On the mechanism of carrier recombination in downsized blue micro-LEDs

Abstract

The mechanism of carrier recombination in downsized μ-LED chips from 100 × 100 to 10 × 10 μm2 on emission performance was systemically investigated. All photolithography processes for defining the μ-LED pattern were achieved by using a laser direct writing technique. This maskless technology achieved the glass-mask-free process, which not only can improve the exposure accuracy but also save the development time. The multi-functional SiO2 film as a passivation layer successfully reduced the leakage current density of μ-LED chips compared with the μ-LED chips without passivation layer. As decreasing the chip size to 10 × 10 μm2, the smallest chip size exhibited the highest ideality factor, which indicated the main carrier recombination at the high-defect-density zone in μ-LED chip leading to the decreased emission performance. The blue-shift phenomenon in the electroluminescence spectrum with decreasing the μ-LED chip size was due to the carrier screening effect and the band filling effect. The 10 × 10 μm2 μ-LED chip exhibited high EQE values in the high current density region with a less efficiency droop, and the max-EQE value was 18.8%. The luminance of 96 × 48 μ-LED array with the chip size of 20 × 20 μm2 exhibited a high value of 516 nits at the voltage of 3 V.

Technology and Applications of Micro-LEDs: Their Characteristics, Fabrication, Advancement, and Challenges

“…In 2021, Chen et al used laser direct writing technology to systematically study the effect of chip size on the performance of μLED chips. 97 The fabrication steps of μLED chips and arrays are shown in Figure 9a. The precision of the photolithography process was improved by the maskless laser direct writing method.…”

“…Those strategies used in conventional red LEDs, including the surface light extraction microstructure, regulating the In content in InGaN, fabricating the passivation layer, and current spreading layers with precise thickness control, may have important implications for enhancing the performance of red μLEDs. 46,50,93,97 For green μLEDs, the lower EQE compared to conventional LEDs is mainly due to a poor direct interband tunneling probability because of the wide band gap. 72,110−112 Therefore, a TJ μLED with a lower device resistance has been proposed, and it exhibits a higher EL intensity and lower efficiency drop compared to conventional μLEDs.…”

“…The leakage current caused by sidewall defects can be improved by depositing a passivation layer of SiO 2 by PECVD. 97 The introduction of ALD technology can also effectively passivate sidewall defects. The effective passivation process is a viable method to reduce the nonradiative recombination centers and therefore diminish nonradiative recombination.…”

“…In 2021, Chen et al used laser direct writing technology to systematically study the effect of chip size on the performance of μLED chips . The fabrication steps of μLED chips and arrays are shown in Figure a.…”

“…(d) Output power density and (e) EQE as a function of current density for different μLED chip sizes. Reprinted with permission from ref . Copyright 2021 Nature/Scientific Reported .…”