When stacking quantum wires yields polarized LEDs

February 08, 2018 // By Julien Happich
In their quest for the manufacture of efficient polarized microLEDs, a team of researchers from the University College of Cork, Ireland, have turned their attention to highly anisotropic wire-like nanostructures, which could combine good carrier confinement (limiting inefficient non-radiative recombinations paths) with a highly anisotropic structure, yielding a strong light emission anisotropy.

Their paper titled "Three-Dimensional Self-Assembled Columnar Arrays of AlInP Quantum Wires for Polarized Micrometer-Sized Amber Light Emitting Diodes" published in the ACS Photonics journal reviews prior art strategies in the optimization of carrier confinement, then unveils a novel light-emitting three-dimensional ordered and self-organized semiconductor system consisting of self-assembled multilayered in-plane AlInP/AlGaInP quantum wires (SMWRs) vertically stacked in regularly spaced columns.

Schematic of an SMWR LED device showing vertically
stacked nanowires (in red) forming self-aligned columns.

Grown through metalorganic vapour phase epitaxy (MOVPE), the stacked nanowires are about 0.5 to 2μm long and elongated orthogonally to the growth plane of the GaAs substrate, from 10 to 25nm wide but only a couple of nanometres thick. Several stacks of these elongated nanowires are concurrently grown side by side, forming vertical columns piling up to 200wires each.

Interestingly, the self-assembled wires were grown without pre-patterned templates (such as V-grooves), their design was optimized for the yellow-orange light emission (580−650nm) in the visible range.

Cross-section TEM of an SMWR with 60 layers,
including an (Al0.5Ga0.5)0.52In0.48P QW on top
of the SMWR (wire direction is perpendicular to
the plane).

Due to the one-dimensionality of their crystallographic structure, the wires possess strong emission anisotropy, with almost 80% of the emitted light polarized along the wire main axis, perpendicular to the growth direction. This makes the new devices particularly suited to modern display applications, as most if not all displays today use polarizers that lower their overall efficiency (blocking most of the originally emitted light).

Through theoretical simulations, the researchers then realized that the internal quantum efficiency of their SMWR LED could be largely improved by coupling a quantum well (QW) to the SMWR system so as to function as a carrier injector.