p-AlGaN Electron Blocking Layer Research Articles

Optimization of AlGaN-based deep UV LED performance by p-AlInGaN/AlInGaN graded superlattice electron blocking layer.

In this paper, we significantly improved the internal quantum efficiency and output power of AlGaN-based deep UV (DUV) LEDs by replacing the conventional p-AlGaN electron blocking layer (EBL) with the p-AlInGaN/AlInGaN graded superlattice (SL) EBL. Simulation results show that the introduction of the p-AlInGaN graded SL EBL improved the carrier distribution while having the lower electric field, thus increasing the radiative recombination rate in multiple quantum wells (MQWs). The highest IQE obtained by p-AlInGaN/AlInGaN graded SL EBL is 96.6%, which is 44.9% higher than the conventional p-AlGaN EBL with no efficiency droop. At the same time, the output power is 4.6 times that of the conventional p-AlGaN EBL. It is believed that the proposed p-AlInGaN graded SL EBL will be helpful in the development of high-performance DUV LEDs.

Enhanced performance in deep-ultraviolet laser diodes with an undoped BGaN electron blocking layer.

Aluminum-rich p-AlGaN electron blocking layers (EBLs) are typically used for preventing overflow of electrons from the active region in AlGaN-based deep ultraviolet (DUV) laser diode (LD). However, these cannot effectively prevent electron leakage and form barrier layers, which affects the hole injection efficiency. Herein, the traditional p-AlGaN EBL in LD is replaced with an undoped BGaN EBL. The undoped BGaN EBL LD increases the effective barrier height of the conduction band to prevent the leakage of electrons and decreases the energy loss caused by the polarization induced electric field, enhancing the hole injection. The slope efficiency of the undoped BGaN EBL LD is 289% higher than that of the highly doped AlGaN EBL LD, and its threshold current is 51% lower. Therefore, the findings of this study provide insights for solving the problems of electron leakage and insufficient hole injection in high-performance and undoped EBL DUV LDs.

Optical Properties of GaN-Based Green Light-Emitting Diodes Influenced by Low-Temperature p-GaN Layer.

GaN-based green light-emitting diodes (LEDs) with different thicknesses of the low-temperature (LT) p-GaN layer between the last GaN barriers and p-AlGaN electron blocking layer were characterized by photoluminescence (PL) and electroluminescence (EL) spectroscopic methods in the temperature range of 6–300 K and injection current range of 0.01–350 mA. Based on the results, we suggest that a 20 nm-thick LT p-GaN layer can effectively prevent indium (In) re-evaporation, improve the quantum-confined Stark effect in the last quantum well (QW) of the active region, and finally reduce the efficiency droop by about 7%.

Fast neutron irradiation effects on AlGaN deep ultraviolet light emitting diodes

AlGaN-based deep ultraviolet (DUV) light emitting diodes (LEDs) are irradiated by fast neutrons, with optical and electrical properties analyzed in detail. Significant enhancement of output power is observed under neutron irradiation fluences of 6.0 × 1012 and 1.5 × 1013 cm−2. However, the device exhibits performance degradation as the fluence increases to 1.0 × 1014 cm−2. As previous observations are limited to degradation. Further analysis reveals that there exist two different competitive mechanisms of neutron radiation effect on DUV LEDs. The enhancement of the output power is attributed to the increased efficiency of the carriers injected into the irradiated multi-quantum wells (MQWs). Meanwhile, neutron irradiation-induced nitrogen vacancies in p-AlGaN electron blocking layer increase the leakage of the current dominating the device performance degradation, in contrast with results from former studies on longer wavelength GaN-based LEDs. Those findings are not only helpful to further enrich the degradation mechanism of neutron irradiated DUV LEDs, but also provide a fresh idea to improve their luminous characteristics.

On the Impact of Electron Leakage on the Efficiency Droop for AlGaN Based Deep Ultraviolet Light Emitting Diodes

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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GaN/In0.15Ga0.85N/n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -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.

