Research On Electrode Design For Improving Wall-plug Efficiency Of AlGaN-based Deep Ultraviolet LED

AlGaN-based deep ultraviolet light emitting diodes(DUV-LED)have the advantages of low voltage,solid-state,anti-impact,thermal stability,anti-radiation,high efficiency,fast response,long lifetime,environmental friendly and tunable emission wavelength.It has huge potential applications and markets in civilian and military fields,such as sterilization and non-line-of-sight communication.However,in the fabrication of flip-chip DUV-LEDs,the conventional electrode will absorb the DUV photons,leading to the decrease of light extraction efficiency(LEE).The application of high-reflective electrode can significantly improve the LEE of the chip,which is the key factor to achieve high LEE DUV-LEDs in the future.Besides,DUV-LED chips suffer from current crowding effect,which results in increase of heat dissipation and reduction of reliability.Presently,optimizing electrode layout is still the main method to improve current injection efficiency.This paper proposes to improve the LEE of AlGaN-based DUV-LED chips by using high-reflective n-type electrode.By theoretically analyzing the polarization and propagation path of the DUV light emitting from the multiple quantum well region,it can be understand that the proportion of DUV light that may escape from the chips after being reflected by the n-type electrode exceeds 30%.Adopting high-reflective n-type electrode is an important method to enhance the LEE.Based on a systematic analysis of metal materials,this paper proposes to use a thin Cr metal layer and a thick Al metal layer as the contact layer,finally obtained a Cr / Al / Ti / Au metal structure as highreflective n-type electrode.Then,a DUV-LED chip was fabricated with high-reflective n-type electrode.The reflectivity of the electrode can reach up to 85.7% at the 280 nm,the light output power of the corresponding DUV-LED chip is increased by 40.9%,and the external quantum efficiency is enhanced by 25.4%.This paper introduces the basic principles and methods for calculating the current spreading effect of AlGaN-based DUV-LED chips by using COMSOL Multiphysics software,and establishes a simulation model in COMSOL Multiphysics software.The voltage distribution,current density distribution,and heat density distribution of three different electrode layout structures(including interdigitated,stripe,and stripe circles)were simulated,and the reason of current crowding was analyzed.The simulation results shows that the current spreading length and the distance between the electrode center and mesa edge determine the current spreading effect,which provides a new idea for optimizing the electrode layout to improve the current injection efficiency.