Browsing by Author "Lu, S."

Now showing 1 - 7 of 7

Open Access
Effect of Mg doping in the barriers on the electrical performance of InGaN/GaN-based light-emitting diodes
(Elsevier B. V., 2018-04) Zhu, B.; Zhang, Z.; Tan, S. T.; Lu, S.; Zhang, Y.; Kang, X.; Wang, N.; Hasanov, N.; Demir, Hilmi Volkan
In this work, we report how the Mg doping in the barriers affects the electrical performance of InGaN/GaN-based light-emitting diodes. When compared with the reference device that does not have Mg doped quantum barriers, the turn-on voltage for the proposed device is reduced and the electrical thermal stability is improved. The superior electrical performance is analyzed through the temperature dependent current-voltage and capacitance-voltage characteristics. Meanwhile a reduced depletion length and increased acceptor concentration are achieved in the control devices which is consistent with the simulated results.
Open Access
High-performance triangular miniaturized-LEDs for high current and power density applications
(American Chemical Society, 2021-08-18) Lu, S.; Zhang, Y.; Zhang, Zi-H.; Zhu, B.; Zheng, H.; Tan, S. T.; Demir, Hilmi Volkan
This work proposes an effective electrode length model and reveals for the first time the relationship between this model and the mesa shape effect. On the basis of this model, we demonstrate high-performance triangular miniaturized-LEDs (mini-LEDs) and benchmark to the conventional square, and circular shapes of the same mesa area. Here, we systematically study the impact of shaping in mini-LEDs both theoretically and experimentally, which is fundamentally different than that of the conventional regular-sized LEDs. We find that, at the current level of 200 mA, the triangular mini-LEDs deliver an enhancement of 36.4% in the optical output power and a decrease of 9.6% for the forward voltage compared to the commonly used square ones, and also an enhancement of 24.6% in the optical output power and a decrease of 14.3% for the forward voltage compared to the circular ones. The superior optical performance is proved to result from longer effective n-electrode length in the case of the triangular mini-LEDs, which suppresses the self-heating effect and thus well preserves the internal quantum efficiency, whereas the light extraction efficiency and the heat dissipation for the triangular shape are not significantly increased for such small mesa sizes, unlike conventional broad-area LEDs. Meanwhile, the reduced voltage is revealed to stem from the decreased n-GaN resistance. Different than conventional LEDs, these findings therefore indicate that the effective n-electrode length matters substantially for the miniaturized-LEDs.
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
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
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
Strain-reduced micro-LEDs grown directly using partitioned growth
(Frontiers Media S.A., 2021-03-10) Lu, S.; Zhang, Y.; Zhang, Z.-H.; Tsai, P. C.; Zhang, X.; Zhang, S. T.; Demir, Hilmi Volkan
Strain-reduced micro-LEDs in 50 μm × 50 μm, 100 μm × 100 μm, 200 μm × 200 μm, 500 μm × 500 μm, and 1,000 μm × 1,000 μm sizes were grown on a patterned c-plane sapphire substrate using partitioned growth with the metal-organic chemical-vapor deposition (MOCVD) technique. The size effect on the optical properties and the indium concentration for the quantum wells were studied experimentally. Here, we revealed that the optical properties can be improved by decreasing the chip size (from 1,000 to 100 µm), which can correspondingly reduce the in-plane compressive stress. However, when the chip size is further reduced to 50 μm × 50 μm, the benefit of strain release is overridden by additional defects induced by the higher indium incorporation in the quantum wells and the efficiency of the device decreases. The underlying mechanisms of the changing output power are uncovered based on different methods of characterization. This work shows the rules of thumb to achieve optimal power performance for strain-reduced micro-LEDs through the proposed partitioned growth process.