DING Bin-bin，ZHAO Fang，SONG Jing-jing，XIONG Jian-yong，ZHENG Shu-wen，YU Xiao-peng，XU Yi-qin，ZHOU De-tao，ZHANG Tao，F(xiàn)AN Guang-han

(Institute of Opto-Electronic Materials and Technology，South China Normal University，Guangzhou 510631，China)

*Corresponding Author，E-mail:gfan@scnu.edu.cn

1 Introduction

The InGaN/GaN multiple quantum well(MQW)light-emitting diode(LED)is promising to replace the conventional incandescent and fluorescent lamps due to its inherent higher energy efficiency and other advantages［1-3］.However，these devices suffer from a rapid efficiency droop with increasing injection of current［4-5］， which is detrimental for high-brightness applications.Various explanations on themechanisms of the efficiency droop have been proposed，such as carrier leakage from the active region［6-8］，Auger recombination［9-10］，poor hole injection［11］，and polarization effect［12］etc.The origin of the efficiency droop，however，is not clearly understood yet.Recently，many approaches，such as the usage of InGaN barriers［13］，AlInGaN barriers［14］，Al-GaN barriers［15］，staggered quantum wells(QWs)［16-18］，AlGaN/GaN superlattice(SL)electron blocking layer(EBL)of gradual Almole fraction［19］，graded electron blocking layer(GEBL)［20］，AlGaN-GaNAlGaN(AGA)electron-blocking layer(EBL)［21］and triangular shaped MQW［22］，are reported for improving the optical performance.

The published simulation results show that the electron leakage may play an important role for the efficiency droop.Therefore，the insertion of an Al-GaN layer between the last QW barrier and p-type hole-injection layer has been suggested with the hope that this wide-band gap material layer would act as an electron-blocking layer(EBL)to suppress the escape of the electrons out of the active region into the p-type hole-injection layer.However，recently published studies point out that the electron confinement by a typical AlGaN EBL is not sufficiently effective to solve the efficiency droop problem.Furthermore，the use of AlGaN EBL can cause some undesired effects such as prohibiting the holes from injecting into the active region，which degrades the luminescence characteristics of the blue LED［23-24］.

In this paper，we proposed AlGaN-GaN-AlGaN EBL with gradual Al composition(GAGA)to improve the optical performance.The light output power，internal quantum efficiency(IQE)，electrostatic field，radiative recombination rate，energy band and carrier concentration of the LEDs with the conventional EBL，AGA EBL and GAGA EBL are investigated numerically with the APSYS(Advance Physical Model of Semiconductor Devices)simulation software，which is capable of dealing with the physical properties of LEDs by solving the Poisson's equation，current continuity equations，carrier transport equation，quantum mechanical wave equation，and photon rate equation.

2 Structure and Parameters

The original blue InGaN/GaN MQW LED used in this paper as a reference was grown on a c-plane sapphire substrate，followed by a 4-μm-thick n-GaN layer(n-doping=2×1018cm－3).The active region consisted of six 2.5-nm-thick In0.15Ga0.85N quantum wells(QWs)，sandwiched by seven 10-nm-thick GaN barriers.On top of the active region were a 20-nm-thick p-Al0.15Ga0.85N EBL(p-doping c=5 ×1017cm－3)and a 0.2-μm-thick p-GaN cap layer(p-doping c=7×1017cm－3).The device geometry was designed into a rectangular shape of 300μm×300μm.For the LED with AGA EBL，the conventional Al0.15Ga0.85N layer was replaced by an Al0.15Ga0.85N-GaN-Al0.15Ga0.85N EBL.For the LED with GAGA，its structure was identical to that of the structure AGA，but the Al0.15Ga0.85N-GaN-Al0.15-Ga0.85N EBL was replaced by an Al0.07Ga0.93N-Al0.15-Ga0.85N-GaN-Al0.15Ga0.85N-Al0.07Ga0.93N EBL.These three structures are shown in Fig.1.The operating temperature is assumed to be 300 K and the internal absorption within the LED device is set to be 500 m－1.Other parameters can be found in Ref.25.

Fig.1 Schematic of original LED with a conventional AlGaN EBL(conventional structure)，LED with a common AlGaN-GaN-AlGaN EBL(AGA structure)，and LED with a gradual Al composition AlGaN-GaN-AlGaN EBL(GAGA structure).

