Design, fabrication and characterisation of ultraviolet light-emitting diodes for biomedical applications

The efficiencies of ultraviolet (UV) light-emitting diodes (LEDs) significantly lag behind those of their blue counterparts, owing to a variety of electrical and optical challenges. There is a marked dip in performance in the wavelength range ∼320 − 340 nm, due to the difficulty in realising a high quality LED template at this composition range. This is precisely the target emission wavelength for LEDs developed in this project, with an application in fluorescence spectroscopy as part of a diagnostic endoscope. Thus, numerous strategies were investigated to improve the output power while eliminating parasitic emission. A novel technique using patterning and overgrowth was built upon to realise a thicker LED template with increased crystal quality. This can result in a significant boost in device performance by improving the internal quantum efficiency and current spreading. Furthermore, by introducing interdigitated contacts and a reduced mesa size, fabricated µLEDs demonstrated increased efficiency through enhanced current spreading. The first demonstration of a boron-containing UV LED was also presented, which indicated the potential benefits of small amounts of BN in the quantum wells. An increase in the wall plug efficiency was observed, although further work is required to understand the growth dynamics. The p-type region of a UV LED was a large focus of this thesis, owing to its poor conductivity limiting device performance. Polarisation doping was investigated as a means to increase the hole concentration. LED structures with various p-type doping conditions were tested, with the best performance observed in an LED with both polarisation doping and impurity doping (‘co-doped’). This is in contrast to studies of UVC LEDs, where polarisation doping only tends to result in the best performance. Additionally, long wavelength parasitic luminescence was eliminated without Mg doping, which is crucial for the biomedical application. A temperature-dependent study was also performed, which, along with room temperature simulations, offered insights into the device physics. Temperature-dependent Hall effect measurements were performed on structures with varied polarisation doping approaches (similar to the aforementioned LEDs) to gain a better understanding of the mechanisms at play. In a p-GaN reference sample, typical carrier freezeout was observed at lower temperatures. A detailed calculation of the carrier scattering mechanisms elucidated the temperature dependence of the mobility. However, polarisation doped structures (co-doped and polarisation doping only) exhibited more complex behaviours. While the samples’ conductivity versus temperature generally agreed with the corresponding temperature dependent LED operating voltages, the co-doped sample exhibited mixed signs of hopping conduction at low temperatures. The presence of this transport mechanism would tend to agree with recent studies examining such effects. Although there is still room for improvement in the output power of the UV LEDs realised in this work - thought to be mainly limited by the dislocation density - significant gains were made in LED performance. Narrow-linewidth UVA emission was achieved, along with the vital removal of parasitic long wavelength luminescence.

Grantor dc:publisher

University College Cork

Year dc:date.issued

2025

Author dc:creator

• Milner, Peter

Advisors dc:contributor.advisor

• Parbrook, Peter James
• Corbett, Brian

• Semiconductor
• Physics
• Ultraviolet
• III-nitrides
• Light-emitting diodes

Statement dc:rights

• © 2025, Peter Milner.

Licence dc:rights.uri

• https://creativecommons.org/licenses/by/4.0/

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/10468/18402

OAI identifier oai:identifier

oai:cora.ucc.ie:10468/18402