SU Shi-chen,PEI Lei-lei,ZHANG Hong-Yan,WANG Jia,ZHAO Ling-zhi

（Institute of Optoelectronic Mɑteriɑl ɑnd Technology,South Chinɑ Normɑl Uniυersity,Guɑngzhou 510631,Chinɑ）*Corresponding Author,E-mɑil:shichensu@126.com

Simulation and Design of High Efficiency InGaN/AlInGaN Based Light-emitting Diodes

SU Shi-chen*,PEI Lei-lei,ZHANG Hong-Yan,WANG Jia,ZHAO Ling-zhi

（Institute of Optoelectronic Mɑteriɑl ɑnd Technology,South Chinɑ Normɑl Uniυersity,Guɑngzhou 510631,Chinɑ）*Corresponding Author,E-mɑil:shichensu@126.com

InGaN-based LEDs with InGaN/AlInGaN and InGaN/GaN multiPle quantum wells（MQWs）were theoreticallY studied and comPared by using the Advanced PhYsical Models of Semiconductor Devices（APSYS）simulation Program，resPectivelY.The carrier concentrations in quantum wells，radiative recombination rate in active region，light-current Performance curves，and the internal quantum efficiencY were investigated.The simulation results show that higher efficiencY realized in the strain-free AlInGaN barrier instead of GaN.

GaN;LED;efficiencY drooP

1　Introduction

InGaN-based light-emittingdiodes（LEDs）have been widelY used for manY imPortant aPPlications，such as full-color disPlaY，LCD back-lighting，illumination，and mobile Platforms[1-5].The conventional InGaN/GaN QW structure has alreadY received extensive studies because it can be used asthe active laYer of the light emitting devices[6-8]. However，the existence of strong electrostatic field maY lead to quantum confined Stark effect（QCSE），Poor overlaP of the electron and hole wave function as well as the degraded radiative recombination rate，internal quantum efficiencY（IQE），and oPtical Performance of the light-emitting diodes[9-10].The Polarization related effect results from the sPontaneous and Piezoelectric Polarization.Polarization is a crucial issue for electrical and oPtical characteristics of LEDs.To imProve the quantum efficiencY of InGaN-based LEDs，Previous rePorts used AlInGaN in the quantum barrier instead of GaN to deal with the Polarization，strain，material qualitY，and interfacial abruPtness issues[11-20].However，the oPtical Performance of the LEDs could be largelY weakened by the Piezoelectric effect，which causes the strong electrostatic field and band bending situation in the activeregion.Thestrainengineeringcouldbe achieved by varYing lattice Parameter of the InxGa1-xN temPlate.For the barrier AlxInyGa1-x-yN with 0＜x＜0.26 and 0＜y＜0.11，the structure In0.08Ga0.92N/ AlInGaN grown would have the barrier having the ɑ-axis lattice constant verY close to that of In0.08Ga0.92N，and the In0.08Ga0.92N QW would be unstrained[21]. Moreover，the strain in the QW can be controlled by the lattice Parameter and thus the comPosition of InxGa1-xN.Strain free QW can be achieved for x＝0.08.If x＞0.08 or x＜0.08，the resulting QWs are under tensile stress and comPressive stress，resPectivelY.In this PaPer，the ProPerties of the strain-free In0.08Ga0.92N/Al0.075In0.045Ga0.88N MQWs LED and the conventional In0.08Ga0.92N/GaN MQWs LED were comPared，the simulation results indicated high efficiencY realized in strain-free In0.08Ga0.92N/ Al0.075In0.045Ga0.88N MQWs LED.

2　Simulation Results

In order to reduce the Polarization effect inside the active region of the original LED structure，the Polarization-matched AlInGaN barriers are ProPosed to rePlace the original barrier laYer.

The simulation InGaN-based LED device was designed on a c-Plane Al2O3substrate.A 3 μm thick Si-doPed n-GaN laYer with a doPing concentration of 5×1018cm-3was used to be the n-tYPe bottom contact laYer.The active region consisted of five Periods of undoPed InGaN/Al0.075In0.045Ga0.88N MQWs with 15-nm barrier and 2-nm well.On toP of the active region was a 200 nm thick Mg-doPed P-GaN with a doPing concentration of 3×1017cm-3.A 20 nm thick MgdoPed P-GaN contact laYer with a doPing concentration of 5×1018cm-3was designed to comPlete the structure.The schematic structures of the simulated samPles are shown in Fig.1.In addition to quaternarY samPles with InGaN wells and AlGaInN barriers，a reference samPle with the conventional In0.08-Ga0.92N wells/GaN barrier active region is taken as reference design with the same structure as the quaternarY samPles.