Monolithic integration of GaN LEDs with vertical driving MOSFETs by selective area growth and band engineering of the p-AlGaN electron blocking layer though TCAD simulation

Based on an InGaN/GaN light emitting diode (LED) structure, monolithically integrated vertical driving metal-oxide-semiconductor field-effect transistors (MOSFETs) were designed and experimentally implemented using a selective area growth (SAG) method. A simple p-GaN/n-GaN stack was selectively regrown on top of the LED wafer to realize an n/p/n structure for the vertical MOSFET fabrication. The integrated vertical MOSFET, which can effectively modulate the injection current through the serially connected LED, exhibited high performance such as an enhancement-mode (E-mode) operation with a relatively high output current density. However, on-resistance (RON) degradation was observed in the fabricated vertical MOSFET at a low drain bias level (VDS < 2 V). Through a 2D TCAD simulation, the origin of the high RON was revealed to be an electron barrier induced by the LED’s p-AlGaN electron blocking layer (EBL). The simulation results also demonstrated that it can be improved by band engineering of the EBL.

The abnormal aging phenomena in GaN-based near-ultraviolet laser diodes

An abnormal aging phenomenon is reported for GaN-based near-ultraviolet laser diodes (LDs). Under an electrical stress for several minutes, the threshold current of the LDs decreased, while the light output power and the operation voltage increased. The amplitude of the abnormal aging phenomena was found to be mainly related to the excess Mg concentration in the p-AlGaN electron blocking layer (EBL). It almost disappeared when the Mg concentration in the AlGaN EBL was reduced to 2 × 1019 cm−3. We propose that this phenomenon was related to the Mg–VN–H complex defects formed in highly doped AlGaN EBL ([Mg] > 3 × 1019 cm−3).

Semipolar InGaN blue laser diodes with a low optical loss and a high material gain obtained by suppression of carrier accumulation in the p-waveguide region

We obtained InGaN laser diodes (LDs) with a low optical loss (1.9 ± 0.6 cm−1) by utilizing an undoped In0.07Ga0.93N p-waveguide (p-WG) and a remote p-AlGaN electron blocking layer. This improvement, however, was accompanied by a low injection efficiency (37%) and a poor material gain. Electroluminescence measurements and energy band diagram simulations revealed excessive carrier accumulation in the p-WG region, which was suppressed by introducing an undoped indium-free GaN p-WG. This enabled LDs with a high material gain, a low optical loss (4.6 ± 0.7 cm−1), an improved injection efficiency (60%), and a peak light output power of 1.6 W at 1.5 A.

Further emission efficiency improvement of a commercial-quality light-emitting diode through surface plasmon coupling.

It is usually believed that surface plasmon (SP) coupling is practically useful only for improving the performance of a light-emitting diode (LED) with a low intrinsic internal quantum efficiency (IQE). In this Letter, we demonstrate that the performance of a commercial-quality blue LED with a high IQE (>80%) can still be significantly improved through SP coupling based on a surface Ag nanoparticle (NP) structure. The performance improvement of such an LED is achieved by increasing the Mg doping concentration in its p-AlGaN electron blocking layer to enhance the hole injection efficiency such that the p-GaN layer thickness can be significantly reduced without sacrificing its electrical property. In this situation, the distance between surface Ag NPs and quantum wells is decreased and hence SP coupling strength is increased. By reducing the distance between the surface Ag NPs and the top quantum well to 66nm, the IQE can be increased to almost 90% (an ∼11% enhancement) and the electroluminescence intensity can be enhanced by ∼24%.

