P.K. Lin

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Outline• Introduction• Experiments• Results and Discussion• Conclusion• References

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Introduction

• Over the past few years, people have proposed numerous physics mechanisms to explain the phenomenon of efficiency droop, such as Auger recombination ,electron leakage , poor hole injection efficiency , polarization effect , and the quantum confined stark effect.

• Up to now, however, the approaches to improve efficiency droop are still mainly to suppress the electron overflow, enhance the hole injection efficiency, and reduce the polarization field.

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Introduction

• Such as the usage of staggered quantum wells (QWs) ,AlGaN barriers , indium graded last barrier , p-InGaN hole reservoir layer , graded electron blocking layer (EBL) , and AlGaN/GaN superlattice EBL of gradual Al mole fraction.

• In this paper, the idea of InGaN barriers and dip-shaped last barrier is proposed with reduced polarization effect, decreased electron current overflow, and increased hole injection efficiency.

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Experiments

C-plan Sapphire

2um-undoped GaN Layer

2μm Si-doped N-GaN(n = 5 x1018cm-3)

MQW

20nm Al0.15Ga0.85NEBL (p = 5x1017cm-3)

170nm Mg-doped P-GaN(p = 7x1017cm-3)

N contact

ITO

P contact

LED MQWComposition In0.16Ga0.84N/

GaNPair six-period

Thickness 3nm/10nm

Chip size: 300x300(um2)

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The structue of the conventional (Original structure)

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FIG. 1. Schematic view of InGaN LEDs with conventional GaN barrier(original structure), InGaN barriers (structure A), and InGaN barriers anddip-shaped last barrier (structure B).

Fig. 2. (a) Light output power, (b) I –V curves, and (c) IQE for the three LEDs.

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Results and Discussion

At 200mA Efficiency droopOriginal structure 40.9%

Structure A 27.7%

Structure B 23.5%

Fig. 3. Electrostatic fields and band wavefunctions of the three LEDs at200 mA.

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Fig. 4. Energy band diagrams of the three LEDs at 200 mA.

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Fig. 5. (a) Electron concentrations, (b) hole concentrations,and (c) electron current density of the three structures at 200 mA.

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Fig. 6. Radiative recombination rate inset with spontaneous emission rate of the three structures at 200 mA.

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• In summary, it is found that the electron leakage is markedly reduced, the hole injection efficiency is greatly enhanced.

• In addition, the electrostatic fields in the MQWs are relieved effectively when the conventional GaN barriers are replaced by InGaN barriers and dip-shaped last InGaN barrier .

• Therefore, the optical and electrical performances of the newly designed LED acquired a significant improvement.

Conclusion

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• R.M.Lin,S.F.Yu,S.J.Chang, T.H.Chiang,S.P.Chang,andC.H. Chen, “Inserting a p-InGaN layer before the p-AlGaN electron blocking layer suppresses efficiency droop in InGaN-based light-emitting diodes,” Appl. Phys. Lett., vol.101,no.8,pp.081120-1–081120-3,Aug.2012.

• T.Lu,S.Li,C.Liu,K.Zhang,Y.Xu,J.Tong,L.Wu,H.Wang, X.Yang,Y.Yin,G.Xiao,andY.Zhou,“Advantages of GaN basedlight-emitting diodes with a p-InGaN hole reservoir layer,” Appl. Phys. Lett.,vol.100,no.14,pp. 141106-1–141106-3,Apr.2012.

• C.S.Xia,Z.M.Simon Li,W.Lu,Z.H.Zhang,and L.W.Cheng,“Droop improvement in blue InGaN/GaN multiple quantum well light-emitting diodes with indium graded last barrier,”Appl. Phys. Lett.,vol.99,no.23,pp.233501-1–233501-3, Dec. 2011.

References

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Thanks for your attention!

References

The advanced physical model of semiconductor devices simulation software

The key parameters Set on The radiative 2.0 x 10-11 cm3/s

Internal absorption 2000m-1

Auger cofficients 1.0 x 10-31 cm3/s

SRH recombination lifetime 100ns

Operating temperature 300K

References

My Designed EBL structure of LED

p-GaNn-GaN

p-AlGaN

MQW(GaN/InGaN)Original structure

p-GaNn-GaN

p-AlGaN

MQW(GaN/InGaN)New structure

透過在MQW與 EBL間插入一層 AlGaN superlattice ，做為緩衝 last barrier 與 EBL lattice mismatch 所帶來能帶傾斜的效應 !!進而增加 EBL有效的能障高度…

My Designed EBL structure

p-GaNn-GaN

p-AlGaN

MQW(GaN/InGaN)Original structure

在 last barrier 做一 n參雜，目的在於行程一空乏區內建電場 !透過 PN面空乏區電場來去抵補極化場 !!

n-GaN

New structure

p-GaN

p-type

n-type

極化場方向內建電場方向

My Designed EBL structure

n-GaN

New structure

p-GaN

p-type

n-type

p-type GaN barrier

n-type EBL

p-type EBL

p-GaN