Investigations of Sidewall Passivation Technology on the Optical Performance for Smaller Size GaN-Based Micro-LEDs

Abstract

Micro-light emitting diodes (Micro-LEDs) based on III-nitride semiconductors have become a research hotspot in the field of high-resolution display due to its unique advantages. However, the edge effect caused by inductively coupled plasma (ICP) dry etching in Micro-LEDs become significant with respect to the decreased chip size, resulting in a great reduction in device performance. In this article, sector-shaped GaN-based blue Micro-LEDs are designed and fabricated. Additionally, the device performance of different size Micro-LEDs with passivation are investigated with respect to those without passivation. Several methods have been applied to minimize the etching damage near the edge, including acid-base wet etching and SiO2 passivation layer growth. The room temperature photoluminescence (PL) results demonstrate that the light emission intensity of Micro-LEDs can be significantly enhanced by optimized passivation process. PL mapping images show that the overall luminescence of properly passivated Micro-LEDs is enhanced, the uniformity is improved, and the effective luminescence area is increased. The recombination lifetime of carriers in Micro-LEDs are increased by the usage of passivation process, which proves the reduction in non-radiative recombination centers in Micro-LEDs and improved luminescence efficiency. As a result, the internal quantum efficiency (IQE) is improved from 14.9% to 37.6% for 10 μm Micro-LEDs, and from 18.3% to 26.9% for 5 μm Micro-LEDs.

Ultra-small InGaN green micro-light-emitting diodes fabricated by selective passivation of p-GaN

“…Using this approach, ultra-small µLEDs could be demonstrated with dimensions of 10 µm or less [9][10][11]. However, the mesa etching process usually introduces surface damage at the sidewalls of InGaN QWs, which causes more non-radiative surface recombination and reduces the external quantum efficiency (EQE) of µLEDs [12,13]. This effect becomes significant, especially when the dimension of the µLEDs shrinks [14].…”

Model for Light Extraction Efficiency of MicroLEDs

“…For the GaN material system, the 𝐴, 𝐵, and 𝐶 coefficients were found to depend on the size of MicroLED and peak emission wavelength (𝜆 𝑝 ) according to the equations ( 19)-( 22) above. These empirical equations were based on data [7][8][9][10][11][12].…”

AlGaN Ultraviolet Micro-LEDs

“…To achieve effective light extraction, several researches have reported that the inclined mesa sidewall of UV LEDs could extract the strong TM-polarized in-plane emission in an the micro-LEDs are formed by dry etching. Several techniques such as wet chemical treatment have been employed to minimize the sidewall effects in GaN based micro-LEDs [42,43]. Chee et al have reported that using post-processing methods based on HCl followed by KOH chemical treatment could recondition the sidewall damage, improving the LEE of (in their case) photonic crystal AlGaN UVC LEDs [44].…”