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Japanese Journal of Applied Physics 51 (2012) 100201
Effect of Internal Electric Field in Well Layer of InGaN/GaN Multiple Quantum Well Light-Emitting Diodes on Efficiency Droop
Sang-Heon Han1;2, Dong-Yul Lee2, Jin-Young Lim2, Jeong Wook Lee2, Dong-Joon Kim2, Young Sun Kim2, Sung-Tae Kim2, and Seong-Ju Park1
1. School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea
2. Samsung Electronics Co., Ltd., Yongin, Gyeonggi 446-711, Republic of Korea

Received June 23, 2012; accepted August 10, 2012; published online September 19, 2012
We investigate the effect of internal electric field in InGaN well layer of InGaN/GaN multiple quantum well light-emitting diodes (LEDs) on efficiency droop behavior. The simulation results show that the internal electric field in InGaN well layers of Ga-polar LEDs is same as the direction of external electric field by forward bias voltage, resulting in a strong efficiency droop. However, N-polar LEDs show that the efficiency droop is drastically improved due to an increase of internal quantum efficiency and carrier injection efficiency by weakening the internal electric field with increasing the forward bias voltage and decrease of electron overflow.

Enhanced Optical Power of InGaN/GaN Light-Emitting Diode by AlGaN Interlayer and Electron Blocking Layer
Sang-Jun Lee, Chu-Young Cho, Sang-Hyun Hong, Sang-Heon Han, Sukho Yoon, Sung-Tae Kim, and Seong-Ju Park
Abstract: We report on the effect of an AlGaN interlayer with a low Al composition, which is inserted between multiple quantum wells (MQWs) and an AlGaN electron blocking layer (EBL) in light-emitting diodes (LEDs). The band diagram shows that this interlayer reduces the hole barrier height between the last GaN barrier in MQWs and the AlGaN EBL to enhance the hole injection. The optical output power at 20 mA of LEDs with interlayer is increased by 20% more than that of LEDs without an interlayer. This improvement is attributed to the enhanced radiative recombination rate, which is due to a more uniform hole distribution and higher hole concentration in MQWs.

COMPOUND SEMICONDUCTOR October 2012 Volume 18 Number 7
See also R –M. Lin et al. Appl. Phys. Lett. 101 081120 (2012)
Inserting InGaN slashes LED droop
Reductions in external quantum efficiency at high current densities diminish when p-doped InGaN is inserted after the electron-blocking layer

Appl. Phys. Lett. 101, 081120 (2012); doi: 10.1063/1.4747802
Inserting a p-InGaN layer before the p-AlGaN electron blocking layer suppresses efficiency droop in InGaN-based light-emitting diodes
Ray-Ming Lin, Sheng-Fu Yu, Shoou-Jinn Chang, Tsung-Hsun Chiang, Sheng-Po Chang et al.
In this study, we observed a dramatic decrease in the efficiency droop of InGaN/GaN light-emitting diodes after positioning a p-InGaN insertion layer before the p-AlGaN electron-blocking layer. The saturated external quantum efficiency of this device extended to 316 mA, with an efficiency droop of only 7% upon increasing the operating current to 1A; in contrast, the corresponding conventional light-emitting diode suffered a severe efficiency droop of 42%. We suspect that the asymmetric carrier distribution was effectively mitigated as a result of an improvement in the hole injection rate and a suppression of electron overflow.
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APPLIED PHYSICS LETTERS 100, 071910 (2012)
Well-to-well non-uniformity in InGaN/GaN multiple quantum wells characterized by capacitance-voltage measurement with additional laser illumination
Tae-Soo Kim,1 Byung-Jun Ahn,1 Yanqun Dong,1 Ki-Nam Park,1 Jin-Gyu Lee,1 Youngboo Moon,2 Hwan-Kuk Yuh,2 Sung-Chul Choi,2 Jae-Hak Lee,2 Soon-Ku Hong,3 and Jung-Hoon Song1
1. Department of Physics, Kongju National University, Kongju, Chungnam 314-701, South Korea
2. THELEDS Co., Ltd., Gwangju 546-12, South Korea
3. Department of Advanced Materials Engineering, Chungnam National University, Daejeon 305-764, South Korea

We experimentally investigated well-to-well non-uniformity in InGaN/GaN multiple quantum well (MQW) structures by using capacitance-voltage measurements with additional laser illumination. By varying the illuminating power of the resonant excitation, well-to-well non-uniformity through the MQWs was clearly revealed. The quantum wells (QWs) close to the n-GaN side show higher carrier accumulations and larger position shift as the excitation power is increased, relative to the p-side QWs. Both results were attributed to the existence of stronger piezoelectric fields in the n-side QWs induced by subsequent partial relaxation of strain through the MQWs.


