Shell-wise oxidation of nanocrystalline silicon observed by X-ray diffraction and TEM

Abstract

The oxidation of nanocrystalline silicon powder (average crystallite size of 87 Å), prepared by laser-driven pyrolysis of SiH₄, has been studied by X-ray diffraction and transmission electron microscopy. In a previous study the Si particles were shown to form perfect isolated near-spherical ‘single crystals’ that are stable up to 1073 K under high vacuum conditions. We have measured the size reduction of the Si nanoparticles in a flow of oxygen as a function of the degree of oxidation. The results are in agreement with model calculations for a shell-wise oxidation of silicon surface layers to amorphous silicon oxide. An oxidation to 50 vol.% implies a size reduction of 20% and tenfold improved visible photoluminescence intensity. Preliminary kinetic studies at 656 Kin oxygen indicate a self-limiting type of oxidation, which is reached for an oxide layer thickness of ≈6.8 Å