Effect of Surface Texture on Light Extraction Efficiency for LEDs

Abstract

The light extraction efficiency of an LED is dependent on its surface texture. However, the surface of the p-GaN layer is not easy to be etch with inverted hexagonal pyramid structures (IHPS) with small top widths and large depths using existing methods. Therefore, it is important to discuss the expected effect of the conditions of thermal annealing and inductively coupled plasma (ICP) reactive ion etching (RIE) for the generation of nano-pin-holes in the photoresist and fabrication of the top surface structure of GaN-based LEDs, in order to enhance the light output power. In this study, the following four items will be discussed: (1) the effect of thermal annealing on the composition of the photoresist; (2) the effect of thermal annealing and ICP RIE on the generation of the nano-pin-holes in the photoresist; (3) the effect of ICP RIE on the IHPS; and (4) the effect of surface texture of the IHPS on the light output power. It has been found that a nano-pin-hole structure in the photoresist etching mask is needed for the fabrication of many IHPS on the LED surface. A maskless via-hole etching technique was used for texturing the photoresist to produce nano-pore structures with diameters of less than 50 nm. The relationship between the light extraction efficiency and the surface texture is discussed in detail. The simulation results show the best light extraction efficiency (LEE) ratio of 358% to be obtained when the distance between two neighboring IHPS patterns (DBNP) is 300 nm. This in turn allowed the formation of IHPS with small top widths and large depths on the LED surface. A LEE ratio of 305% was obtained with the fabrication of IHPS with a top width of 290 nm, a depth of 170 nm and a DBNP of 180 nm on the LED surface.