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Numerical study on efficiency droop of blue InGaN light-emitting diodes

Numerical study on efficiency droop of blue InGaN light-emitting diodes. Yen-Kuang Kuo*, Jih-Yuan Chang, and Jen-De Chen Department of Physics, National Changhua University of Education, Changhua 500, Taiwan *E-mail: ykuo@cc.ncue.edu.tw. Outline. Introduction

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Numerical study on efficiency droop of blue InGaN light-emitting diodes

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  1. Numerical study on efficiency droop of blue InGaN light-emitting diodes Yen-Kuang Kuo*, Jih-Yuan Chang, and Jen-De Chen Department of Physics, National Changhua University of Education, Changhua 500, Taiwan *E-mail: ykuo@cc.ncue.edu.tw

  2. Outline • Introduction • Simulation results and discussion • InGaN barriers • P-doped barriers • P-type last barrier • AlGaN barriers • Varied barrier thicknesses • N-type AlGaN layer • Specific designs on electron blocking layer • Thin last barrier • Conclusion

  3. Introduction In III-N LEDs, the quantum efficiency typically reaches its peakat low current density and then decreases with further increasing drive current, which is commonly referred to as “efficiency droop”.

  4. Introduction Poor hole injection efficiency, electron current leakage, andnon-uniform distribution of carriers in the active region usually play an important role for efficiency droop. In this work, several specific configurations in the active region and electron blocking layer (EBL) of blue InGaN LEDs are investigated numerically with the APSYS simulation program.

  5. Study 1: InGaN barriers

  6. Study 1: InGaN barriers • ※ Original LED (fabricated by EpiStar Inc.) • Active region: five pairs of In0.21Ga0.79N/ • GaN (2 nm/15 nm) QWs • EBL: 20-nm-thick p-Al0.15Ga0.85N • Concentration: n-51018 cm–3; p-1.21018 cm–3 • Device geometry: 300300 m2 • The sloped triangular barriers induced by the polarization effect cause conduction band edge of barriers to be higher than that of the EBL. •  This results in the insufficient electron blocking efficiency and thereby serious electron current leakage. • The electron and hole wavefunctions separate partially. •  This results in the reduction of the radiative recombination rate and IQE. Fig. 1. The energy band diagram of the original InGaN/GaN structure at 150 mA. (Appl. Phys. Lett., vol. 95, p. 011116, 2009)

  7. Study 1: InGaN barriers • The effective barrier height between last barrier and EBL increases dramatically due to lower conduction band energy of the InGaN barrier. • This results in the enhanced electron blocking efficiency of EBL. • Owing to better match of lattice constants between the InGaN barrier and InGaN well, the band bending situation is less severe. • This results in less QCSE and better light-emitting efficiency. Fig. 2. The energy band diagram of the proposed InGaN LED structure with In0.1Ga0.9N barriers at 150 mA .

  8. Study 1: InGaN barriers Fig. 3.Carrier concentrations of (a) original and (b) proposed structures near active region at 150 mA. Fig. 4.(a) IQE and (b) L-I curve for the two LED structures under study. • In Fig. 3: • In the proposed InGaN/InGaN structure, more electrons and holes transport into the active region and contribute to the radiative recombination. • Both electron and hole distributions in the QWs of the proposed InGaN/InGaN structure are more uniform than those of the original structure. • In Fig. 4: • The InGaN/InGaN LED has better light-emitting efficiency (especially at high current) and is almost without efficiency droop.

  9. Study 2: P-doped barriers

  10. Study 2: P-doped barriers • The barriers are partially p-doped in selected regions in order to increase the hole injection and improve the carrier distribution across the MQWs. • The reason of “partial” p-doping in “selected regions” is to avoid the diffusion of Mg into the quantum well during crystal growth. (IEEE Photonics Technol. Lett., vol. 22, p. 374, 2009) Fig. 5. Schematic diagrams of the InGaN/GaN active region for: (a) the original structure, (b) structure A, and (c) structure B.

  11. Study 2: P-doped barriers Fig. 6. (a) L-I curves and (b) IQEs of the blue InGaN LEDs as a function of the forward current density. • When the partially p-doped GaN barriers are employed, the output powers and IQEs of the InGaN LEDs are all enhanced sufficiently. • This is because that structure A and B possess higher concentration of holes inside the QWs due to the improved hole injection efficiency. • When the three barriers near the p-layer are p-doped with a gradually increased doping concentration (structure A), more holes are confined within the QWs, especially the two QWs close to the p-layer, which have the most contribution to the emission power. Fig. 7. Hole concentration of the original structure, structure A, and structure B in the active region at 22 A/cm2 (20 mA).

