Study On The Light Extraction Efficiency Of AlGaN Based Light Emitting Diodes By Designing Nanostructures

AlGaN-based light emitting diodes(LEDs),are new generation solid state light sources,which have many advantages such as energy conversation,long livespan,wide application,tunable wavelength from deep ultraviolet to blue,and so on.It can be appled in various fields such as medical sterilization,industrial curing,plant growth,display and illumination,and so on.Thus,these applications need LEDs have a higher external quantum efficiency(EQE).EQE equals to the product of internal quantum efficiency(IQE)and light extraction efficiency(LEE).IQE of LEDs is decided by the AlGaN crystalline quality,which will not be discussed in this paper.LEE is defined as the proportion of extracted light from device in the total source power.Therefore,there are three problems to enhance LIEE including the total internal reflection(TIR)as AlGaN/air interface,the absorption of both substrate and top metal electrode,and the horizontal propergation of transverse magnetic(TM)modes.The problems of TIR and absorption of electrode are widely solved by using surface roughing approach and fabricating patterned electrode.A metal reflector layer inserted between AlGaN and substrate can solve the problem of substrate absorption.However,the surface morphology of metal reflector would become worse after an annealing process due to its poor stabilization,which leads to a decline of both reflectivity and LEE.The problem of horizontal propagation of TM modes can be solved by fabricating AlGaN nanowire structure.The best structure parameters of nanowire are generally obtained by optimization algorithms.However,these best parameters have small ranges which are not helpful for actual production.Hence,in this paper,the structures of metel reflector,single AlGaN nanwire,and multiple AlGaN nanowires are investigated to enhance LEE of AlGaN-based LEDs,as follows:First,the LEE of Ga N-based blue vertical LEDs has been studied.There are three impacts on wall-plug efficiency(WPE)and LEE:the thickness of Ni,oxygen content in N₂/O₂ mixture annealing ambient atmosphere,the thickness of Ag.The size of Ag grain and and root mean square(RMS)of Ag surface are measured.The impacts of surface topography on optical properties are investigated by Mie scattering theory for Ag grains and surface scattering theory for roughness surface of Ag mirror layer.As a result,a high WPE of 45.7%for Ga N-based blue VLEDs on Si substrates has been achieved at the mean diameter(69 nm)of Ag grains and RMS(2.06 nm)of Ag mirror layers.Second,AlGaN nanowire UV LEDs are studied by using the finite-difference time-domain(FDTD)method,the optical waveguide theory and the Fabry-Perot cavity theory.In order to enhance LEE of UV LEDs,it is necessary to understand the propagating features of light inside a single nanowire.The relationship between the resonance inside the nanowire,the top LEE and the far-field distribution of nanowire UV LEDs,as well as the geometrical parameters of the nanowire and the impact of surrounding index has been discussed.It is shown that a smaller number of the resonant modes is helpful to focus the energy of the guided modes and enhance the induced resonance.However,the impacts of resonance on the far-field patterns can be ignored since the side LEE is still larger than the top LEE due to the optical properties of TM polarized sources.Third,the photonic crystal structure of nanowires is investigated by using planewave expansion method and FDTD method.We analyse the first three optical band gaps of photonic crystals.It is found that,the first photonic band gap can not efficiently inhibit the horizontal propagation of the light,since the radius of the corresponding nanowire are too small to support enough guided modes inside the nanowires.Then only less the radiated modes can be coupled with the guided modes into the vertical direction.As a contrast,both second and third photonic band gaps can be chosen to suppress the horizontal propergation of radiated modes.The study of the number of nanowires shows that,although the radiated modes from the nanowire LED at the edge of the device can not be inhibited by the photonic band gap,the top LEE of device still increase with the increase of the number of nanowires.Finally,this study can improve the fabricated process of blue vertical LEDs and nanowire ultraviolet LEDs.