Green InGan microLEDs beat all efficiency benchmarks
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.
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.
Not only the researchers avoided the use of dry-etching, but the SiO2 micro-hole masks also offer a natural surface passivation around each microLED, greatly simplifying device fabrication. All the μLEDs in the array share a common n-contact, while all the p-contacts are left open, which can then be contacted either individually or across large areas.
As a proof of concept, the authors fabricated a few thousand 3.6μm μLEDs arrayed across a 0.1mm2 surface. They tested the μLED array chip at an injection current of 1 and 3mA, corresponding to a current density of 3 A/cm2 and 9 A/cm2, respectively. Even at the lower current density of 3A/cm2, all the individual 3.6μm μLED pixels exhibited strong green emission. Taken to a single μLED, the authors calculated that an individual 3.6μm μLED could be brightly lit at an ultra-low driving current of 0.3μA under a 2.5V bias. A 640×480 pixels display built around such μLEDs would only draw 0.23W.
injection current density of 3A/cm2 (left) and 9A/cm2 (right).
The authors are keen to note that even an injection current density of 9 A/cm2 for these μLEDs is less than half of a typical current density (22 A/cm2) used for conventional broad area LEDs (in the 330x330μm size range), meaning that the μLEDs should exhibit a lifetime at least as long as today’s broad area LEDs which have an expected operation lifetime exceeding 100 000 hours under normal operation conditions. Finally, the paper reports an ultra-high brightness over 107 cd/m2 and an ultra-high peak external quantum efficiency (EQE) of 6% at about 515nm (green) for the non-optimized design, with measurements showing an internal quantum efficiency (IQE) of 28%.
University of Sheffield – www.sheffield.ac.uk
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