Nanoporous wire-like superstructure of silicon and silicon/germanium solid solution

Naturally grown silicon wire-like crystals have been studied by transmission electron microscopy of high-resolution, scanning tunneling microscopy (STM), ion and Auger spectroscopy. It is known that they are the smallest Si-based heterostructures consisting of a central core of bulk silicon and a nanoporous silicon envelope. Such a structure of the wire-like crystal is formed during its growth by two mechanisms: vapour–liquid–crystal (VLC) and vapour–crystal (VC). The core is created due to the fast growth according to VLC mechanisms, while appearance of the envelope corresponds to a slow thickening of the crystal by VC mechanism. Essential difference in the structure of these parts of the wire-like crystal is possible due to surface reconstruction of the thin wire-like core. It is also shown that morphology and thickness of the envelope depend strongly on the growth conditions.