Pyramidal micro-LEDs output entangled-photons for quantum computing

November 23, 2016 // By Julien Happich
Using scalable and foundry-compatible lithographic processes, researchers from the Tyndall National Institute in Cork have designed pyramid-shaped quantum dot light-emitting diodes able to produce entangled photons (whose actions and states are linked).

The micrometre-sized structures can be replicated in large arrays where each microLED could potentially be individually controlled electrically. This, report the researchers in Nature Photonics under the title "Selective carrier injection into patterned arrays of pyramidal quantum dots for entangled photon light-emitting diodes", could lead to their use and integration into photonics-based quantum computers where polarization-entangled photons could theoretically be used to encode quantum information.

The original manufacturing process includes the epitaxial growth of Quantum Dots (QDs) inside inverted pyramidal recesses originally patterned lithographically patterned on a (111)B GaAs substrate, each with a base approximately 5µm wide. Through numerous metalorganic vapour phase epitaxy (MOVPE) steps, several differently composed III–V (Al)GaAs layers and an InGaAs QD layer self-assemble inside the diminishing pyramidal recess.

According to the paper, complex epitaxial dynamics yield three embedded low bandgap vertical quantum wells (VQWs) and a vertical quantum wire (VQWR) of a diameter around 20nm, respectively.

Scanning electron microscopy image of a sample
with exposed pyramidal structures in an apex-up
Courtesy Roisin Kelly, Tyndall National Institute,
University College Cork

In addition, a thin InGaAs layer forms a group of interconnecting nanostructures: a flat QD at the central axis of the structure, three lateral quantum wires (LQWRs) and three lateral quantum wells (LQWs).

Through back-etching the original substrate, the researchers managed to revert to apex-up pyramidal structures, enhancing light extraction by several orders of magnitude compared to the inward-built embedded devices. Top and bottom contacts were then designed so as to selectively inject current in the single QD at the centre of the pyramid, always using self-aligning tricks that inherently made the devices easy to manufacture at scale.


(Top left) Scanning electron microscopy image of a sample right after the chemical etching step showing exposed pyramidal structures. (Bottom left) Sketch of a p–i–n junction μLED in cross-section view. (Right) a magnified region of the central part of a pyramid with a QD. The epitaxial layers comprise a representative structure with dominant AlGaAs alloys that form a vertical quantum wire (VQWR). Arrows indicate the injection current through the VQWR. Courtesy Roisin Kelly, Tyndall National Institute, University College Cork.