Thesis presented October 19, 2020

Abstract:
III-N semi-conductors, including GaN, AlN, InN and their alloys, are now firmly established as a current solution for solid state lighting and related applications due to their direct band gaps ranging from deep UV to IR (6,14 eV to 0,64 eV). Since the realization of InGaN/GaN quantum wells based blue LEDs, rewarded by the 2014 Nobel Prize in physics, InGaN based visible LEDs have emerged as a prime candidate for lighting applications. However, one of the main challenges for the fabrication of III-N based devices is the large lattice mismatch between the III-N epilayers and the available substrates. Consequently, a high density of extended defects is induced by plastic relaxation, drastically decreasing the LEDs’ efficiency.
Semiconductor nanowires are intensely studied for the realization of high efficacity axial heterostructures, due to the greatly eased elastic strain relaxation resulting from their large aspect ratio. With the aim of realizing red emitting InGaN based LEDs and overcome the green gap issue, this PhD work is mainly focused on the growth of InGaN/GaN nanowires and micro-columns with high In content (approximately 35%In). The growth is achieved by using plasma-assisted molecular beam epitaxy (PA-MBE) on [0001]-oriented GaN wires templates, ranging in diameter from nanowires to micro-columns to provide a better understanding of these key parameters.
Optimization of such devices requires an extensive understanding of GaN and InGaN growth on top of GaN wires. Firstly, GaN nanowires elongation mechanism was shown to be governed by peripheral nucleation. Depending on GaN growth conditions, the top crystallographic planes are tuned from semi-polar planes to c-planes, making possible a surface preparation for the growth of InGaN active region of LED devices. As a new approach, we propose in this work to perform the InGaN growth on a Ga-polar [0001] GaN top surface, in contrast to the InGaN grown on semi-polar facets reported in literature. Taking into account the In incorporation efficacity with respect to the growth orientation, the InGaN epitaxy on a [0001] top facet guarantees a unique InGaN composition on each wire for the realization of monochromatic LEDs. Moreover, the enlargement of GaN wires toward the top was shown to eliminate a parasitic short-circuit and reduce the wire diameters variability. The replacement of the InGaN section by a pyramidal InGaN/GaN superlattice was observed to reduce non-radiative recombinations and increase the InGaN luminescence intensity. Additionally, these structures increase the surface of current injection in the active region of the LEDs, improving their efficiency. Finally, electroluminescence from LEDs realized during this PhD is covering a large range of visible spectrum from 450 nm to 610 nm.

Keywords:
Nitride materials, light emitting diodes, nanowires, InGaN, molecular beam epitaxy

On-line thesis. ​