Ultra-low-current driven InGaN-based blue micro-light-emitting diodes for display application

Abstract

Owing to the increasing demand for new future display technology for augmented reality (AR), virtual reality (VR), hologram, micro-light-emitting diodes (μLEDs) based on III-nitride and AlGaInP have received much attention due to their advantageous properties of high efficiency, high brightness, and high stability. Despite the high potential of μLEDs, the size-dependent efficiency degradation issues, such as the decrease in maximum external quantum efficiency (EQE) and shift of current density showing maximum EQE to higher current region, have acted as a bottleneck for high-efficiency operation of μLEDs. These size-dependent efficiency degradation issues increase the power consumption of display and lower the stability of μLEDs display, and it is widely known that the inductively-coupled-plasma reactive ion etching (ICP-RIE) process for pixelation of μLEDs induces plasma damage in the mesa sidewall, which acts as trap site increasing the Shockley-Read-Hall recombination, decreasing EQE. Size-dependent issues did not arise in LEDs for lighting purposes and in mini-LEDs for backlight purposes due to their low surface-to-volume ratio, however, the dominancy of sidewall effects increases with decreasing size of the μLEDs. To tackle the size-dependent efficiency degradation issues, surface passivation methods to chemically remove the plasma damage, or unconventional pixelation methods to prevent the formation of plasma damage have been reported. However, the reported methods are focused on the post-epitaxial growth process, and could not provide a perspicuous explanation on carrier dynamics in μLEDs. Moreover, previous studies on epitaxy design and optimization are focused on LEDs with large size and high operating current density, which is not optimized to μLEDs due to the difference in operating mechanism and operating current density region. In this thesis, we suggest that new carrier dynamics in μLEDs are introduced by the decreased size of μLEDs, and based on the carrier dynamics, we re-designed the epitaxial structure to increase the external quantum efficiency and mitigate the size-dependent efficiency degradation issues. As a result, the fabricated μLEDs showed a very high external quantum efficiency at a low current density of 0.1~1 A/cm2. We believe that our new carrier dynamics and new epitaxial design is the important milestone to the realization of low-current driven μLEDs display.