3 Results and Discussion

Fig.2 shows the light output power and internal quantum efficiency(IQE)for the LEDs with the conventional EBL，AGA EBL and GAGA EBL.It is evident that the light output power and IQE are significantly enhanced in the LEDswith AGA EBL and GAGA EBL compared with the conventional AlGaN EBL.It also shows that，comparing with the LED with AGA EBL，the light output power and efficiency droop of the LED with GAGA EBL are improved.According to Fig.2(b)，conventional structure has smallest internal quantum efficiency(IQE)and the worst efficiency droop of 22.7%.GAGA structure shows the highest IQE and the significantlymitigated efficiency droop of 3.7%.

Fig.2 (a)Lightoutputpower and(b)internalquantum efficiency(IQE)for the LEDs with the conventional EBL，AGA EBL and GAGA EBL.

Fig.3 shows that the degree of influence on the electrostatic fields and radiative recombination in the active region of different structures is different.As shown in the Fig.3(a) ～ (c)，the electrostatic field in the active region of conventional structure ismuch stronger than that of the other two structures especially for the last QW which is near the EBL，and the electrostatic field in the active region ofGAGA structure are smallest.A stronger electrostatic field in the active region will lead to the bending band，poor overlap of electron and hole wave functions，and hence reduced radiative recombination rate. Therefore，GAGA structure has an advantage over the other two structures in radiative recombination due to the smaller electrostatic field in the active region，which can be seen in Fig.3(d) ～ (f).

Fig.3 (a) ～(c)Electrostatic fields and(d) ～(f)radiative recombination rates of the LEDswith the conventional EBL，AGA EBL and GAGA EBL at200 mA.

Fig.4 shows the band diagrams and carrier concentrations of the LEDs with the conventional EBL，AGA EBL and GAGA EBL.As shown in Fig.4(a)，piezoelectric polarization field along with the spontaneous polarization field pulls down the energy band at the last barrier and AlGaN EBL interface due to the latticemismatch，which also can be seen in Fig.3(a).The lattice mismatch between the last GaN QW barrier and AlGaN EBL can generate a strong piezoelectric polarization field.As a result，the effective potential barrier height for electrons in the conduction band of the EBL is reduced and the electron leakage can not be effectively suppressed.Moreover，because of the band bending effect，the EBL also acts as a potential barrier for holes which may hinder the holes from injecting into the active region.Fig.4(a) ～ (c)illustrates the energy band diagrams of the LEDs with the conventional AlGaN EBL，AGA EBL and GAGA EBL at 200 mA，respectively.As shown in the Fig.4，the LED with the conventional EBL，the effective potential height for holes in the valance band near the lastQW barrier and the AlGaN EBL(735 meV)ismuch greater than that of the LED with AGA EBL(565 meV)and GAGA EBL(513 meV)，which denotes that holes aremore difficult to inject into the active region in the LED with AGA EBL and GAGA EBL.Effective potential height for electrons in the conduction band at the same part(766 meV)is much smaller than that of the LED with AGA EBL(800 meV)and GAGA EBL(850meV).As a result，it can be seen in Fig.4(d) ～ (f)that conventional structure has the worst electron leakage and smallesthole injection efficiency，while GAGA structure acquires the lowest electron leakage and highest hole injection efficiency，especially in the firstQW and the last QW.This can be explained as follows:(1)Because of the tunneling effect［20］，AGA structure and GAGA structure obtain better electron leakage and higher hole injection efficiency over that of conventional structure;(2)Comparing with AGA structure，GAGA structure has the lowest latticemismatch between the last GaN QW barrier and EBL.Consequently，the LED with GAGA EBL show much better performances in the hole injection，electron confinement，and efficiency droop than those with the conventional EBL and AGA EBL.

Fig.4 (a)～(c)Energy band diagrams and(d) ～(f)carrier concentrations of the LEDs with the conventional EBL，AGA EBL and GAGA EBL at200mA.

4 Conclusion

In conclusion，blue InGaN LEDs with the conventional EBL，AGA EBL and GAGA EBL have been investigated numerically.When GAGA structure is used，the hole injection efficiency into the active region can be greatly increased，the electron leakage can be reduced，and the electrostatic field in the active region can be considerably relieved.The InGaN LED with an AlGaN-GaN-AlGaN electron-blocking layer with gradual Al composition has significantly improved optical performance such as much bigger recombination rate and higher IQE.Moreover，the efficiency droop of this structure is markedly alleviated.

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