light-emitting diodes
(AIP Publishing, 2014-04-24) 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.; Teng, J. H.; Wei, S. X.; Demir, Hilmi Volkan
A three-step graded undoped-InGaN layers embedded between the GaN last quantum barrier layer and the p-AlGaN electron blocking layer was proposed and its effect on the performance of InGaN/GaN light-emitting diodes was investigated both experimentally and theoretically. In the proposed structure, the electron leakage is found to be effectively reduced, while the hole injection efficiency is simultaneously increased significantly, hence enabling a greatly enhanced radiative recombination rate within the active region. As a result, improvements of 12.25% in the optical output power and 11.98% in the external quantum efficiency are obtained from the proposed device with the respect to the reference device.