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G.W.Jiang

source:IEEE. G.W.Jiang. Outline. Introduction Experiments Results and Discussion Conclusion. Introduction.

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G.W.Jiang

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  1. source:IEEE G.W.Jiang

  2. Outline • Introduction • Experiments • Results and Discussion • Conclusion 1

  3. Introduction The typical AlGaN EBL is insufficiently effective in blocking the electron leakage and hole injection will be inhibitedinto the active region are generallyrecognized 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. 2

  4. In this letter, we use step-graded AlGaN EBLs with different Al content variation trend to compare with the original AlGaN EBL. 3

  5. Experiments 5x1019cm-3 3x1019cm-3 170nm IEBL 3nm/10nm x6 3um 5x1018cm-3 2um DEBL 25nm Fig. 1. Schematic diagrams of the LEDs with the original. chip size：300 um × 300um 4

  6. Results and Discussion A>B 8% C>A 16% 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

  7. Fig. 3. (a) Enlarged conduction band diagram, (b) Electron concentration distribution near the EBLs of the three samples at 200 mA forward current. 6

  8. Fig. 3. (c) Enlarged valence band diagram, (d) Hole concentration distribution near the EBLs of the three samples at 200 mA forward current. 7

  9. Fig. 4. (a) Electron and (b) hole concentration distribution in the three samples at 200mA forward current. 8

  10. IQE C>A 30% A>B 17% Fig. 5. Simulated IQE as a function of the injection current for the three samples. 9

  11. 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. 10

  12. G.W.Jiang source:IEEE 11

  13. Outline • Introduction • Experiments • Results and Discussion • Conclusion • References 12

  14. 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 effectetc. However, electron current leakage and poor hole injectionefficiency are usually identified to be the major reasonsfor the efficiency droop issue. 13

  15. Unfortunately, the conventional p-type AlGaN EBL cannotusually block the electron in the active region effectively dueto the large band-bending caused by the polarization field. In this paper, we propose a novelsawtooth-shaped EBL to improve the efficiency of electron confinementand hole injection . 14

  16. Experiments 250nm 15% 1.2x1018cm-3 Al% x5 3nm/10nm 0~15% 3um 5x1018cm-3 50nm Fig. 1. Schematic diagram of the LED (left) and the schematic energy band diagrams of the four EBLs. chip size：300 um × 300um 15

  17. Results and Discussion 15% 0% 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.. 16

  18. 距離長 Step 較多 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. 17

  19. Last barrier 較趨緩 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. 18

  20. Step 較多 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. 19

  21. GaN/InGaN接面處 0 GaN/AlGaN接面處 Fig. 4. Electrostatic fields in the five QWs and near last-barrier/EBL interfaces of the four LEDs at 180 mA. 20

  22. 判斷電子confinement的好壞 Fig. 5. Electron current leakage profiles near the activeregion for the four LEDs at 180 mA. 21

  23. Fig. 6. Electron concentrations within the active regionsfor the four LEDs at 180 mA. 22

  24. Fig. 7. Hole concentrations within the active regions for the four LEDs at 180 mA. 23

  25. Fig. 8. Radiative recombination rates within the active region for the four LEDs at 180 mA. 24

  26. Droop% Con EBL=55.2% D:5nm EBL = 35.4% Output power D:5nm / con =2.69 Fig. 9. (color online) Curves of (a) internal quantum efficiency and (b) light output power versus injection current for the four LEDs. 25

  27. 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. 26

  28. References [21] Fiorentini V, Bernardini F and Ambacher O 2002 Appl. Phys. Lett. 801204 [22] Vurgaftman I and Meyer J R 2003 J. Appl. Phys. 94 3675088504- 27

  29. Thanks for your attention. 28

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