OBSERVATION OF A NANOCRYSTALLINE-TO-AMORPHOUS PHASE-TRANSITION IN LUMINESCENT POROUS SILICON

Nanocrystalline silicon aggregates imbedded in a predominantly amorpho us silicon layer have been observed in anodically etched p-Si(100) by using valence band x-ray photoelectron spectroscopy and lattice imaged high-resolution transmission electron microscopy (XTEM). XTEM has ide ntified the as-prepared porous silicon to be a mixed phase of amorphou s and nanocrystalline silicon, with the nanocrystalline aggregates bei ng randomly dispersed throughout the full thickness of a 1 mu m thick amorphous layer and exhibiting a size distribution from 2 to 5 nm in d iameter. The abundance of the nanocrystalline aggregates seems to decr ease as the anodic etching proceeds and as the sample is irradiated by x rays at room temperature in ultrahigh vacuum. Valence band photoele ctron measurements show evidence for a crystalline-to-amorphous phase transition induced by x radiation which may, in part, be activated by photoelectron stimulated hydrogen desorption. The x-ray irradiated sam ples also exhibit a significant reduction in photoluminescence yield, possibly caused by a reduction in the density of nanocrystallites. The observed mixed phase porous silicon and the metastability of the nano crystallites help to explain apparent contradictory descriptions of th e nature of porous silicon. (C) 1995 American Institute of Physics.