Electroluminescence enhancement in InGaN light-emitting diode during the electrical stressing process T. T. Chen,1,* C. P. Wang,1 H. K. Fu,1 P. T. Chou,1,2 and S.-P. Ying3 2
1 Electronics and Optoelectronics Research Laboratories, ITRI, Chutung, Hsinchu 310, Taiwan Department of Engineering Science and Ocean Engineering, Naitonal Taiwan University, Taipei 106, Taiwan 3 Department of Optoelectronic System Engineering, Minghsin University of Science & Technology, Hsinchu 304, Taiwan * [email protected]
Abstract: This study of the optoelectronic properties of blue light-emitting diodes under direct current stress. It is found that the electroluminescence intensity increases with duration of stress, and the efficiency droop curves illustrated that the peak-efficiency and the peak-efficiency-current increases and decreases, respectively. We hypothesize that these behaviors mainly result from the increased internal quantum efficiency. ©2014 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (250.0250) Optoelectronics.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15.
G. Harbers, S. J. Bierhuizen, and M. R. Krames, “Performance of high power light emitting diodes in display illumination applications,” J. Display Technol. 3(2), 98–109 (2007). L. Liu, M. Ling, J. Yang, W. Xiong, W. Jia, and G. Wang, “Efficiency degradation behaviors of current/thermal co-stressed GaN-based blue light emitting diodes with vertical-structure,” J. Appl. Phys. 111(9), 093110 (2012). L.-B. Chang, M.-J. Lai, R.-M. Lin, and C.-H. Huang, “Effect of electron leakage on efficiency droop in widewell InGaN-based light-emitting diodes,” Appl. Phys. Express 4(1), 012106 (2011). Y.-K. Kuo, J.-Y. Chang, and M.-C. Tsai, “Enhancement in hole-injection efficiency of blue InGaN light-emitting diodes from reduced polarization by some specific designs for the electron blocking layer,” Opt. Lett. 35(19), 3285–3287 (2010). Y. Yang, X. A. Cao, and C. H. Yan, “Rapid effciency roll-off in high-quality green light-emitting diodes on freestanding GaN substrates,” Appl. Phys. Lett. 94(4), 041117 (2009). J. Wang, L. Wang, W. Zhao, Z. Hao, and Y. Luo, “Understanding efficiency droop effect in InGaN/GaN multiple-quantum-well blue light-emitting diodes with different degree of carrier localization,” Appl. Phys. Lett. 97(20), 201112 (2010). S.-F. Yu, R.-M. Lin, S.-J. Chang, and F.-C. Chu, “Efficiency droop characteristics in InGaN-based near ultraviolet-to-blue light-emitting diodes,” Appl. Phys. Express 5(2), 022102 (2012). Y. C. Shen, G. O. Mueller, S. Watanabe, N. F. Gardner, A. Munkholm, and M. R. Krames, “Auger recombination in InGaN measured by photoluminescence,” Appl. Phys. Lett. 91(14), 141101 (2007). J. Xu, M. F. Schubert, A. N. Noemaun, D. Zhu, J. K. Kim, E. F. Schubert, M. H. Kim, H. J. Chung, S. Yoon, C. Sone, and Y. Park, “Reduction in efficiency droop, forward voltage, ideality factor, and wavelength shift in polarization-matched GaInN/GaInN multi-quantum-well light-emitting diodes,” Appl. Phys. Lett. 94(1), 011113 (2009). J.-Y. Chang and Y.-K. Kuo, “Influence of polarization-matched AlGaInN barriers in blue InGaN light-emitting diodes,” Opt. Lett. 37(9), 1574–1576 (2012). X. A. Cao, J. M. Teetsov, M. P. D’Evelyn, D. W. Merfeld, and C. H. Yan, “Electrical characteristics of InGaN/GaN light-emitting diodes grown on GaN and sapphire substrates,” Appl. Phys. Lett. 85(1), 7–9 (2004). W.-H. Chang, A. T. Chou, W. Y. Chen, H. S. Chang, T. M. Hsu, Z. Pei, P. S. Chen, S. W. Lee, L. S. Lai, S. C. Lu, and M.-J. Tsai, “Room-temperature electroluminescence at 1.3 and 1.55μm from Ge/Si self-assembled quantum dots,” Appl. Phys. Lett. 83(14), 2958–2960 (2003). J. I. Pankove, Optical Processes in Semiconductors (Prentice-Hall, 1991), Chap. 7. M. Leroux, N. Grandjean, B. Beaumont, G. Nataf, F. Semond, J. Massies, and P. Gibart, “Temperature quenching of photoluminescence intensities in undoped and doped GaN,” J. Appl. Phys. 86(7), 3721 (1999). C.-P. Wang, T.-T. Chen, H.-K. Fu, T.-L. Chang, and P.-T. Chou, “Transient analysis of partial thermal characteristics of multistructure power LEDs,” IEEE Trans. Electron. Dev. 60(5), 1668–1672 (2013).
