| Summary: | <p> UV-C LEDs in the range of 265–280 nm are needed to develop new disinfection and biotechnology applications. The market share for UV-C LED, versus UV-C lamps (Hg discharge and Xe), increased from 8% in 2008 ($240M) to 25% in 2018 ($810M). However, while low-pressure mercury lamps are ~30% energy efficient, the best commercial UV-C LEDs in the 265–280 nm range are ~2% energy efficient; InGaN blue LEDs are 80% energy efficient. Research on AlGaN LEDs has made significant progress into AlGaN material quality (including threading dislocation density and n-AlGaN electrical conductivity) but has lagged regarding light extraction efficiency. Light extraction from UV LEDs is limited by p-GaN absorption because of the lack of p-contact to p-AlGaN with AlN fraction (AlN content > 50%). Furthermore, AlGaN emitters at the 265–280 nm range emit 40–50% of their emissions as transverse magnetic (TM) waves, which are harder to extract than transverse electric (TE) waves. </p><p> SiC is an absorbing substrate that has been largely overlooked in developing UV-C LEDs, even though it has a small lattice mismatch with AlN (~1%) and a similar Wurtzite crystal structure and is more chemically stable. We demonstrate the first lateral thin-film flip-chip (TFFC) ultraviolet (UV) light-emitting diodes grown on SiC. UV LEDs were made at 310 nm, 298 nm, 278 nm, and 265 nm. </p><p> In this dissertation, we discuss the design, epi development, and fabrication of TFFC AlGaN LEDs with reflective p-contacts. The AlGaN:Mg growth temperature and the Mg doping profile in AlGaN:Mg were found to significantly impact the electroluminescence (EL) efficiency of the AlGaN MQWs. KOH roughening enhanced the light-extraction efficiency (LEE) by 100% and by ~180–200% for UV LEDs with 10 nm p-GaN and 5 nm p-GaN, respectively, without affecting the devices’ IV characteristics. The thin-film architecture led to a high LEE of about ~28–30% without LED encapsulation when used with LEDs with 5 nm p-GaN. The best light extraction efficiency in the literature is ~24% (without LED encapsulation) for a 275 nm flip-chip LED grown on PSS sapphire substrate. KOH roughening of AlN is discussed and is compared to KOH roughening of N-Face GaN. To advance LEE further, we attempted to develop LEDs with transparent current n-AlGaN spreading layers as well as highly doped n<sup>+</sup>-AlGaN tunnel junctions on top of UV-C LEDs. Reflective and ohmic n-contacts with low resistivities were developed for the n-Al<sub>.58</sub>Ga<sub>.42</sub>N regrown by MBE. Furthermore, a highly reflective MgF<sub>2</sub>/Al omnidirectional mirror was developed, which can be used with n-contact microgrid to further enhance the LEE in UV-C LEDs with a transparent tunnel junction. </p><p>
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