Novel processes and device technologies for next generation light emitting diodes and micro-displays

Abstract

Remarkable development of light emitting diodes (LEDs) has driven its commercialization in large area displays and general lighting. The use of LEDs in micro-display is rare but recently attracting more attention. LEDs have advantages in terms of efficiency, brightness, lifetime, temperature stability and robustness, compared with other existing micro-display technologies. In the dissertation, novel fabrication processes and device structures of LEDs and micro-displays were explored to overcome their technological bottlenecks. Conventional flip-chip LEDs have problem of current crowding near n-electrode, causing significant brightness degradation at high current injection. We propose a point-contact flip-chip LED, where high density and small n-contacts reduce lateral current spreading distance, improving forward voltage of FCLEDs. Meanwhile the brightness was improved by light extraction enhancement from high density deeply etched holes. Although LED micro-displays have been intensively investigated, high resolution micro-display with decent visual quality is yet to be realized. Non-optimized bumping technology and bowing of micro-LED array causing difficulty in flip-chip bonding are the major issues. Two different approaches were proposed to demonstrate high resolution micro-displays with much better bonding yield. This was successfully demonstrated in a 1700 pixel per inch (PPI) passive-matrix micro-LED array bonded on ASIC. Indium bumps were relocated to the peripheral areas of the chip so that the bumps can be bigger and more spread out. Moreover, significant reduction of bump density in passive-matrix configuration improves bonding reliability. In designing the 400 × 240 active-matrix LED micro-display, the sapphire substrate of the micro-LED array was intended to be as thick as its original to suppress chip bowing, maximizing the coplanarity between micro-LED array and silicon pixel driver. Realization of full color LED micro-display have been a big challenge because it is not practical to selectively grow three different epi-layers emitting at different wavelengths on a single substrate for three primary colors. Dedicated for projection display application, we demonstrated a novel 3LED light engine that can project full color video on screen. 3LED consists of a trichroic prism that combines RGB images generated from three LED micro-displays based on AlGaInP (Red) and GaN (Green and Blue) materials. For near-to-eye display application, RGB quantum dots were printed on micro-LED array by aerosol jet printing, achieving full color LED micro-display with high image quality.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology