Growth and Fabrication of UV LEDs
Tim Wernicke | Technische Universität Berlin, Institute of Solid State Physics
UV-LEDs are exciting light sources allowing for new design concepts and applications due to their small size ideal for beam and emission pattern shaping, their robustness ideal for use in harsh as well as sensitive environments and an emission wavelength that can be tailored to the specific application. This tutorial will provide the essential knowledge to understand the operation principles, fabrication processes and future development of UV-LEDs. First we will review the internal structure of LEDs and their basic functioning mechanisms. Then we will cover the fabrication of LEDs from the semiconductor crystal growth over LED chip fabrication to mounting. Finally we will discuss the novel concepts currently investigated in R&D labs to increase the output power and efficiency in different emission wavelength regions.
LEDs are semiconductor devices based on pn-diodes. The wide bandgap of the AlxGa1-xN material system allows light emission in the UV spectral region that is defined by during the crystal growth process the alloy composition and can reach wavelengths as short at 211 nm. For high emission output powers three main areas need to be optimized: (i) crystal defects in the semiconductor need to be reduced by improved substrate technologies and growth processes to minimize losses during the recombination of charge carriers. (ii) Transport and recombination processes need to be controlled by a proper design of the bandgap and doping. (ii) The escape from the LED-chip by reducing the absorption in the LED semiconductor layers and contacting materials and reducing reflections in the LED’s semiconductor layers. In all three areas basic principles, their practical realization and future concepts will be discussed.