SHELL-WISE OXIDATION OF NANOCRYSTALLINE SILICON OBSERVED BY X-RAY-DIFFRACTION AND TEM

The oxidation of nanocrystalline silicon powder (average crystallite s ize of 87 Angstrom), prepared by laser-driven pyrolysis of SiH4, has b een studied by X-ray diffraction and transmission electron microscopy. In a previous study the Si particles were shown to form perfect isola ted near-spherical 'single crystals' that are stable up to 1073 K unde r high vacuum conditions. We have measured the size reduction of the S i nanoparticles in a flow of oxygen as a function of the degree of oxi dation. The results are in agreement with model calculations for a she ll-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 stu dies at 656 K in oxygen indicate a self-limiting type of oxidation, wh ich is reached for an oxide layer thickness of approximate to 6.8 Angs trom.