#207720 - $15.00 USD Received 27 May 2014; revised 18 Jul 2014; accepted 22 Jul 2014; published 11 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1328 | OPTICS EXPRESS A1328
1. Introduction High-efficiency InGaN-based light-emitting diode (LED) has been used in solid-state illumination and display products [1]. The lifetime test of an LED is important for ensuring its reliability. When the LED is stressed under fixed current and ambient temperature, it is easy to observe a meaningful enhancement of the electroluminescence (EL) spectra which has been reported in previous literature [2]. However, the physical origin of the enhancement phenomenon remains unclear. Another important issue of InGaN-based LEDs is “efficiency droop”, i.e., the efficiency of conventional LEDs reaches its peak at low current density and monotonically drops with increasing the drive current. Since the enhanced EL is inherently an efficiency issue, hence, the physics origin of these phenomena should be highly related. The efficiency droop is mainly caused by the non-radiative recombination processes. Hypothesized mechanisms include electron leakage [3], poor hole injection efficiency [4], electron overflow [5], carrier delocalization [6,7], and Auger non-radiative recombination [8]. Both electron leakage and poor hole injection efficiency have strong relationship with the quantum confined Stark effect (QCSE) induced by the piezoelectric polarization [9,10]. These leakage electrons and not injected holes may recombine outside the active layers through the non-radiative process. This study analyzed optoelectronic properties in direct-current stressed LED samples. Experiments showed that the EL intensity and the reverse leakage current increase simultaneously. In previous reports of Ref. 2, the Mg-dopant activation of p-type GaN increased both hole injection efficiency and conductivity, which respectively caused the EL enhancement and the reverse leakage current increase. However, in our experiments, the identical capacitance-voltage (C-V) curves obtained imply that the increase in carrier concentration is not significant (not shown here). The increased thermal activation energy indicates a change in the active region after application of the stress. Thus, we hypothesize that the EL enhancement of our sample results from the increased internal quantum efficiency (IQE) which is mainly attributable to the annealing effect. The annealing effect reduces the defect density, especially in active regions. Since the non-radiative recombination channels are impeded, free electrons are more easily injected into the radiative states and then overflow the potential minimum of the radiative states (accumulation effect). The linear dependency trend in the integrated EL intensity versus current density curves further support our inferences. Finally, experiments were performed under different stressing conditions to investigate the mechanism of EL enhancement. 2. Experiment The 1 mm2 InGaN-based LED sample used in the experiment was grown on a (0001) sapphire substrate. The LED structure comprised of a 3 um undoped GaN layer, a 2 um highly conductive n-type GaN layer, an InGaN/GaN (~3 nm/10 nm) multiple quantum wells (MQWs) active layer, and a 200 nm p-type GaN layer (Mg doping concentration about 1.0x1017 cm−3). The LED sample was soldered on the Cu-Al base and stressed under fixed direct current in an isothermal chamber. The optoelectronic characteristics were measured before and during the stressing period. The EL measurement was performed with a Keithley 2430 power supply with a ðxed pulse width of 20 ms to provide a high signal-to-noise ratio, and the optical measurement was performed with an integrating sphere (IS;CAS 140B). The current-voltage (I-V) characteristics were measured with a Keithley 236 source measure unit. 3. Results and discussion The LED sample was stressed under fixed direct current of 100 A/cm2 in an isothermal chamber (25 °C) for up to 1574 hours. Before and after different stressing time the sample was measured at different current (0.1, 0.3, 0.5, 5 and 50 A/cm2) at room temperature. Figure 1(a) shows the time-dependent EL intensities, which were normalized to that of the unstressed
#207720 - $15.00 USD Received 27 May 2014; revised 18 Jul 2014; accepted 22 Jul 2014; published 11 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1328 | OPTICS EXPRESS A1329
sample for each current level before and after application of stress. The EL intensity approached saturation during the first 100 hours. The enhancement at low current density exceeded that at high current density. It is because the accumulation effect is particularly evident at low injection current, hence, the degree of the enhancement is diminishes at the higher injection current. Figure 1(b) compares the EL spectra at low and high current density. The main peaks of the EL spectra were attributable to the band-edge transitions, and no defect signals were observed. Notably, although the EL intensity substantially increased, the peak position and line width were unaltered for each current level, which indicates that the radiative recombination channel remain the same after the stress.
