P.K. Lin

1. P.K. Lin 1

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

3. 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. 3

4. 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/GaNsuperlattice 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. 4

5. Experiments P contact The structue of the conventional (Original structure) ITO 170nm Mg-doped P-GaN(p = 7x1017cm-3) 20nm Al0.15Ga0.85NEBL (p = 5x1017cm-3) MQW N contact 2μm Si-dopedN-GaN(n = 5 x1018cm-3) 2um-undoped GaN Layer C-plan Sapphire Chip size: 300x300(um2) 5

6. FIG. 1. Schematic view of InGaN LEDs with conventional GaN barrier (original structure), InGaN barriers (structure A), and InGaN barriers and dip-shaped last barrier (structure B). 6

7. Results and Discussion Fig. 2. (a) Light output power, (b) I –V curves, and (c) IQE for the three LEDs. 7

8. Fig. 3. Electrostatic fields and band wavefunctions of the three LEDs at 200 mA. 8

9. Fig. 4. Energy band diagrams of the three LEDs at 200 mA. 9

10. Fig. 5. (a) Electron concentrations, (b) hole concentrations,and (c) electron current density of the three structures at 200 mA. 10

11. Fig. 6. Radiative recombination rate inset with spontaneous emission rate of the three structures at 200 mA. 11

12. Conclusion • In summary, it is found that the electron leakage is markedlyreduced, the hole injection efficiency is greatly enhanced. • In addition, the electrostatic fields in the MQWs are relieved effectivelywhen the conventional GaN barriers are replaced by InGaNbarriers and dip-shaped last InGaN barrier . • Therefore, the optical and electrical performances of the newly designed LED acquired a significant improvement. 12

13. References • 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. 13

14. Thanks for your attention!

15. References

16. The advanced physical model of semiconductor devices simulation software

19. References

20. My Designed EBL structure of LED p-AlGaN Original structure MQW(GaN/InGaN) n-GaN p-GaN p-AlGaN New structure p-GaN MQW(GaN/InGaN) 透過在MQW與EBL間插入一層AlGaNsuperlattice，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… n-GaN

21. My Designed EBL structure p-AlGaN Original structure MQW(GaN/InGaN) p-GaN n-GaN New structure p-type p-GaN n-GaN n-type 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!!

22. 極化場方向 內建電場方向

24. My Designed EBL structure n-type EBL p-type EBL New structure p-type p-GaN p-type GaN barrier p-GaN n-GaN n-type