G.W.Jiang

source:IEEE

1

Outline

Introduction

Experiments

Results and Discussion

Conclusion

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Introduction The typical AlGaN EBL is insufficiently effective in

blocking the electron leakage and hole injection will be inhibited into the active region are generally recognized as the primary cause of efficiency droop.

Recently, graded AlGaN EBL has been suggested to benefit hole injection and electron confinement but is still under debate which kind of Al content variation trend best favors the reduction of efficiency droop.

In this letter, we use step-graded AlGaN EBLs with different Al content variation trend to compare with the original AlGaN EBL.

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4

Experiments

chip size： 300 um × 300um Fig. 1. Schematic diagrams of the LEDs with the original.

170nm

x63nm/10nm

3um5x1018cm-3

25nm

2um

3x1019cm-3

5x1019cm-3

IEBL

DEBL

Fig. 2. (a) Light output power versus current (L-I) characteristics of the three samples, (b) EQE as a function of the injection current for the three samples. 5

Results and DiscussionA B C

Vf 3.16V 3.15V 3.15V

Ir 0.06uA 0.05uA 0.05uA

A>B 8%C>A 16%

Fig. 3. (a) Enlarged conduction band diagram, (b) Electron concentration distribution near the EBLs of the three samples at 200 mA forward current.

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Fig. 3. (c) Enlarged valence band diagram, (d) Hole concentrationdistribution near the EBLs of the three samples at 200 mA forward current.

Fig. 4. (a) Electron and (b) hole concentration distribution in the threesamples at 200mA forward current.

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Fig. 5. Simulated IQE as a function of the injection current for the threesamples. 9

IQEC>A 30%

A>B 17%

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Conclusion The experimental results indicate that the adoption of step-

graded AlGaN EBLs has little influence on the electrical properties of the fabricated LEDs.

And experiments show that the IEBL gives rise to inferior optical performance, compared to the DEBL and original EBL.

Simulation results, the DEBL can improve performance is attributed to enhanced hole injection without losing the capability of electron confinement.

G.W.Jiang

11

source:IEEE

12

Outline

Introduction

Experiments

Results and Discussion

Conclusion

References

13

Introduction The optical performances of LEDs can be largely weakened

by several mechanisms； including carrier leakage due to the polarization effect、 Auger recombination、 current injection efficiency、 lack of hole injection、 the self-heating effect etc.

However, electron current leakage and poor hole injection efficiency are usually identified to be the major reasons for the efficiency droop issue.

Unfortunately, the conventional p-type AlGaN EBL cannot usually block the electron in the active region effectively due to the large band-bending caused by the polarization field.

In this paper, we propose a novel sawtooth-shaped EBL to improve the efficiency of electron confinement and hole injection .

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Experiments

chip size： 300 um × 300um Fig. 1. Schematic diagram of the LED (left) and the schematic energy band diagrams of the four EBLs.

50nm

3um5x1018cm-3

1.2x1018cm-3

250nm

Al%

0~15%x53nm/10nm

15%

Fig. 2. Conduction energy band diagrams for LEDs with (a) conventional EBL, (b) sawtooth EBL (d =15 nm) at an injection current of 180 mA..

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Results and Discussion

0%

15%

Fig. 2. Conduction energy band diagrams for LEDs (c) sawtooth EBL (d =10 nm), and (d) sawtooth EBL (d = 5 nm) at an injection current of 180 mA.

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距離長Step 較多

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Fig. 3. Valence energy band diagrams for LEDs with (a) a conventional EBL, (b) a sawtooth EBL (d = 15 nm), at an injection current of 180 mA.

Last barrier 較趨緩

Fig. 3. Valence energy band diagrams for LEDs with (c) a sawtooth EBL(d = 10 nm), and (d) a sawtooth EBL (d = 5 nm) at an injection current of 180 mA.

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Step 較多

Fig. 4. Electrostatic fields in the five QWs and near last-barrier/EBL interfaces of the four LEDs at 180 mA.

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GaN/InGaN接面處

0GaN/AlGaN接面處

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Fig. 5. Electron current leakage profiles near the active region for the four LEDs at 180 mA.

判斷電子 confinement的好壞

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Fig. 6. Electron concentrations within the active regions for the four LEDs at 180 mA.

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Fig. 7. Hole concentrations within the active regions forthe four LEDs at 180 mA.

Fig. 8. Radiative recombination rates within the activeregion for the four LEDs at 180 mA.

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Fig. 9. (color online) Curves of (a) internal quantum efficiency and (b)light output power versus injection current for the four LEDs.

Droop%

Con EBL=55.2%

D:5nm EBL = 35.4%

Output power

D:5nm / con =2.69

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Conclusion When used a sawtooth-shaped EBL, the effective barrier

height of the conduction band is increased and the barrier obstacle in the valence band for holes is mitigated due to the correctly modified energy band of the EBL.

The electron confinement is enhanced and more holes can be transported from the p-type region into the MQW.

This effect prevents electron leakage and improves the radiative recombination rate in the QW, leading to a significant improvement in IQE and light output power.

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References [21] Fiorentini V, Bernardini F and Ambacher O 2002

Appl. Phys. Lett. 80 1204

[22] Vurgaftman I and Meyer J R 2003 J. Appl. Phys. 94 3675 088504-

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Thanks for your attention.