Simulation Of N-polar AlGaN-based Deep-ultraviolet LED Device

AlGaN-based deep ultraviolet(DUV)light emitting diodes(LEDs)have the advantages of environmental protection,compact in size,long lifetime and adjustable emission wavelength compared with traditional ultraviolet light.Therefore,AlGaN-based DUV LEDs have huge development potential and market expectation in air and water purification,biomedical detection,solid-state lighting and other fields.However,AlGaN-based DUV LEDs still suffer from low external quantum efficiency(EQE),poor carrier confinement,difficult p-type doping,and complicated fabrication processes,which make the development of nitride-based DUV devices into a bottleneck.Therefore,to improve the luminous efficiency of DUV LED devices,researchers optimize the device performance by improving the device structure.At present,most of the AlGaN-based UV LEDs prepared by researchers are of metal-polar,however,it has been reported that the opposite polarized electric field of N-polar devices is more conducive to carrier injection into the active region and has stronger carrier confinement ability than metal-polar devices.Based on the original experimental of our group,this paper conducts a simulation study on AlGaN-based DUV LEDs.First,a DUV LED structure with a p-type layer consisting of an EBL and a HIL is used as a reference structure,and a graded p-AlGaN structure that simultaneously replaces the EBL and HIL is adopted as a comparative structure.In addition,on the basis of the graded p-AlGaN,three types of superlattice structure and a tunnel junction structure were used to further optimize the optoelectronic properties of the device.The main contents of the paper are as follows:1.Firstly,this paper summarizes the research progress and bottlenecks of AlGaN-based UV LEDs.The main factors restricting the development of AlGaN-based UV LEDs are analyzed and the corresponding solutions are proposed.Next,we introduce the polarization characteristics of the III-nitrides.On this basis,we briefly describe the advantages of N-polar III-nitride light-emitting devices compared to metal-polar light-emitting devices,and then analyze the feasibility of fabrication of DUV LEDs.2.This paper briefly describes the APSYS software used in the simulation in this paper and introduce the physical models and basic equations that mainly includes.In addition,the main use processes of the APSYS software are outlined.Finally,a model of the N-polar DUV LED structure required for the simulation is constructed,and the main device parameters in the simulation are determined.3.N-polar AlGaN-based DUV LED devices with graded p-Al_xGa_1-xN(x=0.65–0.75)structure were studied by APSYS software.The results show that the valence band of the graded p-AlGaN structure is smooth and the conduction band is raised,which is conducive to hole injection and prevent electrons from overflowing the active region.Therefore,the graded p-AlGaN structure can act as both EBL and HIL.Compared with the conventional DUV LED structure using both structures,the active region of DUV LED with a graded p-AlGaN structure has stronger electron binding ability and higher hole injection efficiency at high current density,while its peak IQE is higher and the IQE droop effect is smaller.4.On this basis,this paper uses the superlattice and tunnel junction to further optimize the N-polar AlGaN-based DUV LED.The results show that the active region of the LED with the superlattice structure has a stronger confinement ability for electrons and improves the injection of holes.At the same time,the introduction of the superlattice can reduce the turn-on voltage and operating resistance of the device,and improve the efficiency of the device when it operates at a high current density.The LED device with the tunnel junction has a more uniform distribution of carrier concentration in the active region and better uniformity of light emission.At the same time,this structure effectively suppresses the non-radiative recombination inside the device on the basis of improving the radiative recombination rate of the whole device,so that it has a higher peak IQE and lower efficient droop.In addition,this structure has lower device resistance and lower turn-on voltage.