  12. Study 3: P-type last barrier

  13. Study 3: P-type last barrier Fig. 9. (a) Simulated L-I curves and the experimental data and (b) IQEs of the blue InGaN LEDs. The output power and IQE are both enhanced sufficiently, when the last undoped GaN barrier is replaced by a p-type GaN barrier. Fig. 8. Schematic diagrams of the InGaN/GaN active regions: (a) the original structure, (b) structure with p-doping in the last barrier, and (c) structure with partial p-doping in two-thirds of the last barrier. (IEEE J. Quantum Electron., vol. 22, p. 374, 2009)

  14. Study 3: P-type last barrier As indicated in Fig. 10, both structure A and structure B possess relatively highhole concentration inside the QWs because the hole injection efficiency is effectively improved. Fig. 10. Hole concentrations of the original structure, structure A, and structure B in the active regions at (a) 20 mA and (b) 150 mA.

  15. Study 3: P-type last barrier • The total radiative recombination rates inside the QWs of structure A and B at 20 (150) mA are improved by a factor of 1.35 (2.15) and 1.33 (2.06), respectively, as compared to that of original one. • Therefore, at high current, the optical performance of the blue InGaN LEDs under study can be markedly enhanced when the last barrier is fully or partially p-doped. Fig. 11. Radiative recombination rate of the original structure, structure A, and structure B in the active region at 150 mA. The inset shows the radiative recombination rates at 20 mA.

  16. Study 4: AlGaN barriers

  17. Study 4: AlGaN barriers • The use of an AlGaN barrier instead of a GaN barrier can diminish the polarization charges accumulating in the last-barrier/EBL interface, which accordingly can relax the band-bending of the EBL. Due to this phenomenon, the effective potential barrier height for holes in the valence band decreases and hence the transportation of holes into the active region becomes easier. • In the meantime, the ability of electron confinement in the last-barrier/EBL interface does not degrade under this variation. • As a result, it is expected that the efficiency of hole injection can be enhanced for the structure with AlGaN barrierswithout the cost of reduction of electron confinement. Table 1. Surface Charge Density Between Well Layer and Barrier Layer Table 2. Surface Charge Density Between (last) Barrier Layer and EBL (Opt. Lett., vol. 35, p. 1368, 2010)

  18. Study 4: AlGaN barriers • The effective potential height for holes in the valence band of the InGaN/AlGaN structure is lower than that of the InGaN/GaN one due to slighter polarization effect in the last-barrier/EBL interface. •  Better hole injection efficiency is anticipated. • Besides, the effective potential height for the electrons in the conduction band of the InGaN/AlGaN structure becomes higher than the other structure, which demonstrates the enhancement of electron confinement. Fig. 12. Energy band diagrams of InGaN/GaN and InGaN/AlGaN structures at 150 mA.

  19. Study 4: AlGaN barriers Fig. 13. Carrier concentrations of (a) InGaN/GaN and (b) InGaN/AlGaN structures near active region at 150 mA. Fig. 14. (a) L-I-V (b) IQE performance curves of the InGaN/GaN and InGaN/AlGaN structures. • Ascribing to the enhancement of hole injection and electron confinement, for the InGaN/AlGaN structure, there are more carriers in the active region • Thus, the InGaN/AlGaN structure has higher IQE and output power in the whole range of current injection under study. • The efficiency droop is markedly improved when the traditional GaN barriers are replaced by AlGaN.

  20. Study 5: Varied barrier thicknesses

  21. Study 5: Varied barrier thicknesses • For this study: • The original structure of the blue LED used in the simulation is with equal barrier thickness of 15 nm. • In structure A, the thicknesses of all the three barriers next to the n-side layers are 15 nm, which is identical to the barrier thickness of the original structure. The thickness of the three barriers next to the p-side layer are reduced to 10 nm. • In structure B, the thicknesses of the barriers from the n-side to p-side layers decrease linearly from 15 nm to 5 nm with a step of 2 nm. • In the two redesigned structures, the thicker barriers located close to the n-side layers are used for increasing the electron transport distance while the thinner barriers located close to the p-side layers are used for reducing the hole transport distance. • Based on this rationale, it is expected that the electrons leaking out of the active region will reduce and the holes injecting into the active region will increase. (To be published in IEEE Photonics Technol. Lett.)

