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Sergi Cuesta Arcos

Development of AlGaN nanostructures for the fabrication of electron pumped UV lasers

Published on 13 December 2022
Thesis presented December 13, 2022

There is a growing demand for compact semiconductor-based ultraviolet lasers for application domains such as disinfection, medical treatments, non-line-of-sight communication, Lidar remote detection or 3D printing. This spectral region is mainly covered by excimer lasers, which come with many restrictions due to the use of corrosive halogen gases, or frequency conversion lasers, despite their limitation in terms of wavelength flexibility. Within this context, AlGaN is a promising material due to its direct wide band gap, band engineering capabilities and wavelength tunability. However, the performance of AlGaN-based laser diodes remains limited due to carrier injection problems. The high doping concentrations required to achieve p-type conductivity degrade the material quality and increase the absorption losses, resulting in a sharp increase of the lasing threshold for devices emitting below 370 nm. As an alternative, we propose to directly pump an active region based on AlGaN nanostructures with an electron beam, obviating any need of doping or ohmic contacts.
This thesis is a contribution to the development of AlGaN-based electron beam pumped ultraviolet lasers. AlGaN/GaN heterostructures were designed to be used as active region for electron beam pumped lasers operated with an acceleration voltage ≤ 10 kV and emitting at 355 nm. Separate confinement heterostructures consisting in an AlGaN/GaN multi-quantum-well embedded in an all-AlGaN waveguide were fabricated by plasma-assisted molecular beam epitaxy. We discuss the implementation of graded layers to improve the carrier collection under electron beam pumping, and their impact on the lasing threshold. We find that the lasing threshold is decorrelated from the radiative efficiency of the quantum wells, which is very sensitive to carrier localization. Lasing is demonstrated at room temperature under optical pumping for AlGaN/GaN laser bars of different cavity lengths, emitting at 355 nm. In view of the optical losses coming from cleaved facets, a two-step etching process was developed to fabricate optical cavities with high reflectivity facets, which reduced the optical lasing threshold to 100 kW/cm2 for a 0.3-mm-long cavity at room temperature. From this value, we estimate a lasing threshold for electron beam pumping around 240 kW/cm2 at room temperature. Finally, we present a preliminary study towards the fabrication of AlxGa1-xN/AlyGa1-yN heterostructures targeting emission at 280 nm for germicidal applications, applying the knowledge obtained from AlGaN/GaN structures.

AlGaN, UV, laser, electron pumping, optoelectronics, epitaxy

On-line thesis.