Fig. 1. (a) Normalized EL intensity and (b) EL spectra of the sample measured after different stressing durations.
Figure 2(a) shows the efficiency droop behavior, which was analyzed to identify the enhancement mechanism. The deviation in the efficiency droop curve was large at low current density but small at high current density as the enhancement variation of EL intensity. These behaviors were not attributable to improved extraction efficiency, which showed similar enhancement at all current level. Therefore, these behaviors should be the result from the improved IQE. According to the previous report [2] and the unchanged radiative recombination channels in this study, we infer that the enhanced IQE should be resulted from the dopant activation of p-type GaN and the annealing effect of the active region, respectively. However, the identical C-V curve indicates that the EL enhancement caused by the annealing effect is larger than that caused by dopant activation. The annealing effect reduces defect density, hinders the non-radiative recombination channels, and then enhances electron injection into the radiative states. The electron accumulation process in the potential minima of radiative state is particularly evident in the low current region, which causes the non-uniform improvement in IQE. As stressing time increases, the defect density further decreases, which causes a peak-efficiency-current shift toward to the low drive current and the peak-efficiency increases as shown in the Fig. 2(b). The IQE can be described by the following equation: IQE = Bn 2 / ( An + Bn 2 + Cn 3 ) ,
(1)
where the confections A, B, C are non-radiative, radiative, and Auger non-radiative recombinations, respectively, and where n is the carrier density. Because the peak-efficiencycurrent is smaller than 3 A/cm2. In this low current region (
1 Electronics and Optoelectronics Research Laboratories, ITRI, Chutung, Hsinchu 310, Taiwan Department of Engineering Science and Ocean Engineering, Naitonal Taiwan University, Taipei 106, Taiwan 3 Department of Optoelectronic System Engineering, Minghsin University of Science & Technology, Hsinchu 304, Taiwan * [email protected]
Abstract: This study of the optoelectronic properties of blue light-emitting diodes under direct current stress. It is found that the electroluminescence intensity increases with duration of stress, and the efficiency droop curves illustrated that the peak-efficiency and the peak-efficiency-current increases and decreases, respectively. We hypothesize that these behaviors mainly result from the increased internal quantum efficiency. ©2014 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (250.0250) Optoelectronics.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15.
