Quantum dots deep into hybrid LEDs

May 26, 2016 //By Julien Happich
Boasting record-breaking colour conversion effective quantum yields, an international team of researchers has designed a novel photonic quasi-crystal LED structure which deeply embeds quantum dots within the whole stack of the LED's active layers.

The researchers first grew a LED stack of GaN/InGaN Multi-Quantum Well onto a sapphire substrate, and then used nano-imprint lithography and plasma etching to "punch" the whole stack and structure it as a 12-fold symmetric quasi-crystal (PQC).

Forming an array of 480nm radius cylindrical holes with a lattice pitch of 750nm, the PQC structure was etched deep enough to penetrate through the whole MQW active area, then filled with a blend of quantum dots (QDs) deposited through a spin coating process.

In their Optica paper "Hybrid photonic crystal light-emitting diode renders 123% color conversion effective quantum yield", the researchers see this hybridization and the specific quasi-crystal geometry they chose as key factors.

When used for LED colour conversion, colloidal quantum dots (QDs) are usually dispersed into an encapsulation layer above the active LED structure, missing out the majority of the light emitted by the LED (60% to 80% of which remaining trapped within the epitaxy layers due to total internal reflection).

With this hybrid architecture, the QD emitters are placed in close proximity to the multiple quantum wells (MQWs) of the active area. This, the authors write, improves the out-coupling efficiency between MQWs and QDs, simultaneously allowing for a non-radiative resonant energy transfer between the MQWs and the QDs and near-field radiative coupling of trapped (guided) modes in the LED to the emitters.

(a) Schematic representation, (b) cross-sectional, and (c) top SEM images of a photonic quasi-crystal LED hybridized with QD color converters (Source OSA Publishing).

What's more, due to its highly symmetrical far-field beam shape, the 12-fold symmetric photonic quasi-crystal exhibits long-range order and short-range disorder and possesses semi-random properties, further increasing light extraction compared to traditional photonic crystals.

The researchers report effective quantum yields for the QD emitters reaching 123% for single QD species colour converters, and around 110% for a white blend of three commercially available QDs (emitting at 535, 585 and 630nm) achieving a quasi-perfect 6500 K D65 spectrum. They