Suppression Of Efficiency Droop Research Articles

Suppressing the efficiency droop in AlGaN-based UVB LEDs

The optoelectronic properties of semiconducting aluminum gallium nitride (AlGaN)-based ultraviolet-B (UVB) light-emitting diodes (LEDs) are crucial for real-world medical applications such as cancer therapy and immunotherapy. However, the performance of AlGaN-based UVB LED devices is still poor due to the low hole injection efficiency. Therefore, we have numerically investigated the performance of AlGaN-based UVB LEDs for the suppression of efficiency droop as well as for the enhancement of hole injection in the multiquantum wells (MQWs). The influence of the undoped (ud)-AlGaN final quantum barrier (FQB), as well as the Mg-doped multiquantum barrier electron blocking layer (p-MQB EBL), on the efficiency droop has been focused on specifically. To evaluate the performance of the proposed device, we have compared its internal quantum efficiency (IQE), carrier concentration, energy band diagram, and radiative recombination rate with the conventional device structure. Furthermore, the influence of Al composition in the Al-graded p-AlGaN hole source layer (HSL) on the operating voltages of the proposed UVB LEDs was considered. The simulation results suggest that our proposed structure has a high peak efficiency and much lower efficiency droop as compared to the reference structure (conventional). Ultimately, the radiative recombination rate in the MQWs of the proposed UVB LED-N structure has increased up to ∼73%, which is attributed to the enhanced level of electron and hole concentrations by ∼64% and 13%, respectively, in the active region. Finally, a high efficiency droop of up to ∼42% in RLED has been successfully suppressed, to ∼7%, by using the optimized ud-AlGaN FQB and the p-MQB EBL, as well as introducing Al-graded p-AlGaN HSL in the proposed UVB LED-N structure.

Suppression of efficiency droop in AlGaN based deep UV LEDs using double side graded electron blocking layer

We have envisaged and designed a novel III-V nitride based deep ultraviolet light emitting diode (LED) with reasonably high efficiency at higher current density using a double-side grading in electron blocking layer (EBL). Double-side step- and linear-grading in EBL yield better performance attributable to improved hole injection, stifled electron overflow and diminished electrostatic field in the active region. The performance curves indicate that double sided linear grading in EBL has 5.63 times enhancement in power compared to the conventional LED and the efficiency droop is as low as 15% at the current density of 200 A cm−2 for the emission wavelength of ∼273 nm.

A single InGaN/GaN multiple quantum wells microwire light-emitting diode with high efficiency current injection and spreading

Micro/nanowire light-emitting diodes have inspired considerable research interests due to their wide range of potential applications, such as in micro/nanoscale display, optical communication and optical logics. To realize a single wire light-emitting diode, however, it is usually faced with low-efficiency or wavelength uncontrollable. Herein, we report on an individual coaxial core/shell InGaN/GaN microwire light-emitting diode with tunable dual-/single-wavelength light emission, which can be feasibly realized by controlling growth rate of microwires using MOCVD. The unique cross-sectional morphologies of synthesized microwires can simplify the device fabrication process and are in favor of vertical current injection and spreading simultaneously. Thus, the microwire light-emitting diodes show advantages at the suppression of efficiency droop under large current injection, with a drop ratio of 19.6%, as increasing the injection current from 2.4 A cm−2 to 92.6 A cm−2. This study proposes a core/shell multiple quantum wells microwire structure to fabricate micro light-emitting diodes which may act as a reliable light source for micro/nanoscale integrated photonic systems.

Efficiency droop suppression of distance-engineered surface plasmon-coupled photoluminescence in GaN-based quantum well LEDs

Ag coated microgroove with extreme large aspect-ratio of 500:1 was fabricated on p-GaN capping layer to investigate the coupling behavior between quantum wells and surface plasmon in highly spatial resolution. Significant photoluminescence enhancement was observed when the distance between Ag film and QWs was reduced from 220 nm to about 20 nm. A maximum enhancement ratio of 18-fold was achieved at the groove bottom where the surface plasmonic coupling was considered the strongest. Such enhancement ratio was found highly affected by the excitation power density. It also shows high correlation to the internal quantum efficiency as a function of coupling effect and a maximum Purcell Factor of 1.75 was estimated at maximum coupling effect, which matches number calculated independently from the time-resolved photoluminescence measurement. With such Purcell Factor, the efficiency was greatly enhanced and the droop was significantly suppressed.

