Abstract
In this dissertation, we present several types of light extraction enhancement structures on the backside of light emitting diodes (LEDs) to enhance the efficiency of light output. The first one is auto-cloned photonics crystal (APhC) on the backside of the sapphire wafer of the LED substrate, and the second one is micro mirror array (MMA) structure which was embedded in the gallium nitride (GaN) LED mainbody by the fabrication process of epitaxial lateral overgrowth (ELOG). The first section of the dissertation is related to research of the APhC. Based on the theory of thin film growth, we simulated the growth of the auto-cloned Ta2O5/SiO2 multi-layer photonic crystal with a lateral saw-tooth period under the mechanism of deposition and etching. Ion-beam-sputter (IBS) was applied to deposit the films and RF-bias etching was applied simultaneously with the IBS on the Ta2O5 film. Both simulation and experiment results showed that the quality of the auto cloning can be optimized and well controlled by the RF-bias power. There exists an intermediate power range, within this range, the drop of peak to valley height variation of the saw-tooth profile can be reduced significantly to achieve high degree of auto-cloning. Analysis showed that simultaneous deposition and etching at the proper RF-bias power on the Ta2O5 has the capability to compensate the flattening effect of the SiO2 deposition such that the saw-tooth surface profile can be maintained. In the second section of the dissertation, we introduce the fabrication of three dimensional (3-D) APhC of Ta2O5/SiO2 multi-layers on the backside of the sapphire wafer that has InGaN/GaN multi-quantum wells (MQWs) LED on the front side. 94% light extraction enhancement in comparison to the LED without APhC was obtained. Electrical properties of the LED did not affected by the APhC and its fabrication process. Experimental evidences showed that light extraction enhancement mechanism is in two aspects: for rays that are emitted from the source and incident at lower angle of incidence to the APhC, the APhC acts as a high reflector; for rays incident at higher angle of incidence to the APhC, first order diffracted light from the APhC appears, the diffracted light is concentrated around the surface normal and is therefore capable of escaping. In the third section of the dissertation, we propose a light extraction enhancement structure by using the heat-resistive dielectric MMA embedding in the ELOG GaN. Taking advantages of reducing dislocation density by ELOG together with the capability of diffraction and high reflectance of the patterned structure from the MMA, higher light output power for the LED can be expected. The MMA of Ta2O5/SiO2 dielectric multi-layer with the mirror diameter of 3□m and the array period of 6□m was fabricated on c-plane sapphire substrate. ELOG of GaN was applied to the MMA that was deposited on both sapphire and sapphire with 2.56□m GaN template. The MMA was subjected to 1200oC high temperature annealing and remained intact with high reflectance in contrast to the continuous multi-layer for which the layers have undergone severe damage by 1200oC annealing. The result implies that our MMA is compatible to the high temperature MOCVD growth environment of GaN. In the final section of the dissertation, we propose fabrication of MQWs InGaN/GaN LEDs, 300□m □ 300□m chip size, with Ta2O5/SiO2 dielectric multi-layer MMA embedded in the ELOG GaN on the c-plane sapphire substrate. MQWs InGaN/GaN LEDs with ELOG embedded patterned SiO2 array (P-SiO2) of the same dimension as the MMA were also fabricated for comparison. Dislocation density was reduced for the ELOG samples. 75.2% light extraction enhancement for P-SiO2-LED and 102.6% light extraction enhancement for the MMA-LED were obtained over the standard LED. We demonstrated that the trapped lights can be redirected from the MMA by multiple-diffraction to escape from the LED. Therefore, the light extraction can be enhanced.