나노 구조로 패턴된 InGaN 계열 발광다이오드의 광추출 효율 증가

Abstract

In this thesis, we studied the enhancement of the light extraction efficiency in InGaN-based light-emitting diodes (LEDs) patterned with nano structures. The nano structures in LED were fabricated by following process. A n-type GaN layer were grown on a c-plane sapphire substrate. A SiO2 layer was deposited on the top surface of the n-type GaN layer and then indium tin oxide (ITO) layer was deposited onto the SiO2 layer. The sample was dipped into a HCl solution, which gives rise to oxide self-assembled cluster. The SiO2 layer was then etched. After the patterning of the SiO2 layer, the etching mask of the oxide cluster was removed. After the nano structures of SiO2 were formed on the surface of the n-type GaN layer, n-type GaN layer was regrown. Then the active layer which consisted of five periods of a 2-nm thick InGaN well and an 18-nm-thick GaN barrier was grown. A p-type Al0.2Ga0.8N layer electron blocking layer and a p-type GaN layer were grown onto the active layer, successively. To fabricate LED chips, the surface of the top layer was etched to form a mesa structure and Cr and Au were deposited for n- and p-type electrodes, respectively. For a packaging, the LED chip was die-bonded to the bottom of a plastic leaded chip carrier (PLCC) and then it was encapsulated with a transparent epoxy. To measure the LED properties, we used an integrating sphere coupled with a spectrometer. The output power of nano patterned LED shows a 1.33 time increase compared with that of a normal LED without nano structure when the total output power emitted in all directions from the packaged LED. It was measured under a current injection of 20 mA. The increase of output power from the nano patterned LED depends on the size of the nano structures on the n-type GaN substrate. We performed numerical simulation for nano patterned LED. Numerical simulation on light extraction in LED with nano patterned n-type GaN substrate is presented by using three-dimentinoal finite-difference time-domain (FDTD) method. Reasonable conditions for numerical calculation are investigated, including excitation source conditions, simulation size conditions, and boundary conditions. Effects of the size and distribution of nano patterns on light extraction are investigated and the optimized structures for high extraction efficiency are found. We observed that the light outputs are increased in the nano patterned LEDs, regardless of the height of the nano structures. These simulation results are consistent with our experimental results. We found the optimized nano patterned LED structure through our FDTD simulations.