Thesis presented January 17, 2023

Abstract:
The recent SARS-COV2 pandemic has highlighted one of the many applications of UV radiation: sterilization. Aiming at replacing bulky and environmentally hazardous mercury lamps emitting at 255 nm, research is directed towards the development of solid-state solutions based on semiconductors. Aluminium gallium nitride (AlGaN) light-emitting diodes (LEDs) meet the requirements. In practice, the realization of such devices in planar geometry is limited by low electrical conductivity due to deep dopants and by difficult light extraction. In response, a promising alternative is being explored in my thesis: the nanowire geometry. My thesis work consisted of increasing the knowledge on the physics governing the current transport in AlN nanowire diodes grown by molecular beam epitaxy as well as on the growth techniques and technologies in order to allow the realization of deep UV LEDs.
Firstly, my work focused on the realization and characterization of active areas emitting in the deep UV. AlN nanowires containing ultra-thin GaN quantum wells were grown. After transmission electron microscopy observations and cathodoluminescence (CL) measurements, quantum wells with thicknesses between 1 and 4 GaN monolayers are identified and show emission between 239 and 304 nm. I also studied AlGaN nanowires with very low Ga compositions (< 1 %). The observed optical properties are very different from those of standard alloys with a spectrally broad emission, governed by a strong exciton localization on AlGaN clusters.
Secondly, I was interested in the n-type doping of AlN nanowires with silicon. Using current-voltage characteristics (I-V), the conductance of nanowires was measured. It exhibits a bell-shaped curve as a function of doping, typical of self-compensation phenomena. A detailed study of the electrical transport as a function of temperature also established that a significant part of the dopants remained in Al site with an ionisation energy of 75 meV, in contrast to the deep Si donor with an energy of 270 meV commonly reported.
Thirdly, I studied the electrical transport properties of AlN pn junctions for different doping range and designs, by I-V and electron beam induced current (EBIC) measurements. All diodes show typical rectifying behaviour of pn-junctions. However, the presence of a tunnel barrier limiting the injection of holes at high voltages (V > 6 V) has been demonstrated. Furthermore, by correlating I-V, EBIC, CL and electroluminescence (EL) measurements, optimal structure and growth parameters were identified.
Finally, I realized a broadband LED centered at 285 nm. The different LEDs tested show good reproducibility as well as promising diode behavior. The luminescence intensity is found to be proportional to the injected current and luminescence of defects remains very low even at very high voltage. An external quantum efficiency could be measured at about 5.10^-3 %, mainly limited by the difficult compromise between electrical injection and photon extraction.

Keywords:
Molecular beam epitaxy, UV emission, III- nitrides

On-line thesis.