We studied the influence of the substrate preparation and the growth conditions important to fabricate GaAs nanowires (NWs) with metal-organic vapor phase epitaxy. The growth parameters temperature, precursor partial pressures and growth duration were investigated. The definite choice of the V/III ratio enables NW length growth independent on the diameter. By investigating the temporal evolution of the GaAs-NW growth a diameter-dependent growth rate could be determined. Applying nanosphere lithography arranged GaAs-NW arrays were achieved. The NWs morphology and real structure was investigated using (high-resolution) transmission electron microscopy and selective area diffraction. The twin formation in GaAs NWs was investigated. A crystallographic model is presented. (c) 2006 Elsevier B.V. All rights reserved.