Observation of a nanocrystalline-to-amorphous phase transition in luminescent porous silicon

Abstract

Nanocrystalline silicon aggregates imbedded in a predominantly amorphous 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 identified the as-prepared porous silicon to be a mixed phase of amorphous and nanocrystalline silicon, with the nanocrystalline aggregates being randomly dispersed throughout the full thickness of a 1 μm thick amorphous layer and exhibiting a size distribution from 2 to 5 nm in diameter. The abundance of the nanocrystalline aggregates seems to decrease as the anodic etching proceeds and as the sample is irradiated by x rays at room temperature in ultrahigh vacuum. Valence band photoelectron 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 samples 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 nanocrystallites help to explain apparent contradictory descriptions of the nature of porous silicon.