Enhanced Optical Property of InGaN Light-Emitting Diodes with SiO2 Nano-Bowl Photonic Crystal by Nanosphere Lithography

Abstract

Recently, nitride-based compound semiconductors have been attracting extensive attention in high performance electroluminescent devices due to their broad range of emission wavelength. InGaNbased Light-emitting diodes (LEDs) characterized by small size, high efficiency and long lifetime have been indispensable components in various applications including general lighting, traffic lights, automobile headlights, and backlights in liquid crystal displays. However, it is still necessary to further improve the light output power (LOP) of these modern LEDs to approach the next generation applications such as high-end solid-state lighting. The LOP of the LEDs depends on the external quantum efficiency (EQE) and is severely restricted by poor light extraction efficiency (LEE) due to the total internal reflection (TIR) effect caused by the large contrast in refractive index at the interface between the GaN film and the surrounding air. 1 Various approaches have been reported to improve the LEE of the LEDs by increasing the light scattering at the GaN/air interface including patterned sapphire substrate (PSS), 2,3 surface texturing, 4,5 and graded index structures. 6,7 Lately, many efforts have been taken to demonstrate the fabrication and application of the two-dimensional photonic crystal (2D-PC) structures to improve the LEE of the LEDs by suppressing the unwanted guiding modes. 8‐10 However, previous researches either require complicated process or expensive equipment 11 in the formation of wavelength periodic structures which greatly limited the application of 2D-PC structures to the LEDs. On the other hand, the 2D-PC structures can deteriorate the electrical properties 12 of the LED devices and affect the device lifetime. 13 To avoid these negative effects, 2D-PC structures can be formed onto the indium-tin oxide (ITO) current spreading layer Instead. During the fabrication of these structures in the characteristic length scales on the order of the wavelength, nano-patterning techniques are involved. Compared to nano-patterning using E-beam, 14 nano-imprint 15 lithographic techniques, nano-sphere lithography (NSL) 16‐18 has been demonstrated as a high-efficiency low-cost, and high throughput nano-fabrication method. In this paper, the fabrication and characterization of the InGaN LEDs with SiO2 nano-bowl photonic crystal (SNPC) atop the ITO current spreading layer patterned by self-assembled NSL were reported. The optical and electrical performances of the LEDs with SNPCs were evaluated in comparison with the conventional LEDs, together with an investigation of the mechanisms involved. The optical performances were also confirmed by the numerical simulations using the finite difference time domain (FDTD) method.