Lattice-Matched AlInGaN Electron-Blocking Layer Embedded in High-Performance GaN-based Light-Emitting Devices

We report on high-performance GaN-based laser diodes (LDs) with a lattice-matched AlInGaN electron blocking layer (EBL) instead of a p-AlGaN EBL. In the low injection current region, the intensity of the electroluminescence (EL) of the LDs with a p-AlGaN EBL was slightly higher than that of the LDs with a p-AlInGaN EBL. However, the EL intensity of the LD with a p-AlInGaN EBL was higher than that of the LD with a p-AlGaN EBL with increasing injection current. Further, the lasing performance of the LD with a p-AlInGaN EBL was better than that of the LD with a p-AlGaN EBL at higher injection currents. From these results, we suggest that the lattice-matched AlInGaN EBL enhances the high-power performance by reducing the droop phenomenon in the light-emitting device.

Simulation and fabrication of N-polar GaN-based blue-green light-emitting diodes with p-type AlGaN electron blocking layer

N-polar GaN-based blue-green light-emitting diodes (LEDs) with p-AlGaN electron blocking layer (EBL) were numerically investigated by simulation and experimentally grown on vicinal C-face SiC substrates by metal–organic chemical vapor deposition. By numerical simulation, we can find that p-AlGaN EBL in N-polar LEDs is able to play a more important role in blocking electron overflow than that in Ga-polar LEDs due to the reversed polarization, which leads to a high output power and internal quantum efficiency. Besides, the holes injection efficiency is enhanced in N-polar LED, resulting in a lower turn-on voltage. In experimental studies, N-polar LEDs based on different numbers of quantum wells were grown on vicinal C-face n-SiC substrates. When a forward bias is applied to the epitaxial N-polar LED with two quantum wells, a strong blue-green emission located at 480 nm can be observed. This work indicates that N-polar group-III nitrides have great potential in the application of optoelectronic devices.

Effect of Mg-Preflow for p-AlGaN Electron Blocking Layer on the Electroluminescence of Green LEDs with V-Shaped Pits**Supported by the National Key R&D Program of China under Grant Nos 2016YFB0400600 and 2016YFB0400601, the State Key Program of the National Natural Science of China under Grant No 61334001, the Key R&D Program of Jiangxi Province under Grant No 20165ABC28007, the Natural Science

InGaN-based green light-emitting diodes (LEDs) with and without Mg-preflow before the growth of p-AlGaN electron blocking layer (EBL) are investigated experimentally. A higher Mg doping concentration is achieved in the EBL after Mg-preflow treatment, effectively alleviating the commonly observed efficiency collapse and electrons overflowing at cryogenic temperatures. However, unexpected decline in quantum efficiency is observed after Mg-preflow treatment at room temperature. Our conclusions are drawn such that the efficiency decline is probably the result of different emission positions. Higher Mg doping concentration in the EBL after Mg-preflow treatment will make it easier for a hole to be injected into multiple quantum wells with emission closer to p- GaN side through the c-plane rather than the V-shape pits, which is not favorable to luminous efficiency due to the preferred occurrence of accumulated strain relaxation and structural defects in upper QWs closer to p-GaN. Within this framework, apparently disparate experimental observations regarding electroluminescence properties, in this work, are well reconciled.

Dependencies of surface plasmon coupling effects on the p-GaN thickness of a thin-p-type light-emitting diode.

The high performance of a light-emitting diode (LED) with the total p-type thickness as small as 38 nm is demonstrated. By increasing the Mg doping concentration in the p-AlGaN electron blocking layer through an Mg pre-flow process, the hole injection efficiency can be significantly enhanced. Based on this technique, the high LED performance can be maintained when the p-type layer thickness is significantly reduced. Then, the surface plasmon coupling effects, including the enhancement of internal quantum efficiency, increase in output intensity, reduction of efficiency droop, and increase of modulation bandwidth, among the thin p-type LED samples of different p-type thicknesses that are compared. These advantageous effects are stronger as the p-type layer becomes thinner. However, the dependencies of these effects on p-type layer thickness are different. With a circular mesa size of 10 μm in radius, through surface plasmon coupling, we achieve the record-high modulation bandwidth of 625.6 MHz among c-plane GaN-based LEDs.

