Plessey builds green and blue micro-LEDs on the same die for augmented reality displays

UK LED fab Plessey has achieved native – as opposed to phosphor-converter – blue emission and native green emission from the same wafer, a key step en route to effient monolithic micro-displays for virtual reality (VR) googles and augmented reality (AR) glasses – the firm argues that it is in the best position to achieve sufficient brightness to make compact AR devices that can be used outside.

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“Plessey’s latest growth approach creates both native blue and native green emission layers on the same wafer. The new approach forms micro-LEDs that exhibit high current density operation and long operational lifetime,” according to the firm. “The monolithic formation of two colours significantly simplifies display manufacture. Green micro-LEDs have high efficiency with a narrow spectral width resulting in an excellent colour gamut when operating alongside the high performance blue micro-LEDs.

Among the issues preventing the integration of multiple wavelength diode junctions were magnesium memory effect and diffusion from the p-type cladding of the lower junction into the upper junction.

An additional process challenge is the tuning of the thermal budget during the growth of the second junction to prevent indium phase separation in the blue active region. “Plessey has engineered the thermal budget to maintain high [internal quantum] efficiency, low defectivity and high electrical conductivity required for high brightness display applications,” it said.

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.

The final junctions are vertically separated by a sub-micron layer resulting in reproducible and stable diode performance, well beyond what is typical in the LED industry, claims Plessey.

Electronics Weeky explains the need for native green emission here

Using its historic chip-making expertise and that gathered in the last few years making GaN-on-silicon LEDs, Plessey has been developing micro-displays that have all necessary LED emitters and drive circuits on the same monolithic chip – this is in contract to using pick-and-place process to build such a display out of tiny separate reg, green and blue LED die, or using native blue emission with phosphors to create red and green.

“Our latest breakthrough has a multiplier effect on our previous successes with high efficiency monolithic native blue arrays, native green arrays and hybrid bonding to back-plane by demonstrating a way to synthesise the best of our know-how into a single die,” said Plessey director of epitaxy Wei Sin Tan. “This creates a path to the elusive single RGB panel ultra-high resolution microLED AR display.”

Plessey recently created the world’s first wafer-level-bonded monolithic 3,000pixel/inch GaN-on-silicon micro-LED emissive display with an active matrix CMOS backplane. Its roadmap predicts the production of full RGB micro-LED displays by 2020.