Epitaksiaaliset III-V puolijohdenanolangat

Semiconductor nanowires (NWs) offer versatile possibilities among future electronic and photonic devices. This thesis contains a review of several NW growth methods and the properties of the NWs. The purpose of the practical part of this thesis is to present growth of self-catalyzed GaAs NWs on lithography-free oxide patterns with a strong emphasis on droplet epitaxy and analysis of the structural and optical properties of the NWs. Furthermore, growth of GaAs/AlGaAs/GaAs core-shell heterostructure NWs is presented along with a study of their photonic properties. In terms of the epitaxial growth, focus is on the GaAs droplet epitaxy which is the initial step of the NW growth on lithography-free oxide patterns. Various different growth parameters and two different molecular beam epitaxy reactors were used in order to investigate their influence on the GaAs nanocrystal density and morphology. Uninten-tional Sb exposure was found to cause Si(111) surface roughening and repeatability problems in terms of the GaAs nanocrystal density. High-resolution x-ray diffractometry was used to obtain reciprocal space maps of the self-catalyzed GaAs NWs grown on lithography-free oxide patterns formed by droplet epitaxy. Together with the micro-photoluminescence spectroscopy results they showed that the NWs consist mainly of ZB GaAs except for the WZ segments formed during the crystallization of the Ga catalyst droplets. The micro-photoluminescence also showed impurity related emission possibly due to substrate originated Si atoms. Room temperature photoluminescence spectroscopy of the core-shell NWs showed the necessity of AlGaAs passivation as it increased the PL intensity from the NWs several orders of magnitude. Furthermore, thicknesses of the AlGaAs passivation shell and the GaAs cap protecting it from oxidizing were found to have a significant effect on sample aging. Micro-photoluminescence measurements conducted on a single GaAs/AlGaAs/GaAs core-shell NW showed longitudinal Fabry-Pérot modes. However, thicker NWs are required in order to obtain lasing. A broad understanding was obtained concerning the characteristics of the NWs grown via self-catalyzed method on lithography-free oxide patterns. Simultaneously important experience was accumulated on the possibilities of the NW characterization methods at Optoelectronics Research Centre