Green InGan microLEDs beat all efficiency benchmarks

January 23, 2020 //By Julien Happich
Researchers from the University of Sheffield have developed a novel fabrication method for green InGaN μLEDs, yielding very compact high brightness μLED arrays that could be used for the design of very low power microdisplays.

The fabrication technique published in the ACS Photonics journal under the paper title "A Direct Epitaxial Approach To Achieving Ultrasmall and Ultrabright InGaN Micro Light-Emitting Diodes (μLEDs)” reports 3.6μm-diameter green μLEDs grown at an interpitch of 2μm within pre-patterned SiO2 microhole arrays (compatible with silicon CMOS processing to integrate the necessary drivers).

Instead of combining a standard photolithography technique with subsequent dry-etching processes on a standard III-nitride LED wafer, which are today’s commonly used fabrication steps, the researchers found an alternative method to remove the need for dry-etching processes altogether. Dry-etching processes, as the authors explain, always introduce some level of surface damage know to increase nonradiative recombinations, severely degradation the overall efficiency and optical performance of μLEDs, more so as the device’s size decreases.

Fabrication process of the InGaN μLED arrays: (a) SiO2 mask deposition; (b) SiO2 mask patterning; (c) μLED array overgrowth; (d) plane-view and (e) cross-sectional SEM images of the regularly arrayed μLED wafer.

In their paper, the authors describe a so-called “selective overgrowth method” whereby the InGanN microLED stacks are directly grown within pre-patterned micro-hole arrays through a thin (500nm) SiO2 layer serving as a GaN template over the epitaxial wafer.

Fabricated by metalorganic vapour-phase epitaxy (MOVPE), the individual μLEDs selectively overgrown within each micro-hole consist of a silicon doped n-GaN layer, an InGaN based prelayer (5% indium content), 5 periods of InGaN/GaN multiple quantum wells (MQWs) with 2.5nm InGaN quantum wells and 13.5nm GaN barriers as an active region, and a 20nm p-type Al0.2Ga0.8N blocking layer before the last 200nm p-doped GaN layer.

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