Abstract
Ultraviolet (UV) light emitters with large-energy photons are being developed for emerging biology and medicine applications such as sterilization, water/air purification, and medical treatments, as a replacement for mercury vapor lamps. Solid-state UV light-emitting diodes (LEDs) and lasers based on wide bandgap III-Nitrides offer compact sizes, wavelength tuning, long lifetimes, and sustainability over mercury vapor lamps. While visible LEDs and lasers with longer emission wavelengths find widespread commercial uses for solid-state lighting and display, UV emitters with AlGaN heterostructures struggle with poor external quantum efficiencies of typically less than 10%, which drop further when wavelengths get shorter. Therefore, it is extremely challenging to realize high-efficiency UV emitters despite the demand for compact UV light source, especially for disinfection and sterilization from the pandemic as those high-energy photons can break DNA/RNA bonds and deactivate virus/bacteria. Here, I will discuss our recent developments toward realization of high-efficiency UV LEDs and lasers.To enhance the radiative recombination rate (R sp) from the AlGaN quantum well (QW) active region, we had pursued novel QW design to overcome issues from the existence of valence subbands crossover. Specifically, the arrangement of the three energy-degenerate valence subbands - heavy hole (HH), light hole (LH), and the crystal-field split-off hole (CH) bands for high Al-content and low Al-content AlGaN QWs are completely different. The ordering of the valence bands strongly affects the optical matrix element via the oscillator strength, as well as the direction of the photon emission. Our work has proposed promising solution on the use of delta-QW design to address the valence subbands crossover issue, in order to significantly boost up the R sp. In addition, to enhance the extraction efficiency of UV emitters, we demonstrated the realization and characterization of top-down fabricated, electrically driven UV micropillar array LEDs emitting at 286 nm with a narrow, 9 nm linewidth and output power densities up to 35 mW/cm2. Top-down fabrication allows for formation of perfectly uniform arrays of micropillars with easily tunable diameter and pitch, which cannot be achieved with epitaxially grown nanowires without the use of selective area epitaxy. Dry etch damage induced surface losses were avoided through the use of a two-step etch process combining a Cl dry etch and a hydroxyl-based wet etch to form the micropillar arrays. We also revealed that the unique inverse-taper profile of our micropillars could produce significant enhancements in the extraction efficiency with the increased taper angle. Lastly, our developed III-Nitride wet etch by the use of hydroxyl-based chemicals such as potassium hydroxide (KOH) investigated the effects of temperature, concentration, and the damage recovery aspects of hydroxyl etching of III-Nitride nano and micro structures, leading to important smooth surface profiles for micropillar/nanowire LEDs and optimized mirror facets for ridge-emitting UV lasers.
Published Version
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