Influence of the microstructure geometry of patterned sapphire substrates on the light extraction efficiency of GaN LEDs

Abstract

The influence of the microstructure geometry of patterned sapphire substrates (PSS) on the light extraction efficiency (LEE) of GaN light-emitting diodes (LEDs) is numerically analyzed. Cone structures of various dimensions are studied, along with dome and mixed microstructures. LEE is found to mainly depend on the microstructure surface slope. LEE rises quickly with slope and flattens out when the slope exceeds 0.6. Scaling down the microstructure has little effect on LEE. Light rays are found to travel longer distances in PSS LEDs, as compared with LEDs grown on a flat substrate. Keeping GaN absorption loss low is important for LEE optimization.

Application of patterned sapphire substrate for III-nitride light-emitting diodes

“…155 Additionally, LEE of LEDs with cone-shaped PSS mainly hinged on the microstructure surface slope, which increased with increasing slope and reached a plateau when the slope exceeded 0.6. 156 We have reported that microstructures (cone, truncated cone, hemisphere, and pyramid) on the top surface of sapphire substrate can enhance the LEE of flip-chip blue micro-LEDs via designing their shapes and heights. 157 However, we also suggest that surface texture is less efficient than the PSS for increasing the LEE in an encapsulated LED.…”

“…155 Additionally, LEE of LEDs with cone-shaped PSS mainly hinged on the microstructure surface slope, which increased with increasing slope and reached a plateau when the slope exceeded 0.6. 156…”

Enhanced Performance of InGaN Light-Emitting Diodes via High-Quality GaN and Embedded Air Voids Grown on Hexagonal 3D Serpentine Mask Sapphire Substrates

“…Due to the large refractive index difference between GaN ( n = 2.3) and air ( n = 1), only a small fraction of the light (whose incidence angle is within the critical angle of 23°) can be extracted, while the remaining light will be reabsorbed by the material due to total internal reflection (TIR) . The LEE of GaN-based LED grown on a planar sapphire substrate is only ∼20% without any modification. − Consequently, many efforts have been devoted to increasing the LEE, including surface roughening, patterning of ITO or P-GaN, gradual refractive index antireflection layers, using a patterned sapphire substrate, and embedded metal or air-void structures. − These designs aim to increase the scattering probability of light during propagation, making it more likely to enter the extraction cone. Among these, the formation of air voids within the GaN layer is one of the more widely studied techniques. − Dai et al reported a selective wet-etching process that uses a hot H 3 PO 4 solution to form V-shaped air channels within GaN.…”

Light Extraction Enhancement of InGaN Based Micro Light-Emitting Diodes with Concave-Convex Circular Composite Structure Sidewall

“…To this end, the photon escaped from the gallium nitride (GaN) sidewall can be increased by reducing the TIR, thereby realizing the improvement of light extraction efficiency (LEE) and light efficiency [14][15][16]. Several advantageous approaches have been put forward to increase the LEE, such as patterned sapphire substrates [17][18][19], textured surfaces [19][20][21][22], SiO 2 MS/MP [9,23,24], composite transparent conductive layer [5,25], microlens array [26,27], and photon crystals [16,28,29]. Meanwhile, the LEE can be improved by roughening the bottom [30] or sidewalls of the LED.…”