G. Harbers, S. J. Bierhuizen, and M. R. Krames, “Performance of high power light emitting diodes in display illumination applications,” J. Display Technol. 3(2), 98–109 (2007). L. Liu, M. Ling, J. Yang, W. Xiong, W. Jia, and G. Wang, “Efficiency degradation behaviors of current/thermal co-stressed GaN-based blue light emitting diodes with vertical-structure,” J. Appl. Phys. 111(9), 093110 (2012). L.-B. Chang, M.-J. Lai, R.-M. Lin, and C.-H. Huang, “Effect of electron leakage on efficiency droop in widewell InGaN-based light-emitting diodes,” Appl. Phys. Express 4(1), 012106 (2011). Y.-K. Kuo, J.-Y. Chang, and M.-C. Tsai, “Enhancement in hole-injection efficiency of blue InGaN light-emitting diodes from reduced polarization by some specific designs for the electron blocking layer,” Opt. Lett. 35(19), 3285–3287 (2010). Y. Yang, X. A. Cao, and C. H. Yan, “Rapid effciency roll-off in high-quality green light-emitting diodes on freestanding GaN substrates,” Appl. Phys. Lett. 94(4), 041117 (2009). J. Wang, L. Wang, W. Zhao, Z. Hao, and Y. Luo, “Understanding efficiency droop effect in InGaN/GaN multiple-quantum-well blue light-emitting diodes with different degree of carrier localization,” Appl. Phys. Lett. 97(20), 201112 (2010). S.-F. Yu, R.-M. Lin, S.-J. Chang, and F.-C. Chu, “Efficiency droop characteristics in InGaN-based near ultraviolet-to-blue light-emitting diodes,” Appl. Phys. Express 5(2), 022102 (2012). Y. C. Shen, G. O. Mueller, S. Watanabe, N. F. Gardner, A. Munkholm, and M. R. Krames, “Auger recombination in InGaN measured by photoluminescence,” Appl. Phys. Lett. 91(14), 141101 (2007). J. Xu, M. F. Schubert, A. N. Noemaun, D. Zhu, J. K. Kim, E. F. Schubert, M. H. Kim, H. J. Chung, S. Yoon, C. Sone, and Y. Park, “Reduction in efficiency droop, forward voltage, ideality factor, and wavelength shift in polarization-matched GaInN/GaInN multi-quantum-well light-emitting diodes,” Appl. Phys. Lett. 94(1), 011113 (2009). J.-Y. Chang and Y.-K. Kuo, “Influence of polarization-matched AlGaInN barriers in blue InGaN light-emitting diodes,” Opt. Lett. 37(9), 1574–1576 (2012). X. A. Cao, J. M. Teetsov, M. P. D’Evelyn, D. W. Merfeld, and C. H. Yan, “Electrical characteristics of InGaN/GaN light-emitting diodes grown on GaN and sapphire substrates,” Appl. Phys. Lett. 85(1), 7–9 (2004). W.-H. Chang, A. T. Chou, W. Y. Chen, H. S. Chang, T. M. Hsu, Z. Pei, P. S. Chen, S. W. Lee, L. S. Lai, S. C. Lu, and M.-J. Tsai, “Room-temperature electroluminescence at 1.3 and 1.55μm from Ge/Si self-assembled quantum dots,” Appl. Phys. Lett. 83(14), 2958–2960 (2003). J. I. Pankove, Optical Processes in Semiconductors (Prentice-Hall, 1991), Chap. 7. M. Leroux, N. Grandjean, B. Beaumont, G. Nataf, F. Semond, J. Massies, and P. Gibart, “Temperature quenching of photoluminescence intensities in undoped and doped GaN,” J. Appl. Phys. 86(7), 3721 (1999). C.-P. Wang, T.-T. Chen, H.-K. Fu, T.-L. Chang, and P.-T. Chou, “Transient analysis of partial thermal characteristics of multistructure power LEDs,” IEEE Trans. Electron. Dev. 60(5), 1668–1672 (2013).
#207720 - $15.00 USD Received 27 May 2014; revised 18 Jul 2014; accepted 22 Jul 2014; published 11 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1328 | OPTICS EXPRESS A1328
1. Introduction High-efficiency InGaN-based light-emitting diode (LED) has been used in solid-state illumination and display products [1]. The lifetime test of an LED is important for ensuring its reliability. When the LED is stressed under fixed current and ambient temperature, it is easy to observe a meaningful enhancement of the electroluminescence (EL) spectra which has been reported in previous literature [2]. However, the physical origin of the enhancement phenomenon remains unclear. Another important issue of InGaN-based LEDs is “efficiency droop”, i.e., the efficiency of conventional LEDs reaches its peak at low current density and monotonically drops with increasing the drive current. Since the enhanced EL is inherently an efficiency issue, hence, the physics origin of these phenomena should be highly related. The efficiency droop is mainly caused by the non-radiative recombination processes. Hypothesized mechanisms include electron leakage [3], poor hole injection efficiency [4], electron overflow [5], carrier delocalization [6,7], and Auger non-radiative recombination [8]. Both electron leakage and poor hole injection efficiency have strong relationship with the quantum confined Stark effect (QCSE) induced by the piezoelectric polarization [9,10]. These leakage electrons and not injected holes may recombine outside the active layers through the non-radiative process. This study analyzed optoelectronic properties in direct-current stressed LED samples. Experiments showed that the EL intensity and the reverse leakage current increase simultaneously. In previous reports of Ref. 2, the Mg-dopant activation of p-type GaN increased both hole injection efficiency and conductivity, which respectively caused the EL enhancement and the reverse leakage current increase. However, in our