| In the layered structure of semiconductor light-emitting devices,a large number of photons are bound in the Light-emitting devices in the form of waveguide mode,at which time the microstructure can be used to extract the photon to enhance the luminous efficiency.Here,the microstructure is composed of two or more kinds of materials in space according to a certain pattern,and its characteristic size is quite even smaller than the electromagnetic wavelength.In recent years,microstructures have been applied in many fields,such as efficient utilization of energy,thermal control of spacecraft and new artificial materials.When the microstructure interacts with the radiant photon,the near-field electromagnetic effects,such as surface plasmon polaritons,microcavity resonant effects and photon bandgap effects,are produced.In-depth study of the interaction of different microstructures and radiation photons,and understanding of the mechanism of the near-field electromagnetic effects,can be benefit to us to effectively use microstructures to control the spectral radiation characteristics of optoelectronic devices,to increase the energy efficiency of devices.In this paper,based on the study of the spectral radiation characteristics of the microstructured metal grating,combined with the problem of LED light extraction,the microstructure is applied to the LED chip to increase the light extraction efficiency.The specific research contents mainly include the following aspects:1.Study on spectral properties of microstructure gratingMetal/dielectric multilayer grating microstructures have unique spectral properties.For example,a single-layer metal film grating named two-dimensional hole array(2DHAs)has a multilayer structure characteristic of an insulator/metal/insulator(IMI),which shows extraordinary optical transmission(EOT)phenomenon,and its mechanism is surface plasmon polaritons(SPPs).The metal/dielectric multi-layer structure has many kinds of arrangement,the different arrangement corresponds to different spectral properties,and the mechanism may be different.In this paper,the finite difference time domain(FDTD)numerical modeling for the one-dimensional metal/dielectric grating microstructure is carried out,and the spectral absorption of the grating microstructure and the distribution of the electromagnetic energy in the microstructure are obtained by calculation.Then,compared with the spectral properties of the metal/dielectric layered structure without grating,the grating microstructure shows the enhanced absorption characteristics of the specific wavelength bands.The spectral properties of two-dimensional metal/dielectric/metal(MIM)grating surface microstructure are studied in detail.The effects of several structural parameters on the spectral properties and the mechanism of the specific spectral phenomena are analyzed.It has been found that the MIM grating microstructure shows strong absorption of more than 90%at specific wavelength due to the simultaneous excitation of the Internal SPPs and the external SPPs.2.Numerical study of microstructure grating used to enhance the light extraction efficiency of LEDIn the LED chip,because of the total internal reflection,the photons generated from the electron-hole radiation recombination from the active region are difficult to escape from the chip.Especially for those chips that are made up of materials with very large refractive index differences,photons are more difficult to radiate out of them.Thus,most of the photons are limited in the chip,making the light extraction efficiency of the chip very low.In the existing literatures,single microstructure is used,such as surface roughening,texturing,photonic crystal structure and metal nanoparticles,etc.,and the use of composited microstructures is rarely involved.Based on the analysis of the problems existing in the LED light extraction,a two-dimensional flip-chip LED model with a built-in metal Ag grating in the attenuation length of the evanescent wave in the total internal reflection is proposed firstly,and the efficiency of the light extraction of LED is calculated by FDTD method.By comparing with the light extraction efficiency of the chip without grating,light extraction enhancement factor of the chip with grating is obtained.Furthermore,the metal/dielectric combination grating is used to enhance the the light extraction.It is found that due to the excition of SPPs on the surface of metallic grating and the scattering of SPPs wave on the surface of dielectric grating,in the long visible wavelength bands,the light extraction efficiency of metal/dielectric combination grating chips is several times higher than that of only a metal grating or only a dielectric grating chips.3.The application of microstructured gratings with different metal materials in LED light extraction enhancementAs a necessary condition of excitation of the SPPs,the real part of the dielectric function of the material must be negative.Because abundant free electrons are present in the metal,which leads to the metal having a negative real part of its dielectric function,the metal is the best choice of plasmonic material.However,for different metals,because of the differences in the density of the free electrons,the dielectric parameters,including plasma frequency and damping frequency,are different for each of the metals.These changes make the excitation bands of the SPPs and the attenuation lengths of the SPPs different.Meanwhile,the excitation of SPPs also relates to the structural features of the metals and of the surrounding media.Different micro-structural features can also result in a shift in the SPP band.In this paper,by using fixed dimensions for each structural parameter,different metals have been used as the plasmonic metals to analyze the applicability of each of these metals to different bands.4.The absorption characteristics of microstructured metallic gratings in LED light extraction applicationsFor the surface-plasmon-based light emitting diodes(SP-LEDs)with metal microstructure,it is necessary to excite SPPs around the metal microstructure inside the LED chip.SPPs help to enhance the extraction of waveguide mode in the chip,but if the excited SPPs cannot be coupled to the propagation wave to radiate out of the chip,this will lead to a large amount of metal absorption of photons.Therefore,the absorption of metal in SP-LEDs chip is a very important research content,which needs to be analyzed and discussed in depth.In this paper,based on the FDTD method,the numerical simulations of several three-dimensional LED chips with different microstructures are carried out.The effects of different grating combinations,grating types,metallic materials and source polarization on the LED light extraction efficiency have been discussed.In particular,the effects of various factors on the absorption properties of the metal have been studied.The results show that the metal/dielectric combination gratings significantly increase the light extraction efficiency,in the meanwhile,the metallic absorption is also very large,which is several times of the light extraction efficiency.Therefore,the selection of suitable metal and the appropriate structure for LED light extraction is critical. |
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