III-N semiconductors, including GaN, AlN, InN and their alloys, are the subject of growing interest for the development of optoelectronic devices. Their direct energy gap between the deep UV and IR (6.14 to 0.64 eV) allows a wide range of applications to be exploited. Since the realisation of the first blue LEDs based on InGaN/GaN quantum wells, resulting in the 2014 Nobel Prize in Physics, the InGaN alloy has been particularly studied for the development of visible LEDs. However, the epitaxial fabrication of III-N materials faces many difficulties, including the large lattice mismatch between different III-N materials, as well as with the available substrates. The thesis presented here focuses on the epitaxy of III-N nano- and microwires for the realisation of visible LED structures. In particular, this work seeks to optimise the efficiency of high In content LEDs (around 35% In) for emission in the red range. This thesis led to the filing of 4 patents and the integration of Marion GRUART in the high-tech company Aledia.