We report on the site-selective growth of >90% vertical GaAs nanowires (NWs) on Si (111) using self-assisted molecular beam epitaxy. The influences of growth parameters (pre-growth Ga opening time, V/III flux ratio) and processing conditions (reactive ion etching (RIE) and HF etching time) are investigated for different pitch lengths (200- 1000 nm) to achieve vertical NWs. The processing variables determine the removal of the native oxide layer and the contact angle of Ga-droplet inside the patterned hole that are critical to the vertical orientation of the NWs. Pre-growth Ga-opening time is found to be a crucial factor determining the size of the droplet in the patterned hole, while the V/III beam equivalent pressure (BEP) ratio influenced the occupancy of the holes due to the axial growth of NWs being group-V limited.
Ga assisted GaAs/GaAsSb core-shell structured nanowires were successfully grown on chemically etched p-type Si(111) substrate by molecular beam epitaxy (MBE). The morphology, structural and optical properties of the nanowires are found to be strongly influenced by the shell growth temperature and Sb% in the nanowires. The nanowires exhibit planar defects like twins and stacking faults, with more stacking faults and micro-twins found at the top section. Optical characteristics of the nanowires as measured by 4K photoluminescence (PL) exhibit a red shift to 1.2 eV with increasing Sb incorporation up to 12%. The Raman spectra of reference GaAs nanowires show TO and LO modes representative of the zinc blende structure at 291 cm<sup>-1</sup> and 267.8 cm<sup>-1</sup>, respectively. Red shifts of both modes in conjunction with corresponding asymmetrical peak broadening observed in X-ray diffraction with increasing Sb incorporation are attributed to enhanced strain and disorder within the nanostructures. Nanowires of similar Sb composition but grown at different shell temperatures reveal straight nanowires with better microstructural and optical quality when grown at higher growth temperatures. The presence of GaAs passivation layer significantly enhanced the PL intensity such that PL was observed even at room temperature.