Fig.1　Schematic structure of the simulated InGaN/AlInGaN MQWs LED

The simulated carrier concentrations of the two samPles in multi-quantum wells（MQWs）cut from the n-side to the P-side are Plotted in Fig.2（a）and（b）.

The electron and hole concentrations are verY high in the last quantum well near the P-side indicating the large electron leakage and Poor holes trans-Port efficiencY in InGaN/GaN MQWs LED.However，InGaN/AlInGaNactiveregionreducedthe electron leakage of this LED，which imPlies the enhanced electron confinement in the device.Moreover，as illustrated in Fig.2（b），the overall holes concentration in the MQW region is also increased substantiallY，which suggests that electrons can be confined in MQWs and holes injection efficiencY can be imProved markedlY in InGaN/AlInGaN thanInGaN/GaN MQWs LED.When the strain free In-GaN/AlInGaN MQWs are aPPlied，the electron and holes concentration increases in the QW and the distribution of carrier becomes more uniform than In-GaN/GaN case.In InGaN/GaN MQWs LED，much more severe band bending was induced by the larger Polarization field，leading to the amount of electrons increased in the last well.AlInGaN barrier samPle has better carrier transPortation and this is also more obvious in the hole distribution.The direct consequence is the enhanced radiative recombination rate and light outPut.On the other hand，in samPle with traditional GaN barrier the holes are concentrated locallY in the last QW，which causes the unbalanced hole distribution between different wells and thus leading to the reduction in radiative recombination rate.The distribution of carrier of InGaN/InAlGaN becomes more uniform than InGaN/GaN structures. Under high current densitY，the carrier distribution of both electrons and holes determines how efficient the Photon-emission Process will be.InAlGaN barrier samPle is reduced due to better carrier transPortation and this is also more obvious in the hole distribution.The direct consequence is the increasing radiative in the traditional GaN barrier samPles.Com-Paring electrons with holes，holes suffer more as a result of this nonuniformitY due to their large effective mass and low mobilitY.Thus，our InAlGaN design can reduce the carrier leakage and increase electron-hole Pair radiative recombination，esPeciallY for the distribution of holes.

Fig.2　Distribution of electron（a）and hole（b）concentrations of InGaN/GaN and InGaN/AlInGaN MQWs LED

The curves of the light outPut intensitY υersus the injection current of the two samPles are shown in Fig.3.It is shown that the light outPut Power is significantlY enhanced in this structure with InGaN/ AlInGaN active region.The results show that the light outPut Power of the two devices both increase as forward injection current increasing.

Fig.3　Light outPut Power as a function of forward current densitY for InGaN/AlInGaN and InGaN/GaN MQWs LED

The increase rate of the two structures is just the same at low injection current.The difference light outPut Power between the two samPles is enlarged at high forward current when the injection current further increased.The outPut Power of In-GaN/GaN MQWs LED increases slowlY as the forward current increases，thus AlInGaN/InGaN quantum well LED，the outPut Power increases raPidlY. ComParing with the original LED structure，the new LED structure Possesses higher outPut Power under same current densitY.

Fig.4Presents the simulated IQE curves of In-GaN/GaN and InGaN/AlInGaN MQWs LED.As the forward current increases，IQE of the two LED structures show similar variation trend.As the forward current increases，both samPles show an obvious efficiencY drooP in IQE.However，the efficiencY drooP in LED with AlInGaN barrier is much smaller than that of LED with GaN barrier，which means the efficiencY drooP can be ameliorated by using AlInGaN barrier.

Fig.4　Simulated IQE υs.current of InGaN/AlInGaN and InGaN/GaN MQWs LED

InGaN/GaN MQWs LED exhibits a serious efficiencY drooP.However，the overall oPtical Performances of LED with InGaN/AlInGaN active region are imProved and smaller efficiencY drooP is observed as GaN barriers are changing to AlInGaN.The Possible reason is that AlInGaN barriers can block the electrons effectivelY and increase the holes injection efficiencY.The Polarization induced energY band bending of this device can greatlY affect the carrier transPort，and thus influence IQE.The quantum efficiencY decrease can be understood if the emission mechanism for InGaN/GaN MQWs LED is the excitonic recombination from localized states formed by the indium variation in the QW.In this case，the carrier caPture into localized states followed by emission is faster than the nonradiative recombination. However，since the densitY of these localized states is limited due to strain free，the non-radiative recombination caused by the large Polarization field decreases the quantum efficiencY.This results show that the electron leakage current due to the Polarization field is one of the dominant mechanisms in efficiencY drooP.For InGaN/AlInGaN MQWs LED，it can eliminate the Polarization effect between the barriers and wells，and further reduce the electron leakage.This can be exPlained that the alleviated Polarization field induce band bending and the Poor electron-hole wave function overlaP that leads to the effective blockage of electrons and the radiative recombination was enhanced in the active region.

