Simulation Study Of Weak Polarization Electric Field LEDs

GaN has the advantages of direct band structure,wide bandgap,high electron saturation rate and good chemical stability,so it is the ideal material for the preparation of light emitting diode（LED）.At present,GaN-based LEDs have been widely used in lighting,display and other fields.However,the inherent polarization property of GaN materials will lead to polarization electric field in the order of MV/cm in the materials,which will adversely affect the luminescent performance of LED.For many years,researchers have been working on reducing the polarization electric field intensity in GaN-based LEDs by various methods.Compared with Al GaN,In GaN and other ternary compounds,Al In GaN has one more component regulatory degree and it can be used as the materials of active region in LED to reduce the polarization electric field intensity.At present,there are still many aspects of Al In GaN-based weak polarization electric field LEDs to be further studied,such as the influence of the change of polarization electric field on various recombination mechanisms,the influence of the changed bandgap of quantum barrier on luminescent performance when polarization electric field intensity decreases,the feasibility of Al In GaN as luminescent layers,etc.In this paper,the weak polarization electric field LEDs using Al In GaN as the materials of active region were studied using APSYS software.The main contents are as follows:1.In the blue LEDs with In_0.15Ga_0.85N as the well layers,Al In GaN of different components was used to replace GaN as the barrier layers to construct weak polarization electric field LEDs,and the change of luminescent performance with the polarization electric field intensity was explored.The results show that the luminescent performance of LEDs improves significantly with the decrease of polarization electric field intensity.For the LED structure used in the simulation,with Al_0.1In_0.2Ga_0.7N as the barrier layers,the polarization intensity matching with the well layers can be realized.The polarization electric field intensity decreased from 1.74 MV/cm of the GaN barrier LED to 0.05 MV/cm.The peak internal quantum efficiency of LED increased from 51.7%to 68.5%.The efficiency droop decreased from 21.6%to 6.8%.The reduction of polarization electric field intensity is achieved by increasing In component in the barrier layers,which also leads to the decrease of the bandgap of barrier layers,weakens the restriction effect of quantum wells on carriers,and reduces the carrier concentration and various recombination rates in quantum wells.However,the decrease of auger recombination rate is the most significant,which increases the proportion of radiative recombination in all recombination processes and is also conducive to improving the luminescent performance of LED.2.Based on the blue LED with the optimal barrier layers,the electron blocking layer was optimized.In order to reduce the polarization intensity difference between the barrier layer and the electron blocking layer,compositionally graded Al GaN,Al In GaN and Al In GaN/Al GaN superlattices were respectively used to replace Al GaN as the electron blocking layer.The results show that the luminescent performance of the optimized LEDs is further improved.The LED with Al_0.2In_0.15Ga_0.65N/Al_0.2Ga_0.8N superlattices as the electron blocking layer has the best luminescent performance,the efficiency droop was further reduced from 6.8%to 2.6%.3.In the deep ultraviolet LEDs with Al_0.46Ga_0.54N as the well layers,Al In GaN of different components was used to replace Al_0.6Ga_0.4N as the barrier layers to construct weak polarization electric field LEDs,and the change of luminescent performance with the polarization electric field intensity was explored.The results show that the luminescent performance is affected not only by the polarization electric field intensity,but also by the bandgap of barrier layers.When the bandgap of barrier layers increases,most of the carriers will be concentrated in the quantum well near the p-type region,which increases the intensity and proportion of auger recombination and reduces the luminescent performance.For the LED structure used in the simulation,by using Al_0.64In_0.11Ga_0.25N as the barrier layers,the bandgap of barrier layers can be properly reduced while the polarization intensity matching with the well layers can be realized,and the luminescent performance of the LED can be improved.Compared with the Al_0.6Ga_0.4N barrier LED,the polarization electric field intensity decreased from 0.62MV/cm to 0.01 MV/cm,and the internal quantum efficiency at peak and high current density increased from 54.5%and 35.9%to 57.9%and 40.9%,respectively.4.In the deep ultraviolet LEDs with Al_0.67Ga_0.33N as the barrier layers,Al_0.76In_0.12Ga_0.12N was used to replace Al_0.52Ga_0.48N as the well layers,and the polarization electric field intensity was reduced under the condition of the same wavelength to explore the changes in luminescent performance.The results show that the internal quantum efficiency and light output power of the LED decreases with the reduction of polarization electric field intensity.The reason is that the decrease of polarization electric field intensity leads to Shockley-Read-Hall（SRH）recombination enhancement,which is particularly significant at low current density.With the increase of current density,the internal quantum efficiency gradually approached that of the Al_0.52Ga_0.48N well LED,and the efficiency droop decreased from 29.3%to 20.9%.This is because the effect of SRH recombination is weakened under high current,and the auger recombination and leakage current are inhibited by the decrease of polarization electric field intensity.Therefore,the LED has certain application potential under high current working conditions.