Investigation On Luminescence Dynamics And Failure Mechanisms Of GaN-Based Light-Emitting Diodes

As the core component of semiconductor lighting technology,light-emitting diodes(LEDs)have broad prospects for developments and applications.With the innovations of technologies and industries,LEDs are facing lots of opportunities and challenges.Following the traditional power LEDs,new LED technologies,such as organic light-emitting diodes(OLEDs),Micro-LEDs,high-power LEDs,and deep ultraviolet(DUV)LEDs,will continue to lead the development of semiconductor lighting and display industry.However,some key problems,such as luminescence efficiency,recombination mechanisms,failure mechanisms,packaging technology,and thermal management based on GaN materials.In this work,the photo-thermal-electro characteristics of AlGaN DUV LEDs,InGaN blue and green LEDs,white LEDs covered with peanut-shaped lenses,and 100-watt-level high-power LED array are explored.Also,temperature and current dependent carrier transfer and recombination luminescence dynamics model of GaN-based LEDs is established.Besides,based on failure mechanisms of high-power white LED(1 W),a 100-watt-level high-power LED array light source is designed.The main results and innovations are depicted as follows:1.Study on temperature characteristics of AIGaN-based DUV LED devices.Luminous and electrical properties of 275 nm DUV LED in the temperature range of 20?340 K are analyzed.At 20?300 K,spectral characteristics,EQE-I curves and?-? characteristic curves are investigated.Results show that deep-level radiative recombination within quantum wells and p-type layer is the main factor of the formation of parasitic peaks at 310 and 400 nm.At 280?340 K,considering the relationship between characteristic temperature and external quantum efficiency(EQE),we roughly evaluate effects of defect-related Shockley-Read-Hall(SRH)non-radiative recombination,radiative recombination,Auger recombination,and carrier leakage at different current levels.The leakage carriers,which escape from quantum wells to p-type layer,play an important role at high-temperature and high-current cases.Hence,the leakage current term f(n)is introduced to improve the traditional ABC model.We further calculate the recombination rate and the ratio of four carrier recombination processes,and draw a conclusion that numerous defects within quantum wells are mainly responsible for the low EQE and poor thermal stability.In addition,high-energy and high-concentration carrers will overfly from quantum wells through Auger process and defect-related tunneling effect,resulting in the EQE-droop.Finally,we establish a temperature and current dependent carrier transfer and recombination dynamics model to further summarize the luminescence mechanisms of AlGaN-based LEDs.2.Investigation on mechanisms of the earrier recombination in InGaN LEDs.The discrepancy of luminescence characteristics of blue and green LEDs possibly stems from the difference of concentration of In contents.During current-stress processes,EQEs of In-rich green LEDs decrease significantly with the increasing stress time,while those of In-dilute blue LEDs behave a rising trend,especially in the initial 25 h.The decreasing EQEs can be attributed to the enhancement of non-radiative recombination in the active region when green LEDs are subjected to high-current stress,meanwhile the earrier localization effect caused by the inhomogeneous distribution of In clusters is suppressed.Moreover,the evolution of luminous intensity,peak energy,and full width at half maxima(FWHM)of the micro-area on the surface of blue and green LED within 80 h are determined by using microscopic hyperspectral imaging technology.Point defects and the carrier localization effect contribute together to the InGaN-based LEDs.High EQE of In-dilute blue LEDs is mainly caused by the enhancement of localized states and the reduction in point defects.Finally,a phenomenological model based on the competition between non-radiative recombination centers and localized states in quantum wells is proposed to illustrate the divergent behaviors in the emission of InGaN LEDs.3.Failure mechanisms of white LEDs(1 W)formed by YAG:Ce3+phosphor and incorporation of blue LED source during 3840 h current-stress aging.Peanut-shaped silicone epoxy lens improves the luminous uniformity of white LEDs.Three different current-stress conditions are considered,namely 350 mA(CSA-1),550 mA(CSA-2),and 750 mA(CSA-3).Severe optical power decay under CSA-3 indicates that the comprehensive performance of the chip and packaging materials will degrade most seriously at high current stress.We conjecture that point defects play a negative role in reducing the luminous performance of the chips.However,the Mg dopants reactivation and the annealing effect are of great benefit to improving the optical power in the initial aging stage.The weak variation of voltages means that Ohmic contacts changed insignificantly during the aging process.After 3840 h aging,the maximum visible angles remain at ±60°,but the yellow-browning of silicone lens under CSA-2 performs most severely among those of lenses in all aged cases,leading to a significant drop in transmissivity.Remarkable darkening on the surface of silver-coated reflective layer under CSA-3 exhibits lowest reflectivity in comparison with the others.Results reflect that the silver-coated reflective layer causes more serious optical degradation,because of absorption of the impinging light by dark areas,and elements C,O,Mg and Si associated with the darkening substances.4.Design and analysis of 100-watt high-power LED array source.Based on the study of 1 W white LED,we employ one AlGalnP red LED and white LED excited by blue chip and YAG:Ce+Gd3+ fluorescent ceramic to simulate results of correlated color temperature(CCT),color rendering index(CRI),luminous efficacy of radiation(LER),and wall-plug efficiency(WPE).The ratio of red and white LED chips is set to 1:3,considering the experimental values and setting threshold values of 3000 K<CCT<5000 K,CRI>80,and LER>300 lm/W.The normalized spectra at different currents(50 mA,350 mA,and 750 mA)and the angular distributions of luminous intensity,CCT,CRI,and yellow-blue ratio(YBR)at 350 mA are demonstrated.Results imply that red LED can effectively improve the color rendering for high-power LED light source.Junction temperature of chips,surface temperature of fluorescent ceramic,and surface temperature of transparent quartz glass are discussed in detail.Thermal results reveal that the thermal coupling effect among the multi-chips will significantly elevate the junction temperature in comparison with self-heating,especially for white LEDs encapsulated with fluorescent ceramic.The chip temperature rise in each area of high-power LED array indicates that high temperature locates at the middle units.However,compared with the thermal coupling temperature difference(9?)between red and white LEDs in individual packaging unit,those in all 4x4 array are located at 2.24?5.14?.Besides,the temperature difference(10.4?)on the quartz glass surface of the former is much higher than that(5.14)on the quartz glass surface of the latter.This result states that the partitioning method of high-power LED array can enhance the uniformity of heat distribution for both chips and packaging materials,and in consequence,can optimize the thermal management technology.