Study Of The Mechanism Of LED Efficiency Enhancement By Periodical Nano-scale Structures

With the intensification of the worldwide energy crisis and the environmental pollution,energy saving becomes an important means to address the energy and environmental issues.Light-emitting diode(LED)regarded as the new green lighting sources are gradually leading the development trend of lighting sources.Therefore,the research of LED especially the GaN-based blue LED has drawn much attention of the researchers all over the world.With the continuous improvement of the fabrication technology,many progresses have been made these years,however,it still needs to increase the efficiency of LED before they can completely replace the present lighting sources.In order to enhance the efficiency of LED,two quantities have to be improved:the efficiency of light generation which usually called the internal quantum efficiency(IQE)and the light-extraction efficiency(LEE)from the LED.With the improvement of the material quality and the fabrication technology,the IQE of GaN based blue LED has exceeded 70%and is approaching the limited value.However,the LEE of LED is still very low due to the total internal reflection(TIR)and the Fresnel reflection.Therefore,it is very important to design more reasonable LED structures to improve the LEE of LED.Although various approaches have been used to improve the LEE of LED,such as surface roughing,photonic crystals(PhCs),and many progresses have been achieved.However,we still need to explore some new effective structures to further improve the LEE of LED.Meanwhile,there are still lack of theoretical guidance in the optimization of these structures,and it is very costive and time consuming to optimize the LED structures through designing and fabricating lots of samples.The progress of the computer hardware and the improvement of the simulation algorithm provide us efficient method to solve the above problems,then improve the design efficiency and reduce the development costs.Although the LEE of LED has been improved to some extent through many methods,the influences of these methods on the electrical and thermal properties of LED are rarely considered.However,these properties are very important for the design and fabrication of LED.Firstly,as we know,with the increasing of the current injection,the efficiency of LED decreases drastically,which is the so called efficiency droop,this also restricts the development of high power LED.Secondly,the uniformity of temperature distributions in the active layer will significantly affect the light emission efficiency and the life time of LED.These problems all require us to resolve.Recently,some researchers have demonstrated that the uniform current density distributions in the active layer will be useful for the reduce of the efficiency droop,and uniform current density distributions in the active layer can also lead to uniform temperature distributions.So,it is important to explore some methods to obtain uniform current density distributions and temperature distributions besides the discussion of the improvement of the LEE.Aiming to address the above mentioned problems,many periodical nano-scale structures have been investigated through electromagnetic numerical analysis methods.Based on the finite-difference time-domain(FDTD)method,we established the optical model of LED,then,the effects of hybrid PhCs,embedded PhCs,ZnO nanotubes,and three component hybrid structures on the LEE improvement of LED have been studied in detail.The underlying mechanisms of these structures parameters on the LEE of LED have been deeply analyzed,and then the optimal structure parameters were obtained.For completeness,we established the electrical and thermal model of PhCs LED through the finite element method(FEM),then,the optical and electrical characteristics of normal PhCs LED were investigated simultaneously,while the optical,electrical,and thermal properties of SiO2 PhCs LED have also been discussed.The LEE,the current density distribution,and the temperature distribution in the active layer were analyzed thoroughly.These discussions and results can provide very useful theoretical references in the design and fabrication of high power,high efficiency LED.The main research work of this paper includes:(1)Concerning the PhCs,which is a wildly used method to improve the LEE of LED,in order to make good use of the shallow etched PhCs to obtain as much light extraction as possible without destroying the active layer.The hybrid PhCs with different etching depth were used to increase the LEE of GaN-based LED.The results showed that the structure proposed here could decrease not only the TIR but also the Fresnel reflection,then more LEE could be obtained.With the optimized parameters,over 30%enhancement in LEE has been achieved.The hybrid PhCs with part active layer etched were also investigated in this paper.(2)Considering the low LEE of SiC substrate flip-chip LED and the difficulty of machining the SiC substrate,embedded PhCs were alternatively introduced into the n-GaN layer of LED to improve the LEE of LED.The influences of the embedded PhCs configurations on the LEE of LED were examined to get an optimal structure.The combination effects of the micro-cavity and the embedded PhCs led to a significant enhancement of the LEE by concentrating light emission within the extraction cone and modifying the distribution of the guided modes.With the optimal parameters,the LEE has reached to 20%.The LEE of conventional surface PhCs LED and the double layer PhCs LED were also investigated for comparison.