Fig.5illustrates the radiative recombination rates in the QW for the two LED.As can be seen，the radiative recombination rates in the QW are more uniform for LED with InGaN/AlInGaN active region thanGaInN/GaN MQWs LED.InInGaN/GaN MQWs LED，because of the non-uniform carrier distribution in QWs，most of the radiative recombination haPPens in QWs close to P-side.The electrons and holes in AlInGaN LED distribute more uniformlY than that in InGaN/GaN MQWs LED owing to the imProvement of current sPreading.This indicates that GaInN/AlGaInN MQWs LED can imProve the hole injection and electron confinement comPared to GaInN/GaN MQWs LED.Thus，the radiative recombination rates in InGaN/AlInGaN MQWs LED are enhanced.

Fig.5　Radiative recombination rates of InGaN/AlInGaN MQWs LED and InGaN/GaN MQWs LED

3　SummarY

The strain-free LED with InGaN/AlInGaN MQWs and GaInN/GaN MQWs LED are studied by APSYS simulation Program.The simulation results show that the quaternarY barriers can reduce the current leakage and increase the hole injection by removing the Polarization field in the QWs.Thus，InGaN/AlInGaNMQWsexhibithigherradiative recombination rate and reduced efficiencY drooP at a high injection current.

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宿世臣（1979-），男，黑龍江佳木斯人，博士，副研究員，2009年于中國科學(xué)院長春光學(xué)精密機(jī)械與物理研究所獲得博士學(xué)位，主要從事寬禁帶半導(dǎo)體方面的研究。

E-mail:shichensu@126.com

1000-7032（2016）02-0208-05

高效InGaN/AlInGaN發(fā)光二極管的結(jié)構(gòu)設(shè)計及其理論研究

宿世臣*,裴磊磊,張紅艷,王 佳,趙靈智

（華南師范大學(xué)光電子材料與技術(shù)研究所，廣東廣州 510631）

利用Advanced PhYsical Models of Semiconductor Devices（APSYS）理論對比研究了InGaN/AlInGaN和In-GaN/GaN多量子阱作為有源層的InGaN基發(fā)光二極管的結(jié)構(gòu)和電學(xué)特性。與InGaN/GaN基LED中GaN作為壘層材料相比，在AlInGaN材料體系中，通過調(diào)節(jié)AlInGaN中Al和In的組分可以優(yōu)化器件的性能。當(dāng)InGaN阱層材料中In組分為8%時，可以實現(xiàn)無應(yīng)力的In0.08Ga0.92N/AlInGaN基LED。在這種無應(yīng)力結(jié)構(gòu)中可以進(jìn)一步降低大功率LED的“效率下降”（EffciencY drooP）問題。理論模擬結(jié)果顯示，四元系A(chǔ)lInGaN作為壘層可以進(jìn)一步減少載流子泄露，增加空穴注入效率，減少極化場對器件性能的影響。在In0.08Ga0.92N/AlInGaN量子阱中的載流子濃度、有源層的輻射復(fù)合率、電流特性曲線和內(nèi)量子效率等方面都優(yōu)于InGaN/GaN基LED。無應(yīng)變AlInGaN壘層代替?zhèn)鹘y(tǒng)的GaN壘層后，能夠得到高效的發(fā)光二極管，并且大電流注入下的“效率滾降”問題得到改善。

氮化鎵;發(fā)光二極管;效率下降

2015-10-26;

2015-12-10

國家自然科學(xué)基金（61205037，61574063）;中國博士后基金（2013M53186，2015T80910）;教育部新教師基金;高等學(xué)校博士學(xué)科點(diǎn)專項科研基金（20124407120017）;廣東省省級科技計劃（2015A090905003，2014B040404067）;廣州市產(chǎn)學(xué)研協(xié)同創(chuàng)新重大專項（201508030033）;廣州市越秀區(qū)產(chǎn)學(xué)研項目（2013-CY-007）資助

O484.4;TN383 Document code:A DOI:10.3788/fgxb20163702.0208