Mid-infrared LED efficiency enhancement via the Purcell effect, W-Superlattices, and metalenses
Creators: name: Schrock, Katrina Nicole
Publisher: name: University of Iowa
Year: 2023
Subjects: public abstract

description: Mid-infrared (mid-IR) light emitting diodes (LEDs) have emerged as a viable replacement for thermal broadband sources in a variety of applications due to high bandwidth, and high radiance that does not require heating to the apparent temperature. However, wallplug efficiencies (WPEs) for these devices are below 1%. Both the internal quantum efficiency (IQE) of the device and the extraction efficiency of light are areas where these devices can be improved. The WPE of a device may be improved by engineering the material with which the LED is fabricated to suppress nonradiative recombination mechanisms, positioning emission regions to take advantage of the Purcell effect in a semi-cavity, and adding a metalens to the substrate side of the device, through which light is emitted, to improve extraction. The material with which the LED is fabricated may be engineered to remove states from near the bandgap, suppressing nonradiative Auger recombination. In comparison to a standard InAs/GaSb superlattice, a four-layer superlattice called the W-SL, which contains InAs, GaInSb, and AlAsSb layers, yielded a 2.7 times increase in IQE in optically pumped measurements. The increased confinement of holes in the W-SL structure does lead to concerns about carrier transport. Electrically pumped tests were also carried out, indicating a similar 3 to 4 times increase in radiative output and up to 5 times increase in WPE between similarly fabricated devices. Carrier transport concerns were also investigated through a series of samples with varying emission region thickness. This indicated that beyond approximately 200 nm thick active regions, the carrier transport limits device output.
description: Through positioning emission regions in a standing wave set up by the metal reflector in a semi-cavity, the Purcell effect enhances extraction efficiency as well as radiative recombination in the device. Power in the escape cone is increased through the effect. Thinner emission regions, which benefit the placement of these regions to take advantage of the Purcell effect, also cause an increase in nonradiative Auger recombination. A tradespace between these two effects was explored, indicating that the ideal emission region thickness for taking advantage of the Purcell effect enhancements with the Auger recombination in consideration varied based on the operating conditions of the device.
description: Metalenses can increase the extraction efficiency of the device by introducing a scattering phase gradient at the surface and altering the critical angle, allowing light to escape into air rather than be totally internally reflected. Simulations indicate that mesas of the same size as the metalens array can experience 2-4 times improvement, which was confirmed through optically pumped measurements that yielded 3 times enhancement. Additional confirmation involved unaligned electrically pumped samples with some mesas of the same size as the metalens, resulting in approximately 2 times enhancement. However, simulations also predict that shrinking the size of the mesa will improve enhancement rates, with an expected enhancement of 10 times for the smallest simulated mesa size. A chip was designed to test this hypothesis, and testing began with shrinking mesa sizes, paving the way for experimentally confirming the simulations.