Research on high-efficient LEDs for high-resolution micro-LED displays

Abstract

Micro-LED refers to an LED smaller than 100 µm, unlike LEDs previously used in general lighting. Recently, it has been in the spotlight for displays or devices that require high efficiency and high brightness. In particular, micro-LED is attracting more attention in displays that require ultra-high resolution, such as head-mounted displays and AR glasses, which are areas existing OLEDs cannot replace. The conventional method of realizing the micro-LED display is to arrange red, green, and blue (RGB) color LEDs laterally and operate them individually. The pick-and-place method is mainly used to produce existing micro-LED displays. However, this method is not suitable for application to ultra-high-resolution displays due to its high mechanical dependence and low yield. Therefore, many prior studies have been conducted to implement full-color displays without a pick-and-place process. However, these prior studies have the disadvantage that although each color can be realized, it is difficult to achieve high-efficiency devices. Therefore, adhesive bonding technology was used to implement a display that has high resolution and can control multiple colors independently. Adhesive bonding technology is a technology that uses a polymer material (SU-8) to integrate materials grown on different substrates and remove the substrate by wet etching. This dissertation report covers the design, fabrication, and characterization of a full-color LED device fabricated using only adhesive bonding. First of all, it was confirmed that the laminated thin films were structurally stable. After multiple bonding, each red, green, and blue subpixel was formed using only standard processes. Even after bonding, high efficiency was maintained without performance degradation. The fabricated LED can cover a wide range of color spaces, showing the potential for application in high-resolution micro-LEDs. Additionally, as the LED size becomes smaller in red micro-LEDs, efficiency decreases more due to material-related characteristics, making it challenging to achieve high-efficiency full-color micro-LEDs. Therefore, this dissertation report will explain the results of red micro-LEDs using a double passivation layer. The double passivation layer consists of a thin layer stacked using Atomic Layer Deposition (ALD) and a thick layer stacked using Plasma Enhanced Chemical Vapor Deposition (PECVD). Since productivity is low when forming a passivation layer using only ALD, the double passivation method is suitable in terms of cost. This dissertation report deals with the process of LED applying double passivation and its electrical and optical characteristics.