Browsing by Author "Tian, Kangkai"

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Alternative Strategy to Reduce Surface Recombination for InGaN/GaN Micro-light-Emitting Diodes-Thinning the Quantum Barriers to Manage the Current Spreading
(2020-08-06)
Chang, Le
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Yeh, Yen-Wei
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Hang, Sheng
;
Tian, Kangkai
;
Kou, Jianquan
Owing to high surface-to-volume ratio, InGaN-based micro-light-emitting diodes (mu LEDs) strongly suffer from surface recombination that is induced by sidewall defects. Moreover, as the chip size decreases, the current spreading will be correspondingly enhanced, which therefore further limits the carrier injection and the external quantum efficiency (EQE). In this work, we suggest reducing the nonradiative recombination rate at sidewall defects by managing the current spreading effect. For that purpose, we properly reduce the vertical resistivity by decreasing the quantum barrier thickness so that the current is less horizontally spreaded to sidewall defects. As a result, much fewer carriers are consumed in the way of surface nonradiative recombination. Our calculated results demonstrate that the suppressed surface nonradiative recombination can better favor the hole injection efficiency. We also fabricate the mu LEDs that are grown on Si substrates, and the measured results are consistent with the numerical calculations, such that the EQE for the proposed mu LEDs with properly thin quantum barriers can be enhanced, thanks to the less current spreading effect and the decreased surface nonradiative recombination.
Establishment of the relationship between the electron energy and the electron injection for AlGaN based ultraviolet light-emitting diodes
(2018-07-09)
Zhang, Zi-Hui
;
Tian, Kangkai
;
Chu, Chunshuang
;
Fang, Mengqian
;
Zhang, Yonghui
This work establishes the relationship between the electron energy and the electron concentration within the multiple quantum wells (MQWs) for AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs). The electron energy of different values can be obtained by modulating the Si doping concentration in the n-AlGaN layer and/or engineering the polarization induced interface charges. The modulated Si doping concentration in the n-AlGaN layer will cause the interface depletion region within which the electric field can be generated and then tunes the electron energy. The polarization induced charges and the polarization induced electric field can be obtained by stepwisely reducing the AlN composition for the n-AlGaN layer along the [0001] orientation. We find that the electron concentration in the MQWs can be increased once the electron energy is reduced to a proper level, which correspondingly improves the external quantum efficiency (EQE) for DUV LEDs. According to our investigations, it is more advisable to adopt the n-AlGaN layer with the stepwise AlN composition, which can make both the EQE and the wall plug efficiency high. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Hole Transport Manipulation To Improve the Hole Injection for Deep Ultraviolet Light-Emitting Diodes
(2017-07-01)
Zhang, Zi-Hui
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Chen, Sung-Wen Huang
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Zhang, Yonghui
;
Li, Luping
;
Wang, Sheng-Wen
In this report, we propose to enhance the hole injection efficiency by adjusting the barrier height of the p-type electron blocking layer (p-EBL) for similar to 273 nm deep ultraviolet light-emitting diodes (DUV LEDs). The barrier height for the p-EBL is modified by employing a p-Al0.60Ga0.40N/Al0.50Ga0.50N/p-Al0.60Ga0.40N structure, in which the very thin Al0.50Ga0.50N layer is able to achieve a high local hole concentration, which is very effective in reducing the effective barrier height of the p-EBL for holes. More importantly, besides the thermionic emission, such a p-EBL structure can also favor a strong intraband tunneling process for holes. As a result, we can obtain a more efficient hole injection into the quantum wells, leading to a remarkably improved optical power for the DUV LED with the proposed p-EBL architecture.
Impact of the surface recombination on InGaN/GaN-based blue micro-light emitting diodes
(2019-06-10)
Kou, Jianquan
;
Shen, Chih-Chiang
;
Shao, Hua
;
Che, Jiamang
;
Hou, Xu
In this work, the size-dependent effect for InGaN/GaN-based blue micro-light emitting diodes (mu LEDs) is numerically investigated. Our results show that the external quantum efficiency (EQE) and the optical power density drop drastically as the device size decreases when sidewall defects are induced. The observations are owing to the higher surface-to-volume ratio for small mu LEDs, which makes the Shockley-Read-Hall (SRH) non-radiative recombination at the sidewall defects not negligible. The sidewall defects also severely affect the injection capability for electrons and holes, such that the electrons and holes are captured by sidewall defects for the SRH recombination. Thus, the poor carrier injection shall be deemed as a challenge for achieving high-brightness mu LEDs. Our studies also indicate that the sidewall defects form current leakage channels, and this is reflected by the current density-voltage characteristics. However, the improved current spreading effect can be obtained when the chip size decreases. The better current spreading effect takes account for the reduced forward voltage. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Increasing the hole energy by grading the alloy composition of the p-type electron blocking layer for very high-performance deep ultraviolet light-emitting diodes
(2019-04-01)
Zhang, Zi-Hui
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Kou, Jianquan
;
Chen, Sung-Wen Huang
