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.
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.
By contacting all the μLEDs, the researchers could perform a bulk analysis across approximately 1,300 μLEDs, but did not discard the possibility to control those μLEDs individually for better performance selectivity and to compensate for process inhomogeneity.
Ideally, for quantum information processing, they would like to use the μLEDs as perfectly indistinguishable sources of entangled-photons. Photon extraction efficiency was fairly low too, at around 1%, something the researchers ought to be improving using various tricks of the trade (including built-in material strain and electric fields).
Dr Emanuele Pelucchi, Head of Epitaxy and Physics of Nanostructures and a member of the Science Foundation Ireland-funded Irish Photonic Integration Centre (IPIC) at Tyndall National Institute in Cork sees his team’s results as key steps towards the realisation of integrated quantum photonic circuits for quantum computing processing tasks.
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