experiments, the identical capacitance-voltage (C-V) curves obtained imply that the increase in carrier concentration is not significant (not shown here). The increased thermal activation energy indicates a change in the active region after application of the stress. Thus, we hypothesize that the EL enhancement of our sample results from the increased internal quantum efficiency (IQE) which is mainly attributable to the annealing effect. The annealing effect reduces the defect density, especially in active regions. Since the non-radiative recombination channels are impeded, free electrons are more easily injected into the radiative states and then overflow the potential minimum of the radiative states (accumulation effect). The linear dependency trend in the integrated EL intensity versus current density curves further support our inferences. Finally, experiments were performed under different stressing conditions to investigate the mechanism of EL enhancement. 2. Experiment The 1 mm2 InGaN-based LED sample used in the experiment was grown on a (0001) sapphire substrate. The LED structure comprised of a 3 um undoped GaN layer, a 2 um highly conductive n-type GaN layer, an InGaN/GaN (~3 nm/10 nm) multiple quantum wells (MQWs) active layer, and a 200 nm p-type GaN layer (Mg doping concentration about 1.0x1017 cm−3). The LED sample was soldered on the Cu-Al base and stressed under fixed direct current in an isothermal chamber. The optoelectronic characteristics were measured before and during the stressing period. The EL measurement was performed with a Keithley 2430 power supply with a ðxed pulse width of 20 ms to provide a high signal-to-noise ratio, and the optical measurement was performed with an integrating sphere (IS;CAS 140B). The current-voltage (I-V) characteristics were measured with a Keithley 236 source measure unit. 3. Results and discussion The LED sample was stressed under fixed direct current of 100 A/cm2 in an isothermal chamber (25 °C) for up to 1574 hours. Before and after different stressing time the sample was measured at different current (0.1, 0.3, 0.5, 5 and 50 A/cm2) at room temperature. Figure 1(a) shows the time-dependent EL intensities, which were normalized to that of the unstressed
#207720 - $15.00 USD Received 27 May 2014; revised 18 Jul 2014; accepted 22 Jul 2014; published 11 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1328 | OPTICS EXPRESS A1329
sample for each current level before and after application of stress. The EL intensity approached saturation during the first 100 hours. The enhancement at low current density exceeded that at high current density. It is because the accumulation effect is particularly evident at low injection current, hence, the degree of the enhancement is diminishes at the higher injection current. Figure 1(b) compares the EL spectra at low and high current density. The main peaks of the EL spectra were attributable to the band-edge transitions, and no defect signals were observed. Notably, although the EL intensity substantially increased, the peak position and line width were unaltered for each current level, which indicates that the radiative recombination channel remain the same after the stress.
Fig. 1. (a) Normalized EL intensity and (b) EL spectra of the sample measured after different stressing durations.
Figure 2(a) shows the efficiency droop behavior, which was analyzed to identify the enhancement mechanism. The deviation in the efficiency droop curve was large at low current density but small at high current density as the enhancement variation of EL intensity. These behaviors were not attributable to improved extraction efficiency, which showed similar enhancement at all current level. Therefore, these behaviors should be the result from the improved IQE. According to the previous report [2] and the unchanged radiative recombination channels in this study, we infer that the enhanced IQE should be resulted from the dopant activation of p-type GaN and the annealing effect of the active region, respectively. However, the identical C-V curve indicates that the EL enhancement caused by the annealing effect is larger than that caused by dopant activation. The annealing effect reduces defect density, hinders the non-radiative recombination channels, and then enhances electron injection into the radiative states. The electron accumulation process in the potential minima of radiative state is particularly evident in the low current region, which causes the non-uniform improvement in IQE. As stressing time increases, the defect density further decreases, which causes a peak-efficiency-current shift toward to the low drive current and the peak-efficiency increases as shown in the Fig. 2(b). The IQE can be described by the following equation: IQE = Bn 2 / ( An + Bn 2 + Cn 3 ) ,
(1)
where the confections A, B, C are non-radiative, radiative, and Auger non-radiative recombinations, respectively, and where n is the carrier density. Because the peak-efficiencycurrent is smaller than 3 A/cm2. In this low current region (
