Feb 8, 2018: In order to manufacture efficient polarized microLEDs, the University College of Cork, Ireland, researchers have demonstrated that highly anisotropic wire-like nanostructures can combine good carrier confinement with a highly anisotropic structure, and yield a strong light emission anisotropy.
This has been explained in the paper ‘Three-Dimensional Self-Assembled Columnar Arrays of AlInP Quantum Wires for Polarized Micrometer-Sized Amber Light Emitting Diodes’ published in the ACS Photonics journal.
A three-dimensional ordered and self-organized semiconductor system emitting highly polarized light in the yellow-orange visible range (580–650 nm) is presented, comprising self-assembled in-plane AlInP wires vertically stacked in regularly spaced columns. More than 200 wires per column without detectable defect formation could be stacked. Theoretical simulations and temperature-dependent photoluminescence provided a benchmark to engineer multilayered structures showing internal quantum efficiency at room temperature larger than comparable quantum wells emitting at similar wavelengths. Finally, proof-of-concept light-emitting diodes (LED) showed a high degree of light polarization and lower surface parasitic currents than comparable quantum well LEDs, providing an interesting perspective for high-efficiency polarized yellow-orange light-emitting devices.
The stacked nanowires of about 0.5 to 2μm long are grown through metalorganic vapour phase epitaxy (MOVPE), and elongated orthogonally to the growth plane of the GaAs substrate, from 10 to 25nm wide. The stacks of these elongated nanowires are grown side by side, as they form vertical columns and pile up to 200wires each.
The self-assembled wires were grown without pre-patterned templates, and their design was optimized for the yellow-orange light emission (580−650nm) in the visible range.
“In both devices only a small fraction of the injected current was actually passing through the bulk of the material. In a 50×50μm2 QW LED, only 4.4% of the total current was contributing to the emission, this value decreasing to 1.9% in a 20×20μm2 device. On the other hand, the value of Jbulk/JTOT in the SMWR LED was found to be 30% and 13% for the 50×50μm2 and 20×20μm2 device, respectively, almost 1 order of magnitude larger” reads the paper.
The researchers concluded that the higher the space of confinement and potential fluctuations experienced by carriers in the wires, compared to that in the QWs, reduces the carrier diffusion toward the LED surface before recombination. This leads them to believe that the hybrid QW-SMWR system could be further reduced in size while still achieving a higher current-to-light conversion efficiency. The prototype LED devices operated with a low turn-on voltage of about 1.6V, corresponding to a current density of 0.1A/cm2.
Source: University College of Cork