  22. Study 5: Varied barrier thicknesses • The output powers of structures A and B are enhanced greatly as compared with the original structure for the both cases with 1×10–34 cm6/s and 1×10–30 cm6/s Auger coefficients. • However, the efficiency droop for the original structure under an Auger coefficient of 1×10–30 cm6/s is 72.9%, which is much larger than the value obtained experimentally (in a range of 23–53%). • Therefore, an Auger coefficient of 1×10–34 cm6/s is utilized for subsequent investigations on the optical properties of the blue LEDs under study. Fig. 15. (a) L-I curves and (b) IQEs of the original structure, structure A, and structure B under an Auger coefficient of 1×10–34 cm6/s; (c) L-I curves and (d) IQEs of the original structure, structure A, and structure B under an Auger coefficient of 1×10–30 cm6/s.

  23. Study 5: Varied barrier thicknesses • Decrease in thickness of barriers next to the p-side layers is beneficial for enhancing the injection of holes into the active region and hole transportation in the active region, which would allow more holes to reach the QWs near the n-side layers. • Among the two redesigned structures, structure B possesses the largest amounts of carriers and the most uniform distribution of carriers inside the active region. Fig. 16. Electron and hole concentrations of (a) original structure, (b) structure A, and (c) structure B at 20 mA.

  24. Study 5: Varied barrier thicknesses • As the thickness of barriers close to the p-side layers decreases, more QWs contribute to radiative recombination due to the marked improvement in hole injection efficiency. • For the original structure, the total radiative recombination rate is enhanced by a factor of 3.77 when the injection current increases from 20 mA to 120 mA. On the other hand, for structures A and B, the total radiative recombination rates are enhanced by a factor of 4.15 and 5.67, respectively, when the injection current increases from 20 mA to 120 mA. • The improvement is because that more QWs, especially the three QWs close to the p-side layers, can contribute to radiative recombination. Fig. 17. Radiative recombination rates of (a) the original structure, (b) structure A, and (c) structure B at 20 mA and 120 mA.

  25. Study 6: N-type AlGaN layer

  26. Study 6: N-type AlGaN layer Fig. 18.L-I curves of the LEDs with P and N-AlGaNs. Experimental data are shown by green solid dots. Fig. 19.IQE of the LEDs with P and N-AlGaNs. • The output power of the LED with N-AlGaN at 120 mA is improved by a factor of 2.37 as compared with that with P-AlGaN. • The efficiency droop of the LED with N-AlGaN at 120 mA is markedly improved as compared to that with P-AlGaN. (IEEE Photonics Technol. Lett., vol. 21, p. 975, 2009)

  27. Study 6: N-type AlGaN layer • For the N-AlGaN LED: • More holes transport to the QWs next to the n-side layer due to the absence of P-AlGaN. • The amount of electrons in the active region also increases, despite that the concentration of electrons in the last QW is reduced. • As a result, the radiative recombination rates are more uniform in QWs and almost all QWs can contribute to output power. Thus, the IQE droop at high current may be improved due to the enhancedelectron and holeinjection efficiencies. Fig. 20.(a) Hole and (b) electron concentrations of the LEDs with P-AlGaN and N-AlGaN at 120 mA.

  28. Study 6: N-type AlGaN layer • For the N-AlGaN LED: • The electron distribution in the QWs is more uniform, as indicated in the relative position of the quasi-Fermi levels. • For the P-AlGaN LED: • The strong electric field, caused by the polarization effect, lowers the conduction band energy in the last barrier. • This is a negative effect for the confinement of electrons at high carrier density. • The percentages of electron leakage current for the LEDs with P-AlGaN and N-AlGaN are 46.1% and 4.5%, respectively, at 120 mA. Fig. 21.Band diagrams and quasi-Fermi levels of the LEDs with (a) N-AlGaN and (b) P-AlGaN layers at 120 mA.

