The influence of the substrate temperature and Mg doping on the morphological and optical properties of catalyst-free GaN nanowires grown by plasma-assisted molecular beam epitaxy on Si (111) has been investigated in a large temperaturerange between 665 ° C and 785 ° C. The density and wire sizes in Mg-doped nanowires are found to change with substrate temperature in a similar way as undoped nanowires. Between 725 ° C and 785 ° C a trimodal size distribution and an increase of the wire density from 5.0 x109 cm-2 to 9.5 x109 cm-2 were observed. Transmission electron microscopy indicates that the upper parts of the nanowires are free of structural defects. Raman spectroscopy measurements confirm a high crystalline quality of doped wires, with a line width of the E2 H of 3.3 cm-1 for samples grown at Ts= 785 ° C. Photoluminescence measurements show a strong influence of Mg on the emission properties, namely the increase of thedonor-acceptor pair emission and its phonon replicas.