Metal droplets can be formed in the tho ways (for AIIIBV compounds): 

1) catalyst metal deposition that differs from the growing compounds and do not incorporate (e.g. gold)

2) for the self-catalytic mode metal formed either from the underlying layer\substrate by overheating (and subsequent loss of  more volatile nonmetal element) or by droplet epitaxy mode, when metal deposited alone on the substrate.

Thank you Vasil'evskii and Martin. More specifically I want to know, (in case of foreign catalyst, e.g. Au for III-V growth) how the metal nanoparticles melt to liquid with temperature? Does the nanoparticles make alloy with the substrate or target material??