Appl. Phys. Lett. 98, 121115 (2011); doi: 10.1063/1.3571440
The effect of trimethylgallium flows in the AlInGaN barrier on optoelectronic characteristics of near ultraviolet light-emitting diodes grown by atmospheric pressure metalorganic vapor phase epitaxy
Yi-Keng Fu, Ren-Hao Jiang, Yu-Hsuan Lu, Bo-Chun Chen, Rong Xuan et al.
The letter reports a theoretical and experimental study on the device performance of near ultraviolet light-emitting diodes LEDs with quaternary AlInGaN quantum barrier QB. The indium mole fraction of AlInGaN QB could be enhanced as we increased the trimethylgallium flow rate. It was found the AlInGaN/InGaN LEDs can reduce forward voltage and improve light output power, compared with conventional GaN QB. By using advanced device simulation, it should be attributed to a reduction in lattice mismatch induced polarization mismatch in the active layer, which results in the suppression of electron overflow.
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Efficiency and Droop Improvement in GaN-Based High-Voltage Light-Emitting Diodes
C. H. Wang, D. W. Lin, C. Y. Lee, M. A. Tsai, G. L. Chen, H. T. Kuo, W. H. Hsu, H. C. Kuo, Senior Member, IEEE, T. C. Lu, Member, IEEE, S. C. Wang, Life Member, IEEE, and G. C. Chi
The efficiency and electrical characteristics of GaN-based high-voltage light-emitting diodes (HV-LEDs) are investigated in detail. The spatial distribution of light output and simulation results showed that 100-V HV-LED with smaller microchips had superior current spreading. As a result, under 1-W operation, the luminous efficiency of 100-V HV-LED with smaller microchips was enhanced by 7.8% compared to that of 50-V HV-LED, while the efficiency droop behaviors were reduced from 28% in 50-V HV-LED to 25.8% in 100-V HV-LED. Moreover, smaller microchips exhibited lower series resistance and forward voltage, leading to higher wall-plug efficiency.

Journal of The Electrochemical Society, 158 (9) H908-H911 (2011)
High-Performance Vertical Light-Emitting Diodes with Buried Current Blocking Layer and Non-Alloyed Reflective Cr/Al/Pt/ Au n-type Electrodes
Tak Jeong,a,b Seung Whan Kim,a Sang Hern Lee,a Jin Woo Ju,a Seung Jae Lee,a Jong Hyeob Baek,a and June Key Leeb
a Korea Photonics Technology Institute, Gwangju 500-460, Korea
b Department of Materials Science & Engineering, Chonnam National University, Gwanju 500-757, Korea

A GaN-based vertical light-emitting diode (VLED) with a novel structure, consisting of a buried current blocking layer (BCBL) inserted in p-type GaN and non-alloyed reflective n-type electrodes (Cr/Al/Pt/Au) on N-face n-GaN, is proposed and its enhanced light extraction efficiency is demonstrated. It was found that this new VLED structure could significantly increase the light output power by about 16% compared to that of the conventional VLED chips with n-type Cr/Au electrodes and without the BCBL, although the use of the BCBL slightly increased the forward voltage. The increase in the light output power was attributed to the injection of a large current density within the emission region of the active layers by the BCBL and, thus, the reduction of the optical absorption beneath the n-pad electrodes by the Cr/Al/Pt/Au metals.

40th “Jaszowiec” International School and Conference on the Physics of Semiconductors, Krynica-Zdrój 2011
Interplay between Internal and External Electric Field Studied by Photoluminescence in InGaN/GaN Light Emitting Diodes
G. Staszczaka, A. Khachapuridzea, S. Grzankaa;b, R. Czerneckia;b, R. Piotrzkowskia, P. Perlina;b and T. Suskia
a Institute of High Pressure Physics PAS, Sokołowska 29/37, 01-142 Warszawa, Poland
b TopGaN, Sokołowska 29/37, 01-142 Warszawa, Poland

We have studied a series of polar InGaN/GaN light emitting diodes, consisting of either a blue (440–450 nm) quantum well, or combination of blue and violet (410 nm) quantum wells (with indium content 18% and 10%, respectively). The blue quantum well was always placed close to p-type region of the particular LED. We found that the electroluminescence induced by low current is characterized by light emission from the blue quantum well only. In comparison, optical excitation of our LEDs leads to light emission with energies characteristic either for blue and/or violet quantum wells. The corresponding microphotoluminescence spectra evolve depending on external polarization and variable light intensity of excitation supplied by He–Cd laser. Interplay between built-in electric field and externally applied polarization/screening decides about the band structure profiles and thus radiative recombination mechanisms.

Influence of Mg-doped barriers on semipolar (2021) multiple-quantum-well green light-emitting diodes
Chia-Yen Huang,1,a) Qimin Yan,1 Yuji Zhao,2 Kenji Fujito,3 Daniel Feezell,1 Chris G. Van de Walle,1 James S. Speck,1 Steven P. DenBaars,1,2 and Shuji Nakamura1,2 1 Department of Materials, University of California, Santa Barbara, California 93106, USA
2 Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, USA
3 Optoelectronics Laboratory, Mitsubishi Chemical Corporation, 1000 Higashi-mamiana, Ushiku, Ibaraki 300-1295, Japan
We report the effects of Mg doping in the barriers of semipolar (202 1) multiple-quantum-well light-emitting diodes (LEDs) with long emission wavelengths (>500 nm). With moderate Mg doping concentrations (3  1018–5  1018 cm3) in the barriers, the output power was enhanced compared to those with undoped barriers, which suggests that hole transport in the active region is a limiting factor for device performance. Improved hole injection due to Mg doping in the barriers is demonstrated by dichromatic LED experiments and band diagram simulations. With Mg-doped AlGaN barriers, double-quantum-well LEDs with orange to red emission (k>600 nm) were also demonstrated.


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