Enhancing the Hole-Injection Efficiency of a Light-Emitting Diode by Increasing Mg Doping in the p-AlGaN Electron-Blocking Layer

By increasing the Mg-doping level and hence the hole concentration in the p-AlGaN electron-blocking layer (EBL), particularly around the interface between the EBL and the top quantum barrier (QB), of a light-emitting diode (LED), the polarization field in this layer can be screened for reducing the potential barrier of hole and hence enhancing the hole-tunneling efficiency such that the overall LED emission efficiency is increased. The increase of Mg-doping level is implemented based on an Mg preflow growth technique, in which Mg source is supplied into the metalorganic chemical vapor deposition chamber for several minutes before the growth of p-AlGaN or p-GaN. It is found that by increasing the Mg-doping level by ~20 times near the interface between the EBL and the top QB, LED emission intensity can be enhanced by ~9.4 times. Based on a simulation study, we observe that the energy difference between the valence band edge and the quasi-Fermi level of hole in the EBL is reduced by increasing the Mg-doping level in this layer such that the total hole density in the quantum wells is increased for enhancing the LED emission efficiency.

Improved performance of near UV light-emitting diodes with a composition-graded p-AlGaN irregular sawtooth electron-blocking layer**Project supported by the National Natural Science Foundation of China (Grant Nos. 11474105 and 51172079), the Science and Technology Program of Guangdong Province, China (Grant Nos. 2015B090903078 and 2015B010105011), the Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT13064), the Science

We investigate the performances of the near-ultraviolet (about 350 nm–360 nm) light-emitting diodes (LEDs) each with specifically designed irregular sawtooth electron blocking layer (EBL) by using the APSYS simulation program. The internal quantum efficiencies (IQEs), light output powers, carrier concentrations in the quantum wells, energy-band diagrams, and electrostatic fields are analyzed carefully. The results indicate that the LEDs with composition-graded p-AlxGa1−xN irregular sawtooth EBLs have better performances than their counterparts with stationary component p-AlGaN EBLs. The improvements can be attributed to the improved polarization field in EBL and active region as well as the alleviation of band bending in the EBL/p-AlGaN interface, which results in less electron leakage and better hole injection efficiency, thus reducing efficiency droop and enhancing the radiative recombination rate.

A new structure of p-GaN/InGaN heterojunction to enhance hole injection for blue GaN-based LEDs

A new structure of p-GaN/InGaN heterojunction has been proposed to enhance hole injection for blue GaN-based light-emitting diodes (LEDs). It is demonstrated by the simulation results that a p-GaN (50 nm)/In0.05Ga0.95N (150 nm) heterojunction can make a 25% and 10% increment of hole and electron concentration in the active region, respectively, finally resulting in a 55% improvement on the LED’s radiative recombination intensity. The simulation also reveals that the efficiency droop is alleviated from 32.9% to 21.7% at the current density of 100 A cm−2. The enhanced hole injection is mainly attributed to the increased average background hole concentration of the area between the p-AlGaN electron blocking layer (EBL) to the p-GaN/InGaN heterojunction. The increasing potential barrier of the conduction band, resulting from the introduction of p-GaN/InGaN heterojunction, would also weaken electron leakage and is favorable to the LED’s luminous performance. The experimental results show that the wall-plug efficiency (WPE) of the p-GaN/InGaN LED increases by 26.0% at the injection current of 75 mA, in spite of the increasing electric resistance, which impairs the improvement of the LED’s performance from the enhanced hole injection. The structure of the p-GaN/InGaN heterojunction is novel in the field of p-type region design, and is a simple but effective way to promote the LED’s performance, which is very promising for application in further high-performance LED fabrication.

