Study Of EQE Droop In GaN Based LED

LED has been considered as the third generation of energy efficient green solid-state light source.In recent years,LEDs light source has been successfully commercialized and enters our daily life because of the continually improving efficiency and lowering cost.LED has gradually replaced the incandescent lighting and will eventually eliminate fluorescent lamps and other lighting products.At present,the most serious problem of GaN-based quantum well LED is the gradual decrease of the power efficiency as the injection current increases,which was known as the efficiency droop.Especially for high-power LEDs,the efficiency droop is a severe problem.Therefore,understanding and mitigating efficiency droop is especially critical to achieve LED solid-state lighting applications.Mechanisms for efficiency droop in GaN-based LED are very complex,which can be divided into internal losses and carrier leakage outside quantum well.The internal loss is mainly due to carrier localization,auger recombination and other reasons.The carrier leakage in active regions,was mainly caused by the existence of polarization field,low hole injection efficiency,carrier overshoot,and defect assisted tunneling.In this paper,we seek to investigate the efficiency droop phenomena more comprehensively by examining both the temperature dependence of EQE and emission spectra of ultraviolet to green LEDs based on InGaN/GaN multiple quantum wells.The main works and results are as follows:First,the electroluminescence spectra of a series of LEDs at different temperatures(25K-300K)were systematically tested.For blue and green LEDs,EQE curves shows"S" shape as current densities increase(0.5 A/cm2～550A/cm2)at low temperatures.That is,the EQE descends first and then rises,finally,descends with the increase of injection current density.The first decrease of EQE is caused by the carrier delocalization.With the injection current increasing,carriers begin to fill and occupied energy state in quantum wells,and EQE begins to rise.The last decrease of the EQE is the result of the efficiency droop.Secondly,we propose the carrier recombination model of the Bulk-Material toQuantum-Well in the quantum well.That is,with the increase of injected current,the Fermi level rises to a certain height and then carriers begin to occupy the energy states above quantum wells.Carriers in high energy states will transit to the quantum wells with indirect recombination at a certain probability rather than recombine with holes in the valence band directly.And the most part of the carriers transition to the lower defect state for non-radiative recombination,therefore,leading to EQE droop with the increase of injection current.Finally,for the near-ultraviolet LED,there are two different declining trends in EQE curve at low temperature,that is,it rises sharply at a small current(0.05 A/cm2～75 A/cm2)and then decreases sharply.Then,the eff1ciency decreases slower as injection current continue to increase.There are two different primary mechanisms that determine the efficiency droop at different current region.According to the current and temperature dependence peak energy,we believe that the main reason for the first decreasing is the delocalization of the energy states.Thelocalized states ares sensitive to the temperature,so the delocalization occurs as injection current increase and the subsequent increasing temperature.In the higher current region,the external quantum descent slower relatively,which is suite for Bulk-Material to Quantum-Well indirect combination model.In this paper,the main mechanism of the efficiency droop of near-UV to green InGaN/GaN-based quantum well LEDs is systematically studied.Based on the electroluminescence experimental data,the Bulk-Material to Quantum-Well indirect combination model was established to study the mechanism ofefficiency Droop theoretically.The proposed model is helpful for us to understand the relative radiation and nonradiative recombination probability of carriers in the quantum well.The effects of carrier distribution and transport of on the efficiency droop at high injection current density are elucidated in detail.