A new path to high-efficiency micro-LEDs

A more efficient way to produce micro-light-emitting diodes (micro-LEDs) — the frontrunners to lead next-generation display technology — avoids a damaging plasma etching step. Now, a selective thermal oxidation approach that uses a unique etching-free process, developed at KAUST, is expected to boost device efficiency^[1].

Micro-LEDs are favored for next-gen displays because of their exceptional contrast ratio, brightness, sunlight visibility and longevity. They are multilayered semiconductor optoelectronic devices that consist of a hole injection layer, an indium gallium nitride (InGaN)-based active layer for light emission and an electron injection layer.

Fabricating these micro-LEDs typically relies on plasma etching, which removes the p-GaN and light-active material from nonpixel areas using reactive ions, ensuring that light emission occurs only in the designated pixel areas. The etching process, however, involves ion bombardment and UV radiation, causing defects on the sidewalls of the LEDs, which degrades the device’s performance.

There are several methods that could optimize micro-LED fabrication while reducing sidewall damage, such as chemically removing the damaged layer, passivating the sidewall surface and designing new device architectures. However, each of these approaches occurs after the plasma etching step and introduces more complex processes. Also, the micro-LED sidewall damage remains difficult to fundamentally resolve.

KAUST professor Xiaohang Li, along with his Ph.D. student Zhiyuan Liu and coworkers, have devised a solution that generates micro-LED arrays while avoiding plasma etching. 

The researchers first deposited a silica layer on the surface of a InGaN-based green LED wafer and patterned it. Next, they placed the resulting chip in a tube furnace and heated it at 900 degrees Celsius in the presence of air for four hours.

This annealing process oxidized and destroyed areas that were not covered by silica, causing these areas to lose their original luminescent property. In contrast, silica-coated areas are effectively protected, forming well-defined micro-LED pixels. Silica acts as a protective layer and stops ambient oxygen from diffusing into the LED. Its patterning enables selective thermal oxidation, which defines the final shape and size of the micro-LED pixels.

“Selective silica coverage and thermal oxidation are the core steps in defining pixels,” Liu says, noting that the duration of the oxidation and the thickness of the silica layer are crucial for micro-LED performance.

Ten-micron pixel arrays of InGaN-based green micro-LEDs were found to be more efficient with lower leakage current compared to their plasma-etched analogs.

The team also were able to validate the luminescence of pixels as small as 2.3 micrometers; this means that micro-LEDs fabricated using this method could be applied to future micro-display applications, such as augmented and virtual reality.

Li’s team is working on optimizing their process to further improve micro-LED performance. The researchers are also extending their method to blue and red micro-LEDs to meet the requirements for next-generation micro-display technologies, he says.