;
Shao, Hua
;
Che, Jiamang
It is well known that the p-type AlGaN electron blocking layer (p-EBL) can block hole injection for deep ultraviolet light-emitting diodes (DUV LEDs). The polarization induced electric field in the p-EBL for [0001] oriented DUV LEDs makes the holes less mobile and thus further decreases the hole injection capability. Fortunately, enhanced hole injection is doable by making holes lose less energy, and this is enabled by a specifically designed p-EBL structure that has a graded MN composition. The proposed p-EEL can screen the polarization induced electric field in the p-EBL. As a result, holes will lose less energy after going through the proposed p-EBL, which correspondingly leads to the enhanced hole injection. Thus, an external quantum efficiency of 7.6% for the 275 nm DIN LED structure is obtained. (C) 2019 Chinese Laser Press
Nearly Efficiency-Droop-Free AlGaN-Based Ultraviolet Light-Emitting Diodes with a Specifically Designed Superlattice p-Type Electron Blocking Layer for High Mg Doping Efficiency
(2018-04-24)
Zhang, Zi-Hui
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Chen, Sung-Wen Huang
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Chu, Chunshuang
;
Tian, Kangkai
;
Fang, Mengqian
This work reports a nearly efficiency-droop-free AlGaN-based deep ultraviolet light-emitting diode (DUV LED) emitting in the peak wavelength of 270 nm. The DUV LED utilizes a specifically designed superlattice p-type electron blocking layer (p-EBL). The superlattice p-EBL enables a high hole concentration in the p-EBL which correspondingly increases the hole injection efficiency into the multiple quantum wells (MQWs). The enhanced hole concentration within the MQW region can more efficiently recombine with electrons in the way of favoring the radiative recombination, leading to a reduced electron leakage current level. As a result the external quantum efficiency for the proposed DUV LED structure is increased by 100% and the nearly efficiency-droop-free DUV LED structure is obtained experimentally.
On the Carrier Transport for InGaN/GaN Core-Shell Nanorod Green Light-Emitting Diodes
(2019-01-01)
Kou, Jianquan
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Chen, Sung-Wen Huang
;
Che, Jiamang
;
Shao, Hua
;
Chu, Chunshuang
In this paper, we model and investigate the carrier transport for the core-shell nanorod (NR) structured green light-emitting diodes (LEDs) for which the InGaN/GaN multiple quantum wells are grown on the nonpolar surface of the NR. Our results show the absence of polarization fields in the m-plane quantum wells for the core-shell NR LEDs, which manifest the flat energy band condition, the improved electron injection efficiency, and the high electron-hole wave functions overlap for the quantum wells, leading to the high radiative recombination rate and the enhanced quantum efficiency. We further find that the quantum efficiency is also affected by the vertical charge injection for the core-shell NR LEDs. The vertical charge injection is more sensitive to the doping concentration and the thickness of the p-GaN layer than of the n-GaN layer. Moreover, the increase of the NR height also leads to the nonuniform vertical charge injection.
On the Impact of Electron Leakage on the Efficiency Droop for AlGaN Based Deep Ultraviolet Light Emitting Diodes
(2020-06-01)
Chu, Chunshuang
;
Tian, Kangkai
;
Che, Jiamang
;
Shao, Hua
;
Kou, Jianquan
In this work, we have investigated the origin of efficiency droop for AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). We find that the efficiency droop is likely to be caused by the electron leakage for DUV LEDs studied in this work. The electron leakage arises from the unbalanced electron and hole injection efficiencies. The correlation between the efficiency droop and the electron leakage can be numerically calculated by manipulating the conduction band barrier height of the p-AlGaN electron blocking layer (p-EBL) for the proposed DUV LEDs. For the purpose of demonstrating that the efficiency droop for DUV LEDs can be experimentally decreased by reducing the electron leakage, a p(+)-GaN/In0.15Ga0.85N/n(+)-GaN tunnel junction on DUV LED is grown by using metal organic chemical vapor deposition (MOCVD) technology. The tunnel junction helps to enhance the hole injection, thus decreasing the electron leakage and the efficiency droop. Moreover, the parasitic emission in the p-type hole injection layer is no longer observed thanks to the decreased electron leakage level.
UVA light-emitting diode grown on Si substrate with enhanced electron and hole injections
(2017-11-01)
Zhang, Zi-Hui
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Chu, Chunshuang
;
Chiu, Ching Hsueh
;
Lu, Tien Chang
;
Li, Luping
In this work, III-nitride based similar to 370 nm UVA lightemitting diodes (LEDs) grown on Si substrates are demonstrated. We also reveal the impact of the AlN composition in the AlGaN quantum barrier on the carrier injection for the studied LEDs. We find that, by properly increasing the AlN composition, both the electron and hole concentrations in the multiple quantum wells (MQWs) are enhanced. We attribute the increased electron concentration to the better electron confinement within the MQW region when increasing the AlN composition for the AlGaN barrier. The improved hole concentration in the MQW region is ascribed to the reduced hole blocking effect by the p-type electron blocking layer (p-EBL). This is enabled by the reduced density of the polarization-induced positive charges at the AlGaN last quantum barrier (LB)/p-EBL interface, which correspondingly suppresses the hole depletion at the AlGaN LB/p-EBL interface and decreases the valence band barrier height for the p-EBL. As a result, the optical power is improved. (C) 2017 Optical Society of America