MicroLED tech combines native blue and green epitaxy on single wafer

04 Oct 2019

Plessey’s new patented growth technology achieved by extending capability of proprietary GaN-on-Si process.

Plessey, an embedded technologies developer at the forefront of microLED technology for augmented and mixed reality displays has extended the capability of its proprietary GaN-on-silicon process to enable native blue and native green emission from the same wafer.

The potential of microLEDs is well known, says Plessey, “but several challenges remain before ramping up to large scale consumer applications.” To form RGB microLED displays, typical approaches are to use a pick and place process to transfer discrete R, G and B pixels or to use native blue LEDs as the light source for subsequent colour conversion, to red and green.

Plessey’s latest patented growth approach creates both native blue and native green emission layers on the same wafer. The monolithic formation of two colors significantly simplifies display manufacture. Green microLEDs have high efficiency with a narrow spectral width resulting in what Plessey calls “an excellent color gamut when operating alongside the high performance Blue microLEDs.”

The monolithic integration of both the blue and green microLEDs on the same silicon substrate is the result of a concerted effort aimed at solving several challenges. Among the issues preventing the integration of multiple wavelength diode junctions are, firstly, a magnesium memory effect and diffusion from the p-type cladding of the lower junction into the upper junction.

An additional process challenge to the integration of Blue and Green microLEDs is the precise tuning of the thermal budget during the growth of the second junction to prevent indium phase separation in the blue active region. Plessey has precisely engineered the thermal budget to maintain high efficiency (IQE), low defectivity and high electrical conductivity required for high brightness display applications.

Distinct spectra

A final operation in the formation of GaN microLEDs is a post growth treatment aimed at removing hydrogen atoms that would otherwise compromise the conductivity of p-type layers. The presence of a second junction complicates the removal of hydrogen from the buried device structure negating the effect of standard post-growth activation treatments.

Dr Wei Sin Tan, Director of Epitaxy and Advanced Product Development, at Plessey, said: “Our latest breakthrough has enormous implications and will open the doors towards new innovations across a wide range of display applications. 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. Our achievement demonstrates once more the ability of the engineering team at Plessey to lead in the microLED technology space and deliver truly innovative solutions to well-known problems.”