Effective suppression of efficiency droop in GaN-based light-emitting diodes: role of significant reduction of carrier density and built-in field.

A critical issue in GaN-based high power light-emitting diodes (LEDs) is how to suppress the efficiency droop problem occurred at high current injection while improving overall quantum efficiency, especially in conventional c-plane InGaN/GaN quantum well (QW), without using complicated bandgap engineering or unconventional materials and structures. Although increasing thickness of each QW may decrease carrier density in QWs, formation of additional strain and defects as well as increased built-in field effect due to enlarged QW thickness are unavoidable. Here, we propose a facile and effective method for not only reducing efficiency droop but also improving quantum efficiency by utilizing c-plane InGaN/GaN QWs having thinner barriers and increased QW number while keeping the same single well thickness and total active layer thickness. As the barrier thickness decreases and the QW number increases, both internal electric field and carrier density within QWs are simultaneously reduced without degradation of material quality. Furthermore, we found overall improved efficiency and reduced efficiency droop, which was attributed to the decrease of the built-in field and to less influence by non-radiative recombination processes at high carrier density. This simple and effective approach can be extended further for high power ultraviolet, green, and red LEDs.

Efficiency droop suppression of InGaN‐based blue light‐emitting diodes using dip‐shaped quantum wells

The InGaN‐based blue light‐emitting diodes (LEDs) with dip‐shaped quantum wells (QWs) and conventional QWs were fabricated by MOVPE. The performances of fabricated InGaN‐based LEDs were investigated experimentally and analyzed theoretically. The results reveal that the efficiency droop of InGaN‐based LED is suppressed by using dip‐shaped QWs structure. Firstly, the dip‐shaped QWs LED has higher emission intensity compared with the conventional QWs LED due to the stronger quantum confinement and weaker polarization effect. Furthermore, the effective combination region in dip‐shaped QWs does not reduce obviously which decrease will lead to efficiency droop aggravation. As a result, the efficiency droop at 50 A cm−2 injection current for dip‐shaped QWs LED reduce from 59% for conventional QWs LED to 48%. So, the dip‐shaped QWs can be used to suppress the efficiency droop of InGaN‐based LEDs.

Design of electron blocking layers for improving internal quantum efficiency of InGaN/AlGaN-based ultraviolet light-emitting diodes

In this paper, we designed and simulated InGaN/AlGaN-based near-ultraviolet light-emitting diode (NUV LED) epi-structures to obtain high internal quantum efficiency and low efficiency droop. When the conventional epi-structure of an $$\hbox {Al}_{0.1}\hbox {Gal}_{0.9}\hbox {N}$$ last quantum barrier and an $$\hbox {Al}_{0.21}\hbox {Gal}_{0.79}\hbox {N}$$ electron blocking layer (EBL) was replaced with a graded last quantum barrier and multi-step EBLs, the NUV LED showed 35 % higher internal quantum efficiency and 25 % more suppression of efficiency droop than the conventional NUV LED. Furthermore, a detailed study of the grading effect of the EBL revealed that 10-step EBLs increase performance when compared to other structures. These results are attributed to the polarization-induced effect, which reduces the electron leakage and increases the hole injection efficiency.

Efficiency droop suppression in GaN-based light-emitting diodes by chirped multiple quantum well structure at high current injection**Project suppored by the National High Technology Research and Development Program of China (Grant No. 2014AA032608), the Key Laboratory for Mechanical Behavior of Material of Xi’an Jiaotong University, China (Grant No. 20121201), and the Fundamental Research Funds for the Central Universities, China.

Gallium nitride (GaN) based light-emitting diodes (LEDs) with chirped multiple quantum well (MQW) structures have been investigated experimentally and numerically in this paper. Compared to conventional LEDs with uniform quantum wells (QWs), LEDs with chirped MQW structures have better internal quantum efficiency (IQE) and carrier injection efficiency. The droop ratios of LEDs with chirped MQW structures show a remarkable improvement at 600 mA/mm2, reduced down from 28.6% (conventional uniform LEDs) to 23.7% (chirped MQWs-a) and 18.6% (chirped MQWs-b), respectively. Meanwhile, the peak IQE increases from 76.9% (uniform LEDs) to 83.7% (chirped MQWs-a) and 88.6% (chirped MQWs-b). The reservoir effect of chirped MQW structures is the significant reason as it could increase hole injection efficiency and radiative recombination. The leakage current and Auger recombination of chirped MQW structures can also be suppressed. Furthermore, the chirped MQWs-b structure with lower potential barriers can enhance the reservoir effect and obtain further improvement of the carrier injection efficiency and radiative recombination, as well as further suppressing efficiency droop.

