Au-mediated Si nanowires (SiNW) have been grown at low temperatures (500-560 degrees C) on crystalline Si(100) and amorphous Si surfaces by means of low pressure chemical vapour deposition from Si2H6 in the 0.05-1.2 mbar range. The influence of the substrates on the nanowire (NW) growth and morphology has been investigated by means of x-ray photoelectron spectroscopy and scanning electron microscopy. No NW growth has been observed on the Au covered amorphous Si surfaces. On both substrates, the NW exhibit inhomogeneous sidewalls and a new morphology showing NW entrenchment which has been explained as a consequence of vapour-liquid-solid growth termination due to Au diffusion on the SiNW sidewalls.