Effects of p-AlGaN EBL thickness on the performance of InGaN green LEDs with large V-pits

The effects of the p-AlGaN electron blocking layer (EBL) thickness on the performance of InGaN/GaN multiple quantum wells (MQWs) green light emitting diodes (LEDs) was investigated. It was observed that increasing the thickness of the p-AlGaN EBL could reduce the leakage current and improve the efficiency of green LEDs with large V-pits. It is proposed that increasing the EBL thickness leads to a thicker p-AlGaN on the sidewalls of V-pits, which provides a thicker energy barrier and consequently screens dislocations more effectively. The leakage current (at −5 V) of LEDs with a 40 nm EBL is about an order of magnitude lower than that of LEDs with a 20 nm EBL. With the increase in EBL thickness, at low current densities, the external quantum efficiency (EQE) firstly decreases and then increases afterwards, which could be attributed to the competition between the enhancement of the radiative recombination rate and the reduction of the hole injection efficiency. At operating current density, there is a positive correlation between EQE and the thickness of the EBL. This is attributed to the improved electron confinement in the active region by preventing electrons overflowing to the p-type layer. Meanwhile, the efficiency droop is obviously suppressed when the thickness of the EBL increases from 20 nm to 40 nm. However, further increasing the thickness of the EBL may deteriorate the EQE and efficiency droop. Packaged green LED chips with an optimized EBL emit 260 mW (dominant wavelength: 520 nm) at 350 mA (35 A cm−2), and the EQE reaches 31.2%.

Leakage Current Analysis of GaN-Based Light-Emitting Diodes Using a Parasitic Diode Model

The leakage current of GaN-based light-emitting diodes (LEDs) was analyzed with a parasitic diode model. Arrhenius plots of forward leakage currents revealed that the threshold voltages of the main diode and the parasitic diode were 2.64 and 0.94 V, respectively. The parasitic diode, however, led to an insignificant change in the reverse leakage mechanism, e.g., a predominant reverse leakage mechanism was Poole-Frenkel conduction in the high-temperature range (>300 K) and the variable range hopping in the low-temperature range (<;300 K), as reported previously. The origin of the parasitic diode could be due to the incorporated hydrogen (H) atoms into the top LEDs layers, presumably enhancing the defect-assisted tunneling through the generated H-related deep-level states, particularly generated in the p-cladding or p-AlGaN electron blocking layer.

Effect of p-AlGaN electron blocking layers on the injection and radiative efficiencies in InGaN/GaN light emitting diodes

The effect of electron blocking layer (EBL) on the injection and radiative efficiencies in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) was investigated. In order to identify the dominant mechanism, we measured the external quantum efficiency (EQE), radiative efficiency (RE) and carrier injection efficiency (CIE) experimentally. Through these methods, the origin of efficiency droop and the effect of the EBL were clarified. In addition, the RE and the CIE can be separated quantitatively through our analysis. The insertion of the EBL does reduce the carrier leakage but the RE is also reduced by the insertion, which results in not a significant change in the EQE. The measured piezoelectric field of the QWs significantly increased by the insertion of the EBL, which implies that the insertion of AlGaN layer make a strong effect on the QW strain.

Improved hole distribution in InGaN light-emitting diodes with InGaN-GaN barriers of decreasing indium composition

InGaN light-emitting diodes (LEDs) with InGaN–GaN quantum barriers of gradually decreasing indium concentration along the growth direction are proposed to be used. In the proposed structure, simulation results show that the holes are widely distributed in the active region due to the larger hole injection/transportation efficiency by reducing the effective potential height for holes and the radiative recombination rate is efficiently improved, which leads to the significant enhancement of light-output power and internal quantum efficiency (IQE) over the conventional LED with GaN barriers. Moreover, the optical performance is further improved when the graded InGaN/GaN supperlattice (SL) is introduced between the multiple quantum wells (MQWs) and p-AlGaN electron blocking layer (EBL) in our designed LED structure, permitting more freedom for the optimization.