Effect of Mg Doping in GaN Interlayer on the Performance of Green Light-Emitting Diodes

A light-emitting diode (LED) structure containing a low-temperature (LT) GaN interlayer between active region and AlGaN electron blocking layer is proposed to improve the performance of InGaN-based green LEDs. The experimental and simulated results show that, as the Mg doping depth in the LT-GaN interlayer increases, the hole injection efficiency gets improved and electron current leakage decreases, while defect-related nonradiative recombination increases. With an optimized Mg doping depth in the LT-GaN interlayer, a substantial suppression of efficiency droop can be achieved compared with the conventional LED.

Reducing the efficiency droop by lateral carrier confinement in InGaN/GaN quantum-well nanorods.

Efficiency droop is a major obstacle facing high-power application of InGaN/GaN quantum-well (QW) light-emitting diodes (LEDs). In this paper, we report the suppression of efficiency droop induced by the process of density-activated defect recombination in nanorod structures of a-plane InGaN/GaN QWs. In the high carrier density regime, the retained emission efficiency in a dry-etched nanorod sample is observed to be over two times higher than that in its parent QW sample. We further argue that such improvement is a net effect that the lateral carrier confinement overcomes the increased surface trapping introduced during fabrication.

Efficiency-Droop Suppression by Using Large-Bandgap AlGaInN Thin Barrier Layers in InGaN Quantum-Well Light-Emitting Diodes

The electrical and optical characteristics of InGaN quantum-well light-emitting diodes with large-bandgap AlGaInN thin barriers were analyzed with the consideration of carrier transport effect for efficiency droop suppression. The lattice-matched AlGaInN quaternary alloys with different compositions, thicknesses, and positions were employed as thin barrier layers (1-2 nm) surrounding the InGaN QW in LED structures. The increased effective barrier heights of AlGaInN thin barrier led to suppression of carrier leakage as compared to conventional InGaN QW LEDs with GaN barrier only. The current work provides a comprehensive simulation taking into consideration the carrier transport in self-consistent manner, and the finding indicated the use of thin layers of AlGaInN or AlInN barriers as sufficient for suppressing the droop in InGaN-based QW LEDs. The efficiency of InGaN QW LED with the insertion of lattice-matched Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.82</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> N thin barrier layers showed the least droop phenomenon at high current density among the investigated LEDs. The thickness study indicated that a thin layer (<; 2 nm) of large-bandgap material in the barrier region was sufficient for efficiency droop suppression.

Reduction in the Efficiency-Droop Effect of InGaN Green Light-Emitting Diodes Using Gradual Quantum Wells

The effect of gradual indium gallium nitride (InGaN) quantum wells (QWs) on the suppression of efficiency-droop in green light-emitting diodes (LEDs) is numerically investigated. The presented scheme increases the internal quantum efficiency by 45.5% at I=20 mA and 55.7% at I=100 mA, indicating a considerable reduction of efficiency-droop. This improvement is attributable mainly to the use of the gradual InGaN QW's structure that significantly alleviates band bending in the valence band, improving the transport efficiency of injected holes above that of conventional LEDs. The radiative recombination is thus enhanced as the overlap between electron and hole wave functions is increased. Most importantly, the leakage of injected electrons to p-type region is correspondingly reduced, in turn suppressing the efficiency-droop in the LED.

Stable Temperature Characteristics and Suppression of Efficiency Droop in InGaN Green Light-Emitting Diodes Using Pre-TMIn Flow Treatment

We present experimental results on the improved performance and high stable temperature characteristics of the InGaN green light-emitting diode (LED) with pre-trimethlyindium (pre-TMIn) flow treatment. By using pre-TMIn flow treatment, a relatively large radiative coefficient ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> = 3.34×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) corresponding to a 9.2% enhancement in the internal quantum efficiency, as well as a significant reduction of leakage paths for injected carriers, was obtained. Most important, the pre-TMIn flow treatment evidently reduces the dependence of the external quantum efficiency on temperature and efficiency droop of green LEDs. The improvement is thought to be attributable to the preferential formation of In-rich dots upon pre-TMIn flow treatment, which effectively suppresses the trapping of excitons by threading dislocations and the overflowing of injected carriers outside the active regions at elevated temperatures.