  29. Study 7: Specific designs on electron blocking layer

  30. Study 7: Specific designs on electron blocking layer • There is a side effect of the usage of EBL  the EBL also acts as a potential barrier for holes in the valence band and impedes the transportation of holes into the active region. • The downward band-bending in the last barrier/EBL interface induced by electrostatic polarization makes this problem even more serious. ◎ Without polarization ◎ With polarization enlarged enlarged (Opt. Lett., vol. 35, p. 3285, 2010)

  31. Study 7: Specific designs on electron blocking layer Main goal of this study: To increase the efficiency of hole injection without losing the blocking capability for electrons.  Plan: Reduce the polarization charge density in the last barrier/EBL interface in order to mitigate downward band-bending of this interface.  Method:Some specific designs on EBL of blue InGaN LEDs are proposed. • Structure A is the original structure. • The structures of structure B, C, and D are identical except for the EBLs. Fig. 22. Schematic diagrams of the original structure and the structures with redesigned EBLs.

  32. Study 7: Specific designs on electron blocking layer Table 3. Surface Charge Density in the “Last-Barrier/EBL” and “Front-EBL/Rear-EBL” Interfaces Fig. 23. Electric fields of various InGaN/GaN structures near the EBLs at 150 mA. The original structure (structure A) possesses much stronger electrostatic field in the last-barrier/EBL interface than the redesigned structures because of high surface charge density.

  33. Study 7: Specific designs on electron blocking layer Fig. 24. Enlarged energy band diagrams near the EBLs of various InGaN/GaN structures in (a) conduction band and (b) valence band at 150 mA. Fig. 25. (a) Hole and (b) electron concentrations in the active regions of various InGaN/GaN structures at 150 mA. The insets show the comparison of cases A and D in LOG scale. • The situation of downward band-bending at the interfaces near the EBL of all the three redesigned structures are slighter than that of the original one, which will lead to the enhancement of hole injection efficiency. • The hole concentrations of the redesigned structures increase obviously, which demonstrates the improvement of hole injection efficiency. Besides, the redesigned structures also have higherelectron concentrations, which indicates that the capability of electron confinement does not suffer from the modifications of EBL.

  34. Study 7: Specific designs on electron blocking layer Fig. 26. (a) L-I curves and (b) IQE as a function of current for various InGaN/GaN structures. • The overall performances of the redesigned structures show significant enhancement as compared with that of the original structure. • Structure D seems to be the most superior one among these redesigned structures due to the reduced efficiency droop and higher light output power at elevated level of current injection.

  35. Study 8: Thin last barrier

  36. Study 8: Thin last barrier Fig. 27. Distribution of hole concentrations of the LEDs with (a) 12-nm and (b) 9-nm last barriers at 55 A/cm2. • ※ Original structure: • Active region: ten pairs of In0.21Ga0.79N/ • GaN (2 nm/12 nm) QWs • EBL: 10-nm-thick p-Al0.10Ga0.90N • Concentration: n-41018 cm–3; p-51017 cm–3 • Device geometry: 245560 m2 • A thinner last-barrier could be beneficial for increasing the hole injection efficiency so that holes can inject into more QWs within the active region. • With better hole injection efficiency, electronleakage can be depressed correspondingly. • The radiative recombination and optical power are enhanced accordingly when thinner last-barrier is adopted. (IEEE Photonics Technol. Lett., vol. 22, p. 1787, 2010) Fig. 28. Distribution of electron concentrations of the LEDs with 12-nm and 9-nm last barriers at 55 A/cm2.

  37. Study 8: Thin last barrier Fig. 29. Radiative recombination rates of the LEDs with 12-nm and 9-nm last barriers at 55 A/cm2. Fig. 30. IQE as a function of current density of the LEDs with 12-nm and 9-nm last barriers. • The radiative recombination rates in the QWs next to the n-side layers greatly enhances when the last barrier is 9 nm because more holes can transport to the QWs. • At high current density, the IQE of the LED with 9-nm last barrier is improved sufficiently due to higher hole injection efficiency and more uniform hole distribution.

  38. Study 8: Thin last barrier Fig. 31. (experimental data) Optical power and forward voltage as a function of current density for InGaN LED with the last barrier thicknesses of 12 nm and 9.6 nm. The experimental results show that, when the thickness of the last barrier decreases from 12 to 9.6 nm, the optical power is markedly enhanced, especially at high current density.

  39. Conclusion Some specific designs on band structure near the active region, including the modifications of barrier material or thickness, the redesigns of the electron blocking layer (EBL), etc., in the blue InGaN LEDs are investigated numerically with the APSYS simulation program. Simulation results show that, with appropriate designs, the efficiency droop may be effectively improved due to the increase of hole injection efficiency, enhancement of blocking capability for electrons, or uniform distribution of carriers in the active region. The methods proposed in this paper are advantageous especially for high current injection.

  40. Thank you for your attention !!

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