Nucleation phenomena in a-Si1-xGex films on SiO2 were examined in order to achieve low-temperature solid-phase crystallization. First, film thickness dependence of nucleation was investigated. The nucleation frequency per unit area increased with increasing film thickness, which was attributed to that bulk nucleation was dominant compared with interface or surface nucleation. Next, in order to enhance bulk nucleation in thin films, effects of the initial amorphicity modulation induced by Ar+ irradiation (25 keV, 1 X 10(16) cm(-2)) before annealing were investigated. The incubation time for nucleation in pre-irradiated samples during subsequent annealing at 600 degreesC was significantly decreased to 1/20 of that without pre-irradiation, which was tentatively assigned to enhancement of atomic arrangement induced by densification of a-Si1-xGex films. It is expected that optimization of the irradiation conditions will realize low-temperature (<500 degreesC) formation of poly-Si1-xGex films on SiO2.