OPTICAL-PROPERTIES OF PASSIVATED SI NANOCRYSTALS AND SIOX NANOSTRUCTURES

Thin films of Si nanoclusters passivated with oxygen or hydrogen, with an average size of a few nanometers, have been synthesized by thermal vaporization of Si in an Ar buffer gas, followed by subsequent exposu re to oxygen or atomic hydrogen. High-resolution transmission electron microscopy and x-ray diffraction revealed that these nanoclusters wer e crystalline. However, during synthesis, if oxygen was the buffer gas , a network of amorphous Si oxide nanostructures (an-SiOx) with occasi onal embedded Si dots was formed. Ail samples showed strong infrared a nd/or visible photoluminescence (PL) with varying decay times from nan oseconds to microseconds depending on synthesis conditions. Absorption in the Si cores for surface passivated Si nanocrystals (nc-Si), but m ainly in oxygen related defect centers for an-SiOx, was observed by ph otoluminescence excitation spectroscopy. The visible components of PL spectra were noted to blueshift and broaden as the size of the nc-Si w as reduced. There were differences in PL spectra for hydrogen and oxyg en passivated nc-Si. Many common PL properties between oxygen passivat ed nc-Si and an-SiOx were observed. Our data can be explained by a mod el involving absorption between quantum confined states in the Si core s and emission for which the decay times are very sensitive to surface and/or interface states. The emission could involve a simple band-to- band recombination mechanism within the Si cores. The combined evidenc e of all of our experimental results suggests, however, that emission between surface or interface states is a more likely mechanism.