Plessey Grows Blue and Green MicroLEDs on Same Wafer

British compound semiconductor manufacturer Plessey has grown native blue and native green microLEDs on the same GaN-on-silicon wafer.

Typical implementations for RGB microLED displays today use one of two approaches: either they are made of separate red, green, and blue pixels mounted closely together, or they use all native blue pixels and convert the colour to RGB.

Creating both blue and green pixels with native colour emission on the same wafer will significantly simplify microLED display manufacture, the company said.

Image: Plessey

“This has enormous implications and will open the doors towards new innovations across a wide range of display applications,” said Wei Sin Tan, Director of Epitaxy and Advanced Product Development at Plessey, in a statement. “For mobile and large displays, a high efficiency single RGB tile can now be used for mass-transfer and for micro-displays, this creates a path to the elusive single RGB panel ultra-high resolution microLED AR display. This new process paves the way to commercial, high-performance microLED displays, bringing mass adoption of microLEDs in displays ever closer to reality.”

MicroLEDs promise to revolutionise both large and small format displays. They offer many advantages over today’s OLED (organic light emitting diode) and LCD (liquid crystal display) technologies. This includes the ability to display a perfect black colour as the pixels can be switched off individually, no danger of burn-in or low lifetime as they do not use the organic compounds of OLED, and a good combination of high brightness and power efficiency.

However, there are significant challenges remaining before manufacture can be ramped up to consumer volumes.

TECHNICAL CHALLENGES

Previous attempts across the industry at monolithic integration of red, green and blue microLEDs have failed. Integrating multiple wavelength diode junctions has proved difficult, because of the magnesium memory effect, combined with diffusion from p-type cladding of the lower part of the junction into the upper part.

After GaN growth, there is a treatment step to remove hydrogen atoms that would otherwise reduce conductivity of the p-type layers. Adding a second junction complicates the removal of hydrogen from the buried device structure; Plessey has invented a new way of removing hydrogen from its devices.

Plessey developed a new way of removing hydrogen from its devices to allow fabrication of multiple wavelength diode junctions on the same wafer (Image: Plessey)

The resulting microLED integrates blue and green junctions, vertically separated by a sub-micron layer thickness. Its performance is both repeatable and stable to levels beyond those typical in the LED industry, the company said.

With its new microLED technology, Plessey is targeting high-performance displays in the augmented reality (AR) and mixed reality (MR) display sectors, where the company’s technology will enable smaller, cheaper microLED displays than other microLED technologies.

Plessey’s roadmap envisions the production of full RGB microLED displays before the end of 2020.

Sally Ward-Foxton is a Correspondent for EE Times and EE Times Europe