Fabricating High External Quantum Efficiency Photonic Crystal LED Using Holographic Method

Light-emitting diodes(LEDs) are regarded as the light sources of the 21st Century. Compared with traditional light sources,LEDs have the advantages of power saving, environmental protection and long service life.The main application fields of LEDs are traffic lights,outdoor full-color displays,LCD backlight,general lighting and decorative lighting.They are the high brightness and low power consuming lighting devices which can meet the requirement of future social development.However,there are still several problems prevent LEDs from replacing all other light sources completely,such as low efficiency,high cost,high working temperature,far shorter service life than theoretical value,etc.All these problems are mainly related to the extremely low external quantum efficiency(EQE) of LEDs. The reason of low EQE is that LED's light extraction efficiency(LEE) is very low,which is mainly caused by the total internal reflection(TIR) between the surface of the semiconductor material and the surrounding medium.In recent years several methods have been tried to improve LEE,such as surface roughness,fabrication of Bragg reflector,utilization of two-dimensional(2D) photonic crystals(PCs),etc.Because of its simple fabrication process and high enhancement in EQE,2D PC method is considered to be one of the most effective ways.At present,the lack of a convenient,fast and low-cost method to fabricate large area 2D PCs with small lattice constants(LC) stops PC-LED from industrial production.As fabricating 2D PC patterns on photoresist via holographic lithography has the advantages of low-cost,short fabrication period and being able to produce large area PCs with small LCs at a time,the approach was adopted in this research to fabricate 2D PC masks for high EQE PC-LEDs.In the study,a holographic optical element(HOE) consisting of three identical holographic gratings with 120°separation was used to generate three interference beams so that 2D hexagonal PC photoresist masks could be fabricated through one exposure.The use of HOEs to fabricate PC masks makes the mask fabrication system very simple and stable. Then wet etching technique was used to transfer the patterns into ITO layers of GaN-LEDs, and ICP dry etching technique was applied to transfer the mask structure into sapphire substrate.The experimental results show that this approach not only can produce large area 2D PCs with fast processing and low-cost,but also can increase the EQE of GaN-LEDs effectively.The study includes theoretical and experimental researches on 2D hexagonal PC fabrication technique and the analysis on the mechanism of EQE enhancement.The main research work and results presented in this thesis include:1.Through theoretical analysis,finding out that three beams can interfere to form a 2D hexagonal lattice structure,in condition that the projection angles between every two beams on the interference plane is 120°.Simulating and analyzing the impact on the interference patterns with varying intensity and polarization of the three beams.The results show that only when the three beams have the same intensity and polarization,a 2D hexagonal PC pattern can be formed without distortion.Based on this analysis,positive photoresist was taken as the recording medium to simulate the development process.Theoretical analysis on the selection of optimal parameters in HOE fabrication system is presented.2.Analyzing the two mechanisms of enhancing GaN-LEDs' LEE by fabricating 2D PC structures in LEDs.The analysis and calculation of LEE with 2D hexagonal PCs fabricated on the surfaces of ITO layers and used as the diffraction objects were carried out to obtain the condition for the light being able to diffract out of LEDs.The result shows that lattice constants in the range of 250nm～500nm is the most favorable condition for enhancing LEE.3.HOE holographic method and wet etching technique were used to fabricate 2D hexagonal PCs on the surfaces of ITO layers of GaN-LEDs to increase LEE.The EQE enhancement for the patterns with the same LC(1μm) but different depths and side wall thickness were compared.The results show that the pattern with 200nm thick side wall and 70nm etching depth improved the LED LEE most significantly,with 25%increase of the output intensity.In the same way,patterns with the same depth and side wall thickness but different LCs(1.2μm,1.5μm and 1.8μm) were also compared.As a result,the pattern with small LC(1.2μm) achieved 22.5%increase of output intensity,indicating that small LC is more suitable for increasing LED LEE.4.HOE holographic method and dry ICP etching technique were used to fabricate patterned sapphire substrate(PSS).The pattern on the PSS is 2D hexagonal PC structure with 3.8μm LC and 800nm depth.Then GaN materials were grown on the PSS using MOCVD, followed by other LED device fabrication process.The results of light intensity measurement proved that this is an effective way to enhance GaN-LEDs' EQE.Compared with conventional GaN-LEDs,the average light intensity of PSS-LEDs increased by almost 100 percent,while the highest light intensity increased by 30 percent.The analysis on the reason that PSS can increase EQE significantly without improving the quality of GaN material indicates that the enhancement of EQE is mainly due to the scatter of the light when striking on the PC structure of PSS.