Buried photonic crystals boost LED emission
Switching the position of LED photonic crystal lattices from above to below the active region can cut crystal defects, without losing their light extraction benefits.
That s the key finding of researchers from Samsung and the Gwangju Institute of Science and Technology in South Korea, who detail their results in a June 26 Applied Physics Letters paper. They fabricated a silica photonic crystal (PC) lattice in the n-doped GaN layer of an LED, rather than the more conventional p-doped GaN layer, largely to avoid damage from plasma etching.
The combination of reduced defects and improved extraction boosts external quantum efficiency by 70 percent compared to an equivalent LED without a PC lattice.
This figure, obtained using photoluminescence measurements, is the product of an accompanying 17 percent improvement in internal quantum efficiency and a 45 percent improvement in light extraction efficiency. When the two 300 µm x 300 µm devices' electroluminescence outputs were compared the optical output power at 20 mA was 33 percent higher for the PC LED.
Starting from a sapphire substrate, the Korean team grew its device to include a 130 nm silica layer on top of a 2 µm n-doped GaN layer. They etched the PC from the silica layer using hologram lithography to produce pillars that were also around 130 nm in diameter with a 230 nm period.
The usual plasma dry etching approach used to form the PC from p-doped GaN results in plasma damage to this layer, which degrades the electrical performance of the LED. This damage has been known to reach down far enough through the device to affect the multi-quantum well (MQW) layer. Employing hologram lithography of silica to produce the PC, before the MQW is grown, appears to have circumvented this problem.
The thicker n-GaN layer is better able to accommodate the optimal dimensions of a photonic crystal without disrupting the function of the LED than the thinner p-GaN layer. Also, positioning the silica layer low down in the epistructure allows it to act similarly to silica gratings used in the epitaxial lateral overgrowth method that results in high quality GaN crystal layers (see related stories).
“The increase in internal quantum efficiency can be attributed to the reduction of screw and edge-type threading dislocations in the n-GaN and MQW layer,” wrote Seong-Ju Park and his colleagues. The group also said that this improved crystal quality successfully reduces the leakage currents in the device that are often seen in photonic crystal LEDs.
“The light extraction efficiency of the LED with the embedded photonic crystal was markedly enhanced by the photonic crystal effect without degradation of electrical properties,” they wrote.
