Growth of ordered arrays of heterostructure nanowires

This PhD thesis focused on the fabrication of GaAs/MnAs heterostructure nanowires on Si (111) substrates by molecular beam epitaxy. Nanowires were obtained by self-catalyzed vapor-liquid-solid growth. The possibility to crystallise the catalyst droplet was utilised for the fabrication of MnAs inclusions via sequential crystallisation of a liquid MnGa nanoparticle at the nanowire tip. Resumption of GaAs growth by formation of a new Ga nanoparticle after crystallisation of MnAs was attempted. GaAs/MnAs heterostructures have been grown both on non-patterned and on nanofabricated substrates. Various nanoimprint lithography patterning processes were tested. After continuous improvement of the imprinting procedure, high quality arrays of holes in SiO2 on Si (111) substrates were prepared. GaAs nanowire array growth protocols were optimised and homogeneous nanowire dimensions were obtained on cm-scale substrates together with high reproducibility between different samples. A high yield of vertical nanowire growth was demonstrated. MnAs quantum dots were first synthesised in GaAs nanowires on non-patterned substrates. Systematic study of Mn deposition revealed the existence of an upper limit for Mn incorporation in liquid Ga droplet, beyond which parasitic sidewall nanoparticles were found. MnAs was found to have a polycrystalline structure leading to corrugated surface at nanowire top which in some cases hindered the regrowth. GaAs nanowire regrowth following MnAs quantum dot formation did not reveal presence of Mn, suggesting its evaporation or dissolution after contact with liquid Ga nanoparticle. GaAs/MnAs and GaAs/MnAs/GaAs axial heterostructure nanowire arrays have been grown on diverse patterned substrates. Nanodots few tenth of nm thick were obtained. Confinement of MnAs nanocrystal in GaAs was attempted by means of the growth of a thin GaAs at low temperature. Optimisation of nanowire tips morphology to obtain surface quality suitable for Ga nucleation was performed, but satisfactory growth resumption together with detectable MnAs crystals was not obtained yet.