AlGaN growth and processing for high-efficiency UV LED on SiC
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AlGaN is among the ultra-wide band gap semiconductors that are projected to power a wide range of the next-generation electronic and photonic applications. It has been applied primarily for the development of germicidal UV LEDs that are important for virus disinfection and gained an extra importance in the era of COVID-19 pandemic, with proven efficacy against wide range of viruses - including SARS-CoV-2 - that rivals the most mature disinfection technologies. However, state-of-the-art deep UV LEDs are still lagging behind visible LEDs in terms of wall plug efficiency, lifetime and cost.
Our approach to tackle this issue relies on silicon carbide as a growth substrate in contrast to sapphire. First, the advantages gained by employing SiC as a substrate will be highlighted. Second, growth of Al-rich AlGaN on SiC by metalorganic chemical vapor deposition (MOCVD) will be the main focus of this dissertation, with emphasis on the AlGaN growth kinetics, doping and active region, Detailing simple, yet robust, solutions for AlGaN cracking, doping inefficiency and multiquantum wells (MQW) growth—solutions that enabled the heteroepitaxial growth of up to 5 µm-thick crack-free AlGaN films on SiC (among the highest reported), and the growth of highly conductive Al-rich n-AlGaN films up to 83% Al molar fraction with conductivities that match and exceed the best reported in the literature, as well as the growth of MQW with tunable and monochromatic emission in the UV range of maximal disinfection efficacy (250 nm to 280 nm).
In Al-rich AlGaN, the increased ionic character of Al-N bond leads to dissimilar kinetical behavior between Al and Ga adatoms, as well as precipitates several key challenging issues such as greater deviation from ideal unit cell structure, anisotropic light emission and inefficient doping. This work tackles these limits and explores growth and processing techniques to address these challenges, specifically; optimization of n-doped AlGaN will be illustrated with focus on the effect of the different growth parameters on the conductivity and ohmic contact formation. Studies on the impact of MOCVD growth conditions on the UV LEDs performance will be presented, emphasizing how the growth outcome can be influenced by other external parameters; starting from chamber geometry and conditioning to substrate polishing, preparation and pretreatment. Then, the device performance of UVC LED will be summarized along with new solutions to further improve the overall performance.