(3)Considering the inherent advantages of ZnO nanotubes LED and the complex fabrication progress of ZnO nanotubes especially the big differences in the scale of the nanotubes which fabricated by different research teams,numerical simulations based on FDTD method were carried out to investigate the mechanisms through which ZnO nanotube arrays could effectively improve the LEE.The effects of several parameters,including the inner radius,the wall thickness,the height,and the period of ZnO nanotubes on improving the LEE were studied.An optimization of ZnO nanotubes geometry with respect to the optical properties has been presented.With the optimized structure,more than four times of LEE has been achieved.Furthermore,the LEE over the whole visible spectrum has also been investigated in this paper.(4)In order to solve the problems of low LEE of RGB color mixing white LED,a hybrid structures were used to simultaneously improve the red(R),green(G),and blue(B)LEE of RGB white LED.With the help of 3D FDTD simulations,the effects of the embedded PhCs,the normal surface PhCs,and the nano-rods on the enhancement of red,green,and blue light extraction were investigated.Results from the simulations demonstrated that the maximum LEE for the hybrid structures LED was as much as 2.12,4.27,and 3.84 times of red,green,and blue LEE,respectively,comparing with that of the conventional planar LED.We also compared the proposed structure with the normal surface PhCs LED along the whole visible spectrum.(5)Although PhCs have been used to enhance the LEE of LED in a lot of researches,the effects of PhCs on the electrical properties of high power LED have been rarely analyzed.Considering the droop effect of high power LED and the relation between the droop effect and the current density distribution in the active layer,the electrical and optical characteristics of high power LED have been discussed simultaneously in this paper.Then the influences of several parameters of PhCs on the current density distribution and the external quantum efficiency(EQE)of LED were examined.(6)During the previous researches,most PhCs used to increase the LEE of LED were air-holes,the dielectric PhCs were little concerned,and most of the researches concerning PhCs LED were about the LEE of LED,while the electrical and thermal properties of PhCs LED were little concerned.In this paper,we investigated the optical,electrical,and thermal properties of SiO2 PhCs LED systematically.Numerical simulations based on the FDTD method were carried out to investigate the effects of several parameters of SiO2 PhCs on the enhancement of the LEE.According to our calculations,more than 37%improvement has been achieved in the LEE of the SiO2 PhCs LED with the optimal configurations.The rigorous coupled-wave analysis(RCWA)method was utilized to verify the variation trend of LEE we got from the FDTD algorithm.For comparison,the normal air-hole PhCs LED were also investigated,and the results indicated that SiO2 PhCs LED had higher extraction efficiency.Then the electrical and thermal properties of the optimized SiO2 PhCs LED at high current density injections were investigated and compared with the normal planar LED.To summarize,embroidering on the theme of improving the efficiency of LED,we proposed several periodical nano-scale structures to enhance the LEE of LED.And aiming at the situation of rarely research was about the electrical and thermal properties of LED,we established the optical,electrical,and thermal model of the PhCs LED which were usually used to improve the LEE of LED and systematically investigated the the optical,electrical,and thermal properties of PhCs LED in order to comprehensively understand the mechanisms of periodical nano-scale structures improving the efficiency of LED.Moreover,we have also optimized these structures to get more light harvest for the design and fabrication of high power and high efficiency LED.The main highlights of this paper are:(1)Hybrid PhCs,embedded PhCs,ZnO nanotubes and three component hybrid structures were introduced to improve the LEE of LED,and the mechanisms of these structures improving the LEE of LED were investigated respectively.The influences of these structures configurations on the LEE of LED were examined to get an optimal structure.With the optimized structures,the LEE of LED have been enhanced effectively.(2)The optical and electrical model were established to investigated the optical and electrical properties of PhCs LED simultaneously in this paper.The influences of the PhCs parameters on the current density distribution in the active layer and the EQE of LED were examined in detail.Through optimization,more EQE have been achieved while the current density distribution became more uniform.(3)In this paper,we establish the optical,electrical,and thermal model of SiO2 PhCs LED to systematically investigated the optical,electrical,and thermal properties of SiO2 PhCs LED.The effects of SiO2 PhCs on the LEE,the current density and temperature distributions in the active region were comprehensively discussed.Results indicated that with the optimized SiO2 PhCs structure,the LEE and the uniformity of current density distribution of LED have been improved,while the temperature distribution in the active layer had no noticeable degradation.In summary,Through our researches,we revealed the physical mechanisms of the periodical nano-scale structures on the LEE enhancement of LED.And many optimal periodical nano-scale structures used to increase the efficiency of LED was proposed,the optical,electrical,and thermal properties of PhCs LED were also discussed for completeness.All these conclusions and simulation results drawn in this paper provided very helpful theoretical references in the design and fabrication of high power,high efficiency LED.